Scientists using data from NASA's Lunar Reconnaissance Orbiter, or LRO, have identified bright areas in craters near moon's south pole that are cold enough to have frost present on the surface.
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"We found that the coldest places near the moon's south pole are also the brightest places—brighter than we would expect from soil alone—and that might indicate the presence of surface frost," said Elizabeth Fisher, the lead author of the study."
The frost spotted on moon's surface seems to be thin and patchy in appearance. There are chances that these icy deposits are mixed in with the layer if dust, soil and regolith, which are small rocks.
The researchers say they are not seeing expanses of ice similar to a frozen pond or skating rink. Instead, they are seeing signs of surface frost.
The frost was found deposited in cold traps which were present close to the moon's south pole. Cold traps refer to permanently dark regions which are present on the floor of a deep crater or along a part of the crater which doesn't receive direct sunlight.
Due to the lack of direct sunlight in this area, the temperature remains below minus 163 degrees Celsius (minus 260 Fahrenheit). These prevailing conditions can allow the frost to prevail for extremely long time durations, around millions or billions of years.
The astronomers had guesstimated the presence of ice water in the cold traps more than 50 years ago, but confirming the hypothesis turned out to be challenging for them.
In the late 1990s, hydrogen-rich areas were found near the poles of the moon with the help of NASA's LRO, but it was hard to identify whether hydrogen was present in the form of water or not.
NASA's LRO has been orbiting the moon since 2009 with one of the main aims to understand the nature of these deposits.
The south pole of the lunar surface is cold enough to have frost present on it. The new proof about the presence of water ice in the moon's craters comes from the study comparing the readings of the surface temperatures from LRO's Diviner instrument with the brightness measurements of the lunar surface from the spacecraft's Lunar Orbiter Laser Altimeter (LOLA).
It was found that the coldest regions present around the south pole were more radiant, pointing toward the presence of frost or extremely reflecting materials.
The highest surface temperatures were also analysed by the researchers because water ice wouldn't persist if the temperature rises above a certain extent.
The findings are consistent with another team's analysis of LRO data, reported in 2015. That study compared peak temperatures with ultraviolet, or UV, data from the Lyman-Alpha Mapping Project, or LAMP. Both LOLA and LAMP are able to measure surface brightness without sunlight. LOLA does so by measuring reflected laser light, and LAMP, by measuring reflected starlight and the UV skyglow of hydrogen.
"What has always been intriguing about the moon is that we expect to find ice wherever the temperatures are cold enough for ice, but that's not quite what we see," said Matt Siegler, a researcher with the Planetary Science Institute in Dallas, Texas, and a co-author on the study.
He estimated that water ice and other deposits have been found on Mercury's north pole too, which seems to possess around 400 times more ice than the moon despite Mercury being the closest planet to the Sun. Scientists are still figuring out which scenario is "more normal."
"There's enough evidence now to argue for further investigation. Not only could the moon's ice provide resources for exploration, it also might help us understand the origins of Earth's water," Seigler concluded.
