It's official. There is water on the Moon.
We assumed there was for almost a decade, based on
detections announced in 2009, but the wavelengths used left space for
interpretation. Scientists announce the first unambiguous discovery using a
distinct wavelength peculiar to water. Those 2009 conclusions appeared to be correct.
Because the 2009 detections were produced in the
3-micrometer infrared region, the uncertainty emerged. Water or another
hydroxyl molecule containing hydrogen and oxygen were the only options at this
wavelength.
A team of scientists led by NASA Goddard Space Flight
Centre astronomer Casey Honniball sought to investigate the wavelength that may
validate or refute such conclusions. The 6-micrometer infrared band should
display a line that can only be formed by two hydrogen atoms and one oxygen
atom - the H-O-H bend vibration.
However, establishing an unambiguous detection in that
spectrum is difficult. It necessitates the use of the Stratospheric Observatory
for Infrared Astronomy (SOFIA), a one-of-a-kind telescope flying on an aircraft
above the most of the Earth's atmosphere.
"SOFIA is the only current and planned
observatory capable of these observations," Honniball told ScienceAlert.
"Current lunar spacecraft do not have instruments
that can measure at 6 micrometres, and from the ground, Earth's atmosphere
blocks 6-micron light, and so it cannot be done from ground-based
observatories. SOFIA flies above 99.9 percent of the Earth's water vapour,
which allows 6-micrometre light to pass through and be observed. And luckily
SOFIA's FORCAST instrument can make 6-micrometre measurements and look at the
Moon."
The researchers used FORCAST to thoroughly study the
region where the 3-micrometer detections were made - high southern
latitudes near the south pole. They discovered the emission line they had been
looking for - the one-of-a-kind signature that could only be produced by the
H-O-H bend vibration.
Based on these findings, the team predicts water
abundances of 100 to 400 parts per million, which is compatible with Moon
Mineralogy Mapper 3-micrometre detections.
There are no liquid lakes splashing around on the
lunar surface, and any frozen water would sublimate as soon as it was exposed
to sunlight. However, there are several possibilities for the Moon still
harboring surface water.
"We mainly think the water is in glass,"
Honniball said. "When a micrometeorite impacts the Moon, it melts
some lunar material, which quickly cools and forms a glass. If there is water
already present, formed during or delivered during the impact, some of the
water can be captured in the structure of the glass while it cooled."
Another possibility - patches of persistent darkness
in polar craters - was investigated in a separate article headed by astronomer
Paul Hayne of the University of Colorado Boulder. High crater rims at high
latitudes produce places where sunlight never reaches.
Temperatures in these areas seldom rise above -163
degrees Celsius (-260 degrees Fahrenheit), producing cold traps that might
harbor concealed patches of aqueous ice. Using data from NASA's Lunar Reconnaissance Orbiter,
Hayne and his colleagues determined that the permanently shadowed surface might
cover up to 40,000 square kilometers (15,000 square miles). And 60% of
that is at the South Pole.
"The temperatures are so low in cold traps that
ice would behave like a rock," Hayne said. "A billion years if water
gets in there."
Both papers have far-reaching consequences for future
lunar expeditions. As part of the Artemis mission, NASA plans to create a lunar
base; if an adequate source of water can be located nearby, lunar occupants
might utilize it for drinking, producing crops, and even splitting it
using electrolysis to produce hydrogen for rocket fuel.
Both papers were published in the journal Nature
Astronomy.
Reference(s):
Nature.com | Molecular water detected on the sunlit Moon by SOFIA
Nature.com | Micro cold traps on the Moon
[This is an updated version of the old article]
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