ICE ON THE MOON AND MERCURY - Advances in Geosciences - page 98

a 20 cm thick initial ice deposit from 100 to 500 to 1,000 Myr. In these
simulations, a steady source of water vapor was included at the rate of
1 . 8 10 − 15 g / cm 2 s. 22

The effect of the steady source alone is given in the thick, gray line. 12
As time goes on, the enriched layer thickens, but does not become more
concentrated. One curve (dashed gray) shows the evolution of a 10 cm thick
ice deposit after 1,000 Myr without a steady source of water vapor. Such
an ice deposit would be undetectable by LPNS.
The gray dotted curve on Fig. 2 depicts the depth profile of an ice
deposit initially 10 cm thick. It is possible that several thin ice layers have
been deposited in the PSR of the Moon from cometary impacts. We would
expect this to result in several thin, enriched layers of water ice interspersed
with drier regolith. As long as the total thickness of the ice layers deposited
within the last 500 Myr is < 10 cm, the model simulations are consistent
with the data. 13

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4. Conclusions
Our model suggests that the putative ice deposits in the PSR of Mercury
were emplaced very recently, < 50 Myr ago. Space weathering on the Moon
would not remove the signature of a recent ice layer on the Moon. If rel-
atively pure water ice layers exist in lunar PSRs, our model suggests they
were emplaced quite long ago ( > 500 Myr) and are buried too deep for
detection by neutron spectroscopy. More likely is that any cometary ice
on the Moon began as a thin layer ( < 10 cm).
Gradual accumulation of water vapor (from migration through the
atmosphere) cannot explain the observations of pure ice on Mercury, but
may contribute to the inventory in addition to cometary ice layers. Gradual
accumulation of water vapor is consistent with the observations of < 10%
water ice on the Moon. Further, if gradual accumulation of water is occur-
ring, it limits the thickness of any ice layer that would be consistent with
the lunar observations.
The burial rate of regolith at Mercury (0.43 cm/Myr) exceeds that at
the Moon (0.16 cm/Myr) by a factor of about 3. The slow burial rate at the
Moon allows the slow source to accumulate before being buried whereas the
fast burial rate at Mercury tends to dilute the slow source. Meanwhile, ice
stays near the surface longer at the Moon, thus is subjected to loss from
exposure more than at Mercury.