12min talk, Feb 2017

1. Norbert Schörghofer
University of Hawaii
In collaboration with: E. Mazarico (Goddard), T. Platz
(MPI), S. Schröder (DLR), A. Ermakov (MIT/JPL),
J-P. Combe (Bearfight) and others and the Dawn
Team

2. Small Axis Tilt → Permanent Shadow
• Mercury (0°)
• Earth’s Moon (1.5°)
• Ceres (4°)
Permanently Shadowed Region
(PSR) ⇒ cold (<110K) ⇒ negligible
sublimation loss (<1 m/Gyr)

3. Persistent Shadow on Ceres
Framing
Camera (FC)
images
(140m/px)
animation
82°N
http://photojournal.jpl.nasa.gov/archive/PIA21082.gif

4. Persistently Shadowed Areas
north polar region
of Ceres (>65°N)
based on Survey &
HAMO Images
black = PSR

5. Persistently Shadowed Areas
based on
topography-
derived model
illumination
red = shadowed
all year
PSR area
1,800 km2
0.13% of
hemisphere
Stereographic X (km)
-200 -100 0 100 200
StereographicY(km)
-200
-150
-100
-50
0
50
100
150
200
Ceres North Pole (65-90N)
Maximum incident flux (W/m2
)
0
10
20
30
40
50
60
70
80
90
100

6. Es8mates of Shadow Temperature
based on
analytical
equation for
bowl-shaped
(spherical)
crater by
Ingersoll et al.
(1992) or Buhl et
al. (1968)
D/d = diameter-
to-depth ratio
0 5 10 15 20 25
0
20
40
60
80
100
120
140
160
180
200
Sun elevation (°)
Temperature(K)
D/d=5
H
2
O
NH3
SO
2
CO2
Ceres
Earth’s Moon
Mercury

7. Ceres’ Water Exosphere
Surface-bounded Exosphere
• H2O molecules are gravitationally bound, collisionless,
and travel on ballistic trajectories
• Escape velocity 0.52 km/s; Thermal speed ~0.48km/s
3 hops on
average
1 hop ~ 80
minutes

8. Results from Exosphere Model
0 0.1 0.2 0.3 0.4 0.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Earth’s Moon
trapping efficiency = 1.1
Cold Trap Area (%)
FractionofH
2
OMoleculesTrapped(%)
Ceres
Mercury
Mercury and
Ceres have
comparable
trapping
fraction
~0.1%
Moon ~11%

9. Bright Crater
Floor Deposits
• Bright deposits
are visible in
indirect/scattered
sunlight
• No bright deposits
in most present-
day PSRs
LAMO images:
a, c) unstretched
b, d, e) enhanced ➝
Platz et al. (2016)
Ermakov et al. (2017)

10. Spectroscopic Iden8fica8on
of H2O ice
see also Combe et al., Science (2016), for detection in Oxo Crater
Spectra
relative to
surrounding
area

11. Ermakov et al., GRL, 2017
72˚E
72˚E
76˚E
76˚E
80˚E
80˚E
84˚E
84˚E
88˚E
88˚E
85˚30'N 85˚30'N
85˚45'N 85˚45'N
86˚00'N 86˚00'N
86˚15'N 86˚15'N
86˚30'N 86˚30'N
86˚45'N 86˚45'N
87˚00'N 87˚00'N
0 1 2
km
NP4
S
S
Axis tilt 𝜖
varies from
2° to 19°
(Bills & Scott
2017)
last obliquity
maximum
was 14kyr
ago

12. Water Sources ⇔ Bright Deposits
1. Exogenic sources: infall from comets, IDPs, icy asteroids, ...
2. Endogenic sources: Retreating subsurface ice, Impact-
excavated ice, exposure by avalanches, ...
3. Solar wind – Surface interaction: not likely on Ceres
Retreating ice crust: 0.003 kg/s (Prettyman et al., Science,
2016) ⇒ 1µm of ice trapped in the last 14 kyr ⇒ not
enough to create optically bright deposits since last
obliquity maximum
Lack of optically thick ice deposits (<100 µm) in most PSRs
⇒ <109 kg of H2O were delivered to Ceres in the last
14kyr

13. Mercury – Earth’s Moon – Ceres
• Mercury: all cold traps filled with ice
• the Moon: few faint ice deposits - why different?
• Ceres: ice in some cold traps
Cold-trapping works on Mercury and on Ceres, so it also
ought to work on the Moon. Perhaps the Moon has
almost no supply of water.
Nomenclature:
permanent? persistent? perennial? secular?

14. Conclusions
! Persistently Shadowed Regions (PSRs) on Ceres
mapped with two methods (~1,800 2,100 km2 in the
northern hemisphere)
! Bright crater floor deposits (BCFDs) in a small fraction
of current PSRs, consistent with location of PSRs at
maximum obliquity
! One briefly illuminated BCFD is spectroscopically
identified as H2O ice.
! Cold-trapping works on Ceres (and Mercury). Water ice
deposits provide upper bound on water delivery.
Schorghofer et al., Geophys. Res. Lett. 43, 6783-6789 (2016)
Platz et al., Nature Astronomy 1, 0007 (2016)
Ermakov et al., Geophys. Res. Lett., in press (2017)