Scientists, using an instrument aboard NASA’s Lunar Reconnaissance Orbiter
(LRO), have observed water molecules moving around the dayside of the Moon.
A paper published in Geophysical Research Letters describes how Lyman Alpha
Mapping Project (LAMP) measurements of the sparse layer of molecules
temporarily stuck to the surface helped characterize lunar hydration changes
over the course of a day.
Up until the last decade or so, scientists thought the Moon was arid, with
any water existing mainly as pockets of ice in permanently shaded craters near
the poles. More recently, scientists have identified surface water in sparse
populations of molecules bound to the lunar soil, or regolith. The amount and
locations vary based on the time of day. This water is more common at higher
latitudes and tends to hop around as the surface heats up.
“This is an important new result about lunar water, a hot topic as our nation’s
space program returns to a focus on lunar exploration,” said Dr. Kurt
Retherford, the principal investigator of the LAMP instrument from Southwest
Research Institute in San Antonio, Texas. “We recently converted the LAMP’s
light collection mode to measure reflected signals on the lunar dayside with
more precision, allowing us to track more accurately where the water is and how
much is present.”
Water molecules remain tightly bound to the regolith until surface
temperatures peak near lunar noon. Then, molecules thermally desorb and can
bounce to a nearby location that is cold enough for the molecule to stick or
populate the Moon’s extremely tenuous atmosphere or exosphere, until
temperatures drop and the molecules return to the surface. SwRI’s Dr. Michael
Poston, now a research scientist on the LAMP team, had previously conducted
extensive experiments with water and lunar samples collected by the Apollo
missions. This research revealed the amount of energy needed to remove water
molecules from lunar materials, helping scientists understand how water is
bound to surface materials.
“Lunar hydration is tricky to measure from orbit, due to the complex way
that light reflects off of the lunar surface,” Poston said. “Previous research
reported quantities of hopping water molecules that were too large to explain
with known physical processes. I’m excited about these latest results because
the amount of water interpreted here is consistent with what lab measurements
indicate is possible.
Scientists have hypothesized that hydrogen ions in the solar wind may be
the source of most of the Moon’s surface water. With that in mind, when the
Moon passes behind the Earth and is shielded from the solar wind, the “water
spigot” should essentially turn off. However, the water observed by LAMP does
not decrease when the Moon is shielded by the Earth and the region influenced
by its magnetic field, suggesting water builds up over time, rather than
“raining” down directly from the solar wind.
“These results aid in understanding the lunar water cycle and will
ultimately help us learn about accessibility of water that can be used by
humans in future missions to the Moon,” said Amanda Hendrix, a senior scientist
at the Planetary Science Institute and lead author of the paper. “Lunar water
can potentially be used by humans to make fuel or to use for radiation
shielding or thermal management; if these materials do not need to be launched
from Earth, that makes these future missions more affordable.”
“This result is an important step in advancing the water story on the Moon
and is a result of years of accumulated data from the LRO mission,” said John
Keller, LRO deputy project scientist from NASA’s Goddard Space Flight Center in
Greenbelt, Maryland.
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