Original URL: http://www.theregister.co.uk/2007/07/23/weird_life/
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.
Widening up the field
Not everyone is sure how helpful all this is. Serena Viti, an astrobiology researcher at University College, London, summarises: "If life is not based on what we know, then it becomes more difficult to know what we are looking for."
The further into the report you go, the wider and weirder the possibilities become: the latest experiments in synthetic biology suggest we might not even have to look for life based on DNA. It even raises the possibility of "non Darwinian life". (On being told this, Professor Burchell chuckles wryly, a man laughing at a private joke. The suggestion that some universally agreed on definition of Darwinian life might already exist is obviously amusing.)
The report notes that experiments in synthetic biology have created structures with six or more nucleotides that can also encode genetic information and, potentially, support Darwinian evolution. It also cites studies in chemistry that show how an organism could tap into other energy sources. A reaction of sodium hydroxide and hydrochloric acid, for instance, would make it possible for a life form to have an entirely non-carbon-based metabolism.
Put the paparazzi on the case
The authors recommend that the machinery we send out to the furthest corners of our solar system should carry with it an array of detectors capable of sniffing out life of all kinds. Saturn's moon Titan is a high priority for exploration for the authors. The Huygens probe which landed there in 2005 discovered an environment they consider most fertile for weird life in our solar system.
There are problems with this approach, of course, not least the bewildering array of possible life forms we are looking for.
Professor Burchell argues that if we really want to find alien life, we'll have to do it with cameras. This might not be the most obvious conclusion to come to after reading about how life could be far more varied than boring terran life with its dull as dishwater carbon and water based biochemistry, but it isn't a foolish one.
Cameras will be essential in the hunt for ET, because "there is no single definitive test for life", he says.
Finding carbon-based life anywhere is extremely hard. Even on Earth growing a culture in a petri dish requires care, and a probe that has landed on another planet is not exactly a biology lab. Assuming it exists, there are all kinds of ways you can miss it, not least precisely because you don't know what it will be like.
"Take salt, for instance. Some bacteria need it to grow, but it will kill others," Burchell says. His point is clear: send the wrong test and we could get a negative result.
"Sending a test to find life to another planet is very difficult. For a start the test has to be able to do somthing fairly sophisticated: detect respiration, cell walls or genetic material. Then it has to be very small, not mind being shaken about like a James Bond Martini during a rocket launch, the being frozen in space for a journey of many years before being expected to work right out of the box when it reaches its destination."
A team at the University of Leicester, led by Dr Mark Sims, is developing a chip which could test for some indicators of life. But even this would not be exhaustive.
Add to this the new complexity of looking for life that is unfamiliar, and the already formidable challenge becomes truly daunting. Burchell puts his finger on the problem: it is too complicated to test for life remotely, but you can't send an astronaut to do it because you would, in the best tradition of a twisty CSI plot, contaminate the scene.
This is where the cameras come in.
"A better alternative is to send a camera and look for changes. If you see a change, you can try to assign a reason for it. Are the seasons changing and frost melting, or have you observed something else, maybe a biological process," he says.
Viti seems to agree: she argues that alien life might not look very different, even if it is chemically worlds apart: "I think that when we speak about extra-terrestrial life, one has to remember that the most common form is probably non-intelligent, as defined by us, basic form like bacteria and viruses. So, it is hard to imagine how they would look, or whether to our eye they would look like any different from earth bacteria," she says.
A change, though, might be detectable, regardless of the chemistry of the life that produced it. ®