Sky & Telescope - January 2021 - 36

Lunar Exploration

W ICY FROSTING This composite map, made at three infrared wavelengths by a NASA spectrometer aboard India's Chandrayaan 1 spacecraft, uses blue to denote small amounts of water and hydroxyl (OH) on
the surface of the Moon. Their abundance clearly increases nearer the
north and south poles.

floor. During the Centaur's impact, NASA's Lunar Reconnaissance Orbiter recorded a momentary flash and kept an eye on
the temporary cloud of material. Its findings electrified the
planetary-science community: Water ice and other volatiles
were clearly detected.
Nearly 10 years later, Shuai Li (University of Hawai'i) and

colleagues combed more carefully through M3's data and
found spectroscopic evidence of light reflecting off water ice
in many PSR locations surrounding the lunar poles. While
less than 4% of the total sunless surface exhibits the spectral
features of water ice, here and there up to 30% ice (by mass)
might be mixed with the dusty regolith.
"Nobody believed there would be useful information in
data collected from PSRs," Li explains. After all, how can
you get reflectance spectra from a place without light? But
Li decided to look after Apollo 15 astronaut Dave Scott told
him that the Moon's rough topography can scatter light into
shadows. Studying data with such weak signals was arduous,
but the researchers found what they were looking for.
Meanwhile, other scientists using new laboratory techniques found trapped water inside Apollo samples from the
Moon (S&T: Aug. 2018, p. 26). The Apollo-era paradigm of a
bone-dry Moon was dead.

The Moon's Water Cycle
Water molecules have accumulated in shadowed regions over
eons through direct deposit by other bodies, such as comets,

180°
180

°N

60°

300°

°N

85

85

120°

S
0°

90
9
90°

270°°

90
9
90°

270°
0°°

60°

300°

120°

240°

30°

330°

by M3, LOLA, and Diviner

150°

210°

0°

* Ice exposures constrained

°S

180°

<60

110

160

230

290

>320

Diviner annual maximum temperature (kelvin)

* Ice exposures constrained

by M3, LOLA, Diviner, and LAMP

S TREASURE MAPS Colored dots show where water ice exists in the floors of permanently shadowed craters within 10° of the Moon's north (left)
and south poles. These dots represent positive detections using a combination of instruments on the Lunar Reconnaissance Orbiter and Chandrayaan 1 spacecraft. Curiously, the coldest crater floors don't always have ice deposits - one notable example is the large crater Amundsen, on the 90°
longitude line at the south pole. A closer look at the south pole is on the facing page.

36

JANUARY 2 021 * SK Y & TELESCOPE

INFR A RED M A P: ISRO / N ASA / JPL / BROWN UNIV. / USGS; POLE M A PS:
SHUAI LI E T A L. / PNAS 2018 / CC -BY-NC -ND 4.0

240°

30°

330°

8

80

0
0°
150°

210°



Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021

Contents
Sky & Telescope - January 2021 - Cover1
Sky & Telescope - January 2021 - Cover2
Sky & Telescope - January 2021 - 1
Sky & Telescope - January 2021 - Contents
Sky & Telescope - January 2021 - 3
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