STUART: The main problem, I think, is that we don't really know exactly what we're looking for.
The only known source of life is on Earth. It could be that we have other life in the universe that's
quite similar, but it could be completely different in a different environment. So, we wanted to come up
with a way of looking for life that was kind of agnostic,
where we didn't have to make any assumptions about life at all.
COLE: In order to make any progress here, you have to think about,
you know, what is it about life that seems unique relative to non-living systems?
HEATHER: When you see something that is interesting, that looks like energy went into it ,
that you can't explain through abiotic means, the best way, the most generalized way we could
think of life, is being that extra push of energy that is needed in that environment to make that expression.
A footprint is that little bit of energy in a dusty landscape that tells you
something was alive there, but fossils are also a physical expression of energy that accumulated a
bunch of organic material. And we can do the same thing in chemistry by looking at the complexity
of molecules and saying: is it likely that you would have all of that energy that it took to
assemble a molecule in that way happening without the presence of life?
STUART: What we landed on was basically a complexity measure. The further you go down chemical space,
the larger molecules you make, the more decisions you have to make to get to that molecule,
and the more possible other things you could have made. What we wanted to do was,
we wanted to take molecules and say: how many steps would it take to make this molecule, even if you ignore
all the rules of chemistry, in competition with all the other things that could have been made, and then use that to assess
how likely it is to have been made in a kind of just random process without biological direction behind it.
HEATHER: You know, so this particular idea of molecular assembly was really born in math,
even though it's a chemical method. It was born out of the idea of thinking about building molecules in an algorithmic sense,
and then, recognizing that molecules that are derived from abiotic systems have far less steps
necessary to make them, than the molecules that we associate with biology, which take many more steps.
COLE: So, once we sort of had that, Stu spent a bunch of time writing code so that we could calculate
those quantities for any molecule we cared about. The next step was to think about a way to measure it.
The original idea of using mass spectrometry goes back to this thought experiment and knowing
that NASA had this long legacy of sending mass spectrometers to space, and so, the first thing
that we did was, we went and we got a bunch of molecules that had different complexities,
different molecular assembly values, and we put these in the mass spectrometer and we looked to
see how they broke apart, and we just counted the number of unique pieces they broke into.
But, once you get that right, you can sort of see that the higher the assembly number is, the more pieces
the molecule tends to break into, which was really exciting. Sort of the first time any of these
chemical complexity measures has been something that's detectable in an experiment in the lab.
LEE: Now we have that framework, first of all, we should apply it on Earth.
Secondly, we should go around our solar system and look for complex molecules everywhere.
HEATHER: The MOMA instrument that will be aboard the ExoMars rover has this capability of fragmentation,
and then investigating the sizes and identity of all of those fragments; and there will be a mass
spectrometer that flies with Dragonfly to Titan that will also be able to do that same process.
COLE: I'm also really excited to go to Venus. Well, I'm not going to go to Venus I don't want to die.
But I'm excited that NASA is sending missions there because I don't expect we'll find life there,
but, I think we will learn a lot about the scope of possible abiotic chemistry, so I think
it'll be really good to visit the closest analogue to Earth that we have, and I definitely think we
should be using this technique to learn about the surface of that environment.
LEE: But, I think there's an even more important thing for astrobiology; not just defining what life is,
but also looking in, observationally, looking at exoplanets. So, suddenly you can start to look
for signatures over the entire universe using spectroscopy remotely. But, of course, first things first,
let's get some decent mass spectrometers to Mars, Venus, Titan, Enceladus, and find what we
can, what we can map, and that's really exciting. And I think NASA is super excited and has been great
because without their samples and collaboration, we wouldn't have been able to validate the technique.