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TEMPERATURE OF REGOLITH IN COLD TRAPS ON THE MOON
A.A. Berezhnoi
Sternberg Astronomical Institute
Moscow, Russia

The hypothesis of the existence of the lunar ice in cold traps was
stated in (Watson et al., 1961). Hydrogen-containing compounds in polar
regions on the Moon were detected by Lunar Prospector (Feldman et al.,
1998).
In the paper (Vasavada et al., 1999) the stability of ices on the Moon
and Mercury on the surface of cold traps was studied. But volatiles can
also exist under the surface, because here mechanisms of removal of such
compounds (micrometeoritic bombardment, cosmic rays, sublimation) are less
effective.
Let us consider the question of the dependence of the regolith
temperature in cold traps on depth. We assume that the concentration of
water ice does not vary with the depth. We shall using the following
formula (mix = Cice(ice + (1-Cice)(reg , where (ice is the heat
conductivity of crystalline water ice, (reg is the heat conductivity of dry
regolith, (mix is the heat conductivity of wet regolith, and Cice is the
relative molar concentration of water ice in regolith. We assume that the
heat flow from the lunar interior is equal to 2.2*10-6 W/m2, the character
of dependency of the heat conductivity from the depth is the same in polar
dry and equatorial regoliths (Langseth, Keim, 1977), the heat conductivity
of the polar regolith depends on temperature in accordance to data
(Cremers, Hsia, 1974) for equatorial regolith.
These assumptions may be incorrect because the conditions of polar and
equatorial regolith formation are different.
(Arnold, 1979) estimated the mean temperature on the surface of lunar
cold traps as 40-90 K. We plotted the dependence of regolith mean
temperature on depth for these two values of mean temperature on the
surface (see figure 1). It can be seen that an addition of even 0.1% water
ice to the dry regolith leads to the disappearance of the difference
between mean temperatures on the surface and at 1-2 cm depths.
Let us consider the sublimation of volatiles from cold traps. The
intensity of sublimation was calculated according to (Fanale, Salvail,
1989), the vapour pressure for water, sulfur and carbon dioxides was taken
from (Kikoin, 1976).
It is clear that H2O, SO2, and CO2 ices can exist at 30 cm depth if
the regolith mean temperature here is less than 145, 105, and 80 K
accordingly (see figure 2).
The comparison of data on figures 1,2 leads us to the following
conclusions. If the regoliths mean temperature at 1-2 cm depth is the same
as the surface one, the cold-trap H2O, SO2, CO2 ices are stable.
If the regolith in the upper 1-2 cm layer does not contain water ice
then the mean temperature at 20-30 cm depths is 50-60 K higher than on the
surface and SO2, CO2 ices are unstable in these conditions.
Our results can be checked during the observations of the thermal
emission of polar lunar regolith at 0.1 mm - 10 cm. If the mean radio
temperature of the polar regolith does not increase with increasing
wavelength, this fact can be considered indirect evidence for the existence
of water ice on the Moon.


Arnold J.R. Ice in the lunar polar regions, J. Geophys. Res., V. 84, No.
B10, p. 5659-5668, 1979

Cremers C.J., Hsia A. Thermal conductivity of Apollo 16 lunar fines, Proc.
Lunar Sci. Conf. 5th, V. 3, p. 2703-2708, 1974

Fanale F.P., Salvail J.R. Loss of water from Fobos, Geophys. Res. Lett., V.
16, No. 4, p. 287-290, 1989

Feldman W.C., Maurice S., Binder A.B. et al. Fluxes of Fast and Epithermal
Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles,
Science, V. 281, No. 5382, p. 1496-1500, 1998

Langseth M.G., Keim S.J. In-Situ Measurements of Lunar Heat Flow// The
Soviet -American Conference on Cosmochemistry of the Moon and Planets, ed.
Poeroy J.H., Hubbard N.J., p. 283-293, Washington, NASA, 1977

Vasavada A.R., Paige D.A., Wood S.E. Near-Surface Temperatures on Mercury
and the Moon and the Stability of Polar Ice Deposits, Icarus, V. 141, p.
179-193, 1999

Watson K., Murray B.C., Brown H., The behavior of volatiles on the lunar
surface, J. Geophys. Res., V. 66, No. 9, p. 3033-3045, 1961





Fig. 1. Dependence of the mean regolith temperature in cold traps from the
depth at different mean temperature on the surface and relative molar
concentration of water ice Cice. Dashed line shows the dependence of mean
regolith temperature from the depth; under these conditions water ice with
Cice = 1 % sublimate during 109 years from cold trap.

Fig. 2. Dependence of sublimation time for H20, SO2, CO2 ices from the mean
regolith temperature at different depths. In the all cases the relative
molar concentration of ices in the polar regolith is equal to 1%.


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