Sky and Telescope - February 2018 - 52

FEBRUARY 2018 OBSERVING
Exploring the Moon by Charles Wood

Dry No More
Look closely, and you can spot places where water exists on the lunar surface.

lunar orbiter Chandrayaan 1, we've
known for nearly 10 years that small
amounts of hydrogen-oxygen bonded
molecules - as either water (H2O) or
hydroxyl (OH) - exist in the uppermost
millimeters of the lunar regolith.
A recent detailed reanalysis of these
data by Shuai Li and Ralph Milliken
(Brown University) conﬁrms that
the Moon's water abundance steadily
increases poleward of about 30°, not
counting what might lie frozen on the
shadowed ﬂoors of craters, with a maximum of 500 to 700 parts per million
(ppm) at latitudes above 60° surrounding each pole. Water abundance is below
100 ppm within 30° of the equator.
This uneven distribution strongly
implies that surface temperature
dictates the water's abundance. Probably this latitudinally dependent water

he samples returned by six crews
of Apollo astronauts from 1969 to
1972 showed us, ﬁnally, what the Moon
was made of. Completely unlike the
sopping-wet Earth, those lunar rocks
revealed no evidence of water or even
any minerals remotely related to water.
I vividly recall the ﬁrst science results
from Apollo 11 and hearing the phrase,
"The Moon is bone dry."
But during the last decade teams of
researchers have used improved laboratory instrumentation to reveal water
chemically bound within some lunar
samples, suggesting that it came from
the Moon's interior.
The Apollo samples tell us about
only a half dozen locations on the lunar
surface. However, thanks to the Moon
Mineralogy Mapper (M3), a visibleinfrared spectrometer on the Indian

MARE
IMBRIUM

A17

A15

OCEANUS
PROCELLARUM

is produced when high-energy protons
(hydrogen nuclei) in the solar wind
slam into lunar soils and release oxygen, forming temporary molecules of
H2O that are driven toward the poles by
day-night temperature ﬂuctuations.
The new mapping has also identiﬁed
localized exposures of water, nowhere
near the poles, that seem to be permanent (not solar-wind driven). Nearly all
of these involve volcanic deposits, consistent with water's detection in some
samples collected during Apollo missions. Some of these water-rich volcanics
are easy to observe visually, allowing
your telescopic views to connect you to
the latest lunar discoveries.

Trails of Dark Evidence
Li and Milliken ﬁnd that the most
obvious water-bearing exposures occur

ES A
NT OL
O
M RIC
AG

MARE
TRANQUILLITATIS
Va

l l is

Schrö

Aristarchus

Herodotus

MARE
NUBIUM
250 km

p A map of the Moon's infrared brightness at 2.85 microns reveals the distribution of water (or its radical, OH) in concentrations ranging from less
than 50 parts per million (dark blue) to more than 300 ppm (red). Labels mark locations discussed in the text: A, Aristarchus region; DV, DoppelmayerVitello region; A15 and A17, landing sites of Apollo 15 and 17, respectively; and SG, rilles near Sulpicius Gallus crater. Right: The lunar surface north
and west of Vallis Schröteri contains up to 500 ppm of water either adsorbed by pyroclastic particles or chemically bound to them.

FE B RUA RY 2 018 * SK Y & TELESCOPE

S. LI & R. MILLIK EN / NATUR E GEOSCIENCE (2)

"Cobra
Head"

Sky and Telescope - February 2018

Table of Contents for the Digital Edition of Sky and Telescope - February 2018