The moon’s water secret revealed

“Giant steps are what you take … walking on the moon.” Or so sang the Police in 1979. Of course, now they might sing “splashing on the moon.”

Not really.

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Last week, NASA confirmed for the first time the presence of water on the sunlit lunar surface. Measurements made aboard NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) provided definitive evidence for water molecules in Clavius Crater, one of the largest craters visible of the moon’s face. The concentration of water ranges from 100 part per million to 412 parts per million – or, on average, about 800 mL per cubic metre of surface soil. That’s 10 times drier than the Sahara Desert.

Not enough to really create a splash but free water, nonetheless, and it is an important discovery.

The scientific possibility the moon might harbour liquid water dates back to the early 1960s, prior to the first manned missions. Unlike Earth, the moon has only a slight tilt with respect to its orbit and consequently sunlight cannot penetrate deep into the bowls of the craters near its north and south poles. The interiors of these craters are in constant shadow with surface temperatures well below zero.

Scientists believed these craters would act as cold traps. Any water entering one of these regions would freeze solid resulting in small quantities of water ice. And over the billions of years, large pockets of water ice could have formed.

Water is capable of forming hydrates with minerals, such as silicates and zeolites. Indeed, many of the minerals found on Earth and even some that have been identified on Mars are hydrated species. In addition, water can dissociate to form hydroxyl radicals, which can result in basic minerals such as lime. But free water requires a slightly different chemistry.

On Earth, the origin of our water is still an open question. One theory argues water was an essential component of the planetesimals, which collided with each other to eventually generate our planet. Recent evidence of the isotopic signature of water found in enstatite chondrite meteors, formed in the inner solar system, support this point of view.

But other researchers argue the Hadean Eon – the time when the surface of the planet was still molten rock as a consequence of the collisions of the planetesimals – would have been too hot for the Earth to have retained this early water. They argue our oceans arrive later in the planet’s life as comets from the outer solar system, where water ice is much more readily available, brought water to Earth’s surface during late stage bombardment.

The moon, of course, suffered these same bombardments. Looking up at night, we can see the remnants of comets and other space debris that crashed into the surface in the many craters and scars, marring the moon’s face. Billions of years ago, a massive water bearing comet might have crashed into the moon, resulting in the Clavius Crater and a legacy of water left behind.

Of course the moon’s gravity – only one-sixth of Earth’s – was not enough to hold onto most gaseous molecules. Any free water on the moon’s surface would have boiled away, so we do not find puddles for splashing in as we walk on the moon. But the moon’s weak gravity and thin atmosphere could still provide a mechanism for the transport of water molecules from the sunlit side to the poles where water ice could form. And simulations suggest any crevice of crack protected from direct sunlight – even on the sunny side of the moon – could possibly serve as a hiding place for frozen water.

Over the past 50 years, since the first moon landing, various missions have been launched to detect water ice on the moon. The first direct evidence was provided by the Appollo-14 ALSEP Suprathermal Ion Detector Experiment, which captured bursts of water ions near the landing site in 1971. In 1978, Soviet scientists published a paper claiming to have detected water in rocks (hydrated minerals) returned to Earth by the Luna 24 probe. In 1994, the Clementine probe used bistatic radar to probe the dark regions of the Moon’s south pole. The echoes of these waves were detected using the Deep Space Network on Earth and were consistent with an icy surface. And India’s ISRO spacecraft Chandrayaan-1 released a lunar impact probe into Shackleton Crater in 2008 and found water ice in the subsurface debris kicked up by the impact.

But the SOFIA measurements provide the first definite evidence of water ice across the entire surface of the Moon. Of course, we need more observations if we are to generate a water map of the Moon but the results are hopeful.

If water ice can be found on the moon, a moon base might not be far behind.

And then maybe one day, we will be splashing on the moon.

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