The gravity field and interior structure of Enceladus were determined using Doppler data from three Cassini flybys. There is a negative mass anomaly in the south polar region, largely compensated by a positive subsurface anomaly consistent with a regional subsurface sea at depths of 30-40 km extending to 50° south latitude. Enceladus deviates mildly from hydrostatic equilibrium, with estimated quadrupole coefficients indicating a differentiated body with a low-density core.
Lecture at University of Wisconsin, Madison - April 2, 2018SMJ Mortazavi
Large SPEs, especially when the shielding is inadequate, not only increase the risk of cancer, but also the possibility of occurrence of acute radiation syndrome (ARS). As physical shielding alone cannot solve current space radiation problems, in 2003 we introduced the adaptive response as an efficient model of biological protection. The development of this model is discussed in our recent publications. A recently published paper, authored by 30 scientists from countries such as US, UK, Russia, and Belgium has confirmed the need for selection of astronauts based on their adaptive response (this paper cites our reports on how AR helps choosing the astronauts for a deep space mission). Moreover, A NASA report published in 2016 has cited our early report on the importance of radioadaptive response in space missions and states that cells can be expected to be exposed to multiple hits of protons before being traversed by an HZE particle. However, substantial evidence showing that SPEs are a real concern, indicate that our proposed model is more applicable and evidence-based. Regarding the risk of infection, change of the virulence (ability to cause disease) of microorganisms and astronauts’ dysregulated immune system, significantly increases the infection risk in deep space missions.
Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massi...Sérgio Sacani
Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining1. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability2–5. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.
Detection of an atmosphere around the super earth 55 cancri eSérgio Sacani
We report the analysis of two new spectroscopic observations of the super-Earth 55 Cancri e, in the near
infrared, obtained with the WFC3 camera onboard the HST. 55 Cancri e orbits so close to its parent
star, that temperatures much higher than 2000 K are expected on its surface. Given the brightness
of 55 Cancri, the observations were obtained in scanning mode, adopting a very long scanning length
and a very high scanning speed. We use our specialized pipeline to take into account systematics
introduced by these observational parameters when coupled with the geometrical distortions of the
instrument. We measure the transit depth per wavelength channel with an average relative uncertainty
of 22 ppm per visit and nd modulations that depart from a straight line model with a 6 condence
level. These results suggest that 55 Cancri e is surrounded by an atmosphere, which is probably
hydrogen-rich. Our fully Bayesian spectral retrieval code, T -REx, has identied HCN to be the
most likely molecular candidate able to explain the features at 1.42 and 1.54 m. While additional
spectroscopic observations in a broader wavelength range in the infrared will be needed to conrm
the HCN detection, we discuss here the implications of such result. Our chemical model, developed
with combustion specialists, indicates that relatively high mixing ratios of HCN may be caused by a
high C/O ratio. This result suggests this super-Earth is a carbon-rich environment even more exotic
than previously thought.
Validation of twelve_small_kepler_transiting_planets_in_the_habitable_zoneSérgio Sacani
Artigo descreve a análise dos mais novos exoplanetas descobertos pela missão Kepler, incluindo o Kepler-438b, o exoplaneta mais parecido com a Terra já descoberto até o momento.
Artigo descreve a descoberta do exoplaneta HATS-6b, um exoplaneta parecido com Saturno, porém pesado como Júpiter ao redor de uma estrela anã-M, o tipo de estrela mais abundante na nossa galáxia.
Lecture at University of Wisconsin, Madison - April 2, 2018SMJ Mortazavi
Large SPEs, especially when the shielding is inadequate, not only increase the risk of cancer, but also the possibility of occurrence of acute radiation syndrome (ARS). As physical shielding alone cannot solve current space radiation problems, in 2003 we introduced the adaptive response as an efficient model of biological protection. The development of this model is discussed in our recent publications. A recently published paper, authored by 30 scientists from countries such as US, UK, Russia, and Belgium has confirmed the need for selection of astronauts based on their adaptive response (this paper cites our reports on how AR helps choosing the astronauts for a deep space mission). Moreover, A NASA report published in 2016 has cited our early report on the importance of radioadaptive response in space missions and states that cells can be expected to be exposed to multiple hits of protons before being traversed by an HZE particle. However, substantial evidence showing that SPEs are a real concern, indicate that our proposed model is more applicable and evidence-based. Regarding the risk of infection, change of the virulence (ability to cause disease) of microorganisms and astronauts’ dysregulated immune system, significantly increases the infection risk in deep space missions.
Energetic eruptions leading to a peculiar hydrogen-rich explosion of a massi...Sérgio Sacani
Every supernova so far observed has been considered to be the terminal explosion of a star. Moreover, all supernovae with absorption lines in their spectra show those lines decreasing in velocity over time, as the ejecta expand and thin, revealing slower-moving material that was previously hidden. In addition, every supernova that exhibits the absorption lines of hydrogen has one main light-curve peak, or a plateau in luminosity, lasting approximately 100 days before declining1. Here we report observations of iPTF14hls, an event that has spectra identical to a hydrogen-rich core-collapse supernova, but characteristics that differ extensively from those of known supernovae. The light curve has at least five peaks and remains bright for more than 600 days; the absorption lines show little to no decrease in velocity; and the radius of the line-forming region is more than an order of magnitude bigger than the radius of the photosphere derived from the continuum emission. These characteristics are consistent with a shell of several tens of solar masses ejected by the progenitor star at supernova-level energies a few hundred days before a terminal explosion. Another possible eruption was recorded at the same position in 1954. Multiple energetic pre-supernova eruptions are expected to occur in stars of 95 to 130 solar masses, which experience the pulsational pair instability2–5. That model, however, does not account for the continued presence of hydrogen, or the energetics observed here. Another mechanism for the violent ejection of mass in massive stars may be required.
Detection of an atmosphere around the super earth 55 cancri eSérgio Sacani
We report the analysis of two new spectroscopic observations of the super-Earth 55 Cancri e, in the near
infrared, obtained with the WFC3 camera onboard the HST. 55 Cancri e orbits so close to its parent
star, that temperatures much higher than 2000 K are expected on its surface. Given the brightness
of 55 Cancri, the observations were obtained in scanning mode, adopting a very long scanning length
and a very high scanning speed. We use our specialized pipeline to take into account systematics
introduced by these observational parameters when coupled with the geometrical distortions of the
instrument. We measure the transit depth per wavelength channel with an average relative uncertainty
of 22 ppm per visit and nd modulations that depart from a straight line model with a 6 condence
level. These results suggest that 55 Cancri e is surrounded by an atmosphere, which is probably
hydrogen-rich. Our fully Bayesian spectral retrieval code, T -REx, has identied HCN to be the
most likely molecular candidate able to explain the features at 1.42 and 1.54 m. While additional
spectroscopic observations in a broader wavelength range in the infrared will be needed to conrm
the HCN detection, we discuss here the implications of such result. Our chemical model, developed
with combustion specialists, indicates that relatively high mixing ratios of HCN may be caused by a
high C/O ratio. This result suggests this super-Earth is a carbon-rich environment even more exotic
than previously thought.
Validation of twelve_small_kepler_transiting_planets_in_the_habitable_zoneSérgio Sacani
Artigo descreve a análise dos mais novos exoplanetas descobertos pela missão Kepler, incluindo o Kepler-438b, o exoplaneta mais parecido com a Terra já descoberto até o momento.
Artigo descreve a descoberta do exoplaneta HATS-6b, um exoplaneta parecido com Saturno, porém pesado como Júpiter ao redor de uma estrela anã-M, o tipo de estrela mais abundante na nossa galáxia.
A Search for Technosignatures Around 11,680 Stars with the Green Bank Telesco...Sérgio Sacani
We conducted a search for narrowband radio signals over four observing sessions in 2020–2023 with
the L-band receiver (1.15–1.73 GHz) of the 100 m diameter Green Bank Telescope. We pointed the
telescope in the directions of 62 TESS Objects of Interest, capturing radio emissions from a total of
∼11,860 stars and planetary systems in the ∼9 arcminute beam of the telescope. All detections were
either automatically rejected or visually inspected and confirmed to be of anthropogenic nature. In
this work, we also quantified the end-to-end efficiency of radio SETI pipelines with a signal injection
and recovery analysis. The UCLA SETI pipeline recovers 94.0% of the injected signals over the usable
frequency range of the receiver and 98.7% of the injections when regions of dense RFI are excluded. In
another pipeline that uses incoherent sums of 51 consecutive spectra, the recovery rate is ∼15 times
smaller at ∼6%. The pipeline efficiency affects SETI search volume calculations as well as calculations
of upper bounds on the number of transmitting civilizations. We developed an improved Drake Figure
of Merit for SETI search volume calculations that includes the pipeline efficiency and frequency drift
rate coverage. Based on our observations, we found that there is a high probability (94.0–98.7%) that
fewer than ∼0.014% of stars earlier than M8 within 100 pc host a transmitter that is detectable in
our search (EIRP > 1012 W). Finally, we showed that the UCLA SETI pipeline natively detects the
signals detected with AI techniques by Ma et al. (2023).
The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with...Sérgio Sacani
The ALMA Survey of Star Formation and Evolution in Massive Protoclusters with Blue Profiles
(ASSEMBLE) aims to investigate the process of mass assembly and its connection to high-mass star
formation theories in protoclusters in a dynamic view. We observed 11 massive (Mclump ≳ 103 M⊙),
luminous (Lbol ≳ 104 L⊙), and blue-profile (infall signature) clumps by ALMA with resolution of
∼2200–5500 au (median value of 3500 au) at 350 GHz (870 µm). 248 dense cores were identified, including 106 cores showing protostellar signatures and 142 prestellar core candidates. Compared to
early-stage infrared dark clouds (IRDCs) by ASHES, the core mass and surface density within the
ASSEMBLE clumps exhibited significant increment, suggesting concurrent core accretion during the
evolution of the clumps. The maximum mass of prestellar cores was found to be 2 times larger than
that in IRDCs, indicating that evolved protoclusters have the potential to harbor massive prestellar
cores. The mass relation between clumps and their most massive core (MMCs) is observed in ASSEMBLE but not in IRDCs, which is suggested to be regulated by multiscale mass accretion. The
mass correlation between the core clusters and their MMCs has a steeper slope compared to that
observed in stellar clusters, which can be due to fragmentation of the MMC and stellar multiplicity.
We observe a decrease in core separation and an increase in central concentration as protoclusters
evolve. We confirm primordial mass segregation in the ASSEMBLE protoclusters, possibly resulting
from gravitational concentration and/or gas accretion.
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
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2024.06.01 Introducing a competency framework for languag learning materials ...Sandy Millin
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The gravity field_and_interior_structure_of_enceladus
1. DOI: 10.1126/science.1250551
, 78 (2014);344Science
et al.L. Iess
The Gravity Field and Interior Structure of Enceladus
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2. real-time FTIR as 1-propanol was added to a dilute
[Bu4N][O2CCN]/IL solution. This peak rapidly
diminished during the addition, while concomi-
tantly a peak corresponding to bicarbonate ap-
peared at 1652 cm−1
(39). Over the course of
only 2 min, nasCO2 for cyanoformate disappeared
completely, while the bicarbonate peak reached
a steady absorbance. All of the observed experi-
mental results are entirely consistent with the com-
putationally based proposal of a stepwise process
for the decomposition and hydrolysis of the
cyanoformate anion.
The stability of cyanoformate, a simple anion
of CN–
coordinated to CO2, has been shown to
be dependent on the dielectric constant of its local
environment. We propose that this property al-
lows it to shuttle toxic CN–
away from the low-
dielectric active site of the enzyme ACC oxidase
before its decomposition in higher dielectric me-
dia. In broader terms, the ability to manipulate
solution stability of otherwise unstable or tran-
sient species, through changes to the dielectric
constant, should find other important applications.
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Acknowledgments: We thank the Natural Sciences and
Engineering Research Council of Canada (through the
Discovery Grants Program to J.A.C.C.) and the Academy of
Finland (through its Research Fellowship to H.M.T.). J.A.C.C.
acknowledges generous support from the Canada Research
Chairs Program, the Canadian Foundation for Innovation, and
the Nova Scotia Research and Innovation Trust Fund. H.M.T.
acknowledges generous support from the Academy of Finland,
the Technology Industries of Finland Centennial Foundation,
and the University of Jyväskylä. This work was also supported
in part by GreenCentre Canada, Encana Corporation
(Deep Panuke Education and Training and Research and
Development Fund), Springboard, and the Magnus Ehrnrooth
Foundation (postdoctoral scholarship to J.M.). We are
grateful to NMR-3 (Dalhousie University) for NMR data
acquisition, Canadian Microanalytical Services for elemental
analyses, and the Finnish Grid Infrastructure consortium for
central processing unit time. We also thank E. Plettner for
her helpful discussions and T. S. Cameron and A. Linden for
their crystallographic insights. Metrical parameters for the
structure of tetraphenylphosphonium cyanoformate are
available free of charge from the Cambridge Crystallographic
Data Centre, under reference number CCDC 986380.
Supplementary Materials
www.sciencemag.org/content/344/6179/75/suppl/DC1
Figs. S1 to S33
Tables S1 to S17
References (40–62)
14 January 2014; accepted 24 February 2014
10.1126/science.1250808
The Gravity Field and Interior Structure
of Enceladus
L. Iess,1
* D. J. Stevenson,2
M. Parisi,1
D. Hemingway,3
R. A. Jacobson,4
J. I. Lunine,5
F. Nimmo,3
J. W. Armstrong,4
S. W. Asmar,4
M. Ducci,1
P. Tortora6
The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We
determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler
data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the
south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence
of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about
50°. The estimated values for the largest quadrupole harmonic coefficients (106
J2 = 5435.2 T 34.9,
106
C22 = 1549.8 T 15.6, 1s) and their ratio (J2/C22 = 3.51 T 0.05) indicate that the body deviates
mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2
, where M is the mass and
R is the radius, suggesting a differentiated body with a low-density core.
A
fter Titan, Enceladus has been the most
observed satellite by the Cassini space-
craft during its exploration of the Saturnian
system. Images of this small moon (252 km ra-
dius) revealed a large plume ejected from the
south-polar region, with the source being long
fractures from which vapor and ice emerge as
discrete jets (1). Concurrent observations of so-
dium and potassium salts in the plume (2), and
the measured temperatures within the fractures (3),
strongly argue for the presence of liquid water in
the subsurface source region. The plume exhibits
a time variability well correlated to the predicted
tidal stresses of the body (4).
The endogenic (nonsolar) power emitted from
the south-polar region, derived from Cassini Com-
posite Infrared Spectrometer data, is 15.8 GW, with
a 20% formal uncertainty (5). This is equivalent
to an average surface heat flux of ~20 mW/m2
and is an order of magnitude larger than conven-
tional estimates of tidal heating if Enceladus’ cur-
rent orbital eccentricity represents a so-called
“equilibrium” resonant state with other satellites
(6). It indicates time-variability in its internal prop-
erties (7), in a resonant state with other nearby
moons (8), or in the rate of heat transport. In any or
all of these cases, a plausible internal structure is
that of a liquid water ocean overlain by a (ther-
mally conductive) crust (5).
The design of the Cassini spacecraft does not
allow radio tracking from Earth during remote-
sensing observations. Therefore, only 3 of the
19 flybys of Enceladus completed so far have
been used for gravity measurements. In these close
encounters, the spacecraft was continuously tracked
from ground antennas while flying within 100 km
of the moon’s surface, twice above the southern
1
Dipartimento di Ingegneria Meccanica e Aerospaziale, Sapienza
Università di Roma, via Eudossiana 18, 00184 Rome, Italy.
2
California Institute of Technology, 150-21 Pasadena, CA
91125, USA. 3
Department of Earth and Planetary Sciences,
University of California Santa Cruz, 1156 High Street, Santa
Cruz, CA 95064, USA. 4
Jet Propulsion Laboratory, California
Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA
91109, USA. 5
Department of Astronomy, Cornell University,
Ithaca, NY 14850, USA. 6
Dipartimento di Ingegneria Indus-
triale, Università di Bologna, I-47121 Forlì, Italy.
*Corresponding author. E-mail: luciano.iess@uniroma1.it
4 APRIL 2014 VOL 344 SCIENCE www.sciencemag.org78
REPORTS
3. hemisphere (in the flybys labeled E9 and E19)
and once over the northern hemisphere (E12) (9).
We determined Enceladus’ quadrupole gravity field
and degree-3 zonal harmonic coefficient J3 from
measurements of spacecraft range-rate. With a ra-
dius ~10 times smaller than that of Titan, and
about the same density, the gravitational signa-
ture of Enceladus in Doppler measurements is
much weaker than that of Saturn’s largest moon.
However, the small perturbation due to J3 (about
0.2 to 0.3 mm/s) is still clearly detectable by the
Cassini tracking system, whose accuracy is 0.02
to 0.09 mm/s on a time scale of t ≈ 60 s.
Microwave links between the onboard tran-
sponder and ground stations of NASA’s Deep
Space Network enabled precise measurements of
the spacecraft range-rate. In addition to gravita-
tional forces, our analysis accounts for the main
nongravitational accelerations, most notably neu-
tral particle drag exerted by the substantial gas
plume formed by the jets of the south-polar re-
gion. Flying by the moon at latitudes below –70°,
the spacecraft interacts with the plume at dis-
tances of up to 500 km from Enceladus’ surface,
although most of the effect is localized within
20 s from closest approach. Given the uncertain-
ties in the gas density along the spacecraft flight
path, and the short time scale of the interaction,
the effect of the drag can be modeled as an
unknown, impulsive, vectorial acceleration at
closest approach at E9 and E19 (9). The or-
bital solutions yielded a velocity variation almost
parallel to the spacecraft velocity, as expected for
a drag force, of magnitude 0.25 mm/s for E9 and
0.26 mm/s for E19, which are comparable with
the J3 signature. The inclusion of the neutral
particle drag in the south-polar flybys is therefore
essential for obtaining Doppler residuals free of any
signatures, which is essential for a faithful gravity
solution.
As in previous Cassini gravity analyses (10, 11),
the solution for Enceladus’ gravity field (labeled
as SOL1 in Table 1) was obtained from a multiple-
arc analysis in which all data from the three flybys
were fitted by using separate initial conditions for
the spacecraft state vector at each arc (local pa-
rameters). The Enceladus state vector, the five
degree-2 harmonic coefficients, and J3 were con-
sidered global parameters common to all arcs. To
avoid biased estimates, the a priori uncertainties
on the gravity coefficients were at least 30 times
larger than were the formal uncertainties obtained
from the orbital solution. The nongravitational ac-
celerations due to anisotropic thermal emission
from the three radioisotope thermoelectric gen-
erators and solar radiation pressure were modeled
by using values determined by the spacecraft navi-
gation team from the past 8 years of the Saturn
tour. By processing the data into a multiple-arc
least-squares filter, we were able to estimate local
and global parameters (Table 1). Furthermore, the
solution was proven to be stable with respect to
perturbations of the dynamical model, such as the
estimation of a full degree-3 field (9).
The ratio J2/C22 differs from the value re-
quired for hydrostatic equilibrium (J2/C22 = 10/3),
suggesting that the satellite is not in a fully relaxed
shape. The equipotential surface of the tidal,
rotational, and gravitational potential (the latter
limited to 2,0 and 2,2 harmonics) has semiaxis
differences of a-c = 6.00 km and b-c = 2.07 km.
The tesseral coefficients C21, S21, and S22 are null
within 3s, indicating that the adopted rotational
model (9) is correct and that the orientation of the
principal axes is that expected for a tidally locked
body with a fully damped pole. The estimated
value of J3 implies a 2.5 mGal, negative gravity
anomaly at the south pole (Fig. 1).
The interpretation of Enceladus gravity pres-
ents a greater difficulty and uncertainty than usual,
given the strikingly different appearances of the
northern and southern hemisphere and the appar-
ent confinement of endogenic activity to the high
southern latitudes. Still, the deviation of J2/C22
from 10/3 (the value for a laterally homoge-
neous body) is modest (of order 5%) and the
non–degree-2 gravity is small (of order 2% rel-
ative to J2), suggesting that there is some prospect
of useful inferences.
The topography of Enceladus (12) is not that
of a relaxed hydrostatic body under the action of
tides and rotation in a synchronous orbit. Compar-
isons of the geoid heights with actual topography
for the largest harmonics show differences of up
to 1.2 km (Table 2).
Table 1. Solution for the gravity field of
Enceladus. Estimated gravity harmonic coeffi-
cients from the multiarc fit of E9, E12, and E19
Doppler data.
SOL 1
Coefficient Central value T 1s
J2 (×106
) 5435.2 T 34.9
C21 (×106
) 9.2 T 11.6
S21 (×106
) 39.8 T 22.4
C22 (×106
) 1549.8 T 15.6
S22 (×106
) 22.6 T 7.4
J3 (×106
) –115.3 T 22.9
J2/C22 3.51 T 0.05
Fig. 1. Enceladus’s gravity disturbances. The gravity field due to C21, S21, S22, and J3 (SOL1) is
mapped onto the reference ellipsoid. The negative anomaly at the south pole, representing the asymmetry
between the two hemispheres, is ~2.5 mGal.
Table 2. Comparison between Enceladus’s geoid heights and actual topography. The existing
differences for statistically nonzero harmonic coefficients result in contributions to Enceladus’ gravity
harmonics.
Harmonic Topography* (km)
Equipotential
height†
(km)
Difference
Dh
(km)
Contribution of
difference to
gravity harmonics‡
(×106
)
–C20 (or J2) 3.85 T 0.02 2.680 T 0.010 1.17 T 0.03 1606 f20
C22 0.92 T 0.02 0.780 T 0.007 0.13 T 0.03 183 f22
–C30 (or J3) –0.38 T 0.01 –0.030 T 0.003 –0.35 T 0.01 –349 f30
*From (12), 2s error bars. †From Table 1, 2s error bars (9). ‡The geoid-to-topography ratio (GTR) for uncompensated
topographic relief is 3rc=ð2l þ 1Þ r for degree l, where rc is the crustal density (~0.93 g/cm3
is assumed) and r is the mean density
(1.61 g/cm3
), so GTR (l = 2) ≅ 0.35, and GTR (l = 3) ≅ 0.25. Here, f is the factor (different for each harmonics) by which the gravity due to
the excess topography is compensated by a mass deficit or excess beneath the surface. For Airy compensation, f = 1 – (1 – d/R)l
, where d
is depth of compensation, R = 252 km, and l is harmonic degree. The entries in the final column are given by Dh *GTR* f/(252km).
www.sciencemag.org SCIENCE VOL 344 4 APRIL 2014 79
REPORTS
4. Either J2 and C22 are fortuitously close to hav-
ing ratio 10/3, or the nonhydrostatic contributions
are small because of compensation ( f is small).
The degree-3 gravity, uncontaminated by tides
and rotation, provides an estimate: f30 ≅ 115.3/
349 ≅ 0.33, implying an Airy depth of compen-
sation of ~32 km. This would in turn imply f20 ≅
0.23 (and f22 should equal f20 if compensation is
isotropic); hence, the part of C22 arising from the
topographic excess is ~43 × 10−6
. Assuming that
the hydrostatic contribution to this harmonic is
then the remainder, 1507 × 10−6
, we infer that the
most likely moment of inertia (MOI) of Encela-
dus is 0.336MR2
, where M is the mass and R is
the radius (9). If we choose the value of f20 = f22
such that the hydrostatic part of J2 is exactly 10/3
the hydrostatic part of C22, we obtain f ≅ 0.27,
implying a depth of compensation of ~37 km and
a moment of inertia of ~0.335MR2
.
If we instead use an iterative approach to self-
consistently separate the hydrostatic and non-
hydrostatic parts of both gravity and topography
[(9), section S3.5], we obtain converging esti-
mates of f20 and f22 (~0.25) when the moment of
inertia is 0.335MR2
, suggesting a compensation
depth of ~34 km. The convergent f20 = f22 is close
to the value predicted on the basis of the observed
f30, confirming that the assumption of isotropic
compensation is reasonable.
When we included elastic flexure as a means
of supporting topography in our model, we found
that in order to be consistent with the observed
gravity-to-topography ratios, the elastic thickness
must be less than 0.5 km, a value consistent with
other estimates made from flexural analysis (13)
and relaxation studies (14).
Results in this MOI range are compatible with
a differentiated structure (15). For example, a
value of 0.335MR2
can arise from a model with
a relatively low core density of ~2.4 g/cm3
and a
H2O mantle of density of 1 g/cm3
and thickness
of 60 km (9). The high heat flow and plume ac-
tivity strongly suggests a differentiated structure,
which is compatible with these results.
The data imply a great deal of compensation;
if our assumption of Airy isostasy is correct, the
inferred compensation depth of 30 to 40 km is
most simply explained as the thickness of the ice
shell overlying a liquid water layer. The large
compensation excludes a very different kind of
model, in which the main effect is the tidal and
rotational response of a body with a nonradial
symmetry of material properties. The tidal and
rotational response of a body with a degree-1
variation in properties could introduce topog-
raphy and gravity at degree 3, an example of
mode coupling (16); but if this were responsible
for the topographic distortion, then it would pre-
dict a much larger gravity distortion than is ob-
served (yielding f ~ 1 instead of the observed
much smaller values). For the same reason, a
frozen-in tidal and rotational bulge from an ear-
lier epoch (17) will not explain the observed grav-
ity if that bulge has persisted in the ice mantle. A
frozen-in deformation of the core (18) could in
principle explain the gravity, but the topography
would still have to be highly compensated.
The presence of at least a regional south polar
subcrustal sea suggests a model in which the
mean temperature of the ice beneath the south
pole is warmer than elsewhere, perhaps leading
to a lower mean density of ~1% (corresponding
to 100 K temperature excess and a coefficient of
thermal expansion of ~10−4
K−1
). But to satisfy
the observed gravity, it is then necessary to insert
a region of higher-density material roughly twice
as large as that needed to offset the topographic
depression alone and at a depth at least as great as
the previous compensation depth estimates. If
this material is water and is 8% denser than the
surrounding ice, then a layer ~10 km thick is
required, diminishing in thickness toward the
lower southern latitudes. The total hydrostatic
pressure at the base of this region would be in
balance with the pressure at the same depth,
plausibly at or near the base of the ice shell, in
adjacent non–south polar regions that lack water,
thus reducing the tendency for this layer to spread
laterally. A highly concentrated mass anomaly at
the south pole would predict J3 = –J2,nh. The data
(Table 2) suggest J3 ~ –0.35J2,nh, and this could
be explained by a mass anomaly that extends
from the pole to roughly 50° south latitude (9).
However, the limitations of the data preclude high
confidence in this inference.
Although the gravity data cannot rule out a
global ocean, a regional sea is consistent with the
gravity, topography, and high local heat fluxes
(19) and does not suffer from the thermal prob-
lems that a global ocean encounters (19, 20). A
global ocean would yield larger and potentially
detectable longitudinal librations than are predicted
for a solid body (21). The gravity coefficients and
inferred MOI of Enceladus are not consistent with a
forced 4:1 secondary libration (17, 22) at the 2s
level (9).
References and Notes
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J. Geophys. Res. 116, (E3), E03003 (2011).
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8. K. Zhang, F. Nimmo, Icarus 204, 597–609 (2009).
9. Materials and methods are available as supplementary
materials on Science Online.
10. L. Iess et al., Science 327, 1367–1369 (2010).
11. L. Iess et al., Science 337, 457–459 (2012).
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E11001 (2011).
13. B. Giese et al., Geophys. Res. Lett. 35, L24204 (2008).
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Geophys. Res. Lett. 39, L17204 (2012).
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188, 345–355 (2007).
16. S. Zhong et al., J. Geophys. Res. 39, L15201 (2012).
17. C. C. Porco et al., Science 311, 1393–1401 (2006).
18. W. B. McKinnon, J. Geophys. Res. 118, 1775 (2013).
19. G. Tobie, O. Cadek, C. Sotin, Icarus 196, 642–652
(2008).
20. J. H. Roberts, F. Nimmo, Icarus 194, 675–689 (2008).
21. N. Rambaux, J. C. Castillo-Rogez, J. G. Williams,
Ö. Karatekin, Geophys. Res. Lett. 37, L04202 (2010).
22. J. Wisdom, Astron. J. 128, 484–491 (2004).
Acknowledgments: L.I., M.P., M.D., and P.T. acknowledge
support from the Italian Space Agency. D.H., F.N., and
J.I.L. are grateful to NASA for support through the Cassini
Project. The work of R.A.J., J.W.A., and S.W.A. was carried
out at the Jet Propulsion Laboratory, California Institute of
Technology, under a contract with NASA. The Doppler data
and ancillary information used in this analysis are archived
in NASA’s Planetary Data System.
Supplementary Materials
www.sciencemag.org/content/344/6179/78/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S6
Tables S1 to S7
References (23–36)
8 January 2014; accepted 24 February 2014
10.1126/science.1250551
Geophysical and Geochemical
Evidence for Deep Temperature
Variations Beneath Mid-Ocean Ridges
Colleen A. Dalton,1
*† Charles H. Langmuir,2
Allison Gale2,3
The temperature and composition of Earth’s mantle control fundamental planetary properties, including
the vigor of mantle convection and the depths of the ocean basins. Seismic wave velocities, ocean
ridge depths, and the composition of mid-ocean ridge basalts can all be used to determine variations in
mantle temperature and composition, yet are typically considered in isolation. We show that correlations
among these three data sets are consistent with 250°C variation extending to depths >400 kilometers and
are inconsistent with variations in mantle composition at constant temperature. Anomalously hot ridge
segments are located near hot spots, confirming a deep mantle-plume origin for hot spot volcanism.
Chemical heterogeneity may contribute to scatter about the global trend. The coherent temperature signal
provides a thermal calibration scale for interpreting seismic velocities located distant from ridges.
M
antle convection controls the evolution
of the planet’s interior and results in the
motion of tectonic plates. The charac-
teristics of this convection are governed by the
density and viscosity of the mantle, which are
functions of both temperature and composition.
4 APRIL 2014 VOL 344 SCIENCE www.sciencemag.org80
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