Is the Moon getting more water?
Surprising new sources of water are springing up, aiding hope of sustainable lunar living
Surprising sources are springing up, aiding hopes of sustainable lunar living
Lunar water was a myth before it was a mystery. Like Earth, the Moon formed on the wrong side of the Solar System’s snow line, the imaginary boundary beyond which water ice was available for planetary assembly. Later, the exposed, Sun-scorched surface of our atmosphereless Moon continued to provide a hostile environment for water. It’s perhaps unsurprising that when samples brought back by the Apollo missions were shown to contain trace amounts of the stuff, the verdict was terrestrial contamination.
However, in the 1990s, two NASA orbiters, Clementine and Lunar Prospector, provided strong hints of surface water at the poles. This was confirmed by a series of landmark papers in 2009 analysing data from three missions: India’s Chandrayaan-1 orbiter, Cassini’s flyby on its way to Saturn and NASA’s EPOXI spacecraft. The high-latitude reservoirs of ‘dirty ice’ buried in the lunar soil were vast in scale. A 2010 analysis led by the Lunar and Planetary Institute’s
Paul Spudis using Chandrayaan-1’s radar data estimated as much as 600 million tonnes of water ice could be locked up in just 40 locales around the lunar north pole.
Now evidence is building that not only is lunar water present, but it is being actively supplied to the surface from some surprising sources around our Solar System. Understanding this more exotic lunar hydrocycle is key. Water is an invaluable resource that could open up possibilities of lunar living and cheaper access to Mars and beyond. Understanding how water behaves on the Moon has become mission critical.
“Water is the oil of space,” says George Sowers, a space mining researcher at the Colorado School of Mines. And in the same way the properties and behaviour of fossil fuels are keenly understood by oil prospectors, Sowers believes something similar is needed for lunar water. “It’s a scientific priority to understand the origin and evolution of volatiles like water on the Moon.“
Let’s start with that origin. A 2016 study led by Jessica Barnes, then at the Open University, of the isotopic make-up of lunar water indicated 80 per cent of it was delivered by asteroids, not icy comets as previously suspected. Barnes believes this happened over 4 billion years ago when the Moon’s surface was a magma ocean, and that the
giant outer planets’ migrations into their present orbits was disrupting the asteroid belt.
Once the crust had cooled, volcanic eruptions would periodically bring this water to the surface, as evidenced by water-borne minerals and glasses identified within lunar pyroclastic deposits by the University of Hawaii’s Shuai Li. While comets and micrometeoroids could also add to any surface reservoir, these energetic impacts on solid crust would likely evaporate much of the water away, and could even represent a net loss of surface moisture.
Even if water molecules do make it safely onto the lunar surface, they will struggle to survive anywhere near the equator, where temperatures can reach a blistering 120 degrees Celsius (248 degrees Fahrenheit) due to the lack of atmosphere. Salvation is to be found at both the poles. Here the Moon’s lack of an orbital tilt means the Sun never rises too high in the sky – and never rises at all within its permanently shadowed craters.
At around -230 degrees Celsius (-382 degrees Fahrenheit), these depressional cold traps rival parts of Pluto as some of the coldest places in the Solar System. This has allowed them to steadily accumulate infalling water molecules throughout the history of the Moon, growing them enough to be spotted 100 kilometres (62 miles) up by orbiters like Clementine and Lunar Prospector.
As more satellites were sent to explore the lunar high latitudes, back on Earth a multitude of agencies, space architects and engineers had a new focus for their creative visions of humanity’s long-awaited return to the Moon. Lunar settlement would focus on polar exploration, setting up bases in the shadowy cratered terrain of our celestial companion. The rationale was clear: save mission weight and costs by tapping an abundant in-situ resource. A resource that could not only quench astronauts’ thirst, but also, through electrolysis reactions splitting hydrogen-oxygen bonds, provide breathable oxygen and hydrogen rocket fuel for return or onward missions.
But are missions aiming too high? Last year the Stratospheric Observatory for Infrared Astronomy (SOFIA) discovered vast quantities of watery chemistry bound within regolith particles in the sunbaked lower latitudes. The new data suggested that this type of molecular water could be everywhere across the surface at concentrations as high as 400 parts per million. Perhaps SOFIA’s discovery should broaden our lunar horizons? It certainly challenges our understanding of water’s origin and continued presence on the Moon.
One way to explain the widespread survival of ancient water across the more exposed parts of the lunar surface is to claim it is not ancient at all – rather its newly arrived. An even better way is to make the Sun the source of this new Moon water. That’s exactly what Larissa Starukhina at the Kharkiv National University in Ukraine did back in 2000. She suggested that the protons, or charged hydrogen atoms, raining down on the lunar surface within the solar wind could be freeing up oxygen ions within mineral oxides found in the Moon’s top hundred nanometres. The newly freed-up oxygen could then form OH—, or hydroxyl ions, one of the most common ions in water. As the Sun rises and the surface heats up, any watery chemistry will unbind from its mineral grains, though how much then freely moves across the surface and how much diffuses down further into the regolith is unknown.
Despite this uncertainty, the idea of the solar wind as a hydrating force for those parched equatorial lunar planes has gained traction. Further analysis of data from another Chandrayaan-1 instrument, the Moon Mineralogy Mapper (M3), showed surface water and hydroxyl distributions across a lunar day’s rise and fall with solar flux, as predicted by Starukhina’s model.
“We need to find out if the Sun is a source of replenishable water on the Moon, and if there is a significant source there that could be usable for human exploration,“says NASA’s Orenthal James Tucker. His own recent models on the transport of Moon water across the surface support Starukhina’s predictions of how a hydroxyl signature might change with latitude and across the lunar day.
Of course, one way to conclusively prove the solar wind is refreshing the lunar surface would be to turn it off and watch the signs of equatorial Moon water dry up. As it happens, for a few days each month the Moon does indeed take a break from the Sun and its solar wind, during the period
it ducks behind Earth. Here it feels the shielding effects of Earth’s magnetosphere, the region of space where particles are primarily influenced by Earth’s magnetic field.
Recently two studies have looked for surface dehydration during these lunar holidays out of the sunlight. Both of them drew a blank. In
2019, the Planetary Science Institute’s Amanda Hendrix, analysing ultraviolet data from the Lunar Reconnaissance Orbiter, found that when the
Moon was shielded from the solar wind by Earth, the quantity of water molecules didn’t change.
This finding was reinforced this year by a Chinese and Japanese team led by Shandong University’s Jiang Zhang, who took another look at M3 data for a second period of lunar sheltering at the start of 2009, finding the same null result.
A nail in the coffin for the solar wind hypothesis? Not exactly. In their paper, Zhang and his colleagues explained their data through another of Starukhina’s ideas. The suggestion is that while Earth’s magnetosphere is an effective shield from the proton-rich solar wind, it also contains its own wind: the Earth wind. A mixture of deflected solar wind and escaped material from Earth’s charged upper atmosphere, it delivers a less concentrated but wider variety of charged particles. This includes protons, but also ions of helium, oxygen and nitrogen, travelling at a variety of energies. Starukhina suggests Earth’s magnetosphere can also provide hydrogen atoms for the lunar regolith in the absence of the solar wind.
“For the polar craters permanently shaded from solar wind, the Earth’s magnetosphere is the main source of hydration,” she claims. To back this up, in 2014 modelling led by Kyoto University’s Yuki Harada suggested Earth’s magnetosphere could indeed deliver about the same amount of hydrogen ions as the solar wind, especially towards the lunar poles. The theory was given a further boost
in 2017 when Osaka University’s Kentaro Terada, analysing data from Japan’s SELENE orbiter, found heavy Earth-wind ions such as oxygen in the vicinity of the Moon. Terada believes these oxygen ions could implant into the lunar regolith to form water, while some of the other fast-moving heavy ions could create large vacancies for future proton emplacement. As a result, he was excited to see Zhang’s team invoking Starukhina and the role of the Earth wind in producing the lunar surface water they observed. “I think this is one possibility, but to understand the origin of water more quantitatively, we need ion irradiation experiments and numerical simulations,” says Terada.
Tucker’s preference in terms of future studies are better estimates of the total surface water, which could then be compared with closer approximations of the daytime solar wind source to find the discrepancies that could point to the contribution of the Earth wind. Hendrix, on the other hand, would like to take a closer look at the water load bouncing across the surface through the lunar exosphere:
“The detectable levels are pretty low, so that suggests that the current daytime additions to the polar regions of water are relatively low. Whether it’s enough to sustain any human activity at the Moon, it’s a little too early to say.”
If Starukhina’s original ideas are confirmed, it means on some level the lunar and terrestrial hydrocycles overlap, linked by an invisible wind of charged particles that flows 380,000 kilometres (236,121 miles) across empty space between planet and moon – a wind that rains down charged particles once evaporated off our oceans or released by respiring plants, which now seeds the parched lunar soil with traces of molecular water. It also means the Moon’s water reserve might be increasing. “I think that it probably is getting wetter over time because it has these constant sources,” says Hendrix. “But it’s at a relatively low level.”
As we learn more about a lunar cycle we once thought had dried up billions of years ago, it becomes clearer what a major role water still plays in the surface dynamics of our celestial neighbour, and how little about these processes – critical to any hopes of sustainable lunar living – we know. “We really need to understand that water cycle,” says Tucker. “That’s critical to understanding what’s potentially available to support our operations long term and what type of technologies need to be developed in order to gather that water.” From myth to mystery, lunar water is now a primary motivation, fast tracking our return to the Moon.