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The search for alien life
How do you look for something if you don't know what it looks like?
The parachutes deploy perfectly, and gradually, swinging gently through the tenuous atmosphere, the little probe falls to the ground. Minutes later it jerks slightly as it unfurls its solar panels. Like a butterfly newly emerged from a chrysalis, it soaks in the rays of the sun.
Now its sensors switch on and start to take in the sights, possibly even sounds, of the world on which it finds itself. After a quick internal test, instruments deploy on robotic arms, ready to collect samples of the alien soil, and to test for that most elusive quality of all: life.
But will our probe find anything?
This is a big question, made up of many, many smaller questions. Has life emerged, independently of us, on another planet? Would we be able to spot it, even if we parachuted in right on to what passes, locally, for a head? Could the very worst happen: could we fail to detect life because we looked in the wrong place, or were looking for the wrong thing? And most fundamentally: does life have to be carbon based and dependent on water?
These thoughts were troubling the sleep of NASA's top executives when they asked the National Research Council (NRC) to put together a picture of where life might be found, and what it might look like. What, it asked, are the limits of organic life, or life as we know it?
The NRC does what research councils do best, and set up a committee to answer the question. Their conclusion, detailed in the report The Limits of Organic Life in our Solar System, was clear: life could exist in many forms utterly unfamiliar to us from our experiences on Earth, and if we want to be sure of finding life, we need to expand our ideas of what might qualify.
"Our investigation made clear that life is possible in forms different than those on Earth," said committee chair John Baross, professor of oceanography at the University of Washington. "The search [for life in our solar system] so far has focused on Earth-like life because that's all we know, but life that may have originated elsewhere could be unrecognisable compared with life here. Advances throughout the last decade in biology and biochemistry show that the basic requirements for life might not be as concrete as we thought."
Every carbon atom is sacred
Convention holds that water is an absolute requirement for life, as is carbon, and we have collectively based our strategy for finding alien life on the assumption that this will turn out to be true.
As Professor Mark Burchell, head of the Astrobiology Society in the UK explains, there are very good reasons for this: "People tend to home in on water because it is very common. It is also fairly unreactive, and is liquid across a fairly wide range of temperatures. This suits life well because if a cell is too cold, nothing chemical happens. But if it is too hot, it breaks down.
"Similarly carbon is ubiquitous. Chemically it can store lots of information, and can be manipulated at energies available in sunlight. You can use it to make lots of structures, fold it up and so on. As to why not silicates? Well, the carbon energy band gap is just better and does a little bit more. It may be that this is enough to make it the best choice for life."
NASA is by no means abandoning this strategy; it says the approach has served it well.
"Our 'follow the water' strategy for exploring Mars has yielded a string of dramatic discoveries in recent years about the history of water on a planet where similarities with Earth were much greater in the past than they are today," Doug McCuistion, director of the Mars Exploration Program at NASA said, as the organisation announced the forthcomming launch of Phoenix, a new lander to Mars specifically designed to seek out water.
But this new report suggests that water, at least, might not be essential, after all.
It also says that if we are serious about searching for life we really need to understand how it arises, and that means learning more about life on Earth, particularly in extreme environments. Researchers should also be on the look out for novel biochemistries on Earth, as well.
Of course science fiction writers have played with alternative chemistries for years. (Silicon is a firm favourite in fictional fantasy: Issac Asimov, for instance, had his siliconies, foot wide telepathic silicon life forms, fuelled not by the light of a nearby star, but by gamma rays emitted by radioactive ores.)
But mainstream science has toed a much more conservative line. Life has almost certainly evolved somewhere else in the universe. If it has, there are sound chemical reasons why most of it is likely to be carbon and water based.
Even so, the scientists behind this NASA sponsored investigation concluded that water and carbon are not absolutes. If we set these aside, what are the truly essential ingredients? The report lists, in decreasing order of certainty, four things:
A thermodynamic disequilibrium; an environment capable of maintaining covalent bonds, especially between carbon, hydrogen, and other atoms; a liquid environment; and a molecular system that can support Darwinian evolution.
What this means is that virtually anywhere there is liquid and heat, there could be life.