Using a telescope flying on a 747, astronomers say they have definitive evidence of water molecules on sunlit moon surfaces (Nat. Astronomy 2020, DOI: 10.1038/s41550-020-01222-x).
It wasn’t long ago that scientists thought the moon was completely dry. Then, in 2009, a trio of papers reported spectroscopic evidence of O–H bonds in sunny areas, although researchers couldn’t say if these belonged to water or hydroxyl substituents on other molecules. In 2018, scientists reported ice in shaded craters.
The 2009 results were based on IR signals near 3 µm because that’s what the spacecraft used could measure. Water has an unambiguous IR signal at 6 µm.
Honniball and her colleagues observed the 6 µm signal on the sunlit lunar surface at high southern latitudes but not at lower latitudes. Previously, scientists assumed solar radiation would either destroy water molecules or push them to colder regions. Based on the new data, the team calculated that the moon’s surface is 100–400 parts per million water. But Honniball stresses that the moon is still a very dry place: its surface is probably about 100 times as dry as Sahara Desert sand.
The water they observed is neither ice nor liquid; it exists as lone molecules. The researchers don’t know whether these molecules are trapped inside glasses formed by meteorite impacts or tucked between grains of lunar dust. Honniball says they are planning follow-up observations that might nail that down. Water could be brought to the moon on meteorites, form during meteorite impacts, or come from inside the moon.
The group has applied for more trips on SOFIA to try to map the majority of the moon’s near side at different times. That could reveal more about the water molecules’ location and sources and might also show how water moves around the moon’s surface. “They deserve more time” to answer these questions, Sunshine says.