Moon's a watery world

Photo: smedia.imgix.net

This false color photograph is a composite of 15 images of the moon taken through three colour filter.

For many years astronomers have been puzzling over the question "Why is the Earth so wet?

Two thirds of our planet is covered with water, and the parts of the Earth above the water level are covered with features shaped by water. Astronauts on the moon looked back on a blue world, patched with pure white clouds, evidence of a uniquely wet world. 

However, is it uniquely wet?

The Apollo astronauts brought back a large collection of samples of lunar soil and rocks. One thing they had in common was that they were all very dry. This makes sense.

The moon has almost no atmosphere, so there is no greenhouse effect and all the incoming solar energy hits the ground. The result is daytime temperatures reaching about 130 Celsius, and then falling at night to -170 Celsius.

These temperatures vary with latitude, as they do on the Earth, There are some places, such the depths of craters that never see the Sun where the temperature is as low as -250 Celsius.

This combination of baking and freezing in a vacuum is a good way to dry something, especially if this process has been repeating since the moon and Earth formed, about 4.5 billion years ago.

However, given that both formed from the same ingredients, the moon must also have been a watery world once. Is it still?

If you were to visit the Arctic during the summer, you would first notice the mosquitoes. The second would be that the weather is nice and warm. However, if you were to push a thermometer into the ground, you would find that some distance below the surface, the temperature is below freezing, even in high summer.

This is the permafrost, because it stays frozen throughout the year. Under the insulating layer of soil, the temperature hovers around an average value, staying constant over the year. If this average is below freezing, water down there is always frozen.

On the same basis, if we were to stick a thermometer into the surface of the moon, at some depth we will find the temperature unchanging, around a chilly -40 C.

We have measured the moon's average temperature in other ways, such as using radio telescopes. Short radio wavelengths tell us the temperature of the surface layers. Longer ones tell us the temperatures deeper down, where they are unchanging.

During the early history of the Earth, a lot of water mixed in with the construction material was ejected into the atmosphere, initially as superheated steam. Eventually temperatures fell enough for the first rain to fall, and it rained for a very long time, forming the oceans.

Even today, most of the Earth's water is buried deep in the planet. The moon is smaller than the Earth, leading to the loss of its atmosphere to space, and it cooled off faster.

However, it did form from the same mixture of construction materials, so we should expect there to be quite a lot of water on the moon, somewhere.

Sensitive instruments have detected water molecules at the moon's surface, probably from somewhere inside. Solar radiation would break these up into hydrogen and oxygen, which are lost to space.

However, there are accumulations of ice in deep craters, particularly in the polar regions, in places where the sunlight never reaches. This is certainly encouraging from the point of view of space exploration, because it means people on the Moon can use the local product.

This water would be good for more than just drinking. It can be chemically split into hydrogen and oxygen, yielding components for rocket fuel-and breathable air. If there are large quantities of water ice buried deep in the moon, life for the visitors will be far easier. However, building in permafrost is a challenge.

• After dark, Saturn and Jupiter lie low in the southwest
• Mars is rising in the east.
• Venus lies high in the dawn glow with Mercury below. 
• The Moon will reach First Quarter on the 21st. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.