Ken Tapping, August 15, 2007

In the early 1970s, there was a rather intriguing cartoon in one of the science magazines. It showed an alien creature, crawling across a desert, obviously in serious trouble, and crying "Ammonia, Ammonia!" It was a powerful way to point out that although water is a key substance for our survival, there could be creatures out there whose bodies use some other liquid.

As far as we know, here on Earth life first appeared in the sea. If we take all the biological materials out of our blood, we are left with seawater, a reminder of our early history. Since we have some understanding of what happened here on Earth, we are particularly interested in finding other watery worlds to see if the process happened there too. At the moment, there are two places in the Solar System where we think we might find "life as we know it", using a chemistry based upon liquid water. These are Mars, the fourth planet out from the Sun, and Europa, one of the moons of Jupiter, the fifth planet out from the Sun.

Although Mars is a cold, almost airless desert now, it was not like that billions of years ago. The evidence is very strong that there were lakes, rivers and oceans, all long-since vanished. There could be traces of past life on Mars, or even some hardy remnants hiding beneath the surface.

Europa is so far from the Sun that there is insufficient solar heat to melt the ice. However, the tidal distortions Jupiter inflicts upon Europe are believed to liberate a lot of heat, so underneath the thick layer of surface ice, there could be a deep, dark ocean. The tidal forces would make hydro-thermal vents, black smokers, just like the ones we find at the bottom of our oceans, and which are covered in life forms that owe nothing to the Sun.

Although there may be other liquids that living things could use as a water substitute, water is pretty special. Firstly, it is very common in the universe, and secondly, because of the structure and behaviour of water molecules, it is a really great liquid to 'do chemistry' in. So far we've found nothing else like it. However, that leads us into the old trap of judging things according to our standards. If we accept the possibility of "life not as we know it", we open the door to all sorts of options. On some cold world, where ammonia is a liquid, we might well find our thirsty alien. There are lots of places in our outer Solar System where he or she (assuming these aliens use the same gender arrangements as we do) could be quite happy.

Arthur C. Clarke and Carl Sagan postulated the existence of huge life forms floating in the deep ammonia and methane atmosphere of Jupiter, moistened with occasional showers of ammonia rain. Some scientists have considered more bizarre beings, such as plasma creatures, made of extremely hot gas, tied together by magnetic fields. 

All these ideas are mainly imagination at the moment. However, in a huge universe like ours, with billions of stars and at least as many possibilities, we must explore it with an open mind. We can be pretty sure that when we meet our first extra-terrestrial life forms, they won't look anything like us, or some actor in a rubber suit.

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Ken Tapping is an astronomer at the National Research Council Herzberg Institute of Astrophysics (NRC-HIA), and is based at the Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9 Tel (250) 493-2277, Fax (250) 493-7767,
E-mail: ken.tapping@nrc-cnrc.gc.ca