As I write this column, two spacecraft are racing towards the Moon—one is Russian, the other Indian.
They share the same destination, deep, dark craters near the lunar South Pole where we believe there is water. This is an issue of great importance. If there are useful quantities of accessible water on the Moon, life for us on the Moon will be a lot easier. Otherwise, every drop of water will have to come from somewhere else, probably the Earth.
Almost all of the lunar surface, plus the soils below it, are extremely dry, having been heated and cooled in a vacuum for billions of years,
It is interesting that the Earth and Moon were made around the same time from the same stuff. However, we live in what could be the wettest place in the Solar System, whereas the rock and soil samples brought back from the Moon by the Apollo astronauts were all bone-dry.
Radio telescope studies also indicate a lack of water. Why did these two worlds end up so different?
Basically, as in many areas of astronomy, the mass of an object makes all the difference. The Moon's mass is 1/81 that of the Earth. The gravitational pull on its surface is about 1/6 of that we experience on the surface of the Earth. These numbers meant the Earth would end up as a mild, watery world with an atmosphere, whereas the Moon would end up as an airless, dry rock.
It is widely believed when the Earth was forming, it was hit by a body around the size of Mars. When the material ejected by the impact came together again, we had two bodies, the young Earth and a newly born Moon.
In the beginning, the Moon probably had a molten core, just like the one in the centre of the Earth, and an atmosphere similar to the Earth's before living things modified it. Flows in the core region generated electrical currents, which in turn generated a magnetic field that held the solar wind away from the young Moon.
However, the core cooled, the motions stopped and the magnetic field vanished. The solar wind then set to work scouring away the Moon's atmosphere. Today the solar wind hits the lunar surface. The Apollo astronauts deployed special solar wind catchers when there.
The Moon's weaker gravity did not help. It allowed the atmosphere to extend further into space, where its gravitational hold on it was weaker, which made it easier for it to be scrubbed away or even just drift out into space.
Once the Moon had lost its atmosphere the surface temperature changes during the lunar day became huge. We get a much weaker form of this on Earth. When we have sunny days and clear nights, we get the largest daily temperature differences.
On the Moon, with almost no atmosphere and days that are around 28 of our days long, there is a lot of time for heating up and cooling down. Around local noon, temperatures on the lunar surface at its equator reach around 130 C and at nightfall to around -130 C.
Billions of years of undergoing these temperature variations, in a vacuum, the lunar surface layers have dried out almost completely. Some water comes in from space, as icy meteorites and comet dust. Over most of the lunar surface the daytime heat sends that water back into space very promptly. However, there are deep craters around the poles where there is never any sunlight. These places get very cold and will retain water. Over billions of years, a lot may have accumulated.
Deep below the lunar surface there could be layers of ice or permafrost. Even if that water is there—which is still a big question—it will take a lot of effort to access.
Those accumulations of surface ice in deep craters around the poles would be a start for the first explorers who are building lunar bases. It is possible that is all the water to be had.
• Saturn rises around 9 p.m. and Jupiter around 11 p.m. In hilly places, they will appear later.
• The Moon will reach its first quarter Aug. 24.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.