D. Jewitt, A. Guilbert-Lepoutre
We present optical spectra of (24) Themis and (65) Cybele, two large
main-belt asteroids on which exposed water ice has recently been reported. No
emission lines, expected from resonance fluorescence in gas sublimated from the
ice, were detected. Derived limits to the production rates of water are < 400
kg/s (5{\sigma}), for each object, assuming a cometary H2O/CN ratio. We rule
out models in which a large fraction of the surface is occupied by high albedo
("fresh") water ice because the measured albedos of Themis and Cybele are low
(0.05 - 0.07). We also rule out models in which a large fraction of the surface
is occupied by low albedo ("dirty") water ice because dirty ice would be warm,
and would sublimate strongly enough for gaseous products to have been detected.
If ice exists on these bodies it must be relatively clean (albedo >0.3) and
confined to a fraction of the Earth-facing surface <10%. By analogy with
impacted asteroid (596) Scheila, we propose an impact excavation scenario, in
which 10 m scale projectiles have exposed buried ice. If the ice is even more
reflective (albedo >0.6) then the timescale for sublimation of an optically
thick layer can rival the 10^3 yr interval between impacts with bodies this
size. In this sense, exposure by impact may be a quasi steady-state feature of
ice-containing asteroids at 3 AU.
View original:
http://arxiv.org/abs/1111.3292
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