Astrobiology Magazine (AM): When you look at the first color Pancam image of Gusev Crater, as a geologist, what do you see?

Nathalie Cabrol (NC): You see a flat plain. And you see different populations of rocks. This means that it's a diverse site and has a very complex history. And we had a sense that this would be what we'd see in this place.

As far as interpretation, it's way too early to tell anything. We're still sitting on the lander. But just the diversity of the site is appealing. The color is very appealing. It's a very different site from Viking or Pathfinder, so it's going to tell us a different story.

AM: How is it different?

NC: The distribution of rocks is different. The shapes of some of the rocks are quite different. You have sub-angular, triangular rocks. We were expecting that, because you can see lots of impact craters in images of the site taken by the MGS (Mars Global Surveyor). And you can also see what we are calling "hollows" right now. One of them, right in front of the rover, Sleepy Hollow, looks very much like an impact crater.

And the size of the rocks is different. There are many, many small rocks. Some of them are sub-rounded (partially rounded) to rounded and, more interestingly, they are also smooth and polished. Polish can be from the wind. We know this is a very high-wind region. We can see little tails behind some of the rocks that show that we have wind action. Smooth is a different matter; we'll have to figure out why they are smooth the way they are.

AM: Are there places on Earth where you see a similar distribution of rocks?

NC: We have been talking in the past couple of days about how this could look like places you would see in the Arizona desert, in the California desert, in the Atacama desert (in Chile). Some things could be the result of deposition by flow, with rocks embedded in a matrix of fine sediment, and the whole region deflated (swept clean) by wind.

Volcanic plains also can be very flat with small rocks. But I'm particularly interested by the smoothness and the roundness of some of these rocks.

AM: Why?

NC: Because there are two processes that can lead to very round things: wind and water. On Mars, anything that is larger than 4 millimeters will have a hard time being transported by wind. It will be extremely important to see the shape of small particles to test the hypothesis of water.

We are not seeing, in this landscape, big rocks on top of each other. So whatever put these rocks there, if it was some type of flow - and this still has to be determined - wouldn't be a high-energy outflow (e.g., a flood). If it was a flow, it would be a low-energy flow.

In any case, I'm fascinated by the fact that the distribution of rock is very different from the Viking and Pathfinder sites. So what we're seeing here is another history of Mars. It's definitely a new spot on the map.

AM: Can you tell anything from the colors in the Pancam image about the composition of the rocks and soil?

NC: It would be better to have Mini-TES data. But some of the team members have started playing with the colors, and there seem to be some different shades there. There's probably a dominant material - we don't know what it is yet. But there is some bright material. Some of it looks "plastered" on the rock. Is it just the same material, but finer, that has been plastered there? Or is it something different? Once again, we'll need to look at it with Mini-TES.

AM: Why would the soil be red and the rocks mainly gray?

NC: It depends on the origin of the rocks. If they are volcanic, they could be basalt. Even if they got here through fluvial transport (carried by flowing water), they could still be volcanic rocks, because they would have come from the highlands and that is what you'd expect the very old highlands to be made of.

The soil could be redder because you could have not only erosion from the local rocks, but also airfall deposits that could come from anywhere. We know that the area has been reworked a lot by wind, removing and depositing things from all sorts of places.

And it could be just a difference in grain size. That does a lot to change the color of material. So it might be just the same material, but finer.

AM: In the corner of the Pancam image, where the airbag appears to have scraped along the ground, there's a rock - I think it's a rock - that Steve Squyres said looked "mud-like."

NC: We all said that. When we looked at that we said, It's mud-like. But that's just looking at a picture. We still don't know its composition. The thing is that this material seems to be cohesive, to look like mud. It's going to be very interesting to find out its composition.

I'm thrilled because when you look at it, you can see a patch that has been removed by the scraping. It has been flipped over, but it's still sticking to the rock. This is probably something we haven't seen anywhere else on Mars, and it's going to be really interesting to look at it more closely.

Is there any moisture in this? We don't know. Is there some salt, and we're seeing particles sticking together? Once again, we're just in awe, and looking at strange things that look like things we know on Earth. It doesn't mean that they are.

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You also have, in another part of the panorama, a rock that looks like - and once again it may just be an artifact, but a lot of people on the team don't think so - it has some material plastered on the top.

AM: Couldn't it just be dust?

NC: Well, maybe cohesive dust. But it seems to be the same kind of "mud-like" material you see near the lander.

AM: I notice that the material in the shallow depressions is lighter than the surrounding soil. Could this also be a result of different grains sizes?

NC: Yes, and the reason is pretty straightforward. The wind removes material from the flat plain. But where you have a slight depression, finer material gets trapped.

AM: It's been mentioned that Spirit landed in an area that has been swept by dust devils. How big are these? Are they the size of tornadoes?

NC: No, but they can be different sizes. They can be anywhere from one or two meters (3 to 6 feet) to something that can engulf a building. And they can be pretty energetic. So the biggest ones can remove a lot of stuff.

AM: If you're in a dust-devil region, wouldn't you see dark and light streaks?

NC: That's what we do see in the MOC (Mars Orbital Camera) images.

AM: But why don't you see it in the panoramic images of the landing site?

NC: Well, some of the patterns that we see in the Navcam panorama could be these ground traces of dust devils. For example, a feature that some of us call "the road."

AM: Let's talk a bit about the history of the site. When did you and Edmond first propose Gusev as a landing site?

NC: The first presentation at a site selection workshop was on January 26, 2001.

AM: And it was one of a couple of hundred sites proposed?

NC: One of 185 sites. We proposed Gusev, Gale Crater, and several other possible impact crater lakes. And, to my amazement, at the end of the workshop, not only were Gusev and Gale still in, but also all of the backups. And we were already down to only 20 sites.

AM: Why did you think these locations were good candidates?

NC: Because there were hypotheses that there was water on Mars, and when water flows and you have a big hole, you have a chance for water to pond there. There were several hundred places on Mars where there was evidence that there were dry river beds, or channels - valley networks - flowing into impact craters.

The reason we chose impact craters is that when we started, we started with Viking data. And the elevation data from Viking was very bad. You had plus or minus one kilometer precision. So it was hopeless to try to identify topographic depressions. But impact craters - you're pretty certain that they are holes.

And when you see rivers, or ancient channels, or valley networks flowing into them, you have a chance that something ponded there. We found nearly 200 cases of potential ancient lakes. It wasn't just the topography or morphology, we also found landforms within the basins that indicated that we could have had ponding in these regions.

AM: What kinds of landforms?

NC: Streamlined terraces, for instance; or things looking like shorelines, which are more ambiguous because they are very fragile things; and alluvial fans, things that look like deltas. Now, with MOC images, we have very good evidence that there are some very beautiful deltas on Mars.

AM: But at the time, not everyone agreed that these were water-formed landforms.

NC: No, and they still don't today. This is just an hypothesis. But looking at the evidence, Edmond and I came up with a scenario. Other people looking at the same evidence have come up with other scenarios. Today, what we think is a potential delta in Gusev, even some people on our team disagree with. That's why it's so interesting to be there and to be able to test hypotheses.

But if we leave Gusev with evidence that it's a big pile of volcanic ash, I'm not going to take that personally. Because we didn't go to Gusev because I wanted to. We went because Edmond and I argued that this was an interesting place. Other people came up with other hypotheses than we did, but the bottom line is that it's a darn interesting site. It could be many things. And we're there with the right instrumentation to look at the rocks and test those hypotheses.

We didn't want to go to a site where we knew for sure what we would find. What we know is that it's a basin that has collected material for a long time. Whether the material is glacial, fluvial (formed by flowing water), lacustrine (lake deposits) or aeolian (wind-driven) won't matter in the long run, because we'll learn something about Mars in any case.

AM: What do you think it was about Gusev that finally convinced the community that it was a good place to go?

NC: When you are looking for a site that has evidence of a long history of water, Gusev has the kind of morphological evidence you're looking for. It's fed by a very long channel, Ma'adim Vallis, with a very large watershed 1000 or 1500 kilometers upstream.

One idea is that there was a very large lake up there a long time ago, that suddenly released its water and formed Ma'adim. I don't agree with this hypothesis. I don't believe it was a one-shot thing. I think there is evidence in Ma'adim Vallis that there were several episodes of water flow.

When you look at the watershed upstream today, you see lots of gullies there, lots of things that look glacial. And if they are there now, and they are part of the most recent climate change, it means that when the obliquity was right (thousands of years ago, when Mars was more sharply tilted on its axis, which would have warmed some regions of the planet), something was happening in that region. And obliquity changes cyclically on Mars, which supports the hypothesis that there was some kind of repetitive action here.

So we're looking at the morphological evidence. Many people agree that it's probably one of the best examples of a hydrographic (water-carved) system on Mars. But you also have many people that think that's not the case. Or even if it is the case, that we have Appolinaris Patera (a nearby volcano) that came afterwards and covered up all the evidence, so that what we're seeing today isn't a lakebed but something else.

In our hypothesis, the lakes in Gusev were at the beginning of the history of Gusev, when Ma'adim was very active. Even in that scenario, though, the most recent release of water from Ma'adim wouldn't have been enough to form lakes. All you would see are some very small channels incising what we have called a delta, where Ma'adim flows into Gusev. This would deposit some cobbles and blocks from Ma'adim but it wouldn't form lacustrine features. And, interestingly enough, the landing site is sitting on material from this most recent activity.

So I'm not expecting to find lacustrine evidence at the landing site. Any lacustrine evidence would be down lower. If there is a good impact crater, maybe it would expose things from that earlier time. But I would expect there to be a better chance of finding evidence related to the lacustrine hypothesis in the distant hills that we can see in the panorama.

But to go back to why the site is interesting: We have a lacustrine hypothesis; there's a fluvial hypothesis; there's a volcanic hypothesis; other people say it's global airfall deposits. The reason why this site was selected isn't because Edmond and I have been bugging people for fifteen years about a potential lake there. It's because people came and argued against us and when we had these discussions, what came up was that it's a really interesting site where we can test lots of ideas. The way to test it is to study the rocks, and we have just the right payload to study rocks.

AM: When you get the first Mini-TES images back, what do you hope to see?

NC: What I would be looking for with a lot of interest is clays, salt content. Obviously, if we had gypsum popping up, I would be thrilled, because gypsum forms through evaporation, which would be consistent with the lacustrine hypothesis. And volcanic mud would be very interesting, because it would mean that the material had been sitting in water for some time somewhere upstream.

I'd also like to see microscopic images of the soil. Because if any of that stuff is bigger than 4 millimeters, well-rounded and shiny, I would be the happiest person in the world.

AM: Why? What would that tell you?

NC: That we have rounded, smooth material that is too large to be transported by wind. And the only other process that can form that kind of stuff is water.

AM: So even without mineral information, seeing grains of a certain size and shape would be very important?

NC: Very important.

AM: If you were deciding what to do, first thing, when Spirit gets off the lander, what would it be?

NC: The first thing I'd want to do is check the grain size and rock distribution right in front of the lander. It's a long mission, but things can happen, so you want to have as much science as you can right next to where you are. And getting a good assessment of what you have nearby will help you form your thought process about where to go next.

Once we have explored around - and I guess the idea is running in the team that Sleepy Hollow is a good place to go - but this material that we're calling mud-like is also intriguing. We will probably want to put the microscopic imager, maybe the [Alpha Proton X-ray Spectrometer] APXS and M�ssbauer spectrometer on it.

But, long-term, to test the lacustrine hypothesis against other ideas, we need to find outcrops and we need to find a diversity of material. And these things are definitely exposed somewhere. Not necessarily lacustrine evidence, but different material is exposed, older outcrops are exposed in those hills in the distance.

AM: Can you get there?

NC: That depends on how the rover behaves. And we have to determine within the team whether those hills are something that we really want to investigate, and that will depend very much on what we see (in the Pancam and Mini-TES panoramas) in the coming sols. If we decide that it's something we're interested in doing, and if the rover is healthy, we'll go as far as we can.

But remember that, to get the mineralogy of a relief like that, we don't necessarily need to be right up against it. Mini-TES and Pancam can do wonders. And it's high enough - we think it's about 50 meters high - to start getting some information even from a kilometer away.

Article is courtesy of NASA's Astrobiology Magazine team at Ames Research Center. This article is public domain and available for reprint with appropriate credit.

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