Water, Water, Everywhere?

Penny Wozniakiewicz investigates why astronomers are so concerned with finding water in the Solar System, and why it is vital to our exploration of space

- Penny Wozniakiewicz

Staring up at the Moon with the naked eye, we can forgive early astronomers for assuming the dark patches spread out over its surface were seas - or 'maria' as they were named, after the Latin word for seas. Informed by centuries of ever-improving observations and over 60 years of space exploration, we now know the maria are not seas but rather vast expanses of volcanic basalt that erupted over the lunar surface several billion years ago.

The Moon is in fact very dry: more so than any desert on Earth. Yet despite that, on 23 August 2023 the Indian Space Research Organisation's Chandrayaan-3 mission successfully deployed its lander and rover near the lunar south pole in search of water.

So why search for water in such a dry location? Although there is no liquid water on the Moon, water is present in the form of ice trapped between grains in the lunar soil and incorporated into minerals and glassy beads produced by impacts. The potential for such hidden water was first suggested by remote observations of the surface, and later confirmed by NASA's LCROSS mission, which in 2009 fired an empty rocket stage into a crater on the lunar surface and identified ice in the plume of material flung up from the crash site. Further observations of the surface by the likes of the NASA and German Aerospace Center's SOFIA telescope have since suggested the south polar region of the Moon in particular may host far more water than we ever imagined. As much as 100-400mg of water (about one raindrop) may be present in each kilo of soil.

While this may seem a small amount, it has prompted several space agencies to propose lunar surface missions and instruments to find and characterise lunar water over the next decade.

PROSPECTing for water