PYTS 395B – Detecting Ice on the Moon 1 l Mercury (and the Moon) possesses a tenuous atmosphere Calcium now also seen at Mercury l Sodium emission at the Moon and Mercury shows temporal changes n Stirring of regolith by small impacts

PYTS 395B – Detecting Ice on the Moon 2 l Evidence for ice in polar craters of Mercury n Evidence for ice at lunar poles wClementine bi-static radar wLunar prospector neutron n Evidence for ice at poles of Mercury wVLA radar returns l Sungrazing comets n Kreutz group n Source of water?

PYTS 395B – Detecting Ice on the Moon 3

4 Feldman et al., 1998

PYTS 395B – Detecting Ice on the Moon 5 Bussey et al., 1999

PYTS 395B – Detecting Ice on the Moon 6 Margot et al., 1999

PYTS 395B – Detecting Ice on the Moon 7 l Modeling (Vasavada et al. 1999) shows temperatures in permanently shadowed craters are very low n These cold traps are favored condensation sites Vasavada et al., 1999

PYTS 395B – Detecting Ice on the Moon 8 l Water leaves cold traps by sublimation n 5-15% returns on Mercury n 20-50% returns on the Moon n The rest is lost l Water can be delivered by meteors and comets n For Mercury these rates have been estimated n Balance exists if T ice is ~113K Killen et al., 1997

PYTS 395B – Detecting Ice on the Moon 9 l Lunar cold traps dry compared to Mercury n Recent comet impact? l Polar inconsistency? n More H in lunar north pole n More permanent shadows in lunar south pole n Burial depth differences n Dispersed grains?