Spirit Explores the Columbia Hills
Interview with Steve Squyres
|Principal investigator, Dr. Steve Squyres, Cornell University, describes Mars surface operations Credit: NASA-TV|
Astrobiology Magazine: Can you start by catching us up on what Spirit has been doing for the past week or so?
Steve Squyres: We’ve finished up on Pot of Gold, and we’ve now moved on to a couple of other targets right near by, just to see if they have more or less the same characteristics. And they seem to: the little stalks, the hematite, seem to be fairly common in this stuff. And then we’re going to start looking off into the hills for bedrock.
AM: The last time you held a press conference, you reported that you had seen a novel formation on Pot of Gold. You described the rock’s surface as having thin "stalks" with "nuggets" on the ends of them. Their origin was a big mystery when you announced them. Is it still a mystery?
SS: We’re starting to develop some theories.
AM: And …?
SS: I think I’m not ready to go public with the theories. As you might imagine, some of the theories involve water. Hematite’s pretty hard stuff. It’s good at cementing rocks together. So if you have hematite that is distributed through the rocks in an uneven fashion, then the hematite-rich stuff might stay behind while the hematite-poor stuff gets eroded away. But there’s a lot of details to be worked out. I’m not ready to speculate wildly and openly yet.
AM: If you’re moving on the examine other targets, how are you going to get a better understanding of these odd formations?
|This false-color image taken by the panoramic camera on the Mars Exploration Rover Spirit shows a close-up of the rock dubbed "Pot of Gold" (upper left), which is located near the base of the "Columbia Hills" in Gusev Crater. Scientists are intrigued by this unusual-looking, nodule-covered rock and plan to investigate its detailed chemistry in coming sols. This picture was taken on sol 159 (June 14, 2004). |
SS: We don’t think that the rock was born looking this way. We think that it evolved to this appearance. If you can find rocks that are at various stages of degradation – one that’s pretty much intact, and one where this process has only proceeded part way, and one where it’s totally gone to completion, and things in between – you may be able to piece together what the process is. So that’s something we’re going to try to do.
AM: How likely do you think you are to be able to find that transitional evidence?
SS: Don’t have the slightest clue. We’re only about 10 feet (3 meters) into the Columbia Hills. That’s it. And we found this. We’ve got the whole hill complex in front of us. What else does it hold? I don’t know. We crossed a boundary and literally over a space of 10 or 15 feet (3 to 4.6 meters), everything was different. So there’s much to be learned here.
AM: So you said you’re seeing evidence that the hematite may be distributed throughout the rock unevenly?
SS: No. You could look at the shape of that rock and guess that that might be the case. But there’s no evidence to support that. There’s no evidence on that one way or the other right now.
AM: Is there more evidence of hematite in the nuggets than in the stalks?
SS: Don’t know.
AM: You can’t get in that close?
SS: No, they’re too tiny. The nuggets are only about 3 or 4 millimeters. They’re very tiny things. On these little tiny stalks. We’ve got some remarkable capabilities with the RAT and the instruments that we have. But we can’t carefully slice the thing apart and make measurements on things that are millimeters in size. We just don’t have that capability.
Our one attempt at grinding into Pot of Gold was only marginally successful. This thing is way, way outside the capabilities that we designed the RAT and the IDD for. When we pushed down on it, it moved. It shifted position. That’s tough. Plus it’s got these little stalks, these little spikes, sticking up. When we RATted it, what happened was we lopped the top off of a bunch of the stalks and we brushed the rock clean. Take a look at some of the microscopic imager images after we RATted it. You can see places where the stalks were cut through or lopped off and places where we cut through the nuggets. They get very bright and shiny where they’ve been cut through. And when you look at other places where we’ve driven around on this kind of stuff and crunched it with the wheels, it gets very bright and very shiny. That’s probably telling us something, too.
AM: But you don’t know what…
|Columbia Hills loom with increasing detail at the Spirit rovers’ Gusev Crater site. See Spirit’s images and slideshow |
SS: There’s a form of hematite that’s called specular hematite that’s very bright and sparkly. We may be seeing that.
AM: And is that a form of hematite that forms in water?
SS: We’re asking the same question ourselves. But as I think you’ve learned from the way we’ve done things since the start of the mission, when there’s a potentially important conclusion, we want to be sure before we say that we have concluded it, instead of just guessing.
AM: Yeah, but you usually throw out wild theories a little earlier in the process than you are this time.
SS: Sure. The problem is that it’s hard to even come up with wild theories that make any damned sense. I wouldn’t say that we’ve got a really well formed set of working hypotheses for this thing yet. When you look at it, you get the sense that, when it erodes, it erodes in a very uneven sort of fashion. Some parts of the rock are hard and some parts are soft. You just look at it with your eyes and you get that impression. It’s clearly got hematite in it. And hematite is a very, very hard mineral. The hematite blueberries over at Meridiani [where the Opportunity rover is located] are very tough to grind through. And so it’s a very good material for cementing rock together, making it tough, making it resistant to weathering.
|Schematic of major mission events during entry, descent and landing.|
Credit: NASA/JPL/ Cornell University/ Dan Maas
So a reasonable hypothesis is that the strange shape of this thing is an indicator of the original rock having had some very non-uniform distribution of hematite in it, with some parts receiving lots of hematite and some parts of it receiving less. The stuff that received less has sort of eroded away, and the stuff that received more has been resistant to the erosion and remains as nuggets and as these planar features, and so forth. But what was the exact process that led to the formation of the hematite is not something that I’m prepared to speculate on until we have confirmed, which we have not done yet, a heterogeneous distribution of hematite through this thing. All we’ve got is that, in spots that are the right size for the Mössbauer on this one rock, there was hematite there. Everything else is inference. And I don’t like drawing important scientific conclusions from inference.
Here’s the kind of thing that we’re going to look for. Let’s say that we find another rock like this, or a piece of bedrock like this. And we see nuggets embedded in it, and we see some planar features that embedded in it, that look like they’re resistant, but there’s plenty of that matrix there as well. And let’s say that the planar features are well enough developed that we can actually get the Mössbauer on them.
AM: What’s a planar feature?
SS: If you look at Pot of Gold, it’s got these planar sheets of apparently resistant stuff running through it, at different angles. So suppose somewhere up the hill, I can find a rock that’s Pot of Gold-like, but it’s got a clearly resistant feature, like one of these planar things, that’s big enough that I can actually isolate it with the Mössbauer; and then it’s got some of this matrix material that hasn’t just crumbled away and left the stalks behind; and I can get the Mössbauer on that; and I can measure those two things.
|Pot of Gold with its randomly oriented stems and spheres. Credit: NASA/JPL|
Now suppose I find that hematite is concentrated in the hard resistant planar feature, but is low or absent in the other stuff. Then I have learned something about the heterogeneity of the hematite; and then I have taken my supposition that the morphology of this thing is related to differential cementation and I’ve actually made a measurement that supports that. Until I’ve got a measurement that supports the supposition, I don’t want to draw conclusions from the supposition.
AM: Now that you’ve had a little more time to talk about this formation with your colleagues, has anyone come up with an image somehwere in someone’s library of something at all like this on Earth?
SS: Not that I know of. I think we may be dealing with something uniquely martian here.
AM: But chemistry and physics work the same on Mars as they do on Earth.
SS: That’s right, but the time scales are different. The laws of chemistry and physics are the same, but you can have a rock that has been sitting on the surface in an arid environment for 2 or 3 billion years on Mars. That happens nowhere on Earth. So even though the laws of physics and chemistry are the same, the circumstances in which those laws act are different and they can lead to unique end members once they’ve run to conclusion.
AM: Another question about the hills. Way back at the landing site when you first saw the Columbia Hills off in the distance, you said, "We think we may see layers in there." Now that you’re closer, what are your thoughts??
SS: I wouldn’t say that we see clear layering. I would say that we do see exposures of bedrock that seem to be aligned in horizontal geometry. But whether that is true stratigraphic layering, as opposed to this stuff that’s marking out the location of some kind of fault, or this is some kind of erosional terrace or something, we can’t really tell.
AM: So you don’t know whether hills are the result of a single process or represent a series of different environments?
|Pot of Gold under microscopic examination.|
SS: I have no idea how these hills formed. These are incredibly old. They may be very jumbled up. They might be simple geology; they might be so complex we have no chance of figuring it out. The problem with these hills is that they’re damn big. They’re hilly and they’re steep and dangerous and they’re huge for a vehicle like this. Nobody’s every tried anything remotely like this before. There’s no textbook that you can go to for how to do something like this. And so we’re going to just look up the hill, or look around, and try to find some outcrop and then go for it.
Now one thing that we’ve got to keep in mind is that the power system on this vehicle is really, really getting ragged. We’ve got a lot of dust on the solar arrays; we’re at a pretty far southern latitude; the sun is heading north daily. So every day our solar power drops down and down.
AM: Does being on the hills exacerbate that?
SS: Depends on which way the hills point. If the hills point to the north, it makes it a lot better. If the hills point to the south, it makes it a lot worse. So say I’m trying to climb a hill. I’ve got a hill in front of me, and I’ve got two different routes. A straight-line route, it’s 50 meters (164 feet), easy-going, no problem, and the hill slope faces south. Alternatively, I’ve got a roundabout route, it’s 3 times longer, but the hill slope faces north the whole way. I might be better off going that way, because I’ve got so much more solar power. So in figuring out how to embark on any sort of assault on the hills where you’re looking for bedrock, you may have a particular bedrock outcrop, but the best route to get to it may not be the visually obvious one.
I’ve done a lot of mountaineering. I’ve done a lot of climbing. And I’ve done a lot of route finding. Many aspects of this are familiar. You want to find a slope that’s not too steep for your skills. You want to find terrain that is coherent rock that you can climb on as opposed to crumbly, rubbly stuff that is going to avalanche under you. But I’ve never had to worry about keeping my solar arrays tilted toward the sun. So it’s an interesting exercise.
|Hyperion rover steered by orienting solar arrays to maximize power|
Credit: Carnegie Mellon University
AM: Do you have a near-term target?
SS: No. We’re in the process of gathering the data that we need to pick our route. I don’t know if we’re going to try to climb the hills, or just skirt along the base looking for pieces of bedrock. There’s a lot of ways to skin this cat. And the right way to do it depends on what we see. But the key things are: find bedrock, and keep the solar arrays pointed north. That’s the bottom line.
Related Web Pages
Mars Rovers JPL
Spirit’s images and slideshow
Opportunity image gallery and slideshow
Mars Berries Once Rich in Iron-Water
NASA’s RATs Go Roving on Mars
Pancam- Surveying the Martian Scene
Alpha Proton X-ray Spectrometer