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The authors thank S. S. Jha for useful discussions and comments.
We thank S. Mukhopadhyay, R. Kulkarni, and D. D. Buddhikot
for valuable technical help at the early stages of this work.
This work was financially supported by the Tata Institute of
Fundamental Research, Mumbai, India. The authors declare no
competing financial interests. The project was planned by
S.R. Single crystals were grown and characterized by O.P. and
A. T. All the measurements were conducted by O.P. and A.K.
The data analysis was performed by O.P. and S.R. The
manuscript was prepared by S.R. and O.P. and discussed
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Materials and Methods
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5 April 2016; accepted 11 November 2016
Published online 1 December 2016
Extensive water ice within Ceres’
aqueously altered regolith: Evidence
from nuclear spectroscopy
T. H. Prettyman,1 N. Yamashita,1 M. J. Toplis,2 H. Y. McSween, 3 N. Schörghofer, 4
S. Marchi, 5 W. C. Feldman,1 J. Castillo-Rogez, 6 O. Forni,2 D. J. Lawrence, 7
E. Ammannito, 8 B. L. Ehlmann, 6, 9 H. G. Sizemore,1 S. P. Joy, 8 C. A. Polanskey, 6
M. D. Rayman, 6 C. A. Raymond, 6 C. T. Russell8
The surface elemental composition of dwarf planet Ceres constrains its regolith ice
content, aqueous alteration processes, and interior evolution. Using nuclear spectroscopy
data acquired by NASA’s Dawn mission, we determined the concentrations of elemental
hydrogen, iron, and potassium on Ceres. The data show that surface materials were
processed by the action of water within the interior. The non-icy portion of Ceres’
carbon-bearing regolith contains similar amounts of hydrogen to those present in
aqueously altered carbonaceous chondrites; however, the concentration of iron on Ceres
is lower than in the aforementioned chondrites. This allows for the possibility that
Ceres experienced modest ice-rock fractionation, resulting in differences between
surface and bulk composition. At mid-to-high latitudes, the regolith contains high
concentrations of hydrogen, consistent with broad expanses of water ice, confirming
theoretical predictions that ice can survive for billions of years just beneath the surface.
With a measured bulk density between ice and rock, dwarf planet Ceres is expected to be rich in volatiles, with about 17 to 30 weight (wt %) water ice (1, 2). In- ternal heating generated by the decay
of radioactive elements may have driven aque-
ous alteration and differentiation to form a
rocky interior and icy outer shell ( 3, 4). Orbital
measurements by Dawn’s Visible and Infrared
Mapping Spectrometer (VIR) show that Ceres’
global surface contains aqueous alteration prod-
ucts: ammoniated clays, serpentine, and car-
bonates (1). Localized deposits of surficial water
ice are present but rare ( 5); however, the high-
latitude surface is sufficiently cold that water ice
can survive within a meter of the surface over
Ceres’ 4.5-billion-year lifetime ( 6). Physical differ-
entiation allowed by gravity measurements ( 7)
may have resulted in chemical fractionation, with
surface regions that are not compositionally rep-
resentative of the bulk. We refer to this style of
differentiation as “ice-rock fractionation,” which
can be tested by comparing the composition of
Ceres’ surface to CI and CM carbonaceous chon-
drites. Although often invoked as analogs for
Ceres (1), these primitive meteorites probably
experienced isochemical aqueous alteration on
smaller parent bodies ( 8).
NASA’s Dawn mission aimed to map the chemical composition of Ceres’ uppermost regolith,
providing constraints on origins and evolution.
In December 2015, the Dawn spacecraft entered
a circular polar, low-altitude mapping orbit
(LAMO), with a mean altitude of 0.82 Ceres
body radii. In LAMO, Dawn’s Gamma Ray and
Neutron Detector (GRaND) ( 9) is sensitive to elemental composition. GRaND measures gamma
rays and neutrons produced by the steady interaction of galactic cosmic rays and gamma
rays from the decay of radioelements within a
few decimeters of the surface. Elemental analyses presented here use data accumulated over
5 months in LAMO.
Maps of neutron and gamma-ray counting
rates, corrected for temporal changes in galactic
cosmic ray flux and measurement geometry ( 10),
reveal spatial variations in surface elemental composition (Fig. 1). The rate of 6Li(n,a) reactions
in GRaND’s lithium-loaded glass scintillator depends on the flux of thermal and epithermal
neutrons, which varies inversely with regolith
H content. Neutron capture by Fe in Ceres’ regolith produces a gamma-ray doublet at 7. 6 MeV.
These signatures vary strongly with latitude,
with counts decreasing toward the poles. Longitude variations are comparatively small. The
dynamic range of the composition data is larger
than measured by GRaND at Vesta ( 11).
In Fig. 2, mapped counting data were normalized to globally averaged measurements of Vesta
and compared with models of analog materials.
On Ceres, hydrogen is predominantly in the form
1Planetary Science Institute, 1700 East Fort Lowell, Suite
106, Tucson, AZ 85719-2395, USA. 2Institut de Recherche
d’Astrophysique et Planétologie, CNRS, Université Paul
Sabatier, Toulouse 31400, France. 3Department of Earth and
Planetary Sciences, University of Tennessee, Knoxville, TN
37996-1410, USA. 4University of Hawaii, 2680 Woodlawn
Drive, Honolulu, HI 96822, USA. 5Southwest Research
Institute, Boulder, CO 80302, USA. 6Jet Propulsion
Laboratory (JPL), California Institute of Technology,
Pasadena, CA 91109-8099, USA. 7Johns Hopkins University,
Applied Physics Laboratory, Laurel, MD 20723, USA. 8Earth
Planetary and Space Sciences, University of California, Los
Angeles, Los Angeles, CA 90095-1567, USA. 9Division of
Geological and Planetary Sciences, California Institute of
Technology, Pasadena, CA 91125, USA.
*Corresponding author. Email: email@example.com