Geoscience Reference
In-Depth Information
distribution of volatiles as a function of time by Monte-Carlo calculations
of a large number of independent columns of regolith through time as space
weathering processes modify them. The profile obtained by averaging over
all Monte-Carlo runs of a single initial condition can be viewed as a large-
area footpoint average of the depth distribution. The model is described in
detail in Refs. 12-14. Although our previous work on the Moon has followed
the evolution of H content, all lunar calculations are converted to equivalent
H
2
O concentration, [H
2
O], for presentation in this paper.
3. The Observations
Mercury and the Moon have different signatures of ice in the polar regions.
This is due in part to the techniques used to measure the regions. However,
real differences in the contents of the PSR probably also exist.
One common observation is radar. Mercury's north polar region
observed with the Arecibo radar shows evidence for possible water ice in
PSRs
4
extending down to 72
◦
latitude.
15
The polarization signature is con-
sistent with relatively pure water ice of several tens of cm thickness covered
by
30 cm of dry regolith (Personal communication). Whereas Mercury
data indicate pure ice deposits buried by dry regolith; lunar data suggest
that no pure ice is there.
16
In addition, neutron measurements have been conducted in orbit of the
Moon and are planned for Mercury. A decrease in epithermal neutron flux
is an indicator of the presence of hydrogen, although the chemical form of
the hydrogen cannot be determined.
3
Hydrogen concentration in approxi-
mately the top meter of the lunar regolith has been determined from Lunar
Prospector Neutron Spectrometer (LPNS) measurements.
17
The highest
hydrogen concentration, and thus the highest potential for water, is out-
side of the largest craters in northern hemisphere and inside small craters in
the southern hemisphere. The data are consistent with
<
10% (H
2
O) spread
throughout top 1 m of regolith.
18
,
19
∼
3.1.
Mercury
The Mercury observations put some strict constraints on the ice deposits
there. Figure 1 shows example profiles from the Mercury simulations. Three
solid lines show the evolution with time of an ice layer initially 50 cm thick.
The thinnest line depicts the profile after 20 Myr. The medium and thick
lines are from 50 to 80 Myr, respectively. The broken lines contrast profiles
from ice layers of different initial thicknesses at 50 Myr for comparison with
