Near-Surface Temperatures on
Mercury and the Moon and the
Stability
of
Polar
Ice
Deposits
Ashwin R. Vasavada, David A. Paige,
Stephen E. Wood
Icarus (October 1999)
Tim Wasserman
TERPS Conference
Outline
• Background
• Thermal Modeling of
– Flat Surfaces
– Craters
• Results
• Comparison With Observations
• Summary & Conclusions
Background
• Ice delivered to Moon and Mercury by
comets, asteroids, dust, outgassing, etc.
• Water molecules in sunlit area hop around
until they dissociate, ionize, get lost…
• Some will fall into permanently shadowed
areas inside of craters near the polar
regions
• May be cold enough to prevent
sublimation over long timescales
Background, continued…
• Arecibo radar observations of Mercury
show ice-like radar features in craters
• Clementine spacecraft found radar
features that could be explained by water
ice near the lunar poles
• Lunar Prospector findings suggest large
amounts of Hydrogen present on Moon
• Purpose of this study was to model the
near-surface temperature environments
and determine if water, if present, could
survive for substantial time periods
Flat Surface Thermal Modeling
• Two layer model
– Top: 2cm thick, highly insulating
– Bottom: denser, more conductive
• Define density, thermal conductivity,
albedo, infrared emissivity for each layer
• Internal heat flux, orbit position, orientation
• Run this model through timesteps
Daily Surface Temperatures
• Equator Surface
Temperature over the
course of a day on
Mercury and the
Moon
Variations With Latitude
• Maximum surface
temperature (solid)
• Maximum
temperature at depth
(dashed)
• Varies with latitude
Crater Thermal Modeling
• Bowl shaped & flat-floored
• 32 x 32 square grid of oriented flat
surfaces
• Takes into account topography
– Sunlight scattering off of crater walls & floor
– Infrared radiation from walls & floor
– Curvature of planet
• For each timestep calculate flux on each
element due to sun and other elements
Crater on Mercury
OK, Temperatures…Now what?
• At 110K, it takes 1 billion years for a 1
meter layer of water ice to evaporate
• Also at 110K, influx of water from
meteorites & asteroids balances global
loss rate
• Therefore, assume ice at 110K is stable
Mercury
Max
Average
Moon
Max
Average
Summary
• Modeled the temperature environments of
flat surfaces and craters on Mercury and
the Moon
• Determined if water ice is stable in these
regions
• Compared to actual crater observations
Conclusions
• Unshaded, surface ice deposits are not stable on
either body
• Unshaded, subsurface ice deposits are stable within
2º latitude of lunar poles
• Ice deposits in permanently shaded portions of
craters are stable 10º from pole of Mercury and 13º
from pole of the Moon
• Ice deposits are stable in all craters on Mercury
observed to have ice-like radar responses, although
some deposits must be buried under regolith to
escape diurnal temperature swings. Lunar
References
• Vasavada, A.R., et al. 1999. Near Surface Temperatures on
Mercury and the Moon and the Stability of Polar Ice
Deposits. Icarus. 141, 179-193.
• Clementine Website. http://www.cmf.nrl.navy.mil/clementine/.