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water ice is neither unique nor conclusive. 5 - 7 This paper assumes that the
observations are indicative of water ice and interpret them in that context.
There are differences in the observed deposits at the Moon and Mercury
indicating some diversity in the histories of the two bodies. In this paper, we
compare the observations at the Moon and Mercury and model simulations
of the deposits to understand the distinctions and their implications.
2. Volatile Evolution in Permanent Shadowed Regions
The current depth distribution of volatiles in a PSR is partly a result of the
source of volatiles. The amount of time over which emplacement occurred is
of particular importance. An episodic source of ice, e.g., a cometary impact,
would deposit a relatively pure layer of water ice in a short time. In contrast,
a continuous source of water ice, like migration of water vapor through the
exosphere after release from the surface by micrometeoroid bombardment
or solar wind sputtering, would slowly and steadily add volatiles to the
PSR over time. Since there is always new material arriving on the Moon,
the migrating water would be mixed into the regolith. The expected depth
profile is a thick, impure deposit.
Space weathering also affects the depth distribution of volatiles in the
PSR. Space weathering is a term for the collection of processes that mod-
ify the properties of a planetary surface from exposure to radiation and
impact. After the volatiles condense in the PSR, they are retained nei-
ther completely nor without interaction with the surrounding regolith.
Some volatiles are lost due to incident starlight, scattered sunlight, and
sputtering. 8 However, impacts are the primary modifying factor by moving
material both vertically and laterally. Each successive impactor excavates
material in one locality and buries material in an adjacent locality. The
depth of alteration varies with impactor size and energy. Small impactors
are more frequent than large impactors; so most impacts affect only a small
area. Thus, every piece of the surface has undergone its own unique impact
history and will have a different concentration profile with depth. Material
that may be vaporized from an impactor can recondense when it arrives
in an adjacent location in the PSR. 9 In this way an existing ice layer gets
mixed with the surrounding regolith with time. The crater production func-
tion has a very similar shape for Mercury and the Moon; however, Mercury's
flux is 5.5 times greater than the Moon's. 10 , 11
Our model simulates the evolution of volatiles in a column of regolith in
aPSR. 12 It considers various sources of volatiles by slowly adding volatiles
over time or by starting with a pure ice layer. 13
It predicts the depth
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