At the most recent NASA Astrobiology Science Conference, a panel of scientists discussed different types of planets where we might find alien life. In part six of this series, the panelists answer audience questions about the origin and evolution of life elsewhere.

Q: We believe life on Earth spontaneously arose in a liquid environment. If we find subsurface alien life in a more or less solid environment, does that imply it first may have started in liquid on the planet's surface and then, when conditions got extreme, it found refuge beneath the surface?

T.C. Onstott: Right, but the alternative is the possibility that there are multiple origins of life that could occur in different environments and combinations. If life could have originated in the subsurface, would it have looked a lot different from life that we see here on Earth today? That's still a question we don't know the answer to. Some of the processes that are responsible for generating pre-biotic compounds would occur in a subsurface environment. But certain types of compounds might not be as prevalent in the subsurface as you might find in a surface environment.

Q: Do thermodynamics favor a liquid environment for the evolution of life or a more solid environment?

T.C. Onstott: It depends on which environment is more benign. If life does originate in the subsurface and then later colonizes the surface as the surface environment becomes more habitable, that may produce a different sequence of evolutionary events. For instance, as the luminosity of the sun increases the planets that are farther away outside the habitable zone, such as Mars, begin to warm. But you are also getting desperately close to when the sun goes into a red giant phase. You don't have as much time to evolve. But if you had 5 billion years of evolution in the subsurface, then you don't need that much more time to evolve. So you could come up with different scenarios as to how life can evolve if there are multiple origins of life. Q: Could there be sort of an extreme version in the scale of life? Something so large that it makes our universe look microscopic, or something so small that the tools we use to look are simply the wrong scale to detect anything.

Jill Tarter: Yes. We could spend an afternoon talking about whether we live inside a computer simulation, in which advanced technology on a different scale could warp our appreciation of our environment. When my daughter was growing up, they used to tell ghost stories about maybe we're just a small thing in the bloodstream of a giant. We don't have tools for measuring those scales, except the strange things that we're seeing in the universe as we look farther out and farther back in time. I guess if I wanted to be very extreme, maybe we could consider dark energy or dark matter as being manifestations of some else's technology that we haven't even thought about because the scale is so impossible.

Q: To find a Slime World, why not use the same technology as scientists use to find the neutrino and then zoom out?

Tori Hoehler: In a way that's kind of what we're doing. When we're looking for a Slime World, we're looking for a place that we can't reach out and touch physically. Also we look for some kind of signal that's coming to us from a long, long way away. In this case, what we get to see is light. But there are all kinds of things about light that tell us something. There are different sorts of light that allow us to see chemical information. So, for example, I could see what I'm breathing out right now while I'm talking if I look in a different sort of light. And in a sense what we're doing is exactly what you're suggesting. We're looking with the right kind of detector and the right kind of technique to see the sort of information that we think might represent light. So when we look for neutrinos, we're looking for evidence of a certain sort of process. And when we look for the sorts of chemicals that life might breathe out, then we're looking for evidence of life that we think might make those chemicals.

Q: Is it possible that there may be life embedded in the ice somewhere on Jupiter's moon Europa, since ice on Earth generally has micro-organisms in it?

T.C. Onstott: I was thinking of that the other day, because of the difficulty of penetrating the crust of Europa with a space vehicle in order to look for life beneath the icy crust. There have been periods of time when Europa's icy crust has floundered. You can see chunks of the ice that were frozen in place, and twisted on its ends. Conceivably, if the bottom face of the ice is now tilted upwards, then it would be readily accessible. If you could bring a chunk of that back to Earth, perhaps by blasting it off and catching it, then you could look for organisms in that ice. Or perhaps you can use telescopic methods, looking for signature in the ice itself, some type of pigment perhaps.

Q: The microbial communities in Earth's lithosphere can be very diverse. It's not a monoculture of one species. And so the question is, if there is subsurface microbiota on Mars, can you speculate on what you would expect the diversity of that ecosystem might be, given that evolution has carried on for 3.5 to 4 billion years on Mars? Are we just talking about the level of cyanobacteria, or could Mars have subterranean complexity that moves up the ladder to multi-cellular subsurface organisms?

Tori Hoehler: That's a very complicated question. I think that the diversity that we see around us on Earth is a product of a couple of things. It's a product of how many useful spaces there are to expand into, which means how many different sorts of energy and how many different ranges of conditions under which that energy can be exploited. It's also a function of how rapidly evolution allows those spaces to be populated with different organisms. So I think that if life has been exclusively in the subsurface on Mars, which maybe has a subset of the niches that we have here. The range of conditions certainly isn't as broad as what we have on Earth, taking together the atmosphere and oceans and subsurface.

But to me the more interesting question is the pace at which evolution could occur there. Evolution happens as a function of growth. We think about the pace of evolution being rapid in our own world because we have abundant energy. Stuff grows like mad, and there's a lot of chance for evolution to populate all of those different spaces. One of the things that characterize a subsurface is that there's little energy to go around. That implies that growth rates would be very low. You see a much lower biomass and much lower turnover rates of micro-organisms in the subsurface than the surface. So I think there probably are somewhat fewer niches into which life could expand on Mars, and I would guess that maybe the rate at which that expansion could happen would be quite a bit slower than here, orders of magnitude slower, maybe.

Q: So, the diversity would be orders of magnitude simpler than what we see in even our own subterranean biosphere.

Tori Hoehler: My guess is certainly lower. Whether lower than in our subsurface biosphere, I think that's a question of if there were life on Mars, whether it had begun on the surface and retreated into the subsurface, or if it was indigenous to the subsurface. If the latter, then yes, perhaps the diversity on Mars would be even lower than in our subsurface.