Building an Alien Biochemistry
Ben - Finally this week, a new way to think about the search for extra terrestrial life. Here's Cambridge University's William Bains...
William - I come at this from a very indirect historical route but in summary, I became interested in astrobiology because of some particular chemistry I was doing and as a biochemist, I started out by asking, "Well, what is life and what is biochemistry?" from a very fundamental level. And that seemed to be quite a productive line of research to follow. Most of the material I was reading about the origins of life was written by chemists who looked at it chemically or astronomers who looked in terms of creating planets. They won't look at it in terms of somebody who's been used to trying to discover new drugs, looking at organisms that are very complicated, very poorly understood collections of stuff. So I thought I'd start with that and see what the stuff is and what could we say about life from the very basic level, via a basic understanding of the process of living.
Ben - We obviously have a very good example of life here on earth. It's very diverse, it's very interesting but in a way, biochemically, that sort of makes it just a sample of one. Is it possible that this blinkers the way that we do look for life?
William - Well, it certainly does. It has to, doesn't it? We look around life on earth and it's so powerful, it's so diverse that it's very easy to be overwhelmed by that. There is no other possible way you could put life together. Actually, I was talking to another conference attendee a bit earlier. One of the great advantages as a biochemist talking to physicists, is that they think about things completely differently. This physicist said "well actually we don't have any samples of life at all because you need to discount the earth because of observer bias". So once you discount the earth, how many examples of life do we have? We might have life on Mars but we don't know anything about it. Apart from that, we have no examples of life at all. And if you start from that position, you realise we don't really know much about what life would be like outside the earth at all. So, we're both overwhelmed by the fact there's life here but also, we're conned into thinking that we are typical. We might be. We've no idea, but we don't know.
Ben - So, do you have to look at this, try and be objective, and look at the possible biochemical combinations that we could have that could create life?
William - Absolutely and that's the overall thrust of the research that I'm doing. It's a really difficult problem and I don't pretend to have done more than nibble at the edges of it. But yes, the idea is to try to think from a chemical basis what sorts of chemistry could you put together to do the sort of fundamental things that life does.
Ben - We have, from years of studying chemistry, a very good idea of how different molecules will react together, what possible combinations we have. But it very quickly builds up into an enormous network of possible interactions. How do you whittle this down to the ones that could be plausible for life?
William - Yeah. It's a huge problem in combinations of options. There are two answers to that. The first is there are some very general rules you can apply. So, life has to work in quite thick solutions. If you look at the difference between a fish and the sea, the fish is full of chemicals, full of stuff. Protein, metabolites, all sorts of things, and the sea is clean. So, life is going to have to be dissolved in something. So, if you try to build life out of chemicals that simply don't dissolve, it's not going to work. They have to be reasonably stable. So, trying to build life out of molecules that will instantly react with water and expecting that life to work on earth just isn't going to work. And there are a number of slightly more indirect and sophisticated filters you can put in but the same sort of basic logic. The difficulty is, does that then leave you with one or two possibilities of biochemistry or half a dozen different types of possibilities, or does it leave you with hundreds of billions of possibilities? At the moment, we have no idea.
Ben - Is it actually easier to look at a planet or a moon and say, "Well this is the chemical environment we have. These are the options for life?"
William - That's what I'd hope. That's the overall aim to say, not only are these the options for life but these are the likely signs that that sort of chemistry is going on in that environment. And then of course we'd look for it. This is a fascinating intellectual puzzle but there's not much point doing it unless it comes out with something you can test, something that the astronomers can look for in a spectra or the spacecraft can look for when they land.
Ben - I understand that you have been considering Titan as an option and looking at the chemistry on Titan?
William - Yes. Titan is the coldest body in the solar system which we know has definitely got liquid on it. The surface is incredibly cold. It's only about 90 Kelvin, so 90 degrees above absolute zero. It's definitely got ponds of liquid methane and ethane mixture on the surface. In some ways, it's like earth but of course, the chemistry is bound to be completely different. And that's why I've chosen it. Not because I think there's likely to be life there but because it is an extreme example that pushes your assumptions and therefore says, "Can we say anything sensible about that?"
Ben - So, if life does has to have solubility involved, then it must be a series of chemicals that are soluble in this liquid methane and ethane. Do we know of any that would fit the bill?
William - We know of a few very small simple molecules definitely. I mean, liquid methane and liquid ethane are important to the gas industry so people have measured the solubility of things like carbon dioxide and water in liquid methane. People have measured the solubility of a number of small molecules in very cold solvents, so, liquid methane, liquid nitrogen. So we've got some data on what is soluble, on the type of molecule that's soluble. They all tend to be small and this is a problem for designing a biochemistry because you do need things that will dissolve and you need lots of different molecules that dissolve. And whether we can get enough different types of molecule dissolved, there's still hope in question.
Ben - What are the core jobs that we need biochemistry to do in order to get what we think of as life?
William - Oh Gosh! Well, you have to be able to have some sort of large molecules that can act as catalysts. This isn't just to make the chemistry happen fast. It's also to make sure that the chemistry goes in the right direction. You need some sort of information storage. Life works from an internal plan, a coded description. This doesn't completely describe life but life emerges from that code. Clearly, life has to handle energy. It needs energy to make something happen to drive the chemistry forward in the right direction. You need structural components. You need to contain the chemistry in some sort of bag or sack. The problem we're stating, the requirements for chemistry in this terms is incredibly general. Somebody came along and say, "Could you do these things with the chemistry of silicon?" And the answer is, "Not under terrestrial conditions." Water, in particular, just dissolves most silicon chemicals and turns the silicon back into sand. But in Titan conditions, where the water is all completely frozen out as rock, and you're dissolving it in liquid methane and liquid ethane, the answer is possibly. Do I think there's silicon base life swimming around in the lakes of Titan? Extremely unlikely, but we can't yet rule it out.
Ben - Cambridge University's William Bains on a biochemical approach to finding life on other planets.