Investigating Endurance Crater
Interview with Steve Squyres
|Steve Squyres in front of Mars Exploration Rover test unit
For the past three weeks, Opportunity has been exploring a bedrock outcrop known as Karatepe inside Endurance Crater. Karatepe is of interest to scientists because it lies below a rock layer whose characteristics match those of Opportunity Ledge, a smaller outcrop in Eagle Crater. The rover spent the first months of its mission studying that outcrop. Because Karatepe lies lower stratigraphically than Opportunity Ledge, it is older; it represents an earlier era in Mars’s history. Opportunity has been slowly working its way down the Karatepe outcrop; it has detected sulfates, salts that formed through a process involving liquid water, as far down as it has explored.
Astrobiology Magazine (AM): Have you found the bottom of the sulfates yet?
Steven Squyres (SS): No.
AM: How far down have you gone?
SS: We are I think about 7 meters (about 23 feet) now past the lip of the crater, as you would draw a ruler across the ground. So how deep down we have gone? To be honest with you, I haven’t done the math, but it’s 2 or 3 meters (7 to 10 feet) below the lip.
AM: And it’s sulfates all the way down?
SS: It’s sulfates all the way down. We haven’t found anything that isn’t sulfates. Not a trace.
AM: And just for comparison, Opportunity Ledge, the outcrop in Eagle Crater, was how deep?
|A feature called "Karatepe" within the impact crater known as "Endurance." Scientists believe this layered band of rock may be a good place to begin studying Endurance because it is less steep and more approachable than the rest of the crater’s rocky outcrops, such as Burns Cliff shown in the banner image.
SS: Forty centimeters (16 inches).
AM: So 3 meters and counting versus 40 centimeters?
SS: I think we can extrapolate with some confidence over to Burns Cliff and we see that the section at Burns Cliff is at lest 4 meters (13 feet) high. So we have increased our estimate of the amount of sulfates here by an order of magnitude. And how deep it really goes, we don’t know. There’s a lot more of this stuff than I would have originally guessed.
AM: And water had to be present to form all that sulfate?
SS: Yes. And if we can get to the bottom of it, if we can find really how much sulfate there is, then we can perform a calculation, based on how much water you need to evaporate away to make that much sulfate, you can attach quantitative numbers to it. If we never get to the bottom of it, all we can say is, it must have been at least this much water and it could have been more. So we’re trying to find the bottom.
There’s one outcrop, named Namib, that has exposed in its lower portion some very dark rock that could be basaltic sandstone. We’ve tried to isolate it with Mini-TES, but we’re so far away from it right now and it’s such a narrow layer, it’s very, very hard to isolate just the rock. If you look right downhill from the rock, there’s this apron of dark sand. And we’ve looked at that with Mini-TES and that’s a perfect basalt spectrum. So that’s basaltic sand.
The question is, was that shed from the outcrop, or did it get blown down from above and funnel through cracks in the outcrop and spray out there? It turns out, because of the prevailing wind direction, that it could have gotten blown over the lip of the crater. So we can’t tell. One of the things that we want to do when we finish up at Karatepe is try to get closer to that stuff.
|A feature called "Burns Cliff", part of the rocky outcrop in Endurance Crater.
AM: This darker layer that you see on Namib, does it lie below layers of sulfate that correspond to what you’re looking at on Karatepe?
SS: I believe that the darker rocks at Namib are at a stratigraphic level that corresponds to some of the stuff we’re on at Karatepe and some of the stuff at the stuff we see at Burns Cliff. So, if it really is something different, if it’s really not our old friends the sulfates, but it’s some other basaltic sandstone, or what have you, that means that not only do you have vertical variations in the stratigraphy over distances of centimeters and 10s of centimeters (about an inch or 2 to about a foot), but you’ve got lateral variations of the stratigraphy over distances on the order of 100 meters (about 300 feet). That would be very interesting, too. That’s not uncommon in some kinds of sedimentary rocks. It’s not at all uncommon in some kinds of sedimentary rocks to vary laterally over distance of 10s, 100s of meters.
AM: When you presented the results of your initial close-up look at Karatepe, you delineated several layers in the rocks and you were going to look for chemical differences among the layers. Have you found anything there yet?
SS: A few differences. But they’re very subtle. It’s much more homogeneous than I would have guessed. And that could be a hint that there’s been some kind of stirring, some kind of mixing, that has taken place. If you lay down some sediments and they have some kind of compositional stratification, but then some process comes along – water flow, wind – and it just stirs it up and jumbles it – you put the whole thing in a blender and let it settle out again – it’s going to have the same composition all the way through. So, it may be an indication that that’s the case. We have seen some subtle variations in chemistry, but they’re not major, they’re not profound by any means.
|Namib. Click image to enlarge overhead view of Endurance Crater Credit: Mars MGS/NASA/JPL|
AM: Not enough to reach conclusions about different processes that might have taken place?
SS: I think it’s enough to reach conclusions that there’s nothing like the [variations we saw at Eagle Crater]. For example, we saw this variation in the chlorine-to-bromine ratio over distances of 10s of centimeters (a few inches to a foot) back at Eagle Crater. And that’s a clear indicator of an evaporative process, because the chlorides and the bromides have different solubilities and they precipitate out [under different conditions]. But if you took that section back at Eagle and put it in a blender and mixed it all up, then you would see a uniform chlorine-to-bromine ratio everywhere. And so this may be a hint that there has been some kind of stirring or mixing process that has gone on here. It could be water; it could be wind.
AM: You also said that you were going to look for evidence of large crossbeds in Endurance Crater.
SS: There is a massive, huge crossbed right at the base of Burns Cliff. And it’s the kind of thing that you would expect from wind. So we see these 4 meters (13 feet) of sulfates at Burns Cliff, and we see tilted sediments underneath that. We don’t think the sediments were laid down flat and then there was tectonic tilting. We think they were laid down at an angle. And when you see sediments laid down on a big scale – meters across – at an angle of 20 or 30 degrees, that says pretty unambiguously that it’s a dune. So we think that the big thick sequence at Burns cliff was preceded by some stuff that was moved around by the wind.
Now, that could have been basaltic sand, but it could have been sulfate sand. And evaporative process is one where the water goes away; it dries out. So you can have an evaporative process that leaves a bed of sulfate sand grains behind, and those can be blown around by the wind. You can have dunes made of sulfate just like you can have dunes made of any other particles of the right size. And then they can cement later. So we think we have clear evidence for an aeolian episode at the base of Burns Cliff. We’re looking through the stack of sediments [at Karatepe] that corresponds to Burns Cliff and trying to look for evidence of wind versus water there.
AM: How much farther down can you go at Karatepe?
|Layers A and E resemble the tilted, wavy lines predicted from sedimentation in water and B-D layers may be either wind- or water-eroded/deposited.
Credit: NASA/JPL/ Cornell University/ Dan Maas
SS: It’s starting to look like we can go as far as we want and be able to climb back out. That remains to be seen. We’re on a 25-plus-degree slope and just yesterday we went over a step about half a meter (a foot and a half) high that was at more than a 35-degree angle, and we climbed back up over it just to see if we could. And we did.
These vehicles have remarkable climbing capability. So we’re going to proceed down this section at Karatepe until either we get to some topographic point of no return, which frankly may not exist – the vehicle climbing capability may be so good it’s just not an issue – or until we run out of intact stratigraphy. If we get deep enough and we find out that we’re just into a jumble of rocks that are not in place any more, then there’s no point in treating it like stratigraphy, because it isn’t. At that point, we have to start looking at other things that we should do, because you’re always prioritizing science, and there are some very important high-priority science objectives out on the plains.
AM: You mentioned those before you went down into Endurance Crater and said you might decide to look at them before you entered the crater.
SS: Getting out after we went in looked easy enough that we concluded that by going in we were not taking a significant risk that we’d never be able to get to these [targets out on the plains]. So the idea is, do the highest priority stuff at Karatepe, which is work our way down this intact stratigraphy, and then having done that, go on to the next high-priority item, which is some of the odds and ends out on the plains.
There are these little cobbles, these little fist-sized rocks, scattered out on the plains. We don’t know what those are. We haven’t looked at a single one of them yet. We just went blowing by them. So we think it’s time to figure out what’s going on with those things.
There’s the heat shield. The heat shield is really interesting and important, both from an engineering perspective and from a science perspective. So there’s that. We’ve got this tick list of things we want to do out on the plains. When we finish up at Karatepe, we’ll probably go take care of those things.
Once we’ve done that, then we face a fundamental decision. At that point, the question is, do we go back into the crater, and try to do more work there, or do we say, we’ve done most of what we can do here, let’s go someplace else? And that’s a decision that we’ll make a month, a month and a half from now.
AM: Are you having solar-energy-budget problems with Opportunity similar to those you’re having with Spirit?
SS: It’s not as bad, for two reasons. One is that we’re not at nearly as extreme a latitude. Gusev is at 15 or 16 degrees south, whereas Meridiani is right close to the equator. So what that means is that it’s just more favorable generally. And then, what we’ve been able to do at Meridiani –what we have not yet done at Gusev – is to find ourselves a slope that will tilt the solar arrays even more towards the sun. When we first looked at driving down into Endurance Crater, we had two possible ingress points. One was Karatepe, the other was Larry’s Leap. One of the things that ultimately made us favor Karatepe was that Larry’s Leap would have pointed the arrays south; Karatepe pointed them north. We got much more energy that way. And so right now Opportunity is definitely doing better energy-wise than Spirit is because of having that advantage.
AM: How long do you think this mission can go?
SS: I think it is entirely possible that we will make it through the martian winter with at least one of these vehicles. The depths of martian winter come in late September of this year. Then it starts to get better again. And if we can survive the deepest part of the winter in September, and the situation starts to improve, conceivably we could go months beyond that. So it’s not inconceivable that we could go into 2005.
Related Web Pages
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