Remote observations of planetary surfaces from orbiting spacecraft have produced a plethora of detailes and much exciting new information. But this technique, being remote and ‘hands-off’, still has nearly everything in common with earth-based observations. A unique aspect of space exploration comes when we actually land on other bodies. So far we have managed this on the Moon, and on planets Venus and Mars.

The Moon has become the best known celestial body in the solar system after Earth itself. It is the only body from which rock samples have been obtained, and some 381 kg of rock have been returned, the vast majority from the six landing Apollo Missions but some very much smaller samples from USSR robotic vehicles. Much was revealed about the age of the Moon and of specific lunar features. Also the rocks provided vital clues as to the tectonic and magnetic development of our nearest neighbour and the conditions on its surface. Most of the analysis techniques were non-destructive and used only very small sample masses. In fact the vast majority of the returned Moon rock is still available for the scientists of the future to experiment with. All that is needed are new ideas and more sensitive analysis techniques.

Samples have also come from Mars, in the form of meteorites such as Shergotty (this rock falling in India in 1865), Nakhla (an Egyptian fall in 1911), Allan Hills 84001 (that was picked up on a glacier in Antarctica in 1984) and Chassigny (that hit France in 1815). These rocks were formed volcanically between 1300 and 200 million years ago and have been blasted off the surface of Mars by asteroidal impact cratering events. They subsequently spent many millions of years in space before suffering a scorching and erosive retarding transit through the Earth’s atmosphere. The problem with these exciting Martian meteorites is that we have little idea as to exactly where they came from on the surface of Mars. That they actually came from Mars was confirmed by the Viking landers, these finding that the nitrogen isotopes in the Martian atmosphere are similar to those found absorbed in the meteoritic rocks.

The history of lunar landing is rather sad, because history is very much the apposite word. The seven USA Surveyor probes were working on the surface between May 1966 and January 1968. The six USA Apollo missions landed between July 1969 and December 1972. The USSR’s Luna 20 and Luna 24 sample return missions brought back some 30 and 170 gm respectively of rather randomly selected soil, in 1972 and 1976. There have been no landers in the last 28 years. And this is not for want of good scientific justification. The mechanism that led to the formation of the Earth-Moon system is still essentially uncertain. Even though the suggested theories have vociferous advocates they have many aspects that are difficult to accept. Much more needs to be understood about the structure and interior of the Moon. We are not even sure whether it initially became so hot that it differentiated and now has a small iron core. The dynamic history of the lunar orbit is also problematic. The change in the Earth-Moon distance (and thus the rotation speed of the Earth) seems to have been discontinuous, with periods of fast evolution separated by epochs of stability. The Moon, through its tides, has played a critical role in the evolution of the Earth’s surface and on the origin and speed of the development of life.

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