Lunar Volatiles: New Perspectives from Diviner Observations

Paul Hayne1, Oded Aharonson2,1, David Paige3, and the Diviner Lunar Radiometer Team

1California Institute of Technology2Weizmann Institute of Science

3University of California, Los Angeles

June, 2012

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

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)

Ice Sublimation and Lag Formation

• 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

0p

( )v Tp p

IR emission to space

heat

H2O (g)

solar

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”

Diviner Observations

Polar Temperatures and Distribution of Cold Traps

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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

Diviner’s independent two-axis actuators allow targeting independent of the spacecraft

~ 4 km footprint

100

101

102

103

0

0.2

0.4

0.6

0.8

1T = 100 K blackbody

wavelength (mm)

scal

ed ra

dian

ce

Paige et al. (2010)

Mean annual temp.

Paige et al. (2010)

The LCROSS Mission

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• LCROSS Shepherding Spacecraft (SSc) equipped with a suite of remote sensing instruments, including UV/VIS and NIR spectrometers

Goal of the LCROSS mission: probe the subsurface of a lunar cold trap and see what comes out

Stability of Volatiles at the Lunar Poles

(Paige et al., 2010)

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, )

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, ?

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

Obliquity Effects: Mean Annual Temperature

Present day: 1.5 4

8 12Siegler et al. (2011)