Water in moon rocks key to understanding lunar history

Water in moon rocks key to understanding lunar history

A new review of hundreds of chemical analyses of Moon rocks indicates that the amount of water in the Moon’s interior varies regionally – revealing clues about how water originated and was redistributed in the Moon. These discoveries provide a new tool to unravel the processes involved in the formation of the Moon, how the lunar crust cooled, and its impact history. This is not liquid water, but water trapped in volcanic glasses or chemically bound in mineral grains inside lunar rocks.

Rocks originating from some areas in the lunar interior contain much more water than rocks from other places. The hydrogen isotopic composition of lunar water also varies from region to region, much more dramatically than in Earth. The present consensus is that the Moon formed as the result of a giant impact of an approximately Mars-sized planetesimal with the proto-Earth. The water in the Moon is a tracer of the processes that operated in the hot, partly silicate gas, partly magma disk surrounding Earth after that impact.

The source of the Moon’s water has important implications for determining the source of Earth’s water, which is vital to life. There are two options: either, water was inherited by the Moon from the Earth during the Moon-forming impact, or it was added to the Moon later by comets or asteroids. It might also be a combination of these two processes.

Katharine Robinson, lead author of the study and Graduate Assistant at the University of Hawai’i – Manoa (UHM) School of Ocean and Earth Science and Technology, and Researcher G. Jeffrey Taylor, both at the UHM Hawai’i Institute of Geophysics and Planetology, compiled water measurements from lunar samples performed by colleagues from around the world, as well as their own.

Specifically, they measured hydrogen and its isotope, deuterium (hydrogen with an extra neutron in its nucleus) with ion microprobes, which use a focused beam of ions to sputter ions from a small rock sample into a mass spectrometer. The ratio of hydrogen to deuterium can indicate the source of the water or trace magmatic processes in the lunar interior.