By Duncan Steel 27/10/2020

[avatar user=”duncansteel” size=”thumbnail” align=”right” /]

The space news this week is largely focused on an announcement from NASA regarding the discovery of water on the Moon. Not liquid water – the lunar surface is far too cold for that – but apparently ice deposits in the surface layers in near-polar regions, and perhaps deeper below the surface too.

Finding water on the Moon in an accessible form would be important for our future exploration and exploitation of Earth’s natural satellite, perhaps involving permanently-crewed scientific outposts. The water itself would be essential for drinking, washing, and growing food; it could be electrically separated to produce oxygen for breathing; and when split into its constituent hydrogen and oxygen it could be used as rocket fuel. Transporting the necessary water from Earth would be hugely expensive, because we are in a gravity well, whereas the gravity of the Moon is only one-sixth as strong. Finding water there in quantities making it viable to ‘mine’ the Moon for this essential might make living there feasible, in much the same way as astronauts now live for months on the International Space Station.

Several spacecraft missions sent to the Moon by the US, the Soviet Union, India and China have given us indications that water might persist on the lunar surface, in the form of frost or ice near the surface. I was involved with one: the Clementine mission in the mid-1990s. The radar experiment on Clementine rendered data suggestive of ice in the surface layers close to the poles of the Moon, the South Pole in particular, but later doubts were cast on this for reasons that may well be countermanded by the new understandings just announced. But read on…

Finding water on the Moon might seem surprising because in the vacuum of space water sublimates (transforms directly from solid to vapour) at a very low temperature – below minus 160 degrees Celsius – and so might not be expected to persist when exposed to sunlight on the lunar surface. The thing is, though, that there are places where sunlight never intrudes, and so in theory water ice might exist. In recent years we have found water in some unexpected places in the solar system: the interior planet Mercury, and the dwarf planet/asteroid Ceres.

The information from the previous space probes, however, could be interpreted in more than one way: essentially what was detected was the hydroxyl radical (OH) rather than water itself (hydrogen hydroxide!). That OH could be bound in minerals, might be in some crystalline form, or might indeed be water in molecular form. I do not know a huge amount about chemistry, but recall from school seeing that copper sulphate in its crystalline (hydrated) form contains a lot of water and is a bright blue (indeed it is sometimes called bluestone or blue vitriol), whereas in its anhydrous form it is a light grey powder, and so it is feasible to lock up water molecules as water of crystallisation.

One idea was that there could be near-surface ice on the Moon hidden from the Sun in craters close to the poles, in particular in the huge Aitken Basin, which lies on the lunar farside near the South Pole. Such locations are termed Permanently Shadowed Regions (PSRs). There being no atmosphere on the Moon, no substantial amount of heat could get to any ice in a PSR, the only mechanism for energy transfer being radiation (sunlight, starlight) or perhaps high-speed solar wind particles, because with no air there is no convective or conductive heat transfer. In theory such ice might remain stable for millions of years.

In part due to this interest in the tantalising evidence for ice on the Moon, the Chinese Change’e 4 probe was landed in the Aitken Basin in January 2019.

Lunar southern pole illumination map, built up from images obtained by the Lunar Reconnaissance Orbiter satellite. The dark area near-centre is a 19-km wide crater named Shackleton (for the Antarctic explorer), and the South Pole is near the nine o’clock position on its rim. The black areas receive little or no sunlight, and so remain frigid, therefore likely locations for ice to persist. For more information, see here.

Two papers now published in Nature Astronomy look at the question of lunar ice in quite distinct ways. Rather than restricting the possibility of PSRs to large craters, Paul Hayne (University of Colorado) and colleagues investigated the possibility of there being much smaller areas on the lunar surface where there is permanent shadowing, on scales from kilometres down to centimetres, terming these micro cold traps. One can think of these as being similar to ice patches experienced on the road in the shadows after a frosty night: where the early sunlight has melted and dried the tarmac, all is good… but you come around a corner into a shaded section and the surface is treacherous. It happens that on the Moon there are regions at high latitudes (where the Sun is always low in the sky) where no sunlight warms the surface. It is only now, with the Moon mapped at high spatial resolution by NASA’s Lunar Reconnaissance Orbiter, that scientists have the data to perform such an analysis.

The other paper, by Casey Honniball (NASA-Goddard Space Flight Center) and co-workers, involves the direct detection of water molecules on (or in) the lunar surface. Previous observations in the infra-red part of the spectrum (at around 3 microns wavelength) had confirmed the hydroxyl (OH) presence, but Honniball and colleagues used a longer wavelength (six microns) at which a water molecule is resonant, knowing that detecting emission at that wavelength would be an unambiguous signal of water.

Whilst we might represent a water molecule as H-O-H, in fact the molecule is not linear, being shaped more like a flattened letter V (with an angle between 104°-105°), and the angle in the V will oscillate at a specific frequency, and therefore emit radiation at a specific wavelength. I wrote above that I claim no expertise in chemistry, and so will leave it to someone else to explain the vibrational spectroscopy of water.

To observe the Moon at 6 microns, Honniball and colleagues needed a telescope above the large amount of water vapour in the lower terrestrial atmosphere, and so used SOFIA (the Stratospheric Observatory For Infrared Astronomy). This is a Boeing 747 equipped with a large telescope, and is a joint project of the space agencies of the USA (NASA) and Germany (DLR). Note that most austral winters SOFIA is based for a couple of months at Christchurch International Airport, and flies out over the Pacific and down over the Antarctic Ocean. (In this case the data collected by Honniball and colleagues did not come from an NZ-based flight.)

The NASA/DLR airborne observatory SOFIA. A telescope with aperture 2.7 metres (visible in the dark open hatch on the side of the aircraft) enables astronomers to make observations in the infra-red part of the spectrum by flying high above the main water vapour content of our atmosphere.

Honniball’s team found the signature of water molecules in data collected pointing SOFIA’s telescope close to the lunar South Pole, but not nearer the equator, consistent with the idea that water ice might persist in PSRs. More than that, though, they suggest that in fact rather being located in macroscopic reservoirs (like craters), in fact the ice could be present on a microscopic scale, between the grains of the lunar ‘soil’ which we term regolith. The interpretation of Honniball and her team is that the water is hiding from sunlight as ice between and within the grains.

If their figures are correct, then two or three tonnes of regolith (about a cubic metre) might contain a litre of water. That could make it feasible to extract the water, using it both for crewed outposts on the lunar surface, but also perhaps to employ as rocket fuel as we push on to Mars and beyond. Sounds like science fiction at present, but it may be the science fact of the not-too-distant future.

Locations where Honniball et al. collected infrared data using SOFIA. Clavius, the second-largest crater on the lunar nearside, is close to the South Pole. The chosen Mare Reference area is at a far lower latitude, where water ice would not be anticipated. (I must mention a peculiarity here: the lunar maria [singular mare] got that description because ancient astronomers thought they were water, appearing blue to the eye; maria is the Latin word for ‘seas’.) Aficionados of science fiction will know the name of the crater Clavius extremely well.

An obvious question that arises from the discovery of water on the Moon is this: from where does it come? What is its origin? The answer might well be the same as the answer to the question of where Earth’s water originated, given that it was extremely hot and fiery after its formation a little over 4.5 billion years ago. That is, we think that the proto-Earth was desiccated, and water around at that time was steaming into our early atmosphere and then being photo-dissociated by the harsh, extremely active initial Sun, so that the free hydrogen and oxygen would be wafted off into space. Indeed, essentially all volatiles would have been lost at that time, in our planet’s Hadean beginnings.

Earth, we think, accumulated water and other volatiles mostly over the following several hundred million years, as a multitude of asteroids and comets delivered such chemicals: sometimes impacts by such bodies would blow off more material than they delivered, but gradually our atmosphere and oceans agglomerated as our planet cooled. Doubtless much water and other gases were liberated subsequently by volcanos, but apparently much of the water of which you are made (around 60 per cent of the human body is water) arrived in comets and asteroids. At present material is still arriving from space: about 40,000 tonnes a year as small meteoroids and interplanetary dust, with the overall mass influx being dominated by the very-occasional large body arrival.

It seems that much the same applies to the Moon. When an asteroid or comet hits, forming a crater, a lot of rock may be thrown into space (and we now know of hundreds of meteorites that are free samples from the Moon) but some of the material of which the impactor is composed may remain there. In the past astronomers have done calculations about how long ice might be stable if suitably hidden within the lunar regolith, and how much is arriving, but now those calculations will need to be revisited in terms of the amount of ice that seems to be present.

To conclude, I must point out that this discovery is linked to something that is astonishingly obvious, and yet few people realise it to be true: at full moon (when the whole nearside of the Moon is illuminated by the Sun), our natural satellite appears to be about nine or ten times brighter than at first or last quarter (when half of the lunar disk appears illuminated).

How come? Why is it not just twice as bright, if twice the solar-illuminated area is visible? The answer is that the lunar surface is rough – not like a ball-bearing or even a disco ball – and so the microstructure of the grainy surface makes a big difference to how much sunlight gets scattered to us on Earth. At full moon some of the sunlight is scattered directly back from the tops of grains of the regolith, or perhaps enters a cavity between two grains but is reflected straight back out. At first or last quarter the solar photons need to be scattered broadly at 90 degrees compared to the direction they have come from, and this might take multiple bounces between grains of rock, on each bounce there being a finite probability that the photon will be absorbed. This relative brightening when the Moon is close to full (that is, in opposition) we term the opposition effect (or surge). You might witness a similar thing when flying in a small aeroplane: look down at the plane’s shadow on the ground, and often you will see a bright halo around it, causes by direct backscatter of sunlight.

The relevance of this for the identification of water on the Moon is that it may well be the fine-scale structure of the lunar surface that makes the persistence of the ice feasible. The cracks and crevices between the grains, coupled with the lack of an atmosphere, means that there are many small-scale havens where ice, once lodged, can remain for aeons, because they are shaded from sunlight: the only thing (more-or-less) that might elevate their temperatures and cause the ice to vaporise into space.

Photograph at the head of this blog post: The Apollo 16 landing site as imaged by the Lunar Reconnaissance Orbiter. LM: Lunar Module (the four-legged platform on which the manned capsule landed, and then blasted off again, hence the black surrounds due to the disturbed regolith). LRV: Lunar Roving Vehicle. RTG: Radioisotope Thermoelectric Generator. ALSEP: Apollo Lunar Surface Experiment Package. Geophone line: part of an active seismic experiment aimed at understanding the lunar interior (and the ‘Geo’ is obviously a misnomer, is it not?). For more information about this image, see here.