More recent tests reveal no thick ice sheets on the Moon

In the late 1990's the Prospector spacecraft orbited the Moon and returned data that indicated there were sheets of ice in the crater depths around the lunar poles. Now, more recent data seems to refute that.

In the 1990's the Clementine and Lunar Prospector spacecraft discovered there were craters on the Moon which were permanently in shadow. This was particularly exciting because scientists believe ice could exist in such places. Prior to the crater's discovery it was believed that the airless Moon could not contain ice because the Sun would have evaporated it. But ice could exist in the floors of deep craters near the Moon's poles where sunlight never reached. The reason the presence of lunar ice was exciting was because it would prove vital if a lunar base was established in the future.

Unfortunately the fanfare over lunar ice has been rather short-lived because more recent results using radar have concluded that lunar ice, if present, will be more difficult to extract than some have proposed.

The new theory is that although it is still possible to have ice on the Moon, it won't be the sheets of ice that previous spacecraft hinted at. At the Moon's poles, the Sun only rises two degrees above the horizon, so it is correct that the deepest craters have areas which are permanently in shadow. It would be possible for water molecules ejected from cometary impacts could remain trapped in the lunar soil on the floors of these polar craters. Another possibility is that hydrogen from the solar wind could combine with oxygen atoms in the lunar soil and form water molecules. Something like these processes must also have happened on Mercury, where radar has revealed sheets of ice at the planet's poles.

These recent quests for lunar ice were conducted using radar with the powerful Arecibo radar transmitter in Puerto Rico. While the tests from the 1990's only probed to a depth of about an inch below the lunar surface, the tests using Arecibo went down to a depth of just over two feet deep below the surface. The observations looked into permanently dark crater floors and showed that they do not have thick sheets of ice like Mercury.

This dry outlook doesn't discredit the previous missions results, however, it just means they need to be interpreted differently. Clementine's data came primarily from the rugged side of one crater wall, not its smoother floor. The rougher the terrain, the more likely that a reflected signal will mimic ice. Using Arecibo, and probing deeper into the soil gives a more accurate picture.

Whatever ice there is in these crater floors is likely in frozen patches, spread out in low concentrations. If there are any ice sheets at all on the Moon, they would be less than an inch thick and inter-layered within the lunar soil. Added up though, the Moon still could have a lot of ice.

The likely scenario is that the ice is thoroughly mixed in with the soil in these crater floors. It gets mixed in so well because the Moon's soil is constantly being "gardened" or turned over. Since the Moon has no atmosphere to speak of, meteors don't burn up on entry as they do on Earth. Instead, all the meteors strike the Moon's surface, churning up the soil mixing it with the ice.

With ice mixed intermixed with lunar soil, it will be more difficult to extract. Even so, it will most likely be more practical to collect lunar ice than to transport water from Earth. Astronomers want to get a better look at those crater floors around the Moon's pole to learn how much ice there is and how easy or hard it will be to get to. A lunar base would be a big step towards taking humans to Mars, a goal that may believe will be achieved before 2025.

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