Moon crater: Water ice younger than expected?

Micrometeorite erosion could spread and renew lunar ice

Permanent in the shade: there are water ice in the polar craters of the moon - but this could be younger and more volatile than previously thought. © NASA / GSFC
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From Ancient: The water ice in the polar lunar craters is probably much younger than expected. Instead of millions or even billions of years, the ice deposits are probably only a few thousand years old, as researchers have determined. Because by impacts of micrometeorites water molecules are constantly ejected from the crater floor, at the same time fresh water is added. This could be an advantage for future lunar missions.

The earth's moon was long considered extremely dry. But meanwhile, measurements of moon probes and analyzes of rock samples from the Apollo missions prove that there is water on the moon. Part of it is bound in the moon rock, but there are also water ice in the craters of the lunar polar regions. Its reason lies in deep shade all year round and therefore acts as a kind of cold trap for water molecules. This could have accumulated in some craters meter-thick layers of ice.

False-color image of the south pole region of the moon - here there could be water ice in craters. © NASA / Scientific Visualization Studio

Ancient stocks?

But how persistent are these water ice supplies? According to popular theory, it is so cold at the bottom of the lunar crater that water droplets once deposited there remain almost indefinitely long. The extreme frost of less than minus 200 degrees ensures that the water molecules can neither melt nor evaporate. The ice in these polka-craters could therefore be millions to billions of years old at least that's what they thought so far.

However, a new study raises doubts about this ancient ice. Because the polar lunar craters are cold, but not completely isolated. "Their surface is hit by particles of the solar wind and meteoroids and that can promote reactions that normally only occur at higher surface temperatures, " explains William Farrell of the Goddard Space Flight Center of NASA in Greenbelt.

Bombed by solar wind and meteorites

The consequences of this bombardment for the lunar crater ice have now been studied in detail by Farrell and his colleagues in a model simulation. It turned out that the solar wind alone ensures that water molecules are repeatedly blasted out of the moonshine. More water mixed with moon dust could be stirred up by the impact of small meteorites. display

Once released from the ice, these tiny ice lumps can fly up to 30 kilometers, the researchers report. As a result, water is distributed on the crater floor, but it is also carried to the crater rim and into other, warmer crater areas. "Each of these impacts spreads a gauzy layer of ice creamer on these surfaces, exposing them to solar heat and the space environment, " says co-author Dana Hurley of Johns Hopkins University.

Hazed nner water veil over the craters

Part of the ejected water ice evaporates and can thus escape completely from the crater. According to researchers, a 40-kilometer lunar crater releases about ten trillion water molecules per second. "We have determined that there must be a water exosphere over the polar craters, " Farrell and his team report. This gossamer vapor haze over the lunar polyps could be detectable in future space probes. Because per cubic centimeter there should be one to ten water molecules.

More importantly, the surface of these ice deposits changes constantly due to this constant erosion. "It takes less than 2, 000 years for a half-micron of the ice surface to erode, " say the researchers. While the inclines and the solar wind are constantly removing part of the ice, water-containing meteorites also add new water ice. "We can no longer see these craters as dead water reservoirs, " says Farrell.

A few thousand years instead of millions of years

But that means: the ice in the lunar craters is probably not millions or billions of years old, but much younger. "The largest part of this frosty regolith should be less than 2, 000 years old, " Farrell and his team report. The hope to find ice deposits from the early days of the solar system in the lunar craters is therefore in vain.

However, the new findings also have something positive: In future moon missions, astronauts may not have to drive down to the extremely cold bottom of the lunar craters in search of water. "Our results tell us that meteorites are doing us some work and transporting material from the coldest crater areas to its outskirts, " says Hurley. "There, astronauts could reach the water-ice with solar-powered rovers."

"We need first-hand data"

Whether or not water ice is actually to be found on the crater edges, will probably only show forthcoming missions to the polar craters. Rovers or astronauts could then explore how old the lunar ice actually is and whether there is a kind of water vapor ice cycle on the Earth satellite. "We need first-hand data to understand what's happening there, " says Hurley. (Geophysical Research Letters, 2019; doi: 10.1029 / 2019GL083158)

Source: NASA / Goddard Space Flight Center

- Nadja Podbregar