1) Measurements from MESSENGER's Mercury Laser Altimeter revealed regions of unusually dark and bright surface deposits near Mercury's north pole.
2) These deposits are concentrated on pole-facing slopes and spatially correlate with areas exhibiting high radar backscatter, suggesting the presence of near-surface water ice.
3) Analysis found the bright deposits are consistent with surface water ice, while the dark regions likely overlie buried ice and provide thermal insulation, such as from complex organic materials delivered by comets or asteroids.
High-resolution UV/Optical/IR Imaging of Jupiter in 2016–2019Sérgio Sacani
Imaging observations of Jupiter with high spatial resolution were acquired beginning in 2016, with a cadence of 53
days to coincide with atmospheric observations of the Juno spacecraft during each perijove pass. The Wide Field
Camera 3 (WFC3) aboard the Hubble Space Telescope (HST) collected Jupiter images from 236 to 925 nm in 14
filters. The Near-Infrared Imager (NIRI) at Gemini North imaged Jovian thermal emission using a lucky-imaging
approach (co-adding the sharpest frames taken from a sequence of short exposures), using the M′ filter at 4.7 μm.
We discuss the data acquisition and processing and an archive collection that contains the processed WFC3 and
NIRI data (doi:10.17909/T94T1H). Zonal winds remain steady over time at most latitudes, but significant
evolution of the wind profile near 24°N in 2016 and near 15°S in 2017 was linked with convective superstorm
eruptions. Persistent mesoscale waves were seen throughout the 2016–2019 period. We link groups of lightning
flashes observed by the Juno team with water clouds in a large convective plume near 15°S and in cyclones near
35°N–55°N. Thermal infrared maps at the 10.8 micron wavelength obtained at the Very Large Telescope show
consistent high brightness temperature anomalies, despite a diversity of aerosol properties seen in the HST data.
Both WFC3 and NIRI imaging reveal depleted aerosols consistent with downwelling around the periphery of the
15°S storm, which was also observed by the Atacama Large Millimeter/submillimeter Array. NIRI imaging of
the Great Red Spot shows that locally reduced cloud opacity is responsible for dark features within the vortex. The
HST data maps multiple concentric polar hoods of high-latitude hazes.
Observation of Bose–Einstein condensates in an Earth-orbiting research labSérgio Sacani
Quantum mechanics governs the microscopic world, where low mass and momentum
reveal a natural wave–particle duality. Magnifying quantum behaviour to
macroscopic scales is a major strength of the technique of cooling and trapping
atomic gases, in which low momentum is engineered through extremely low
temperatures. Advances in this feld have achieved such precise control over atomic
systems that gravity, often negligible when considering individual atoms, has
emerged as a substantial obstacle. In particular, although weaker trapping felds
would allow access to lower temperatures1,2
, gravity empties atom traps that are too
weak. Additionally, inertial sensors based on cold atoms could reach better
sensitivities if the free-fall time of the atoms after release from the trap could be made
longer3
. Planetary orbit, specifcally the condition of perpetual free-fall, ofers to lift
cold-atom studies beyond such terrestrial limitations. Here we report production of
rubidium Bose–Einstein condensates (BECs) in an Earth-orbiting research laboratory,
the Cold Atom Lab. We observe subnanokelvin BECs in weak trapping potentials with
free-expansion times extending beyond one second, providing an initial
demonstration of the advantages ofered by a microgravity environment for
cold-atom experiments and verifying the successful operation of this facility. With
routine BEC production, continuing operations will support long-term investigations
of trap topologies unique to microgravity4,5
, atom-laser sources6
, few-body physics7,8
and pathfnding techniques for atom-wave interferometry9–12
A giant galaxy in the young Universe with a massive ringSérgio Sacani
In the local (redshift z ≈ 0) Universe, collisional ring galaxies make up only ~0.01% of galaxies1 and are formed by head-on galactic collisions that trigger radially propagating density waves2–4. These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe5–9. However, not much is known about distant (z > 0.1) collisional rings10–14. Here we present a detailed study of a ring galaxy at a look-back time of 10.8 Gyr (z = 2.19). Compared with our Milky Way, this galaxy has a similar stellar mass, but has a stellar half-light radius that is 1.5–2.2 times larger and is forming stars 50 times faster. The extended, dif- fuse stellar light outside the star-forming ring, combined with a radial velocity on the ring and an intruder galaxy nearby, provides evidence for this galaxy hosting a collisional ring. If the ring is secularly evolved15,16, the implied large bar in a giant disk would be inconsistent with the current understand- ing of the earliest formation of barred spirals17–21. Contrary to previous predictions10–12, this work suggests that massive col- lisional rings were as rare 11 Gyr ago as they are today. Our discovery offers a unique pathway for studying density waves in young galaxies, as well as constraining the cosmic evolution of spiral disks and galaxy groups.
NGTS-1b: A hot Jupiter transiting an M-dwarfSérgio Sacani
We present the discovery of NGTS-1b, a hot-Jupiter transiting an early M-dwarf
host (Teff,∗=3916 +71
−63 K) in a P = 2.647 d orbit discovered as part of the Next Generation
Transit Survey (NGTS). The planet has a mass of 0.812 +0.066
−0.075MJ and radius
of 1.33 +0.61
−0.33 RJ , making it the largest and most massive planet discovered transiting
any M-dwarf. NGTS-1b is the third transiting giant planet found around an M-dwarf,
reinforcing the notion that close-in gas giants can form and migrate similar to the
known population of hot Jupiters around solar type stars. The host star shows no
signs of activity, and the kinematics hint at the star being from the thick disk population.
With a deep (2.5%) transit around a K = 11.9 host, NGTS-1b will be a strong
candidate to probe giant planet composition around M-dwarfs via JWST transmission
spectroscopy.
Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents,
including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter (DM) and
the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio
Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields
cluster MACS J0416.1-2403 (z = 0.396), which consists of NE and SW subclusters whose cores are separated on
the sky by ∼250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a
cool core. Approximately 450 kpc south–southwest of the SW subcluster, we detect a density discontinuity that
corresponds to a compression factor of ∼1.5. The discontinuity was most likely caused by the interaction of the
SW subcluster with a less massive structure detected in the lensing maps SW of the subclusterʼs center. For both
the NE and the SW subclusters, the DM and the gas components are well-aligned, suggesting that MACS J0416.1-
2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The
halo has a 1.4 GHz power of (1.3 ± 0.3) × 1024WHz−1, which is somewhat lower than expected based on the
X-ray luminosity of the cluster if the spectrum of the halo is not ultra-steep. We suggest that we are either
witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.
The Next Generation Transit Survey (NGTS)Sérgio Sacani
We describe the Next Generation Transit Survey (NGTS), which is a ground-based
project searching for transiting exoplanets orbiting bright stars. NGTS builds on the
legacy of previous surveys, most notably WASP, and is designed to achieve higher
photometric precision and hence find smaller planets than have previously been de-
tected from the ground. It also operates in red light, maximising sensitivity to late
K and early M dwarf stars. The survey specifications call for photometric precision
of 0.1 per cent in red light over an instantaneous field of view of 100 square degrees,
enabling the detection of Neptune-sized exoplanets around Sun-like stars and super-
Earths around M dwarfs. The survey is carried out with a purpose-built facility at
Cerro Paranal, Chile, which is the premier site of the European Southern Observatory
(ESO). An array of twelve 20 cm f/2.8 telescopes fitted with back-illuminated deep-
depletion CCD cameras are used to survey fields intensively at intermediate Galactic
latitudes. The instrument is also ideally suited to ground-based photometric follow-up
of exoplanet candidates from space telescopes such as Gaia, TESS and PLATO. We
present observations that combine precise autoguiding and the superb observing con-
ditions at Paranal to provide routine photometric precision of 0.1 per cent in 1 hour
for stars with I-band magnitudes brighter than 13. We describe the instrument and
data analysis methods as well as the status of the survey, which achieved first light
in 2015 and began full survey operations in 2016. NGTS data will be made publicly
available through the ESO archive.
The discovery of_lensed_radio_and_x-ray_sources_behind_the_frontier_fields_cl...Sérgio Sacani
We report on high-resolution JVLA and Chandra observations of the Hubble Space Telescope (HST) Frontier Cluster
MACSJ0717.5+3745. MACSJ0717.5+3745 offers the largest contiguous magnified area of any known cluster,
making it a promising target to search for lensed radio and X-ray sources. With the high-resolution 1.0–6.5 GHz
JVLA imaging in A and B configuration, we detect a total of 51 compact radio sources within the area covered by the
HST imaging. Within this sample, we find sevenlensed sources with amplification factors larger than two. None of
these sources are identified as multiply lensed. Based on the radio luminosities, the majority of these sources are
likely star-forming galaxies with star-formation rates (SFRs) of 10–50 M: yr−1 located at 1 1 z 1 2. Two of the
lensed radio sources are also detected in the Chandra image of the cluster. These two sources are likely active galactic
nuclei, given their 2–10 keV X-ray luminosities of ∼1043–44 erg s−1. From the derived radio luminosity function, we
find evidence for an increase in the number density of radio sources at 0.6 z 2.0, compared to a z 0.3 sample.
Our observations indicate that deep radio imaging of lensing clusters can be used to study star-forming galaxies, with
SFRs as low as ∼10Me yr−1, at the peak of cosmic star formation history.
High-resolution UV/Optical/IR Imaging of Jupiter in 2016–2019Sérgio Sacani
Imaging observations of Jupiter with high spatial resolution were acquired beginning in 2016, with a cadence of 53
days to coincide with atmospheric observations of the Juno spacecraft during each perijove pass. The Wide Field
Camera 3 (WFC3) aboard the Hubble Space Telescope (HST) collected Jupiter images from 236 to 925 nm in 14
filters. The Near-Infrared Imager (NIRI) at Gemini North imaged Jovian thermal emission using a lucky-imaging
approach (co-adding the sharpest frames taken from a sequence of short exposures), using the M′ filter at 4.7 μm.
We discuss the data acquisition and processing and an archive collection that contains the processed WFC3 and
NIRI data (doi:10.17909/T94T1H). Zonal winds remain steady over time at most latitudes, but significant
evolution of the wind profile near 24°N in 2016 and near 15°S in 2017 was linked with convective superstorm
eruptions. Persistent mesoscale waves were seen throughout the 2016–2019 period. We link groups of lightning
flashes observed by the Juno team with water clouds in a large convective plume near 15°S and in cyclones near
35°N–55°N. Thermal infrared maps at the 10.8 micron wavelength obtained at the Very Large Telescope show
consistent high brightness temperature anomalies, despite a diversity of aerosol properties seen in the HST data.
Both WFC3 and NIRI imaging reveal depleted aerosols consistent with downwelling around the periphery of the
15°S storm, which was also observed by the Atacama Large Millimeter/submillimeter Array. NIRI imaging of
the Great Red Spot shows that locally reduced cloud opacity is responsible for dark features within the vortex. The
HST data maps multiple concentric polar hoods of high-latitude hazes.
Observation of Bose–Einstein condensates in an Earth-orbiting research labSérgio Sacani
Quantum mechanics governs the microscopic world, where low mass and momentum
reveal a natural wave–particle duality. Magnifying quantum behaviour to
macroscopic scales is a major strength of the technique of cooling and trapping
atomic gases, in which low momentum is engineered through extremely low
temperatures. Advances in this feld have achieved such precise control over atomic
systems that gravity, often negligible when considering individual atoms, has
emerged as a substantial obstacle. In particular, although weaker trapping felds
would allow access to lower temperatures1,2
, gravity empties atom traps that are too
weak. Additionally, inertial sensors based on cold atoms could reach better
sensitivities if the free-fall time of the atoms after release from the trap could be made
longer3
. Planetary orbit, specifcally the condition of perpetual free-fall, ofers to lift
cold-atom studies beyond such terrestrial limitations. Here we report production of
rubidium Bose–Einstein condensates (BECs) in an Earth-orbiting research laboratory,
the Cold Atom Lab. We observe subnanokelvin BECs in weak trapping potentials with
free-expansion times extending beyond one second, providing an initial
demonstration of the advantages ofered by a microgravity environment for
cold-atom experiments and verifying the successful operation of this facility. With
routine BEC production, continuing operations will support long-term investigations
of trap topologies unique to microgravity4,5
, atom-laser sources6
, few-body physics7,8
and pathfnding techniques for atom-wave interferometry9–12
A giant galaxy in the young Universe with a massive ringSérgio Sacani
In the local (redshift z ≈ 0) Universe, collisional ring galaxies make up only ~0.01% of galaxies1 and are formed by head-on galactic collisions that trigger radially propagating density waves2–4. These striking systems provide key snapshots for dissecting galactic disks and are studied extensively in the local Universe5–9. However, not much is known about distant (z > 0.1) collisional rings10–14. Here we present a detailed study of a ring galaxy at a look-back time of 10.8 Gyr (z = 2.19). Compared with our Milky Way, this galaxy has a similar stellar mass, but has a stellar half-light radius that is 1.5–2.2 times larger and is forming stars 50 times faster. The extended, dif- fuse stellar light outside the star-forming ring, combined with a radial velocity on the ring and an intruder galaxy nearby, provides evidence for this galaxy hosting a collisional ring. If the ring is secularly evolved15,16, the implied large bar in a giant disk would be inconsistent with the current understand- ing of the earliest formation of barred spirals17–21. Contrary to previous predictions10–12, this work suggests that massive col- lisional rings were as rare 11 Gyr ago as they are today. Our discovery offers a unique pathway for studying density waves in young galaxies, as well as constraining the cosmic evolution of spiral disks and galaxy groups.
NGTS-1b: A hot Jupiter transiting an M-dwarfSérgio Sacani
We present the discovery of NGTS-1b, a hot-Jupiter transiting an early M-dwarf
host (Teff,∗=3916 +71
−63 K) in a P = 2.647 d orbit discovered as part of the Next Generation
Transit Survey (NGTS). The planet has a mass of 0.812 +0.066
−0.075MJ and radius
of 1.33 +0.61
−0.33 RJ , making it the largest and most massive planet discovered transiting
any M-dwarf. NGTS-1b is the third transiting giant planet found around an M-dwarf,
reinforcing the notion that close-in gas giants can form and migrate similar to the
known population of hot Jupiters around solar type stars. The host star shows no
signs of activity, and the kinematics hint at the star being from the thick disk population.
With a deep (2.5%) transit around a K = 11.9 host, NGTS-1b will be a strong
candidate to probe giant planet composition around M-dwarfs via JWST transmission
spectroscopy.
Merging galaxy clusters leave long-lasting signatures on the baryonic and non-baryonic cluster constituents,
including shock fronts, cold fronts, X-ray substructure, radio halos, and offsets between the dark matter (DM) and
the gas components. Using observations from Chandra, the Jansky Very Large Array, the Giant Metrewave Radio
Telescope, and the Hubble Space Telescope, we present a multiwavelength analysis of the merging Frontier Fields
cluster MACS J0416.1-2403 (z = 0.396), which consists of NE and SW subclusters whose cores are separated on
the sky by ∼250 kpc. We find that the NE subcluster has a compact core and hosts an X-ray cavity, yet it is not a
cool core. Approximately 450 kpc south–southwest of the SW subcluster, we detect a density discontinuity that
corresponds to a compression factor of ∼1.5. The discontinuity was most likely caused by the interaction of the
SW subcluster with a less massive structure detected in the lensing maps SW of the subclusterʼs center. For both
the NE and the SW subclusters, the DM and the gas components are well-aligned, suggesting that MACS J0416.1-
2403 is a pre-merging system. The cluster also hosts a radio halo, which is unusual for a pre-merging system. The
halo has a 1.4 GHz power of (1.3 ± 0.3) × 1024WHz−1, which is somewhat lower than expected based on the
X-ray luminosity of the cluster if the spectrum of the halo is not ultra-steep. We suggest that we are either
witnessing the birth of a radio halo, or have discovered a rare ultra-steep spectrum halo.
The Next Generation Transit Survey (NGTS)Sérgio Sacani
We describe the Next Generation Transit Survey (NGTS), which is a ground-based
project searching for transiting exoplanets orbiting bright stars. NGTS builds on the
legacy of previous surveys, most notably WASP, and is designed to achieve higher
photometric precision and hence find smaller planets than have previously been de-
tected from the ground. It also operates in red light, maximising sensitivity to late
K and early M dwarf stars. The survey specifications call for photometric precision
of 0.1 per cent in red light over an instantaneous field of view of 100 square degrees,
enabling the detection of Neptune-sized exoplanets around Sun-like stars and super-
Earths around M dwarfs. The survey is carried out with a purpose-built facility at
Cerro Paranal, Chile, which is the premier site of the European Southern Observatory
(ESO). An array of twelve 20 cm f/2.8 telescopes fitted with back-illuminated deep-
depletion CCD cameras are used to survey fields intensively at intermediate Galactic
latitudes. The instrument is also ideally suited to ground-based photometric follow-up
of exoplanet candidates from space telescopes such as Gaia, TESS and PLATO. We
present observations that combine precise autoguiding and the superb observing con-
ditions at Paranal to provide routine photometric precision of 0.1 per cent in 1 hour
for stars with I-band magnitudes brighter than 13. We describe the instrument and
data analysis methods as well as the status of the survey, which achieved first light
in 2015 and began full survey operations in 2016. NGTS data will be made publicly
available through the ESO archive.
The discovery of_lensed_radio_and_x-ray_sources_behind_the_frontier_fields_cl...Sérgio Sacani
We report on high-resolution JVLA and Chandra observations of the Hubble Space Telescope (HST) Frontier Cluster
MACSJ0717.5+3745. MACSJ0717.5+3745 offers the largest contiguous magnified area of any known cluster,
making it a promising target to search for lensed radio and X-ray sources. With the high-resolution 1.0–6.5 GHz
JVLA imaging in A and B configuration, we detect a total of 51 compact radio sources within the area covered by the
HST imaging. Within this sample, we find sevenlensed sources with amplification factors larger than two. None of
these sources are identified as multiply lensed. Based on the radio luminosities, the majority of these sources are
likely star-forming galaxies with star-formation rates (SFRs) of 10–50 M: yr−1 located at 1 1 z 1 2. Two of the
lensed radio sources are also detected in the Chandra image of the cluster. These two sources are likely active galactic
nuclei, given their 2–10 keV X-ray luminosities of ∼1043–44 erg s−1. From the derived radio luminosity function, we
find evidence for an increase in the number density of radio sources at 0.6 z 2.0, compared to a z 0.3 sample.
Our observations indicate that deep radio imaging of lensing clusters can be used to study star-forming galaxies, with
SFRs as low as ∼10Me yr−1, at the peak of cosmic star formation history.
The Variable Detection of Atmospheric Escape around the Young, Hot Neptune AU...Sérgio Sacani
Photoevaporation is a potential explanation for several features within exoplanet demographics. Atmospheric escape
observed in young Neptune-sized exoplanets can provide insight into and characterize which mechanisms drive this
evolution and at what times they dominate. AU Mic b is one such exoplanet, slightly larger than Neptune (4.19 R⊕).
It closely orbits a 23 Myr pre-main-sequence M dwarf with an orbital period of 8.46 days. We obtained two visits of
AU Mic b at Lyα with Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph. One flare within the
first HST visit is characterized and removed from our search for a planetary transit. We present a nondetection in our
first visit, followed by the detection of escaping neutral hydrogen ahead of the planet in our second visit. The outflow
absorbed ∼30% of the star’s Lyα blue wing 2.5 hr before the planet’s white-light transit. We estimate that the
highest-velocity escaping material has a column density of 1013.96 cm−2 and is moving 61.26 km s−1 away from the
host star. AU Mic b’s large high-energy irradiation could photoionize its escaping neutral hydrogen in 44 minutes,
rendering it temporarily unobservable. Our time-variable Lyα transit ahead of AU Mic b could also be explained by
an intermediate stellar wind strength from AU Mic that shapes the escaping material into a leading tail. Future Lyα
observations of this system will confirm and characterize the unique variable nature of its Lyα transit, which,
combined with modeling, will tune the importance of stellar wind and photoionization.
We present long-baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations of
the 870 m continuum emission from the nearest gas-rich protoplanetary disk, around TW Hya, that
trace millimeter-sized particles down to spatial scales as small as 1 AU (20 mas). These data reveal
a series of concentric ring-shaped substructures in the form of bright zones and narrow dark annuli
(1{6AU) with modest contrasts (5{30%). We associate these features with concentrations of solids
that have had their inward radial drift slowed or stopped, presumably at local gas pressure maxima.
No signicant non-axisymmetric structures are detected. Some of the observed features occur near
temperatures that may be associated with the condensation fronts of major volatile species, but the
relatively small brightness contrasts may also be a consequence of magnetized disk evolution (the
so-called zonal
ows). Other features, particularly a narrow dark annulus located only 1 AU from the
star, could indicate interactions between the disk and young planets. These data signal that ordered
substructures on AU scales can be common, fundamental factors in disk evolution, and that high
resolution microwave imaging can help characterize them during the epoch of planet formation.
Keywords: protoplanetary disks | planet-disk interactions | stars: individual (TW Hydrae)
XUE: Molecular Inventory in the Inner Region of an Extremely Irradiated Proto...Sérgio Sacani
We present the first results of the eXtreme UV Environments (XUE) James Webb Space Telescope (JWST)
program, which focuses on the characterization of planet-forming disks in massive star-forming regions. These
regions are likely representative of the environment in which most planetary systems formed. Understanding the
impact of environment on planet formation is critical in order to gain insights into the diversity of the observed
exoplanet populations. XUE targets 15 disks in three areas of NGC 6357, which hosts numerous massive OB stars,
including some of the most massive stars in our Galaxy. Thanks to JWST, we can, for the first time, study the effect
of external irradiation on the inner (<10 au), terrestrial-planet-forming regions of protoplanetary disks. In this study,
we report on the detection of abundant water, CO, 12CO2, HCN, and C2H2 in the inner few au of XUE 1, a highly
irradiated disk in NGC 6357. In addition, small, partially crystalline silicate dust is present at the disk surface. The
derived column densities, the oxygen-dominated gas-phase chemistry, and the presence of silicate dust are
surprisingly similar to those found in inner disks located in nearby, relatively isolated low-mass star-forming
regions. Our findings imply that the inner regions of highly irradiated disks can retain similar physical and chemical
conditions to disks in low-mass star-forming regions, thus broadening the range of environments with similar
conditions for inner disk rocky planet formation to the most extreme star-forming regions in our Galaxy.
Similar to Bright and dark_polar_deposits_on_mercury_evidence_for_surface_volatiles (20)
Observation of Io’s Resurfacing via Plume Deposition Using Ground-based Adapt...Sérgio Sacani
Since volcanic activity was first discovered on Io from Voyager images in 1979, changes
on Io’s surface have been monitored from both spacecraft and ground-based telescopes.
Here, we present the highest spatial resolution images of Io ever obtained from a groundbased telescope. These images, acquired by the SHARK-VIS instrument on the Large
Binocular Telescope, show evidence of a major resurfacing event on Io’s trailing hemisphere. When compared to the most recent spacecraft images, the SHARK-VIS images
show that a plume deposit from a powerful eruption at Pillan Patera has covered part
of the long-lived Pele plume deposit. Although this type of resurfacing event may be common on Io, few have been detected due to the rarity of spacecraft visits and the previously low spatial resolution available from Earth-based telescopes. The SHARK-VIS instrument ushers in a new era of high resolution imaging of Io’s surface using adaptive
optics at visible wavelengths.
Earliest Galaxies in the JADES Origins Field: Luminosity Function and Cosmic ...Sérgio Sacani
We characterize the earliest galaxy population in the JADES Origins Field (JOF), the deepest
imaging field observed with JWST. We make use of the ancillary Hubble optical images (5 filters
spanning 0.4−0.9µm) and novel JWST images with 14 filters spanning 0.8−5µm, including 7 mediumband filters, and reaching total exposure times of up to 46 hours per filter. We combine all our data
at > 2.3µm to construct an ultradeep image, reaching as deep as ≈ 31.4 AB mag in the stack and
30.3-31.0 AB mag (5σ, r = 0.1” circular aperture) in individual filters. We measure photometric
redshifts and use robust selection criteria to identify a sample of eight galaxy candidates at redshifts
z = 11.5 − 15. These objects show compact half-light radii of R1/2 ∼ 50 − 200pc, stellar masses of
M⋆ ∼ 107−108M⊙, and star-formation rates of SFR ∼ 0.1−1 M⊙ yr−1
. Our search finds no candidates
at 15 < z < 20, placing upper limits at these redshifts. We develop a forward modeling approach to
infer the properties of the evolving luminosity function without binning in redshift or luminosity that
marginalizes over the photometric redshift uncertainty of our candidate galaxies and incorporates the
impact of non-detections. We find a z = 12 luminosity function in good agreement with prior results,
and that the luminosity function normalization and UV luminosity density decline by a factor of ∼ 2.5
from z = 12 to z = 14. We discuss the possible implications of our results in the context of theoretical
models for evolution of the dark matter halo mass function.
THE IMPORTANCE OF MARTIAN ATMOSPHERE SAMPLE RETURN.Sérgio Sacani
The return of a sample of near-surface atmosphere from Mars would facilitate answers to several first-order science questions surrounding the formation and evolution of the planet. One of the important aspects of terrestrial planet formation in general is the role that primary atmospheres played in influencing the chemistry and structure of the planets and their antecedents. Studies of the martian atmosphere can be used to investigate the role of a primary atmosphere in its history. Atmosphere samples would also inform our understanding of the near-surface chemistry of the planet, and ultimately the prospects for life. High-precision isotopic analyses of constituent gases are needed to address these questions, requiring that the analyses are made on returned samples rather than in situ.
Multi-source connectivity as the driver of solar wind variability in the heli...Sérgio Sacani
The ambient solar wind that flls the heliosphere originates from multiple
sources in the solar corona and is highly structured. It is often described
as high-speed, relatively homogeneous, plasma streams from coronal
holes and slow-speed, highly variable, streams whose source regions are
under debate. A key goal of ESA/NASA’s Solar Orbiter mission is to identify
solar wind sources and understand what drives the complexity seen in the
heliosphere. By combining magnetic feld modelling and spectroscopic
techniques with high-resolution observations and measurements, we show
that the solar wind variability detected in situ by Solar Orbiter in March
2022 is driven by spatio-temporal changes in the magnetic connectivity to
multiple sources in the solar atmosphere. The magnetic feld footpoints
connected to the spacecraft moved from the boundaries of a coronal hole
to one active region (12961) and then across to another region (12957). This
is refected in the in situ measurements, which show the transition from fast
to highly Alfvénic then to slow solar wind that is disrupted by the arrival of
a coronal mass ejection. Our results describe solar wind variability at 0.5 au
but are applicable to near-Earth observatories.
Gliese 12 b: A Temperate Earth-sized Planet at 12 pc Ideal for Atmospheric Tr...Sérgio Sacani
Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the
atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets
receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric
composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet
transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period (Porb) of 12.76 days.
The planet, Gliese 12 b, was initially identified as a candidate with an ambiguous Porb from TESS data. We
confirmed the transit signal and Porb using ground-based photometry with MuSCAT2 and MuSCAT3, and
validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as
well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope
and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host
star is inactive, with an X-ray-to-bolometric luminosity ratio of log 5.7 L L X bol » - . Joint analysis of the light
curves and RV measurements revealed that Gliese 12 b has a radius of 0.96 ± 0.05 R⊕,a3σ mass upper limit of
3.9 M⊕, and an equilibrium temperature of 315 ± 6 K assuming zero albedo. The transmission spectroscopy metric
(TSM) value of Gliese 12 b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12 b to the small
list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.
Gliese 12 b, a temperate Earth-sized planet at 12 parsecs discovered with TES...Sérgio Sacani
We report on the discovery of Gliese 12 b, the nearest transiting temperate, Earth-sized planet found to date. Gliese 12 is a
bright (V = 12.6 mag, K = 7.8 mag) metal-poor M4V star only 12.162 ± 0.005 pc away from the Solar system with one of the
lowest stellar activity levels known for M-dwarfs. A planet candidate was detected by TESS based on only 3 transits in sectors
42, 43, and 57, with an ambiguity in the orbital period due to observational gaps. We performed follow-up transit observations
with CHEOPS and ground-based photometry with MINERVA-Australis, SPECULOOS, and Purple Mountain Observatory,
as well as further TESS observations in sector 70. We statistically validate Gliese 12 b as a planet with an orbital period of
12.76144 ± 0.00006 d and a radius of 1.0 ± 0.1 R⊕, resulting in an equilibrium temperature of ∼315 K. Gliese 12 b has excellent
future prospects for precise mass measurement, which may inform how planetary internal structure is affected by the stellar
compositional environment. Gliese 12 b also represents one of the best targets to study whether Earth-like planets orbiting cool
stars can retain their atmospheres, a crucial step to advance our understanding of habitability on Earth and across the galaxy.
The importance of continents, oceans and plate tectonics for the evolution of...Sérgio Sacani
Within the uncertainties of involved astronomical and biological parameters, the Drake Equation
typically predicts that there should be many exoplanets in our galaxy hosting active, communicative
civilizations (ACCs). These optimistic calculations are however not supported by evidence, which is
often referred to as the Fermi Paradox. Here, we elaborate on this long-standing enigma by showing
the importance of planetary tectonic style for biological evolution. We summarize growing evidence
that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern
plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated
emergence and evolution of complex species. We further suggest that both continents and oceans
are required for ACCs because early evolution of simple life must happen in water but late evolution
of advanced life capable of creating technology must happen on land. We resolve the Fermi Paradox
(1) by adding two additional terms to the Drake Equation: foc
(the fraction of habitable exoplanets
with significant continents and oceans) and fpt
(the fraction of habitable exoplanets with significant
continents and oceans that have had plate tectonics operating for at least 0.5 Ga); and (2) by
demonstrating that the product of foc
and fpt
is very small (< 0.00003–0.002). We propose that the lack
of evidence for ACCs reflects the scarcity of long-lived plate tectonics and/or continents and oceans on
exoplanets with primitive life.
A Giant Impact Origin for the First Subduction on EarthSérgio Sacani
Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. Itremains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact(MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to theaccumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, withsome of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here,we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subductioninitiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMBtemperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements inLLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications forunderstanding the diverse tectonic regimes of rocky planets.
Climate extremes likely to drive land mammal extinction during next supercont...Sérgio Sacani
Mammals have dominated Earth for approximately 55 Myr thanks to their
adaptations and resilience to warming and cooling during the Cenozoic. All
life will eventually perish in a runaway greenhouse once absorbed solar
radiation exceeds the emission of thermal radiation in several billions of
years. However, conditions rendering the Earth naturally inhospitable to
mammals may develop sooner because of long-term processes linked to
plate tectonics (short-term perturbations are not considered here). In
~250 Myr, all continents will converge to form Earth’s next supercontinent,
Pangea Ultima. A natural consequence of the creation and decay of Pangea
Ultima will be extremes in pCO2 due to changes in volcanic rifting and
outgassing. Here we show that increased pCO2, solar energy (F⨀;
approximately +2.5% W m−2 greater than today) and continentality (larger
range in temperatures away from the ocean) lead to increasing warming
hostile to mammalian life. We assess their impact on mammalian
physiological limits (dry bulb, wet bulb and Humidex heat stress indicators)
as well as a planetary habitability index. Given mammals’ continued survival,
predicted background pCO2 levels of 410–816 ppm combined with increased
F⨀ will probably lead to a climate tipping point and their mass extinction.
The results also highlight how global landmass configuration, pCO2 and F⨀
play a critical role in planetary habitability.
Constraints on Neutrino Natal Kicks from Black-Hole Binary VFTS 243Sérgio Sacani
The recently reported observation of VFTS 243 is the first example of a massive black-hole binary
system with negligible binary interaction following black-hole formation. The black-hole mass (≈10M⊙)
and near-circular orbit (e ≈ 0.02) of VFTS 243 suggest that the progenitor star experienced complete
collapse, with energy-momentum being lost predominantly through neutrinos. VFTS 243 enables us to
constrain the natal kick and neutrino-emission asymmetry during black-hole formation. At 68% confidence
level, the natal kick velocity (mass decrement) is ≲10 km=s (≲1.0M⊙), with a full probability distribution
that peaks when ≈0.3M⊙ were ejected, presumably in neutrinos, and the black hole experienced a natal
kick of 4 km=s. The neutrino-emission asymmetry is ≲4%, with best fit values of ∼0–0.2%. Such a small
neutrino natal kick accompanying black-hole formation is in agreement with theoretical predictions.
Detectability of Solar Panels as a TechnosignatureSérgio Sacani
In this work, we assess the potential detectability of solar panels made of silicon on an Earth-like
exoplanet as a potential technosignature. Silicon-based photovoltaic cells have high reflectance in the
UV-VIS and in the near-IR, within the wavelength range of a space-based flagship mission concept
like the Habitable Worlds Observatory (HWO). Assuming that only solar energy is used to provide
the 2022 human energy needs with a land cover of ∼ 2.4%, and projecting the future energy demand
assuming various growth-rate scenarios, we assess the detectability with an 8 m HWO-like telescope.
Assuming the most favorable viewing orientation, and focusing on the strong absorption edge in the
ultraviolet-to-visible (0.34 − 0.52 µm), we find that several 100s of hours of observation time is needed
to reach a SNR of 5 for an Earth-like planet around a Sun-like star at 10pc, even with a solar panel
coverage of ∼ 23% land coverage of a future Earth. We discuss the necessity of concepts like Kardeshev
Type I/II civilizations and Dyson spheres, which would aim to harness vast amounts of energy. Even
with much larger populations than today, the total energy use of human civilization would be orders of
magnitude below the threshold for causing direct thermal heating or reaching the scale of a Kardashev
Type I civilization. Any extraterrrestrial civilization that likewise achieves sustainable population
levels may also find a limit on its need to expand, which suggests that a galaxy-spanning civilization
as imagined in the Fermi paradox may not exist.
Jet reorientation in central galaxies of clusters and groups: insights from V...Sérgio Sacani
Recent observations of galaxy clusters and groups with misalignments between their central AGN jets
and X-ray cavities, or with multiple misaligned cavities, have raised concerns about the jet – bubble
connection in cooling cores, and the processes responsible for jet realignment. To investigate the
frequency and causes of such misalignments, we construct a sample of 16 cool core galaxy clusters and
groups. Using VLBA radio data we measure the parsec-scale position angle of the jets, and compare
it with the position angle of the X-ray cavities detected in Chandra data. Using the overall sample
and selected subsets, we consistently find that there is a 30% – 38% chance to find a misalignment
larger than ∆Ψ = 45◦ when observing a cluster/group with a detected jet and at least one cavity. We
determine that projection may account for an apparently large ∆Ψ only in a fraction of objects (∼35%),
and given that gas dynamical disturbances (as sloshing) are found in both aligned and misaligned
systems, we exclude environmental perturbation as the main driver of cavity – jet misalignment.
Moreover, we find that large misalignments (up to ∼ 90◦
) are favored over smaller ones (45◦ ≤ ∆Ψ ≤
70◦
), and that the change in jet direction can occur on timescales between one and a few tens of Myr.
We conclude that misalignments are more likely related to actual reorientation of the jet axis, and we
discuss several engine-based mechanisms that may cause these dramatic changes.
The solar dynamo begins near the surfaceSérgio Sacani
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating
region of sunspot emergence appears around 30° latitude and vanishes near the
equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations
closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary
to theories suggesting deep origins of these phenomena, helioseismology pinpoints
low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface
shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with
a poloidal magnetic field strongly implicates the magneto-rotational instability5,6,
prominent in accretion-disk theory and observed in laboratory experiments7.
Together, these two facts prompt the general question: whether the solar dynamo is
possibly a near-surface instability. Here we report strong affirmative evidence in stark
contrast to traditional models8 focusing on the deeper tachocline. Simple analytic
estimates show that the near-surface magneto-rotational instability better explains
the spatiotemporal scales of the torsional oscillations and inferred subsurface
magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these
estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo
resulting from a well-understood near-surface phenomenon improves prospects
for accurate predictions of full magnetic cycles and space weather, affecting the
electromagnetic infrastructure of Earth.
Extensive Pollution of Uranus and Neptune’s Atmospheres by Upsweep of Icy Mat...Sérgio Sacani
In the Nice model of solar system formation, Uranus and Neptune undergo an orbital upheaval,
sweeping through a planetesimal disk. The region of the disk from which material is accreted by
the ice giants during this phase of their evolution has not previously been identified. We perform
direct N-body orbital simulations of the four giant planets to determine the amount and origin of solid
accretion during this orbital upheaval. We find that the ice giants undergo an extreme bombardment
event, with collision rates as much as ∼3 per hour assuming km-sized planetesimals, increasing the
total planet mass by up to ∼0.35%. In all cases, the initially outermost ice giant experiences the
largest total enhancement. We determine that for some plausible planetesimal properties, the resulting
atmospheric enrichment could potentially produce sufficient latent heat to alter the planetary cooling
timescale according to existing models. Our findings suggest that substantial accretion during this
phase of planetary evolution may have been sufficient to impact the atmospheric composition and
thermal evolution of the ice giants, motivating future work on the fate of deposited solid material.
Exomoons & Exorings with the Habitable Worlds Observatory I: On the Detection...Sérgio Sacani
The highest priority recommendation of the Astro2020 Decadal Survey for space-based astronomy
was the construction of an observatory capable of characterizing habitable worlds. In this paper series
we explore the detectability of and interference from exomoons and exorings serendipitously observed
with the proposed Habitable Worlds Observatory (HWO) as it seeks to characterize exoplanets, starting
in this manuscript with Earth-Moon analog mutual events. Unlike transits, which only occur in systems
viewed near edge-on, shadow (i.e., solar eclipse) and lunar eclipse mutual events occur in almost every
star-planet-moon system. The cadence of these events can vary widely from ∼yearly to multiple events
per day, as was the case in our younger Earth-Moon system. Leveraging previous space-based (EPOXI)
lightcurves of a Moon transit and performance predictions from the LUVOIR-B concept, we derive
the detectability of Moon analogs with HWO. We determine that Earth-Moon analogs are detectable
with observation of ∼2-20 mutual events for systems within 10 pc, and larger moons should remain
detectable out to 20 pc. We explore the extent to which exomoon mutual events can mimic planet
features and weather. We find that HWO wavelength coverage in the near-IR, specifically in the 1.4 µm
water band where large moons can outshine their host planet, will aid in differentiating exomoon signals
from exoplanet variability. Finally, we predict that exomoons formed through collision processes akin
to our Moon are more likely to be detected in younger systems, where shorter orbital periods and
favorable geometry enhance the probability and frequency of mutual events.
Emergent ribozyme behaviors in oxychlorine brines indicate a unique niche for...Sérgio Sacani
Mars is a particularly attractive candidate among known astronomical objects
to potentially host life. Results from space exploration missions have provided
insights into Martian geochemistry that indicate oxychlorine species, particularly perchlorate, are ubiquitous features of the Martian geochemical landscape. Perchlorate presents potential obstacles for known forms of life due to
its toxicity. However, it can also provide potential benefits, such as producing
brines by deliquescence, like those thought to exist on present-day Mars. Here
we show perchlorate brines support folding and catalysis of functional RNAs,
while inactivating representative protein enzymes. Additionally, we show
perchlorate and other oxychlorine species enable ribozyme functions,
including homeostasis-like regulatory behavior and ribozyme-catalyzed
chlorination of organic molecules. We suggest nucleic acids are uniquely wellsuited to hypersaline Martian environments. Furthermore, Martian near- or
subsurface oxychlorine brines, and brines found in potential lifeforms, could
provide a unique niche for biomolecular evolution.
Continuum emission from within the plunging region of black hole discsSérgio Sacani
The thermal continuum emission observed from accreting black holes across X-ray bands has the potential to be leveraged as a
powerful probe of the mass and spin of the central black hole. The vast majority of existing ‘continuum fitting’ models neglect
emission sourced at and within the innermost stable circular orbit (ISCO) of the black hole. Numerical simulations, however,
find non-zero emission sourced from these regions. In this work, we extend existing techniques by including the emission
sourced from within the plunging region, utilizing new analytical models that reproduce the properties of numerical accretion
simulations. We show that in general the neglected intra-ISCO emission produces a hot-and-small quasi-blackbody component,
but can also produce a weak power-law tail for more extreme parameter regions. A similar hot-and-small blackbody component
has been added in by hand in an ad hoc manner to previous analyses of X-ray binary spectra. We show that the X-ray spectrum
of MAXI J1820+070 in a soft-state outburst is extremely well described by a full Kerr black hole disc, while conventional
models that neglect intra-ISCO emission are unable to reproduce the data. We believe this represents the first robust detection of
intra-ISCO emission in the literature, and allows additional constraints to be placed on the MAXI J1820 + 070 black hole spin
which must be low a• < 0.5 to allow a detectable intra-ISCO region. Emission from within the ISCO is the dominant emission
component in the MAXI J1820 + 070 spectrum between 6 and 10 keV, highlighting the necessity of including this region. Our
continuum fitting model is made publicly available.
WASP-69b’s Escaping Envelope Is Confined to a Tail Extending at Least 7 RpSérgio Sacani
Studying the escaping atmospheres of highly irradiated exoplanets is critical for understanding the physical
mechanisms that shape the demographics of close-in planets. A number of planetary outflows have been observed
as excess H/He absorption during/after transit. Such an outflow has been observed for WASP-69b by multiple
groups that disagree on the geometry and velocity structure of the outflow. Here, we report the detection of this
planet’s outflow using Keck/NIRSPEC for the first time. We observed the outflow 1.28 hr after egress until the
target set, demonstrating the outflow extends at least 5.8 × 105 km or 7.5 Rp This detection is significantly longer
than previous observations, which report an outflow extending ∼2.2 planet radii just 1 yr prior. The outflow is
blueshifted by −23 km s−1 in the planetary rest frame. We estimate a current mass-loss rate of 1 M⊕ Gyr−1
. Our
observations are most consistent with an outflow that is strongly sculpted by ram pressure from the stellar wind.
However, potential variability in the outflow could be due to time-varying interactions with the stellar wind or
differences in instrumental precision.
X-rays from a Central “Exhaust Vent” of the Galactic Center ChimneySérgio Sacani
Using deep archival observations from the Chandra X-ray Observatory, we present an analysis of
linear X-ray-emitting features located within the southern portion of the Galactic center chimney,
and oriented orthogonal to the Galactic plane, centered at coordinates l = 0.08◦
, b = −1.42◦
. The
surface brightness and hardness ratio patterns are suggestive of a cylindrical morphology which may
have been produced by a plasma outflow channel extending from the Galactic center. Our fits of the
feature’s spectra favor a complex two-component model consisting of thermal and recombining plasma
components, possibly a sign of shock compression or heating of the interstellar medium by outflowing
material. Assuming a recombining plasma scenario, we further estimate the cooling timescale of this
plasma to be on the order of a few hundred to thousands of years, leading us to speculate that a
sequence of accretion events onto the Galactic Black Hole may be a plausible quasi-continuous energy
source to sustain the observed morphology
X-rays from a Central “Exhaust Vent” of the Galactic Center Chimney
Bright and dark_polar_deposits_on_mercury_evidence_for_surface_volatiles
1. Bright and Dark Polar Deposits on Mercury: Evidence for Surface
Volatiles
Gregory A. Neumann et al.
Science 339, 296 (2013);
DOI: 10.1126/science.1229764
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2. REPORTS
4. O. Gasnault et al., Geophys. Res. Lett. 28, 3797 28. W. C. Feldman et al., Science 281, 1496 (1998). much shorter than impact gardening processes (35).
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5. W. C. Feldman et al., Geophys. Res. Lett. 34, L05201 that a limited fraction of the area of permanent shadow data represent an upper limit.
(2007). and radar-bright regions contain surficial water ice
6. J. O. Goldsten et al., Space Sci. Rev. 131, 339 (2007). (23, 30). The neutron data, however, do not have the Acknowledgments: We thank the MESSENGER team
7. D. J. Lawrence et al., Planet. Space Sci. 59, 1665 (2011). spatial resolution to distinguish regions of surface ice for their contributions to the development and operation
8. Supplementary online material describes the full NS data from the larger areas of shallowly buried ice. Furthermore, of the spacecraft, P. G. Lucey and two anonymous
reduction and analysis. multiwavelength radar studies (2) suggest that polar reviewers for comments that improved the manuscript,
9. D. J. Lawrence et al., Icarus 209, 195 (2010). deposits in the three largest north polar craters [Chesterton, D. Delapp and D. Seagraves of Los Alamos National
10. W. C. Feldman et al., Nucl. Instrum. Methods A245, 182 Tolkien, and Tryggvadóttir (2)] that make a large Laboratory for early help in the data reduction and
(1986). contribution to the overall neutron signal are, on average, calibration, respectively, and D. Hurley for discussions
11. L. R. Nittler et al., Science 333, 1847 (2011). buried beneath a thin cover of dry soil or other regarding surface modification models. This work was
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15. L. G. Evans et al., J. Geophys. Res. 117, E00L07 (2012). 31. N. L. Chabot et al., J. Geophys. Res. 10.1029/2012JE004172 Physics Laboratory and NASW-00002 to the Carnegie
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1991), pp. 475–594. thermal effects (23) that can operate on time scales 10.1126/science.1229953
sits of nearly pure water ice up to several meters
Bright and Dark Polar Deposits thick lie at or near the surface. Analysis of al-
timetry and roughness measurements from the
on Mercury: Evidence for Mercury Laser Altimeter (MLA) (10, 11) on the
MErcury Surface, Space ENvironment, GEoche-
Surface Volatiles mistry, and Ranging (MESSENGER) spacecraft
(12) indicates that craters hosting radar-bright de-
posits at high northern latitudes are not anom-
Gregory A. Neumann,1* John F. Cavanaugh,1 Xiaoli Sun,1 Erwan M. Mazarico,2 alously shallow, nor do they display distinctive
David E. Smith,2 Maria T. Zuber,2 Dandan Mao,3 David A. Paige,4 Sean C. Solomon,5,6 roughness properties in comparison with craters
Carolyn M. Ernst,7 Olivier S. Barnouin7 that lack such deposits (13). Consequently, the
radar-bright material does not form a thick layer
Measurements of surface reflectance of permanently shadowed areas near Mercury’s north pole overlying regolith (13). A thinner surficial layer
reveal regions of anomalously dark and bright deposits at 1064-nanometer wavelength. These containing substantial concentrations of ice would,
reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with however, be optically brighter than the surround-
areas of high radar backscatter postulated to be the result of near-surface water ice. Correlation of ing terrain (14) and should be detectable by ac-
observed reflectance with modeled temperatures indicates that the optically bright regions are tive remote sensing.
consistent with surface water ice, whereas dark regions are consistent with a surface layer of We report here measurements with MLA of
complex organic material that likely overlies buried ice and provides thermal insulation. Impacts surface reflectance in permanently shadowed north
of comets or volatile-rich asteroids could have provided both dark and bright deposits. polar regions of Mercury. The MLA instrument
illuminates surface spots 20 to 80 m in diameter
ercury’s near-zero obliquity and impact- vective or conductive sources of heat, the perma- at 350- to 450-m intervals (10). The receiver sys-
M roughened topography (1) prevent di-
rect sunlight from reaching substantial
portions of its polar regions. Lacking major con-
nently shadowed, near-surface regolith experiences
temperatures similar to those of the icy Galilean
satellites (2). It has long been believed on theo-
tem measures threshold-crossing times of the
received pulse waveforms at two voltages (15).
A single low-threshold crossing provides sur-
retical grounds that such conditions are favor- face elevation, and the timing of the rising and
1
NASA Goddard Space Flight Center, Code 698, Greenbelt, able to the accumulation of volatiles (3, 4). Even falling signal levels for strong returns at both
MD 20771, USA. 2Department of Earth, Atmospheric, and with Mercury’s close proximity to the Sun, ex- low and high thresholds enables MLA to esti-
Planetary Sciences, Massachusetts Institute of Technology,
tremes of daytime temperature are not expected mate the received pulse energy and make active
Cambridge, MA 02139, USA. 3Sigma Space Corporation,
Lanham, MD 20706, USA. 4Department of Earth and Space to penetrate regolith to substantial depth, allow- measurements of surface reflectance, rs, via the
Sciences, University of California, Los Angeles, CA 90095, USA. ing near-surface water ice, if present, to remain lidar link equation (16, 17) and preflight sensor
5
Department of Terrestrial Magnetism, Carnegie Institution of stable against sublimation for billions of years calibrations (10).
Washington, Washington, DC 20015, USA. 6Lamont-Doherty (2). Such hypotheses were renewed when Earth- During its primary mapping mission, MES-
Earth Observatory, Columbia University, Palisades, NY 10964,
USA. 7The Johns Hopkins Applied Physics Laboratory, Laurel, based radar observations of Mercury, at wave- SENGER orbited Mercury in an eccentric orbit
MD 20723, USA. lengths from 3.6 to 70 cm (5–9), revealed regions with a 12-hour period and a ~200- to 400-km
*To whom correspondence should be addressed. E-mail: of high backscatter and depolarization at both periapsis altitude at 60° to 70°N. In this orbit,
gregory.a.neumann@nasa.gov poles. Radar observations suggested that depo- the MLA ranged to Mercury from 29 March 2011
296 18 JANUARY 2013 VOL 339 SCIENCE www.sciencemag.org
3. REPORTS
to 16 April 2012, densely sampling the north A map of radar cross section in the north RB region behind its steep (17° slope) north-
polar region in nadir mode northward to 83.5°N polar region at S-band (12.6-cm wavelength) (9) facing wall, just south of 85°N (Fig. 1B). With a
and sparsely in off-nadir mode at more norther- (Fig. 1B) shows many regions of high backscat- depth-to-diameter ratio of 0.025, typical for a
ly latitudes (Fig. 1A) (1). More than 4 million ter cross section; other such regions extend be- complex crater of this size, only a portion of its
topographic and 2 million reflectance measure- yond the limits of the map to latitudes as low as floor can lie in permanent shadow, consistent
ments were collected at latitudes greater than 67°N. The polarization characteristics of these with the shape of the RB region. An unnamed
65°N in the first year of mapping. Of 700 orbital regions are suggestive of cold-trapped volatiles 1.5-km-deep, 18-km-diameter crater “Z” lies on
profiles, 60 targeted latitudes higher than 84°N (5, 6, 18). These radar-bright (RB) features gen- the central floor of Prokofiev and is RB. The
with off-nadir ranges, some yielding energy mea- erally coincide with high-latitude, steep-walled 62-km-diameter crater Kandinsky (formerly “J”)
surements and some not (fig. S1). Orbital geome- craters of which the southern floors are perma- to the north has a nearly circular RB region (Fig.
try and power and thermal constraints precluded nently shadowed from direct sunlight because of 1B). These and similar regions may now be sub-
observations of many polar craters, and measure- Mercury’s near-zero obliquity. The largest RB jected to illumination models that use detailed
ments of those that were accessible at oblique features lie north of 85°N, whereas the 108-km- polar topography (19).
incidence returned noisier measurements than at diameter Prokofiev crater [previously given the A plot of the maximum illumination flux over
nadir orientation. informal name “K” (18)] has a crescent-shaped 10 solar days is shown in Fig. 1C. We modeled
the primary shadowing of the finite disk of the
Sun with the orbital and rotational geometry of
A B Mercury following an earlier methodology (20).
-4 -2 0 2 0.00 0.02 0.04 0.06 0.08 0.10
Downloaded from www.sciencemag.org on January 23, 2013
180˚ 180˚ Zero flux corresponds to areas of near-permanent
shadow that receive only scattered light. Mercury’s
orbital eccentricity and 3:2 spin–orbit resonance
result in lower average solar flux near longitudes
of 90° and 270°E. Shallow, degraded craters and
0˚
0˚
12
12
24
24
0˚
0˚
craters lying near the 0° and 180°E longitudes
of Mercury’s equatorial “hot poles” have higher
average illumination. Except for relatively fresh
craters on the northern smooth plains (1), there
are few RB features along these azimuths south
of 85°N.
The reflectance measurements binned at
1 km by 1 km resolution are shown in Fig. 1D.
30
30
˚
˚
60
60 The log-normally distributed quantity rs has a
0˚
0˚
mean of 0.17 T 0.05 (SD), and 98% of returns
have rs < 0.3 (fig. S1). For comparison, the broad-
band geometric albedo of Mercury from space is
0˚ 0˚
0.142 (21). About 7% of returns comprise a sec-
C D ondary “MLA-dark” (MD) mode distinguished
0 20 40 60 80 100 0.0 0.1 0.2 0.3 0.4 by rs < 0.1. This mode is seen in regions that are
180˚ 180˚ markedly darker than their surroundings. These
regions coincide with areas where many received
pulses do not trigger at the high threshold (fig.
S2), although weak laser output, oblique inci-
0˚
0˚
12
12
dence, steep terrain, and/or extreme range, as
24
24
0˚
0˚
well as low reflectivity, can lead to poor signal
recovery. The deficit of energy measurements
in many MD regions indicates that the mea-
Kandinsky sured rs values are upper bounds for surface
Prokofiev albedoes that are lower by factors of 2 to 3 than
their surroundings.
Many of the MD regions are associated with
30
30
˚
˚
polar craters containing RB material (Fig. 2).
60
60
0˚
0˚
The larger MD regions generally enclose the RB
features. MD returns lie mainly within regions of
very low peak illumination, although not neces-
0˚ 0˚ sarily permanent shadow. The reflectance is low
Fig. 1. Maps of topography, radar cross section, solar illumination, and reflectance in polar stereo-
graphic projection southward to 75°N. Kandinsky and Prokofiev craters are outlined in three of the four Table 1. Classification of 175 craters according
panels. (A) Topography (color scale in km) and shaded relief; the datum is a sphere of radius 2440 km. to radar and optical characteristics of associated
(B) Earth-based radar image (9) displayed as a dimensionless radar cross section per unit area. (C) Maximum deposits.
incident solar flux over a 10-year period as a percentage of the solar constant at 1 astronomical unit
(AU) from an illumination model. The red box outlines the region shown in Fig. 2. (D) The 1064-nm MLA MLA MLA MLA
Radar
bidirectional reflectance from MLA low- and high-threshold measurements in near-nadir directions, dark bright/mixed normal undetermined
median-averaged in 1 km–by–1 km bins. At latitudes poleward of 84°N, MLA obtained only a limited Bright 96 9 0 24
number of off-nadir profiles, and the projected reflectance data in this region are interpolated by a
Dark 28 0 15 3
nearest-neighbor weighted average only within 2 km of data whose incidence angles were less than 10°.
www.sciencemag.org SCIENCE VOL 339 18 JANUARY 2013 297
4. REPORTS
over the southern floors and the northward-facing lar outline. The correspondence of dark material (b5 and f5) (Fig. 2 and fig. S3) are relatively
walls of virtually all craters at latitudes between with pole-facing slopes and the lack of such dark- pristine (>1 km deep), so their interiors may not
75° and 84°N. Darkening also occurs on some ening in most craters southward of 70°N ap- be visible to Earth-based radar. Twelve such craters
poleward-facing exterior rim slopes of craters in pears to rule out instrumental effects or observational are <14 km in diameter. Those craters with MD
the otherwise smooth plains within the 320-km- geometry as a cause of the surficial darkening. material that lack a RB signature and are 14 km
diameter Goethe basin. Such darkening extends To assess the relations between MLA-dark or larger in diameter are at latitudes south of
into regions that are partially illuminated. features, RB deposits, and illumination, we ex- 80°N. As with the RB regions, MLA-dark de-
The asymmetric distribution of MD regions amined (22) 175 regions of low illumination iden- posits are more prevalent near 90° and 270°E,
with respect to terrain slope direction does not tified as lying within craters varying in size from longitudes that receive less average illumina-
simply result from observing geometry, surface ~7 to 108 km in diameter (23) and from 65°N tion as a result of Mercury’s spin-orbit reso-
roughness, or the magnitude of the surface slope. poleward (Table 1). All craters with RB deposits nance and eccentric orbit, and in fresh craters
The pulses returning from the MD portions are and sufficient MLA sampling show at least some on the smooth plains. At latitudes north of 75°N,
not noticeably wider or narrower than those from MD features in their poleward-facing portions. 15 similar shadowed regions (putatively small
the illuminated portions, nor do equator-facing por- Of 128 RB craters with RB deposits, 96 contain craters) with neither a RB signature nor MD
tions of the floor show lower reflectance. If surface collocated MD portions, whereas there are 28 material are located mainly on an elevated area
slope or roughness were causing reduced energy additional craters with MD material that lack a surrounding Purcell crater between longitudes
return, the darker regions would have a circu- corresponding RB signature. Two such craters 170° and 230°E. Radar coverage may be partial-
Downloaded from www.sciencemag.org on January 23, 2013
A pct B C rs
100 0.10 0.4
b5
f5
80 0.08
Yoshikawa 0.3
120
60 e5 0.06
0.2
40 0.04
c5 80
g5 0.1
20 0.02
0 0.00 0.0
250 300 350 400 250 300 350 400 250 300 350 400
km km km
Fig. 2. Regional view of the area outlined in Fig. 1, in polar stereographic area. The projected radar map (9) has been shifted by 4 km to account for
projection. Red circles show the outlines of six craters. (A) Maximum incident solar differences in projection and to achieve optimal registration with the MLA-based
flux, as a percentage of the solar constant at 1 AU. (B) Radar cross-section per unit maps. Regions of interest (22) are labeled. (C) MLA reflectance (colored dots).
A B -2
1.0
-3
km
-4
-5 0.5
-6
rs
-7
0.0
C
-2
1.0
-3
km
-4
-5 0.5
-6
-7
0.0
rs
D
-2
1.0
-3
km
-4
-5 0.5
-6
rs
-7
0.0
-50 0 50
Distance (km)
Fig. 3. (A) MLA reflectance measurements (colored dots) of the north polar height (black lines) and reflectance (red dots) through Prokofiev acquired on
region from longitude 0° to 90°E and latitude 82.5° to 90°N. Background is 22 through 24 March 2012 starting at 0308 UTC on each day, at a 5° to 7°
a mosaic of MDIS (34) frames at different illumination geometries and has a nadir angle. Vertical exaggeration is 10:1. The profiles are centered at
nonlinear contrast stretch for visibility. Three profiles through Prokofiev (b-b′, longitude 60°E and traverse the poleward-facing wall of Prokofiev crater in
c-c′, and d-d′) were acquired at near-nadir orientation. Profiles through an approximate west-to-east direction. The blue lines show the modeled
Kandinsky were acquired at ~30° off-nadir orientation. (B to D) Profiles of extent of low average solar flux (<50 W m−2 or <0.04 of terrestrial).
298 18 JANUARY 2013 VOL 339 SCIENCE www.sciencemag.org
5. REPORTS
ly obscured by rough terrain in this sector, but the pothesis that water ice is exposed at the surface 10. J. F. Cavanaugh et al., Space Sci. Rev. 131, 451 (2007).
lack of RB features more likely has a thermal in areas where surface temperatures are never 11. The MLA is a time-of-flight laser range finder that
uses direct detection and pulse-edge timing to determine
origin at these “hot pole” longitudes in locations sufficiently high for substantial loss by subli- precisely the range from the MESSENGER spacecraft to
where partial illumination might preclude stabil- mation. The surface measurements are aver- Mercury’s surface. MLA’s laser transmitter emits 6-ns-long
ity of near-surface water ice (fig. S4). ages over footprints that are dozens of meters pulses at an 8-Hz rate with 20 mJ of energy at a wavelength
Although the MLA-dark regions are more in extent and could represent a thin or unevenly of 1064 nm. Return echoes are collected by an array
of four refractive telescopes and are detected with a
abundant and extensive than RB regions, there distributed layer of optically bright material single silicon avalanche photodiode detector. The timing
are at least nine areas within the largest RB re- that has not been covered by dust or regolith. of laser pulses is measured with a set of time-to-digital
gions at very high latitudes in which the MLA However, to the extent that MLA-bright and converters linked to a crystal oscillator for which the
reflectances are optically bright. The nine cra- RB characteristics are sampling the same ma- frequency is monitored from Earth.
12. S. C. Solomon, R. L. McNutt Jr., R. E. Gold, D. L. Domingue,
ters hosting RB material, at latitudes between terial, the associated deposits must have a thick- Space Sci. Rev. 131, 3 (2007).
82.5° and 88.5°N, have portions with rs > 0.3 ness of at least several meters. The reflectance 13. M. J. Talpe et al., J. Geophys. Res. 117, E00L13
as well as areas that are anomalously dark or measurements presented here strongly suggest (2012).
that return no reflectance measurements. The that one of the largest and deepest regions of 14. G. B. Hansen, T. B. McCord, J. Geophys. Res. 109, E01012
(2004).
two most prominent such craters are north of permanent shadow in crater Prokofiev is a host
15. The MLA measures the threshold crossing times of the
84.9°N latitude. for water ice deposits exposed at the surface. received pulses at two discriminator voltages simultaneously,
Craters Kandinsky and Prokofiev, for which The existence of these dark and bright sur- a low threshold for maximum sensitivity and a threshold
high radar cross sections suggest thick, near- faces and their association with topography in- about twice as high to give four sample points of the
Downloaded from www.sciencemag.org on January 23, 2013
surface ice deposits (18), are shown in Fig. 3. dicates that their formation processes operated received pulse waveform. A laser pulse may result in
triggers at one or both thresholds or not at all. Ranging
Their regions of permanent shadow (Fig. 1C) during geologically recent times and may be with low-threshold detections is possible at ranges up
have many reflectance values in excess of 0.3 active on Mercury today. The rates of darkening to 1500 km, but steady returns that cross both low and
(pink or white symbols), especially along the and brightening must be higher than those for high thresholds are obtained mostly at altitudes less
southern portion of Prokofiev. Three profiles processes that act to homogenize surface reflec- than ~600 km and with near-nadir (<20°) incidence.
When a pulse is detected by a pair of discriminators, its
crossing the RB region are plotted along track in tance, such as impact gardening. Were vertical energy and duration may be inferred from a model
Fig. 3, B to D. Profile 3B grazed the uppermost mixing by impact gardening dominant at the waveform that accounts for the dispersion in time of
kilometer of the crater wall and recorded no meter scale, we would expect that the polar de- return pulses as a result of surface slope and/or
high-threshold detections in regions of shadow. posits would have reflectance values (and radar roughness. To estimate the pulse energy, we adopted a
simple triangular model that fits the rising and falling
Profile 3C passed 2 km into the interior along backscatter characteristics) more similar to those edges of the trigger at each threshold. This model
the north-facing wall and shows many strongly of surrounding terrain. generates values nearly equal to a Gaussian model for
reflective returns (red symbols) up to the edges Detailed thermal models (25) suggest that well-constrained pulses. Energy is a nonlinear function
of the crater, where such returns dropped out for surface temperatures in the majority of the high- of pulse timing measurements and tends to have a
long-tailed or approximate log-normal distribution, as
several seconds. Profile 3D reached portions of latitude craters with RB deposits that MLA has
illustrated in the supplementary materials.
the crater floor that are in permanent shadow observed to date are too warm to support per- 16. C. S. Gardner, IEEE Trans. Geosci. Rem. Sens. 30, 1061
and recorded variable reflectance. These profiles sistent water ice at the surface, but the temper- (1992).
are the only ones to date obtained over the shad- atures in their shadowed areas are compatible 17. The lidar link equation is Erx = Etxhr(Ar/R2)(rs/p), where
owed interior of Prokofiev at the relatively small with the presence of surficial dark organic ma- Erx is the received signal pulse energy, Etx is the
transmitted laser pulse energy, hr is the receiver optics
incidence angles (6° to 7°) for which reflectance terial. Modeled subsurface temperatures in these transmission, Ar is the receiver telescope aperture area,
measurements are most reliable. Two profiles dark regions are permissive of stable water ice R is range, and rs is the target surface reflectivity (relative
nearest to crater Z (Fig. 3A) also include re- beneath a ~10-cm-thick layer of thermally in- to Lambertian). The ratio rs of reflected energy to incoming
turns with rs > 0.3, as do several traversing sulating material. In contrast, thermal modeling energy (i.e., irradiance/solar flux, often simply written
I/F) would be unity for a perfect diffusive reflector for
crater Kandinsky to the north, but the measure- of the bright areas is supportive of surface water which the transmitter and receiver orientation are
ments are noisier owing to incidence angles ice. This interpretation of the surface reflectance perpendicular to the surface. Mercury’s reflectivity at
greater than 25°. at 1064 nm is fully consistent with the radar re- optical wavelengths normally lies in a range from 0.08
The observations of 1064-nm reflectance from sults as well as with neutron spectroscopic mea- to 0.12 (30–32), but because of the opposition effect
(33) the average 1064-nm reflectance is about 50%
laser altimetry thus fall into three categories: Most surements of Mercury’s polar regions (26). The
higher, or about 0.17.
are typical of Mercury reflectivity as a whole; a bright and dark areas can be ascribed collectively 18. J. K. Harmon, P. J. Perillat, M. A. Slade, Icarus 149,
subset is much darker; and a smaller subset is to the deposition of water and organic volatiles 1 (2001).
substantially brighter. The association of MD re- derived from the impacts of comets or volatile- 19. The topography derived from 700 MLA profiles (29 March
gions with RB regions in near-permanent shadow rich asteroids on Mercury’s surface and migrated 2011 to 1 May 2012) provides a near-complete topographic
map of the northern hemisphere northward to 84°N at a
suggests that a thin, radar-transparent layer of to polar cold traps via thermally stimulated ran- resolution of 0.5 km. Craters Prokofiev and Kandinsky were
optically dark material overlies and surrounds dom walk (27–29). sampled by several off-nadir profiles, from which radial
the postulated polar ice deposits. If water ice averages of topography were constructed and used to fill
were present in the ground as a matrix between in the unsampled interior after adding pseudo-random
References and Notes noise, with a root variance of 70 m, and decimating and
mineral grains, it could lower the reflectance rela- interpolating with the blockmedian and surface programs
1. M. T. Zuber et al., Science 336, 217 (2012).
tive to dry ground but would sublimate rap- 2. A. R. Vasavada, D. A. Paige, S. E. Wood, Icarus 141, 179 of the Generic Mapping Tools (http://gmt.soest.hawaii.
idly and lose optical contrast if exposed to (1999). edu). We modeled the average and maximum illumination
high temperatures. The presence of MD regions 3. K. Watson, B. C. Murray, H. Brown, J. Geophys. Res. 66, conditions over a Mercury day by using an approach (20)
3033 (1961). developed to assess illumination conditions of polar
in many smaller craters without RB deposits,
4. J. R. Arnold, J. Geophys. Res. 84, 5659 (1979). regions of the Moon.
areas where scattered light raises average tem- 5. M. A. Slade, B. J. Butler, D. O. Muhleman, Science 258, 20. E. Mazarico, G. A. Neumann, D. E. Smith, M. T. Zuber,
peratures (2, 24), indicates the presence of vola- 635 (1992). M. H. Torrence, Icarus 211, 1066 (2011).
tiles that are both darker than water ice and 6. J. K. Harmon, M. A. Slade, Science 258, 640 (1992). 21. A. Mallama, D. Wang, R. A. Howard, Icarus 155, 253
stable to higher temperatures. 7. B. J. Butler, D. O. Muhleman, M. A. Slade, J. Geophys. Res. (2002).
98, 15,003 (1993). 22. We selected 175 representative regions of interest
The identification of optically bright regions 8. G. J. Black, D. B. Campbell, J. K. Harmon, Icarus 209, from maps of permanent shadow derived from MLA
associated with large RB features at the highest 224 (2010). topography, radar cross section, and MLA-dark regions,
(>84.9°N) latitudes is consistent with the hy- 9. J. K. Harmon, M. A. Slade, M. S. Rice, Icarus 211, 37 (2011). as shown in the supplementary materials. Because
www.sciencemag.org SCIENCE VOL 339 18 JANUARY 2013 299
6. REPORTS
many craters are not resolved by MLA, we also selected Imaging System (34) image mosaics. Locations are 32. W. E. McClintock et al., Science 321, 62 (2008).
craters with diameters ≥7 km from MESSENGER images. less certain for smaller features inadequately sampled 33. T. Gehrels, Astrophys. J. 123, 331 (1956).
Smaller RB deposits were not considered because most by MLA. Diameters of craters sampled ranged from 7 to 34. S. E. Hawkins III et al., Space Sci. Rev. 131, 247 (2007).
appear from images to lie in small secondary craters, at 108 km, not including the 320-km-diameter Goethe
the foot of poleward-facing scarps, or in rough terrain basin. Not included are several degraded and partially Acknowledgments: The MESSENGER project is supported
and are inadequately sampled by MLA. The radar-bright flooded craters, such as a 133-km-diameter degraded by the NASA Discovery Program under contracts NAS5-97271
deposits were mapped with a threshold of 0.075 in the crater that encloses Purcell but for which the relief to the Johns Hopkins University Applied Physics Laboratory
MATLAB image processing toolbox and correlated with does not create an area of permanent shadow. and NASW-00002 to the Carnegie Institution of Washington.
craters identified in MLA topography and MESSENGER 24. D. A. Paige, S. E. Wood, A. R. Vasavada, Science 258, We are grateful for the myriad of contributions from the
images. Labels assigned in uppercase are consistent with 643 (1992). MLA instrument and MESSENGER spacecraft teams and for
previous nomenclature (15); lowercase letters and 25. D. A. Paige et al., Science 339, 300 (2013); comments by P. Lucey and two anonymous referees that
numerals were assigned to provisional features. Regions 10.1126/science.1231106. improved the manuscript.
with MLA energy measurements were classified as dark, 26. D. J. Lawrence et al., Science 339, 292 (2013);
Supplementary Materials
normal, or bright/mixed according to their contrast 10.1126/science.1229953.
www.sciencemag.org/cgi/content/full/science.1229764/DC1
in brightness with those of surround areas; gaps in 27. B. J. Butler, J. Geophys. Res. 102, 19,283 (1997).
Supplementary Text
high-threshold returns were also taken to indicate darker 28. J. A. Zhang, D. A. Paige, Geophys. Res. Lett. 36, L16203
Figs. S1 to S5
material. Bright regions are surrounding those for which (2009).
Reference (35)
more than half of the returns have rs > 0.3. 29. J. A. Zhang, D. A. Paige, Geophys. Res. Lett. 37, L03203
23. Diameters of large craters were fit to the maximum MLA (2010). 5 September 2012; accepted 14 November 2012
topographic contours of the rims, whereas the diameters 30. T. B. McCord, J. B. Adams, Science 178, 745 (1972). Published online 29 November 2012;
of smaller craters were estimated from Mercury Dual 31. F. Vilas, Icarus 64, 133 (1985). 10.1126/science.1229764
Downloaded from www.sciencemag.org on January 23, 2013
estimates. On Mercury, biannual average temper-
Thermal Stability of Volatiles in the atures can be interpreted as close approximations
to the nearly constant subsurface temperatures
North Polar Region of Mercury that exist below the penetration depths of the
diurnal temperature wave [about 0.3 to 0.5 m for
David A. Paige,1* Matthew A. Siegler,1,2 John K. Harmon,3 Gregory A. Neumann,4 ice-free regolith, and several meters for ice-rich
Erwan M. Mazarico,4 David E. Smith,5 Maria T. Zuber,5 Ellen Harju,1 areas (5, 9)]. The latitudinal and longitudinal sym-
Mona L. Delitsky,6 Sean C. Solomon7,8 metries in surface and near-surface temperatures
result from Mercury’s near-zero obliquity, eccen-
Thermal models for the north polar region of Mercury, calculated from topographic measurements tric orbit, and 3:2 spin-orbit resonance (11, 12).
made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Comparison between the areal coverage of
spacecraft, show that the spatial distribution of regions of high radar backscatter is well model-calculated biannual maximum and aver-
matched by the predicted distribution of thermally stable water ice. MESSENGER measurements age temperatures and the thermal stability of a
of near-infrared surface reflectance indicate bright surfaces in the coldest areas where water range of candidate volatile species (Fig. 2) pro-
ice is predicted to be stable at the surface, and dark surfaces within and surrounding warmer vides strong evidence that Mercury’s anomalous
areas where water ice is predicted to be stable only in the near subsurface. We propose radar features are due dominantly to the presence
that the dark surface layer is a sublimation lag deposit that may be rich in impact-derived of thermally stable water ice, rather than some
organic material. other candidate frozen volatile species. Within the
region sampled, the vast majority of locations
arth-based radar observations have yielded Measurements of surface reflectance at a within which biannual average temperatures are
E maps of anomalously bright, depolarizing
features on Mercury that appear to be lo-
calized in permanently shadowed regions near
wavelength of 1064 nm, made with the Mercury
Laser Altimeter (MLA) onboard the MESSEN-
GER (MErcury Surface, Space ENvironment,
less than ~100 K are radar-bright, whereas for
areas with biannual average temperatures of greater
than 100 K there are almost no radar-bright de-
the planet’s poles (1, 2). Observations of similar GEochemistry, and Ranging) spacecraft, have posits (Fig. 2C). This distribution suggests that
radar signatures over a range of radar wavelengths revealed the presence of surface material that the radar-bright features are due to the presence
imply that the radar-bright features correspond collocates approximately with radar-bright areas of a volatile species that is not thermally stable at
to deposits that are highly transparent at radar within north polar craters and that has approxi- temperatures higher than ~100 K. Because of the
wavelengths and extend to depths of several me- mately half the average reflectance of the planet, exponential dependence of vacuum sublimation
ters below the surface (3). Cold-trapped water ice as well as bright material within Kandinsky and loss rates with temperature, the thermal stabilities
has been proposed as the most likely material to Prokofiev craters that has approximately twice of the candidate volatile species shown in Fig. 2A
be responsible for these features (2, 4, 5), but other the average planetary reflectance (7). MLA mea- over time scales of millions to billions of years are
volatile species that are abundant on Mercury, surements have also provided detailed maps of well separated in temperature. As shown in Fig. 2A
such as sulfur, have also been suggested (6). the topography of Mercury’s north polar region and fig. S8, 1 mm of exposed water ice—or 1 mm
(8). Here, we apply this information in conjunc- of water ice buried beneath a 10-cm-thick lag
1
Department of Earth and Space Sciences, University of Cali- tion with a ray-tracing thermal model, previously deposit—would sublimate to a vacuum in 1 billion
fornia, Los Angeles, CA 90095, USA. 2Jet Propulsion Lab- used to predict temperatures in the polar regions years at temperatures of 100 to 115 K, which we
oratory, Pasadena, CA 91109, USA. 3National Astronomy and of Earth’s Moon (9), to calculate the thermal sta- interpret as strong evidence that Mercury’s anom-
Ionosphere Center, Arecibo, PR 00612, USA. 4NASA Goddard
bility of volatile species in the north polar region alous radar features are due dominantly to the
Space Flight Center, Greenbelt, MD 20771, USA. 5Department
of Earth, Atmospheric and Planetary Sciences, Massachusetts of Mercury. presence of thermally stable water ice. If the radar-
Institute of Technology, Cambridge, MA 02139, USA. 6Cali- Maximum and average modeled temperatures bright deposits were composed primarily of a
fornia Specialty Engineering, Flintridge, CA 91012, USA. 7De- (10) over one complete 2-year illumination cycle material with a higher or lower volatility than
partment of Terrestrial Magnetism, Carnegie Institution of for the north polar region of Mercury are shown water ice, we would expect them to be thermally
Washington, Washington, DC 20015, USA. 8Lamont-Doherty
Earth Observatory, Columbia University, Palisades, NY 10964, in Fig. 1, A and B. The topography model north stable in areas with lower or higher annual aver-
USA. of 84°N latitude has been extrapolated from only age temperatures than we observe. As illustrated
*To whom correspondence should be addressed. E-mail: a few off-nadir data tracks, so model tempera- in Fig. 2, B and C, the fractional areal coverage of
dap@moon.ucla.edu tures within this circle should be taken only as radar-bright regions that are also just sufficiently
300 18 JANUARY 2013 VOL 339 SCIENCE www.sciencemag.org