Amount of water on the Moon varies by location and time of day
Two Earth-based studies confirm water molecules are present on the Moon, but the amounts vary depending on location and time of day, Casey Honniball of NASA’s Goddard Space Flight Center reported in a Jan. 21 online seminar titled “A Tale of Lunar Water,” sponsored by the Lunar and Planetary Institute (LPI) in Houston, Texas.
Lunar samples returned by NASA’s Apollo missions and by the former Soviet Union’s Luna missions contain low levels of volatile elements though a few had high water contents, Honniball said. However, these samples represent only localized areas on the lunar surface and not the whole Moon.
In 2009, three separate probes—India’s Chandrayaan-1 spacecraft, NASA’s Deep Impact spacecraft on its way to study the interior of Comet Tempel 1, and the joint NASA-ESA Cassini spacecraft to Saturn—all unexpectedly revealed hydration features on the Moon’s surface.
Water ice was recently detected in permanently shadowed lunar regions. Significantly, hematite, which requires water to form, was found in sunlit areas at the Moon’s poles.
Honniball used IRTF‘s SpeX instrument, a 0.7 to 5.3 micron medium resolution spectrometer and imager to investigate variations in the levels of water ice present at various times during the lunar daytime on the entirety of the Moon’s near side and at various lunar latitudes.
Impacting micrometeorites generate heat that converts hydroxyl, an entity composed of one hydrogen atom bonded to one oxygen atom (HO), on the Moon’s surface into water.
Lunar water is stored either as ice particles or in glass particles formed when the heat from impacts melts lunar regolith. Melted surface material forms a glass-like structure into which water can be captured.
IRTF data revealed that higher latitudes on the Moon have higher abundances of water and greater variations in water content depending on time of lunar day. In contrast, lower latitudes have lower amounts of water but experience fewer variations based on time of day.
The data also showed that surface areas with lower temperatures have higher levels of water while those with higher temperatures have lower levels of water. The causes of these variations remain uncertain, Honniball said.
SOFIA observed the Moon’s surface for the first time in August 2018, focusing on two different locations. It found evidence of water in Clavius Crater, located in the Moon’s southern hemisphere, as well as in some mare at mid-northern latitudes.
Clavius marks the first unambiguous detection of water on the Moon’s sunlit surface. The region holds an estimated 100 to 400 parts per million of water, Honniball estimated.
During the SOFIA observations, she and her colleagues expected water abundance to increase with latitude, but this did not occur. This phenomenon could be localized and not typical of the Moon as a whole, she explained.
More SOFIA observations will be conducted during the summer of 2021. These are part of a larger project in which SOFIA will map all molecular water on the Moon.
Additional IRTF observation will also be done, with the goal of better understanding of the ways hydroxyl and water behave differently on the lunar surface. Current plans call for observing during the various lunar phases as well as focusing on the polar regions.
Diurnal variations—differences in abundance based on time of day—could indicate there is a source of hydration at the Moon’s poles though it is unclear whether the substance moving is hydroxyl or molecular water, Honniball said.
If lunar water originates in micrometeorite impacts, it should be scattered all over the Moon rather than concentrated in specific locations, she said.
The ultimate goal of all these studies is understanding the source of lunar water, the way the water moves across the Moon’s surface, and whether it is concentrated at specific geologic locations.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.