Space exploration and the discovery of evidence for extraterrestrial life

When humans first landed on the moon in 1969, there were no signs of extraterrestrial life. This was unsurprising, as earlier satellite missions had confirmed what had appeared to earth-based astronomers to be a completely dead world without an appreciable atmosphere. Similarly, the Voyager missions to Mars and the Venera missions to Venus during the 1970s revealed hostile, barren landscapes. Venus, often regarded in the past as earth’s ‘sister planet’, turned out to be one of the most hostile environments so far discovered, with sulphuric acid rain, temperatures in which lead is liquid and constant high winds. By the mid-1970s, a consensus had developed among astronomers that life in the Solar System was restricted to the Earth.

However, developments in the 1990s saw a number of claims for life on our near neighbours. The most well known of these was the announcement on 16 August 1996 that a fragment of meteorite from Antarctica (known as ALH 84001), shown after its discovery to be of Martian origin, contained fossil traces of bacterium-like structures and telltale formations deriving from the biological activity of bacteria. Critics were quick to point out that some of the tell-tale traces could have formed in a super-heated environment such as that the meteorite would have experienced in passing through the earth’s atmosphere, that the structures could be artefacts of the gold coating applied to the surface to prepare for electron micrography and that the bacterium-like structures were too small to accommodate DNA. Nevertheless, the popular media seized on the evidence and the idea that life may once have existed on Mars is now firmly ingrained in the popular consciousness. Further announcements in early 2001 about discoveries in other meteorite samples add weight to the ‘pro-life’ argument by providing cumulative evidence for the biogenic origin of the features observed in these mineral samples.

Other satellite explorations have suggested possible locations for the development of life in the solar system, most notably the Jovian moon Europa. Although covered in a thick layer of ice, this moon nevertheless appears to possess oceans of liquid water beneath the ice and to have an internal temperature caused by geological activity. This is similar (but not identical) to the conditions under which life is now thought to have developed on earth. No evidence for the presence of life has yet been found, but there are plans for future missions to drop probes through the ice into the oceans beneath in an attempt to identify organic activity.