O
Keith Cooper
Water on the Moon
ur Moon is quite the paradox. It contains many seas – Australe, Crisium, Imbrium,
Tranquillitatis, to name just a few – but very little water. e lunar rocks that the intrepid
Apollo astronauts brought back with them contained so little water that most scientists assumed at
the time that the water must have contaminated the rocks after arriving on Earth.
Yet now we know that the Moon isn’t quite as bone-dry as had been assumed. In October, NASA
made what they claimed was a huge discovery – water on the sunlit surface of the Moon. It wasn’t a
huge amount, the equivalent of a Coke can’s worth per football pitch-sized area, or between 100 and
412 parts per million. But hadn’t we been here before? is is not the rst time that NASA, or other
space agencies, have discovered water on the Moon, so what’s going on?
Here is the story of water on the Moon – where it comes from, what it can tell us about the Moon’s
birth and evolution, and how it could be used by humans in the future.
PERMANENT SHADOWS
ose aforementioned lunar seas, or maria, were so named because, in the ancient past, Greek and
Roman astronomers mistook them for bodies of water on the distant Moon. Today we know that
indeed they did used to be oceans, but oceans of lava, not water.
Instead, the water present on the Moon seems to come in three forms. One is as ice surviving in
permanently shadowed craters at both lunar poles. Unlike Earth with its 23.5-degree tilt, the Moon’s
obliquity is fairly low, just 6.7 degrees. As a result, the Moon doesn’t lean over that much relative to
the Sun, so at the poles the Sun lurks constantly on the horizon and deep craters never see sunlight.
e temperatures in them plunge as low as –250 degrees Celsius. e water present within them,
locked away in eternal darkness – at least until some astronaut or rover comes along with a torch – is
thought to have come from countless cometary impacts over billions of years. One theory is that the
permanently shadowed regions act as cold traps for vaporised water molecules that migrate across the
sunlit lunar surface until nding sanctuary in the deep freezers at the poles. Multi-spectral infrared
observations, as well as radar, suggest that the water-ice at the lunar north pole is scattered among
40 permanently shadowed craters containing an estimated 600 million metric tonnes of ice, whereas
the ice at the south pole seems to be more concentrated, particularly inside Shackleton, a 21-
kilometrewide, 4.2-kilometre-deep crater.
Several space missions have even deliberately driven impactors into the permanently shadowed
craters in an effort to unearth the water present there. NASA’s LCROSS (Lunar Crater Observation
and Sensing Satellite) mission received a lot of hype in October 2009, when the upper stage of the
Centaur rocket that had powered the spacecraft towards the Moon months earlier crashed into the
crater Cabeus at the lunar south pole. e impact sent a plume of debris above the crater rim,
allowing LCROSS to spectrally analyse it, nding that it contained 5.6 per cent, ± 2.9 per cent,
water by mass. It was deduced that this water came in small chunks maybe 10 centimetres across, or
as a thin coating over grains of regolith (the dusty dirt lying on the surface), as opposed to a large,
thick glacier. Nevertheless, results from the laser altimeter on NASA’s Lunar Reconnaissance Orbiter
conclude that 22 per cent of nearby Shackleton’s surface is covered in ice.
Yet NASA was not the rst to accomplish this feat. A year earlier, the Indian Space Research
Organisation’s Chandrayaan-1 lunar orbiter deployed an impact probe that slammed hard into the
shadows of Shackleton, releasing a plume of sub-surface debris that showed evidence of water. e
ndings, however, were not made public until later. By then, NASA had already announced that
Chandrayaan-1 had discovered water on the Moon, but via a different instrument – the Moon
Mineralogy Mapper, or M3 for short, which had been built and was operated by NASA, but which
ew on board the Indian spacecraft. M3, supported by data from the Cassini spacecraft as it ew past
the Moon in 1999 while on its way to Saturn, and the EPOXI spacecraft that encountered the
Moon in 2009 on the way to the comet Hartley 2, had detected the spectral signature of water and
hydroxyl (made from one atom of oxygen and one of hydrogen) molecules in the upper few
millimetres of the lunar surface, even in broad sunlight, and with the highest abundances in the
polar regions. Whereas the water in the permanently shadowed craters probably came from comets,
the water detected by M3 is formed when hydrogen ions – simply protons – in the solar wind hit the
Moon’s unprotected surface and react with oxygen atoms locked inside silicate minerals. Four years
later, M3 also showed that there was another type of water on the surface: magmatic water, which is
present in magma deep underground, but which has found its way to the surface following
excavation by asteroid impacts.
DAYLIGHT WATER
Which brings us to the here and now in this potted history of the detection of water on the Moon.
In October 2020, NASA announced that water molecules had been discovered on the oor of the
giant 231-kilometre-wide crater Clavius, while in direct sunlight. is discovery was made by
SOFIA, the Stratospheric Observatory For Infrared Astronomy, which is a 2.5-metre telescope
contained within a Boeing 747SP jet.
Of course, SOFIA was telling us what we already knew – that small quantities of water molecules
exist out in the open on the Moon. What it truly did for the rst time is distinguish between water
and hydroxyl – the previous observations by M3 had seen both types of molecule, but had struggled
to distinguish them. However, since hydroxyl is just one hydrogen atom short of a water molecule, it
made for an excellent proxy for water.
ere is a mystery here. As cold as the permanently shadowed regions become, the sunlit surface can
reach high temperatures of 230 degrees Celsius (although you wouldn’t consider it ‘hot’, since there’s
no atmosphere to contain the heat). Water molecules on the surface should evaporate in double-
quick time, and either retreat into cold traps or escape into space. e water therefore must be
replenished at a relatively high rate. Hydrogen ions in the solar wind, as mentioned earlier, could
contribute, but other mechanisms of water formation are also required. It’s not coming from ice
below the visible surface, as exists on Mars – observations by neutron spectrometer instruments on
various spacecraft in lunar orbit have not been able to positively identify sub-surface ice beyond that
in the permanently shadowed craters. One possibility is that micrometeorites raining down onto the
lunar regolith could carry small amounts of water with them. ese micrometeorite impacts could
then ash heat dust grains, forming tiny beads that trap water molecules, allowing them to remain
on the surface in daylight without vaporising.
WHAT THE WATER TELLS US
e presence of the three varieties of water – the water brought by comets, the water brought by
micrometeorites or formed by the solar wind hitting the surface, and the magmatic water contained
deep within the Moon, each tell a story.
In the water-ice that has been delivered to the Moon by comets is a record of the impact history of
the Solar System, spanning billions of years. Studying that ice, if we can reach it in the depths of
the permanently shadowed craters, can teach us about the intensity of impacts in the early Solar
System – how many comets crashed into the inner planets, and when? It could also help tell us
where Earth got its water from – while comets are a contender, most comets studied so far have far
different ratios of hydrogen to deuterium in their water compared to that on Earth. If the isotope
ratio of the Moon’s water is the same as Earth’s or different, it will provide a vital clue as to what
brought water to Earth. Was it comets or, as the scienti c consensus is now beginning to favour,
carbonaceous chondrite asteroids that have been shown to have the same isotopic ratio as Earth’s
water?
e thin lm of water molecules detected on the surface by M3 and SOFIA teach us about the solar
wind, and how it interacts with planetary bodies and the complexity of the resulting chemistry. If
the solar wind can create water molecules on the Moon, could it do the same on other airless bodies,
such as asteroids?
en there’s the magmatic water. When the Moon formed from debris ripped from the young
Earth’s crust by the collision of a Mars-sized protoplanet, scientists’ expectation had been that the
intense heat of the collision would have boiled away any easily evaporated materials such as water,
and thus the Moon would have been born dry. is picture has begun to change, thanks to improved
simulations that replicate the collision between Earth and the protoplanet, and more sophisticated
analysis of lunar rocks returned from the Moon.
As mentioned at the top of this article, the Apollo rock samples are incredibly dry. In 2011, a team
led by Erik Hauri of the Carnegie Institution For Science was able to con rm the presence of tiny
amounts of water molecules native to the Moon in the rocks, as opposed to the water being
terrestrial contamination. Further to that, Hauri’s team studied so-called ‘melt inclusions’ contained
within the lunar rocks. Melt inclusions are tiny micron-sized blobs of magma trapped inside crystals,
which in the case of the studied Moon rocks were crystals of the mineral olivine. It turns out that
these melted blobs of magma contain water molecules as well as volcanic gases, and Hauri found
that the isotope ratio of hydrogen to deuterium in the Apollo Moon rock water is the same as the
water in Earth’s mantle, and must have come from there. is suggests that water did somehow
survive the collision that formed the Moon – perhaps the collision was less violent, and slower, than
we thought it was – and wound up buried deep inside the Moon. It should be pointed out that the
isotopic ratio of the Moon’s magmatic water need not be the same as the water in the ice within the
shadowy craters.
WATER AS A RESOURCE
So, to sum up, there’s quite a bit of water on the Moon, but it’s mostly thinly spread. SOFIA may
have detected water inside Clavius, but the crater is still 100 times drier than the Sahara Desert. It is
also hard to reach – a spacecraft landing near the poles would not be able to rely on sunlight to
provide power, and in deep, permanently shadowed craters, communications would be cut-off from
Earth (at least without a relay satellite in geocentric orbit around the Moon).
is could pose a problem, because plans for humans to return to the Moon on a more permanent
basis, such is the aim of NASA’s Artemis programme that intends to send a man and a woman to
the Moon this decade, to be followed by the development of lunar outposts, rely on using water.
Obviously, astronauts could drink the water, but H2O could also be split to provide oxygen and
hydrogen for rocket propellant, or oxygen for breathing. Mining that water, however, could be
difficult if there isn’t a chunk of solid ice on hand.
So the next stage is to understand the distribution of water across the lunar surface, and perhaps
below it too. Large, permanently shadowed craters might be difficult to reach, but new research by
scientists at the Planetary Science Institute in Arizona has found that micro cold traps could also
exist on the Moon, inside small craters, or behind large boulders. ey might be small, maybe just a
metre across, but add it all up, say the scientists, and these micro cold traps could harbour up to a
fth of all the frozen water on the Moon.
ese micro cold traps would be far more accessible. Rather than clambering over a steep crater rim
and tumbling down the other side, a rover or an astronaut could enter one of these smaller cold traps
via a much gentler slope, while avoiding the utterly cold temperatures and engul ng darkness of the
larger craters.
Several new missions now have mapping lunar water as one of their priorities. NASA have
announced the Lunar Trailblazer, which will be a small spacecraft that will launch in 2025 to peer
deep into the permanently shadowed regions, seek out micro cold traps and study the lunar water
cycle, showing how the surface abundance of water molecules changes in concentration and
distribution throughout a lunar day (which lasts 29 Earth days, with two weeks of daylight and two
weeks of night at any given location on the Moon). Lunar Trailblazer will supplement VIPER
(Volatiles Investigating Polar Exploration Rover), a robotic rover that will launch in 2023 to explore
the permanently shadowed regions and drill into the surface in search of water.
e Indian Space Research Organisation has already headed back to the Moon with Chandrayaan-
2, which carries a radar to search for water-ice unseen in the shadows, and it will be joined in this
effort by another NASA mission, Lunar Flashlight, which is a cubesat that will use infrared lasers
and a spectrometer to map the ice when the spacecraft launches later in 2021.
e Moon was never wet like, say, Mars was billions of years ago. It never had real oceans, it never
had rivers or lakes, and it certainly never rained or snowed. What water is present has been snatched
from the Earth, or swept up from the detritus in the Solar System. And its location may dictate
future plans for a human presence on the Moon. But there’s no doubt that if it can be identi ed,
accessed and then mined, then the lunar water will be our gateway both to the Solar System at large,
and to the Moon’s past. One thing’s for sure – future lunar astronauts need not take their swimming
costumes with them.
The seas - or maria - on the Moon are not seas of water, but were once oceans of lava. What water survives on the Moon is ice
hidden in permanently shadowed craters, or a thin lm of molecules hopping across the daylight surface.
Get ready to do a deep dive into the story of freezing water on the Moon, which could one day
reveal secrets about the Moon’s origins and provide a means for astronauts to live there,
writes Keith Cooper.
A visualisation of Shackleton, based on observations and laser altimetry data from the Lunar Reconnaissance Orbiter, showing the
deep pool of permanent shadow that lls the crater.
Data from the Lunar Reconnaissance Orbiter indicates that the craters Cabeus (which LCROSS observed water-ice in), Shoemaker and
Faustini, all near the lunar south pole, probably all contain large amounts of water-ice.
An oblique view of the crater Clavius, inside which SOFIA detected water molecules in broad lunar daylight.
New observations by SOFIA, as well as missions such as Lunar Trailblazer, will seek to determine how the abundance of water on the
surface of the Moon varies as the lunar day progresses and the terminator between night and day moves slowly across the surface.
In December, NASA gave the go ahead to the Lunar Trailblazer cubesat mission, which will map water on the Moon.
An artist’s impression of NASA’s VIPER rover, which will drive into permanently shadowed craters.
ViewWater on the Moon
January 2021
Astronomy Now