High resolution mosaic of lunar south pole region
Place to be: probes have found evidence of water at the lunar south pole, though in what form remains unclear © Alamy

When early modern astronomers looked at the moon, in the mid-17th century, they interpreted its large dark areas as bodies of water — an erroneous view that lives on today in the Latin nomenclature of the lunar surface, which features many maria (seas) and even one oceanus.

By the late 19th century however, scientific opinion had changed completely, seeing the moon as being devoid of water as well as atmosphere. This perception of an entirely desiccated moon persisted through the 20th century, reinforced by analysis of samples brought home by Apollo astronauts, which did not contain a hint of water.

But observations in the 21st century have pushed the pendulum of expert opinion back towards the existence of some lunar water — if only a minuscule fraction of the amount imagined 400 years ago.

Hints that water ice might persist in the permanent shadow of crater rims close to the lunar south pole were confirmed in 2008 by an infrared mapping instrument on India’s Chandrayaan-1 orbiter. Then, in 2009, water was detected in the impact plume when Nasa deliberately smashed a probe into a shaded crater. And, in 2020, another Nasa instrument detected the presence of water molecules even in sunlit areas.

Access to water will be essential for the lunar settlements that several space agencies envisage for the coming decade. In addition to its familiar terrestrial uses — drinking, cooking, washing, nurturing plants and so on — water will be split at the molecular level by solar electricity into its atomic components — oxygen and hydrogen — for breathing and for fuel, respectively.

Some senior figures believe this need to secure water supplies has triggered a new “space race”, as Nasa chief Bill Nelson put it last year. In an interview with El País newspaper, Nelson said the US wanted to “prevent China from coming in and saying that the [south pole] water is theirs”. China dismissed his concerns, declaring that it “advocates the peaceful use of outer space”.

How accessible any water might be remains to be seen. Observations, so far, leave much uncertainty about the quantity of ice or water lying close to the lunar surface and how easily it can be extracted and purified.

Thomas Zurbuchen, professor of space science at ETH Zurich, who ran Nasa’s science missions until 2022, says: “I’ve heard all kinds of numbers out there for the amount of water on the moon, from hundreds of millions to hundreds of billions of tonnes. We went out and asked scientists but no consensus has emerged. We need in situ measurements.”

Equally uncertain is the form in which the water exists. “Are we talking about sheets of water ice beneath the surface, or a fine mixture of ice and soil, from which you’d have to dig out a lot of soil to extract the water?” asks Mahesh Anand, professor of planetary science at the UK Open University. “We won’t know until an instrument has landed on the moon that can directly image the surface and drill into it.”

Some answers are expected from Nasa’s Volatiles Investigating Polar Exploration Rover, or Viper, which is due to land near the moon’s south pole later this year on a 100-day roving mission to collect data on lunar water. With three instruments and a 1-metre drill, Viper will investigate environments at different depths and temperatures.

An engineering model of the Volatiles Investigating Polar Exploration Rover
Space utility vehicle: Viper, Nasa’s Volatiles Investigating Polar Exploration Rover, is due to land on the moon this year © Alamy

Analysis of nuclear isotopes in the ice will provide evidence for the origins of lunar water. Planetary scientists have proposed three likely sources.

First, it may be left over from water present when the moon was formed 4.5bn years ago, probably the result of a protoplanet smashing into the young Earth. Second, it could be the result of water-bearing comets and asteroids bombarding the moon over billions of years. Third, it may be derived from the solar wind: the protons (hydrogen ions) emitted by the sun, reacting with oxygen in minerals on the lunar surface to produce water molecules.

Across the moon’s non-polar regions most, or all, of this water is likely to be bound chemically to dust and rocks. But it could still be an exploitable resource, says Anand, if equipment is developed to drive out the water with heat generated by solar power.

On Mars, the next destination envisaged for human exploration after the moon, evidence for substantial ice deposits is much stronger. In January, scientists released an analysis of radar data from Europe’s Mars Express orbiter showing deposits up to 3.7km thick, buried beneath the Medusae Fossae Formation in the planet’s equatorial region. If melted, this would cover the whole of Mars in water 2 metres deep.

The history of scientific opinions about water on Mars bears some similarities with those about water on the moon. The famous 19th-century view of Martian canals and oceans turned out to be a mirage, and 20th-century astronomers saw a dead desert. But that began to change when images from spacecraft showed polar ice caps, dried-up river valleys and ancient lake beds — signs that water had once flowed abundantly.

Further out in the solar system, the icy moons of Jupiter and Saturn are believed to hold deep oceans of liquid water beneath their frozen surfaces. Saturn’s Enceladus has particularly captured the scientific imagination with its spectacular eruptions of water vapour, within which Nasa’s Cassini mission detected organic molecules that hinted at a potentially habitable environment.

Next to be examined by robotic spacecraft will be the Jovian moon Europa, with its cracked icy crust also covering a subsurface ocean. Two orbital missions, Nasa’s Europa Clipper and the European Space Agency’s Juice, are due to explore Europa in the early 2030s.

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