T.C. Onstott: I'll be talking about Subsurface World. Jupiter's moon Europa is one of those places within our solar system where we might expect to find Subsurface World.

By analyzing the DNA within a bacterium, we can figure out the metabolic pathways an organism can have. We discovered one particular organism 3 kilometers beneath the surface in South Africa. This organism is capable of living without any interaction with the photosphere above.

It's deriving all of its nutrients from the surrounding minerals, and it contains all that it needs to survive within that environment. It doesn't need any other micro-organisms.

It is a one-world environment. The other interesting aspect about this is that the radiation from the environment, just the normal decay of radiogenic isotopes, is sufficient to maintain this organism indefinitely. So it never does achieve any type of entropic death.

There is enough chemical energy provided by the decay of uranium to oxidize water and the production of reducing compounds to keep the system going.

So we have proof that life can exist without photospheric influence, and it can exist for billions of years, and that means the chances are good that life could exist beneath the surface of Mars.

The environment on the surface of Mars at one time may have supported life, but now it's a difficult place for life to exist. Beneath the surface, where we anticipate finding liquid water, organisms such as this could exist. But what happens when the surface has never been habitable, as in the case of Europa or Saturn's moon Titan?

When we look at the bacteria deep beneath the surface on Earth, it turns out that life is deeply rooted from an evolutionary point of view. That means it's very primitive. If we look at the history of life on Earth, we have fossils of microorganisms from 3.45 billion years ago.

A billion years prior to that, some abiogenic formation of pre-biotic compounds was occurring. In between those two events, it was either a fortunate series of extraordinary events that led to life, or it may be a very common occurrence. If you believe it was a common occurrence, then we should anticipate finding life on other planets and other solar systems, even other planets within our own solar system.

The question is, can this life occur within the subsurface with no input from, for instance, meteorites or sunlight? One early process important for life is compartmentalization - the formation of some type of organic vesicles. On the surface of the planet, all hell is breaking loose, and it's hard for life to evolve there. What if the nursery is at depth?

In South Africa, you find hydrocarbon vesicles occurring from 3 kilometers depth, and if you take a look at these in detail, the sizes of the vesicles are small but variable. If you look at them on the inside, you see their mineralized surfaces sandwiching a layer of organic carbons. So there are processes taking place beneath the crust of South Africa which appear to be abiogenic.

In fact, the deeper we go, the more we lose the signature of DNA life forms, and the more we see abiogenic processes occurring. A question that remains is, could life originate in the subsurface? We haven't proven this, but it's a line of investigation that we can pursue for the next few years.

James Kasting defined the 'habitable zone' of a solar system as a function of the distance away from different types of stars. The 'habitable zone' is where liquid water exists on the surface of a planet - it's where we would expect to find life existing. But in a subsurface universe, where life can originate and remain in the subsurface, that habitable zone is much larger.

So, in fact, dark life, or life that exists without photosynthesis, could be the dominant life form in the universe. But the only problem, of course, is how do you see it or detect it? The planets are truly not visible. In that regard, there's not much we can do other than detect a few trace gasses.

It is difficult to determine whether or not you're seeing atmospheric biomarkers that reflect some microbial process at depth. The only way we can find Subsurface World is to actually go there and collect samples.

Seth Shostak: Our next speaker is Peter Ward from the University of Washington. He is a paleontologist, and a popular science author.

Peter Ward: I want to talk about planets that would have complex life, and ask what is it about them that got them to that state. How long do they last, and how does it end? I wrote a book called "Rare Earth" with Don Brownlee, and the follow-up book was called "The Life and Death of Planet Earth."

The first one sold a bunch of copies. The second one only sold about three because it was so depressing. I mean, who wanted to know about the end of the Earth? And yet, both books are really about one thing: The Earth in space and time.

If we're trying to find what a complex life-inhabited world might look like in space, we immediately think of our own planet. We have continents, we have oceans, and we believe that continents and oceans combined are necessary to maintain that habitability which we all enjoy. But we know that there are other aspects that are needed as well, such as the geometry of our solar system.

For instance, you need a good Jupiter to reduce the impact rate of comets and asteroids on our particular planet. But in other solar systems we find bad Jupiters, because they have migrated in close to their star and disrupted the inner solar system by doing so, or have a highly elliptical orbit around a star that is also disruptive.

So maybe the systems that have complex life would have a Jupiter in a less than highly elliptical orbit at a great distance from the star itself.

No one is talking about manned missions to Venus, and why is that? Because the temperatures there are hundreds of degrees Centigrade above what any sort of complex life on this planet could survive. Venus is a case where we would not expect complex life. And Mars - we hope there are microbes, but we certainly don't think there will be higher complexity.

If we are ever lucky enough to get a signal from other complex life, I think it's going to be so difficult to figure out that particular signal. Just consider the difficulties we encounter communicating between Macs and PCs.

But we're not just talking about complex life in space. We need to think about it over time as well. Early in the Earth's history the conditions would not have allowed for complex life.

This is a very strange squeeze that any complex life-inhabited planet is going to have. Over time, the gaseous atmospheres change, and what we call the habitable zone narrows and shrinks because of changes in the atmosphere. And so you're running a race. You only have a finite amount of time in which a planet can have complexity.

For Earth, we expect our solar system will change radically as our sun becomes a red giant about 5 billion years from now. After our star goes red giant and then collapses upon itself, we end up with a very different type of system of life on Earth.

I like to think of life on Earth as an Oreo, and complex life is the white middle, but you've got this nasty black world of the first microbial age, and we expect to be followed by another slime world in a second microbial age. So while planets like Earth seem ideal for complex life, there is a time limit for that life, and before and after that time you wouldn't expect to have complexity.

The time of complex life on Earth has had numerous mass extinctions. Most of the creatures that have ever lived on our planet have gone extinct. The age of animals seems to be about halfway over.

By old age, we think planets with complex life can also suffer accidents, just like you and I can. We see this in Earth's history.

The famous K-T mass extinction was caused by an enormous asteroid colliding with the planet 65 million years ago. If comet Hale-Bopp had hit us when it passed by a decade ago, there would have been an extinction far greater than the one that killed the dinosaurs.

It is not only asteroids and comets that are dangerous to planets. The planets are perhaps dangerous to themselves. Tectonic processes that lead to huge eruptions of lava on the Earth's surface, so-called flood basalts, also can cause mass extinctions.

250 million years ago one species survived such a catastrophe, and that particular reptile led to the evolution of mammals.

Tectonics can be fatal, but plate tectonics also helps keep our particular planet stable and allows for complexity. Venus is as nasty as it is because it doesn't have plate tectonics. It has a stagnant lid and the surface melts every 300 to 500 million years.

All kinds of complexity are necessary to keep our planet within the series of constraints that allows complex and fragile animals such as us to exist. As we head out into space, we realize that not only is the number of possible Earths small, but we have to find the possible Earth in that brief Oreo of time when it can have complexity.

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