near-surface temperatures on mercury and the moon and the stability of polar ice deposits ashwin r....

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 2004

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 observations are also consistent with the results

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/.