Earth may be the only planet in the solar system to have life on it, but it has emerged that life may be possible on at least two moons of the outer planets.
The universe is so vast that most of us find it hard to believe that we are the only living things in it. The belief that there must be intelligent life on Mars was so widespread that when HG Wells’ sci-fi story “War of the Worlds” was first read on the radio it caused widespread panic in America as it was mistaken for a news report. Scientists initially believed that Venus, Earth and Mars alone made up what they called “the habitable zone.” This is the zone where the distance from the sun is just right to allow a suitable range of temperatures for life – on Earth most life exists between 50℃ and −60°C, so this is regarded as the temperature range within which life can exist. The availability of liquid water is also taken to be essential for all life.
Mercury, due to its proximity to the sun, is too hot. After Mars, it gets too cold. Could Venus and Mars be “just right?” For a long time, we thought so. However decades of space exploration and research have led scientists to conclude that Earth is a very special place indeed – Venus may be the brightest star in our sky at night, but prospects for life there are bleak. The atmospheric pressure is 95 times ours (95 bars vs. 1 bar), Venus’ surface is hot enough to melt lead, and what little liquid exists there isn’t water, but acid.
Life on Mars seemed more likely, and it was in fact widely believed that there was intelligent life. This is partly due to a translation error – when an Italian astronomer reported seeing “canali,” which means “channels” in Italian, the Americans misinterpreted it as “canals.” As canals are manmade, these simple geological features were misinterpreted as proof of intelligent life. In fact a US astronomer developed a theory that Martians were using the canals to transport water from the polar ice caps to the desert areas for purposes of irrigation, and soon it was generally accepted that there must be intelligent life on our neighbour planet.
However, we now know that the surface pressure is far too low for water to exist, and that it is inhospitably cold – night-time temperatures can reach -128℃. Also, Martian soil is rich in peroxides, which are highly toxic to life as we know it. More recent voyages to Mars have found evidence that surface water did once exist, possibly billions of years ago, and scientists now hope to find fossils rather than living creatures on Mars.
Maybe the theory that Earth is the only oasis in space’s great desert holds true – or, at least, maybe life in the universe is very rare indeed, and that other life-bearing planets are simply too far away for us ever to have any contact. It looked like the search for other life was dead in the (lack of) water – but lately this search has been opened up again, with the exciting possibility that at least two of the moons in our solar system might be suitable for life.
Water is common in the outer regions of the solar system, where, due to the extreme cold, it exists as ice which is as solid as ice. Many if not most of the moons in the outer reaches of the solar system also contain hydrocarbons and organic compounds, which are essential to life. Theoretically, the only reason these moons could not support life is that the vast distance between them and the sun just makes them too damn cold. But, as it turns out, the sun might not be the only way to heat a planet – and maybe life is not confined to planets. In fact, at least two moons are warm enough for liquid water to exist.
Satellite pictures of Europa, one of Jupiter’s moons, have shown cracks in the moon’s icy surface, suggesting that a liquid ocean exists beneath the icy layer. Even more exciting, on Enceladus, which orbits Saturn, water has actually been seen. The reason these moons are warm enough to have liquid water is called tidal heating – even though they are too far from the sun to be effectively warmed by it, the gravitational pull of their planets and of other moons close to them cause a tidal effect, just like we see in our oceans. The energy generated by these tides heats up the interior of the moon.
For a while, it looked like the search for other life in the solar system was at an end and E.T. would never phone home from anywhere but Hollywood. Maybe the search is just about to begin.