At the most recent NASA Astrobiology Science Conference, a panel of scientists discussed different types of planets where we might find alien life. In the fifth segment of this series, the panelists address questions from the audience about the search for life on Mars and elsewhere.

Tori Hoehler: I think the question of manned exploration crystallizes around Mars. That's the one that lies within the realm of feasibility. If you look at what we understand about the potential for life on Mars, NASA has had a program of robotic exploration that has a nice sequential progression to it.

Every couple of years there is an opportunity to send something to Mars that lets us learn more about it. I suspect the most significant path forward to do that is with continued robotic exploration. Eventually we'll select the right sort of sample that has the best chance for containing evidence of life and we'll bring it back here. As far as I know, all of that could be accomplished robotically.

There are different issues that come with human exploration. Some of them are hard to ignore, and it's simply that having a human hand involved in these things lends an aspect of significance to it, of public engagement. But from a purely scientific perspective, most of what we would need to do to make reasonable statements about the prospects for life on Mars could be done with robots.

T.C. Onstott: I agree with Tori, and part of the reason is because our physiology prevents us from spending a long time in space and making the trip to Mars. So either we change ourselves so that we're more space portable, or we rely upon this increasing ability to develop in next 10 or 20 years autonomous robotic units that can have nearly as much intelligence as a human mission would have.

That also gives us a certain advantage in that we can cleanly observe the system without seriously impacting it at the same time. So there's a lot of appeal to sticking to a continued advancement of robotic exploration.

Seth Shostak: Suppose we were to go to Mars, either robotically or as humans, and find some living stuff underneath the ground. But we want to terraform Mars and turn it into a place where we can build condos. How do the panelists feel about that? Do you think that we should develop Mars even if it has native inhabitants, or should we just leave it as a national park?

Jill Tarter: Let me take this question, because I'm the most outrageous of the panelists. I think that we would do ourselves a disservice if we didn't take the opportunity to learn about something new and different, assuming that any life found there has an independent genesis. If we missed the opportunity to study another biology because we preferred our condos, that would be an enormous disservice to the human race. However, once we have done a reasonable study - you'll never learn it all - you'd have to weigh the benefits after the initial discovery against having a second place for humanity to continue.

Seth Shostak: Other panelists? I'll assume that we study the Martians first. After we've got them all catalogued, can we obliterate them?

Tori Hoehler: I think it does boil down to that choice. The assumption is that by moving in we would essentially obliterate them. I think there's a moral choice to be made. Personally, I don't feel that you can do something like that. But I also can see a situation in the future where you need to worry about what's going to happen here at home.

I could see that maybe we're forced into a situation where things have gone so terribly wrong here and we haven't managed to stop what's happening on our planet now, that you simply are forced into that decision. So I don't think it's a "never say never" kind of situation. But I think it would take a lot of resources and a lot of effort to colonize Mars. I think those same resources and effort can be put into figuring out how to keep Earth habitable.

Q: My question ties into some of this. Partly to subsurface probing of local moons or planets in the solar system, and also to sending robotic explorers to Mars, which every now and then we tend to crash a few into Mars.

So what are the chances that we've already seeded Mars with life from Earth? How would we distinguish it if we were to find life elsewhere with a robotic probe from something that hasn't already come from Earth? And what would we do to mitigate the chances of seeding another body with Earth life forms?

Peter Ward: Can I defer to Andy Schuerger? The world's expert is sitting right there.

Andy Schuerger: I wanted to ask a question anyway, so thanks, that gets me to the front of the line. I work at the Kennedy Space Center, and for about 8 or 9 years I have been looking at how micro-organisms that are likely to be on space crafts might survive on Mars and whether or not they can potentially grow there.

It's almost certain that viable terrestrial micro-organisms have been either crashed or successfully landed on Mars. The only two vehicles that were ever sterilized were the two Viking landers. Everything else has been very clean, assembled under very excellent conditions, but there is a small finite amount of micro-organisms that have made the trip.

The real question in my mind is not that we've sent viable microbes and they're likely to have been landed or crashed on Mars, but what are they doing now that they're there? All the work I've done suggests that it's really hard to achieve any growth and proliferation at the surface of Mars.

If you put odds on it, I would say that the viable microbes are there because we've sent some of them, but they're not overtly contaminating the planet. It's just so dry, so cold, and has a very high UV flux, and about ten other things that are limiting their growth.

Seth Shostak: Would anyone else on the panel like to comment on contamination?

Tori Hoehler: I would only add that this is an issue that is taken very seriously by the global space faring community. NASA, for example, has a planetary protection officer whose job is to prevent both forward and backward contamination.

Peter Ward: I can see three possibilities about life on Mars. The worst is we get there, and we don't find any life. That's horrible. We want to live in a diverse universe, at least I do. Number 2, we get there and we find life that is so demonstrably different than Earth life, we've found a separate creation.

Unless, of course, it came from Earth a long time ago and evolved, or we came from Mars a long time ago, and back and forth. Number 3 is we find more life and it's just like Earth life. Now, is it just like Earth life because of convergent evolution, because there's really only one way to make DNA? Or did we crash it there? That's the miserable one - then you're really unhappy. But most unhappy is finding nothing.