Lunar orbiters recently detected water frost on the floor of some of the south polar craters, regions that are known to be very cold and can trap water. The floor of these craters are also exposed to the space environment including incoming meteors and solar wind particles. In a paper published in the journal Geophysical Research Letters, planetary researchers show that these external environmental processes can erode the water frost — with a layer of 1/2 of a micron of frosty soil being eroded on time scales of less than 2,000 years; the products of this erosion are ejected into space above craters and could be detected by properly-instrumented polar orbiting spacecraft.
A high-resolution free-air gravity map based on data returned from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission, overlaid on terrain based on NASA’s Lunar Reconnaissance Orbiter (LRO) altimeter and camera data. The view is south-up, with the south pole near the horizon in the upper left. The terminator crosses the eastern rim of the Schrödinger basin. Gravity is painted onto the areas that are in or near the night side. Red corresponds to mass excesses and blue to mass deficits. Image credit: NASA’s Scientific Visualization Studio.
“People think of some areas in these polar craters as trapping water and that’s it,” said lead author Dr. William Farrell, a plasma physicist at NASA’s Goddard Space Flight Center.
“But there are solar wind particles and meteoroids hitting the surface, and they can drive reactions that typically occur at warmer surface temperatures. That’s something that’s not been emphasized.”
Unlike Earth, with its plush atmosphere, the Moon has no atmosphere to protect its surface. So when the Sun sprays charged particles known as the solar wind into the Solar System, some of them bombard the lunar surface and kick up water molecules that bounce around to new locations.
Likewise, wayward meteoroids constantly smash into the surface and uproot soil mingled with frozen bits of water.
Meteoroids can hurtle these soil particles, which are many times smaller than the width of a human hair, as far as 19 miles (30 km) away from the impact site, depending on the size of the meteoroid.
The particles can travel so far because the Moon has low gravity and no air to slow things down.
“So every time you have one of these impacts, a very thin layer of ice grains is spread across the surface, exposed to the heat of the Sun and to the space environment, and eventually sublimated or lost to other environmental processes,” said Dr. Dana Hurley, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.
While it’s important to consider that even in the shadowed craters water is slowly seeping out, it’s possible that water is being added, too.
Icy comets that crash into the Moon, plus the solar wind, could be replenishing it as part of a global water cycle; that’s something planetary researchers are trying to figure out.
Additionally, it’s not clear how much water there is. Is it sitting only in the top layer of the Moon’s surface or does it extend deep into the Moon’s crust, scientists wonder?
Either way, the topmost layer of polar crater floors is getting reworked over thousands of years, according to the team’s calculations.
Therefore, the faint patches of frost at the lunar poles could be just 2,000 years old, instead of millions or billions of years old as some might expect.
“We can’t think of these craters as icy dead spots,” Dr. Farrell said.
“To confirm our calculations, a future instrument capable of detecting water vapor should find, above the Moon’s surface, one to 10 water molecules per cubic centimeter that have been liberated by impacts.”
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W.M. Farrell et al. The Young Age of the LAMP-observed Frost in Lunar Polar Cold Traps. Geophysical Research Letters, published online July 1, 2019; doi: 10.1029/2019GL083158
