Australian Sky & Telescope - April 2018

54 AUSTRALIAN SKY & TELESCOPE April 2018

EXPLORING THE MOON by Charles Wood

The Moon’s watery secrets

Look closely, and you can spot places where water exists on the lunar surface.

T

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 half a dozen locations on the lunar

surface. However, thanks to the Moon

Mineralogy Mapper (M^3 ), a visible-

infrared spectrometer on the Indian

lunar orbiter Chandrayaan 1, we’ve

known for nearly 10 years that small

amounts of hydrogen-oxygen bonded

molecules — as either water (H 2 O) or

hydroxyl (OH) — exist in the uppermost

millimetres 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 is produced when high-energy

protons (hydrogen nuclei) in the solar

wind slam into lunar soils and release

oxygen, forming temporary molecules

of H 2 O that are driven toward the poles

by day-night temperature fluctuations.

The new mapping has also identified

localised 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 in broad pyroclastic deposits

(involving gas-driven eruptions) —

and the biggest, containing up to

500 ppm water, covers the northern

half of the Aristarchus Plateau.

Frothy fire fountains erupted from

the ‘Cobra Head’ volcanic cone when

A17

DV

OCEANUS

PROCELLARUM

MARE

NUBIUM

MARE

IMBRIUM

MARE

TRANQUILLITATIS

A

SG

A15

Aristarchus

Herodotus

“Cobra

Head”

250 km

MONTES

AGRICOLA

Val

lis^ Schröter

i

SA map of the Moon’s infrared brightness 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, Doppelmayer-Vitello

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. S. LI & R. MILLIKEN /

NATURE GEOSCIENCE

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