Sky & Telescope - January 2021 - 37

°S

X WHERE THE DARK IS A shaded-relief
topographic map of the terrain within 5° of the
Moon's south pole reveals how many craters
have deep floors that are hidden from direct
sunlight (gray shading). Note that some floors
(such as in Cabeus at upper left) are only
partly in permanent shadow.

Elevation (meters)
85
°S

LUN A R COLD -TR A P PROCESSES: G REGG DINDER M A N / S&T, SOURCE: K E VIN
CA NNON; SOU TH POLE M A P: JULIE STOPA R & HE ATHER ME Y ER / LUN A R &
PL A NE TA RY INSTIT U TE

and from volcanic gas that still leaks from the lunar interior. Water ice bonds tightly to bits of lunar dust and rock a
few centimeters below the surface. Micrometeorite impacts
release the water into the Moon's barely-there exosphere;
sunlight's warmth and the solar wind can also send the molVolcanic
OH/H2O from
solar wind
outgassing
ecules hopping about the lunar surface or launch them into
Asteroid and
the exosphere. Some molecules migrate randomly to PSRs,
comet impacts
where the water apparently remains stable. The craters are
very cold, about −240°C (−400°F).
Cold-trapped
Ariel Deutsch (then at Brown University) and colleagues
volatiles
extended the analysis of Li's team by estimating the ages of
craters near the lunar south pole. Most of the permanently
shadowed real estate lies within 20 large craters at least 3.1
S CRYO-CACHING Water can reach the Moon's surface via the impact
of water-rich asteroids or comets, the escape of gas from its deep intebillion years old that contain patchy deposits of surfacerior, or the interaction of energetic solar-wind particles with lunar rocks.
exposed ice. But, puzzlingly, some ancient craters that should
Over time, much of this water vapor migrates to permanently shadowed
have ice deposits do not, possibly having baked during a reori- areas near the lunar poles, where it freezes on contact with the frigid
entation of the Moon's spin axis and a resultant wandering of surface and forms deposits.
the lunar poles long ago.
up to 50 meters shallower than equivalent sites closer to the
Another mystery is why only about a quarter of 100
equator. The likely culprit? Water ice filling the bottoms.
younger craters, all less than 15 km wide, show detectable
Strangely, the shallowing trend does not hold with northexposures of water ice on their hidden floors. This ice is
polar-region craters.
perhaps less ancient, having been delivered relatively recently
All told, Rubanenko and colleagues suggest that there
by micrometeorites (instead of larger impactors in the distant
might be up to 100 billion metric tons of lunar water ice -
past) and by the solar wind stirring up water molecules
roughly the volume of Lake Tahoe, and much higher than
elsewhere on the surface. Or perhaps the water originated
others have estimated.
in some of the large ancient craters nearby. Some researchers suggest that the ice in both older
and younger craters should be roughly
Malapert
0°E
the same age, because much older ice
would have been destroyed over time
by impacts and other causes. No one
knows for sure.
Cabeus
There's a lot we don't know. Says
Deutsch: "What are the origins of the
Haworth
ice? What processes are responsible for
the delivery, modification, and destruction of ice, and what are the relative
Shoemaker
Sh
strengths of these processes? What is
Ibn Bajja
Amundsen
the physical form of the ice (thickness,
abundance, texture, etc.)? How does
Faustini
de Gerlache
Gerlac
che
ice vary with depth [and] on lateral
270°E
90°E
Shackleton
Shackleto
on
length scales?"
Slater
There may even be more icy craters
89
than scientists have suspected. Lior
Rubanenko (Stanford) and colleagues
Sverdrup
evaluated the depth and slope of
thousands of lunar craters and found
that the ones near the south pole are
Idel'son L
7,010 (high)

Wiechert P

−105
−7,220 (low)

Wiechert
iechert J
Kuhn

180°E

sk yandtelescope.org * JANUARY 2 021

37


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Sky & Telescope - January 2021

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