by Leslie Mullen for Astrobiology Magazine

Nathalie Cabrol, a planetary geologist with NASA Ames and the SETI Institute, and a member of the Mars Exploration Rover team, has been testing the rover prototype, "Zoe."

The field tests have taken place in Chile's Atacama Desert, one of the driest places on Earth, and, consequently, a region where life is extremely scarce. In the latest field test, which took place last September and October, Zoe managed to find life in the Atacama.

The rover will be heading back to the desert this fall for further tests.

In this interview with Astrobiology Magazine editor Leslie Mullen, Cabrol explains how Zoe could put the search for martian life on the fast track.

Astrobiology Magazine(AM): You've said that Zoe, the rover you've been testing in the Atacama Desert, represents the next generation of rovers.

Nathalie Cabrol (NC): The Mars Exploration Rovers are about the size of a table, while Sojourner was the size of a shoebox. Zoe is bigger - over 2 meters long.

So Zoe would be a small car, and she's a fast lady. Opportunity has been going a little shy of 200 meters in one traverse, and that's spectacular for that vehicle.

Well, Zoe can do so much more - on the last day of the most recent operation in the Atacama, we went 1.2 kilometers in one traverse.

Zoe has much more mobility than the MER rovers, and by going far, you're giving yourself a good chance to see more things. And this is where I become happy as a field scientist, because when you're in the field, you never, ever, walk around in small circles.

When you get out of your car, you don't just stay there and look around, you know?

But I had to wonder: in the search for life in extreme environments, where life, if any, is going to be scarce and located in tiny oases, is going far and fast the way to find more of these micro-oases, or to miss more?

AM: Right, because you've cruised past them.

NC: One day, (after we had downloaded all the data Zoe had collected), a member of the science team said, "Oh shoot, look at that, there is something really interesting that (the rover passed in the middle of its traverse), which is now hundreds of meters or even kilometers behind us. It's too bad that we cannot go back."

And I said, "Why can't we go back?" I realized the only thing preventing us from going back was the fact that we had never done it before. Usually you keep the rovers going forward!

AM: You didn't need to go back before because the rovers have always moved so slowly. But going faster created a new problem of needing to go back to places the rover passed before you had a chance to download the data.

NC: Exactly. So we had to demonstrate that our rover is capable of going back. To do that, we choose a long traverse, going back over a kilometer. We came to within a few meters (of our starting point) just by triangulation and looking at features.

The science team wrote a traverse that was translated into Zoe, since there's no software on Zoe that could help her go back. She has no pattern recognition software, for instance, to follow her tracks back.

But now we are thinking about adding that. So this has triggered a snowball effect, an avalanche of ideas.

I'm looking forward to developing new techniques using autonomy. Today rovers are going from point A to point B, with the science team telling the rover, "Go to B because there is something interesting there that I want you to see."

But if your rover is capable of understanding what the science priorities are - the rovers are the ones spending the time in the field, while you are very far away - then the rover would know that along this path there is something that is even more interesting than B.

If Zoe was capable of making decisions, she could scan her environment and decide if what she sees as she's going towards the destination can be a better payoff for the mission success. Then Zoe can go to a new target on her own, do some work there, and say, "Look what I found. What should I do next?"

AM: So Zoe is a work in progress, and you're still adding things.

NC: Oh, yes, we are just into the second year of the experiment. Zoe is trying to find life in the Atacama Desert, and to do that, you have to understand the weather and the day-to-day climate, as well as the microclimates (of the habitats).

So we needed to add something onboard the rover to provide that information. You also need to have something that will help you understand if what you are looking at is living or not.

In our tests in the Atacama, the first trigger in the search for life was the visual imaging, where we'd see an odd morphology, a shape that was out of the ordinary.

Then we'd use the fluorescent imager to test for chlorophyll. We also sprayed the target with dyes that, when used with the fluorescent imager, light up when they bind to nucleic acid or protein.

A big question is: when can we say, unambiguously, that we have found life? Is it life only when we get positive results for all the tests, or would just one or two positives be enough?

We stopped at forty sites, and not once did we see all positives. The morphological evidence alone is striking - I mean, all of a sudden you see this little bulb of orange on a rock.

AM: That orange bulb was lichen?

NC: Yes, that was obvious when you were looking at it. But it was obvious because this was the Atacama, and you knew that this was possible on Earth. But as a geologist I can tell you that there are crystal formations that look like that. So even if the morphology is striking, it might not be the only thing.

For some samples, we had two positives, and we were seeing the DNA and the protein matching at the same place, and we said, "Ooh, there is something out there," but at no point was the science team compelled to say that we had unambiguous evidence for life.

AM: When you sprayed the lichen, how many positive results did you get?

NC: We met lichen in different places. Let's say one of the lichens was positive for chlorophyll. But why wouldn't it also be positive for DNA or protein?

The difficulty we met was that these organisms are protecting themselves from the harsh environment with a shell, and the dyes we were using had a hard time penetrating that shell.

AM: So that's why it might not be positive for DNA, even though lichen obviously have DNA.

NC: Exactly. But then the team added vinegar to the spray, and that seemed to solve the problem. It literally dissolved the problem, which was the calcium carbonate in the shells. But if you have organisms on Mars, you can bet they will have even better protection. It's going to take a lot more than vinegar to get through that!

The samples that had more positives weren't necessarily the ones that showed the best results when they were cultured in the lab. Even the one-positive ones showed beautiful cultures. So that goes back to our question, how many positives add up to the identification of life?

Well, if you have all positives that's great, but if you have only one, it doesn't mean it's not life. So there's a lot of thinking to be done there, to fine-tune what we'll be adding to Zoe in the next year. In our upcoming field tests, we will be using dyes to also test for lipids and carbohydrates.

AM: Is Zoe going to have the ability to try to culture things on Mars? Or is going to be, "Well, that had two positives, so it could be life, but let's move on?"

NC: What I'm most interested in is the methodology that will bring my rover and my payload, whatever the payload is, to the right spot. We're just giving the biologists pointers on the instruments and the methodology.

In future years, maybe we'll put some other instruments on Zoe to look for the type of life we can find on Mars, if any. But Zoe is just a wonderful platform to learn how to do this.

AM: So the actual development of biology detection is in the hands of the biologists, who will be adding their instruments to your payload.

NC: Exactly. But what Alan Waggoner has been doing (with the fluorescent dyes) is fantastic, because a goal of our ASTEP-funded project is to characterize the Atacama.

The Atacama's habitats and environments are not very well understood, and Alan and the science team now have put life on the map there. Their results also prove that looking for life with a rover is a technique that works.

AM: When do you find out if Zoe gets to be the rover that goes to Mars?

NC: Zoe is not going to go to Mars, ever.

AM: I mean the model, the design.

NC: We are hoping that the things we've been developing will someday make their way into the Mars Science Lander-type of rover. That could be the science, the technology, the hardware, or even the science team.

One huge benefit that I've found in the past year was how the simulation of Mars missions can train planetary scientists. Many didn't have a clue about how to use a rover when they started.

As a scientist, I wanted to understand how rovers work, because then I could not only talk to the engineers about what I wanted to see, but I could help them make a better design.

You can have the best site in the world, but if the site is not safe, if you cannot land there, if you can't traverse there, what's the point? If you don't land safely, you don't have a mission.

I've been working with Carnegie Mellon University since 1997, and we try to find solutions together to make the best rover for a successful mission. Before, you had some sort of robotic vehicle off the shelf that was produced by an engineer, and they'd say "Here's what we have, try to use it."

It's not that world anymore, and MER is a fantastic example of that. It's probably not foreign to the fact that these rovers are still alive today and performing so well. Once you learn the other side's perspective, it's an enrichment, really. It's something that is benefiting everybody.

Mars Odyssey recently detected water ice near the surface in the high latitudes, and in 2007 the Phoenix Mars Lander will investigate those regions. This August, the Mars Reconnaissance Orbiter will be launched. What it discovers will determine the fate of the Mars Science Laboratory, which is scheduled for launch in 2009.

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