Ceres' Darkest Secret: Ice in Permanently Shadowed Craters
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
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
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)
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)