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Lunar Volatiles: New Perspectives from Diviner Observations

Lunar Volatiles: New Perspectives from Diviner Observations. Paul Hayne 1 , Oded Aharonson 2,1 , David Paige 3 , and the Diviner Lunar Radiometer Team 1 California Institute of Technology 2 Weizmann Institute of Science 3 University of California, Los Angeles June, 2012.

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Lunar Volatiles: New Perspectives from Diviner Observations

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  1. Lunar Volatiles: New Perspectives from Diviner Observations Paul Hayne1, Oded Aharonson2,1, David Paige3, and the Diviner Lunar Radiometer Team 1California Institute of Technology 2Weizmann Institute of Science 3University of California, Los Angeles June, 2012

  2. Inefficiency of Jeans Escape • Maxwell-Boltzmann velocity distribution: • Gravitational escape: Watson, Murray and Brown (1961, 1963) showed that gravitationally-bound volatiles will migrate to “cold traps” after N non-destructive hops  Water strongly bound to the Moon by gravity, < 10-6 molecules escape per hop

  3. Cold Trapping • Sublimation rates highly non-linear with temperature • Loss from sunlit areas extremely fast; shadowed areas, extremely slow • WMB1961 showed that even with solar wind sputtering and UV photolysis, water molecules only need hops to reach “permanent” shadow (where K is fractional shadowed area of the Moon) Vasavada et al. (1999)

  4. Ice Sublimation and Lag Formation solar IR emission to space H2O (g) Ice table moves downward as ice sublimates and diffuses through desiccated regolith layer Quasi-steady state can result if sources balance sinks, or if sublimation slow Depth of ice table depends on insolation, regolith composition and porosity heat

  5. Stability of Buried Ice • Lag deposit insulates ice from sublimation and reduces equilibrium vapor pressure • Schorghofer (2008) estimates water ice sublimation temperature rises to 145 K when buried by > 1 m regolith • We can use Diviner measurements to map the depth of the “ice table”

  6. Diviner Observations Polar Temperatures and Distribution of Cold Traps

  7. Diviner Spectral Channels: • 2 solar channels: 0.35 – 2.8 mm • 7 infrared channels: • 7.80 mm • 8.25 mm • 8.55 mm • 13-23 mm • 25-41 mm • 50-100 mm • 100-400 mm Diviner typically operates in “push-broom” mode ~ 4 km footprint Diviner’s independent two-axis actuators allow targeting independent of the spacecraft

  8. Mean annual temp. Paige et al. (2010)

  9. Paige et al. (2010)

  10. The LCROSS Mission Goal of the LCROSS mission: probe the subsurface of a lunar cold trap and see what comes out • LCROSS Shepherding Spacecraft (SSc) equipped with a suite of remote sensing instruments, including UV/VIS and NIR spectrometers

  11. Stability of Volatiles at the Lunar Poles (Paige et al., 2010)

  12. LCROSS Results • Water ice ~6% (3%) abundance by mass • Many other volatiles: Ca, Mg, Na • Also mercury (don’t drink the water!), and silver (Ag, )

  13. LCROSS Results • Majority of observed volatiles predicted by theory along with Diviner temperature measurements • Some surprises: • Methane (CH4), carbon monoxide (CO), • Molecular hydrogen (H2), from LAMP, ?

  14. Obliquity Effects • Siegler et al. (2011) showed polar volatiles must be younger than the “Cassini state transition”, when Moon’s obliquity reached nearly 90 • We do not know when (in time) this occurred unstable time

  15. Obliquity Effects: Mean Annual Temperature 4 Present day: 1.5 8 12 Siegler et al. (2011)

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