June 17, 2005
The Martian Hot Spots
In their explorations of Mars, both the Spirit and Opportunity rovers found evidence that liquid water was once on the planet's surface. Joy Crisp, project scientist for NASA's Mars Exploration Rovers, discussed the rovers' long journey and their surprising discoveries at a public lecture on May 19, 2005. In this contribution, Crisp fields questions about the future of Mars exploration.
In this final part of the edited transcript, Joy Crisp answers questions from the audience.-----
Q: Have you found any fossils on Mars?
A: We have not found any fossil evidence so far. A lot of folks in the public have asked us about things that they've seen in the NASA images. We are looking for compelling evidence for fossils, and would love it if we saw it. But the scientists on the team who are experts in fossils say everything that we have seen so far could be explained by non-biologic processes. Minerals can create interesting shapes -- we know that from Earth.
Q: Is there a correlation between hematite and elevation on Mars?
A: Not really. This coarse-grained hematite is just in a couple of spots on Mars. The elevation of the Meridiani Plains, where we find hematite, is kind of a medium elevation on Mars. Hematite probably forms based on time and chemistry -- where this kind of water was, and where these kinds of conditions were, rather than elevation.
Q: In the places where you found evidence for water, how long was it around? Are we talking about days, or years?
A: We have a difficult time pinning precise numbers to that. We did not bring age-dating equipment with us. We may need to bring samples back from Mars to the Earth to determine how long the water had to be around. It was probably there for hundreds of years, minimum. It could be much more, but we don't have the equipment to say how much more.
Q: Could you obtain water from the rocks of Mars, for future explorers to access?
A: In theory you could. We have estimated that the Meridiani rocks could possibly contain seven-weight percent water. You'd have to heat up the rocks to release that water, though, and that might require a lot of energy.
If you were going to use that as a resource for a future human mission, you'd want to make sure how much water the rocks really contain. We don't directly analyze the water content. We're making indirect assessments of it. It's possible that we might find other deposits that would be even better resources for water.
Q: When is it martian summer, and when are you going to have to shut down for the winter?
A: Right now it's the middle of spring in the southern hemisphere, so we made it through our first winter. When we landed, it was late summer.
We thought we'd have to hibernate in the winter. But then we drove our rovers onto slopes that kept our solar panels tilted towards the sun. When we did that, we did not have to hibernate. So it's possible that we could keep going and going without hibernating in the winters.
But we don't know how long these rovers could last. They could die at any moment. They're way beyond their tested lifetimes. Also, if we get a global dust storm, we're not sure if we'll get enough incoming sunlight to keep our electronics warm. But so far, so good.
Q: What's the current situation with the RAT - the rock abrasion tool?
A: On Spirit, it may be at the end of its life. The last time we used it to grind into a rock, it chattered quite strongly. We've done some tests of a fairly worn down rock abrasion tool, and gotten that same kind of chatter. If it is at the end of its grinding teeth, it could hurt the arm if we continue to use it. But we can still use it to clear dust, spinning the brush around to remove dust. Spirit's RAT has done 15 grindings so far, and about 50 brushes.
Opportunity's rock abrasion tool has done 22 grindings. We ground a lot of the volcanic hard rock with Spirit, and wore the teeth down a lot faster than the sulfate rock, which is softer, that Opportunity encountered. So it has a lot more life to it.
Q: What's the next Mars mission to help answer some of these questions?
A: The mission after ours is a landed mission, the Phoenix Lander, but it's not a rover. It's just going to have a robotic arm that digs, it'll have some ovens that can analyze soil and ice, a wet chemistry lab, a meteorology mast, and it'll have cameras.
The next rover mission is called the Mars Science Laboratory. It might launch in 2009 or 2011, and its design is a much larger rover that can carry more instruments. It basically has a "stomach" in it, with a couple of instruments that are like analytical chemistry labs. One does X-ray diffraction, which identifies minerals and X-ray fluorescence, which can tell you the elemental abundances in the sample. Another can measure organic compounds and isotopes. If you can look at certain isotopes of carbon, hydrogen, oxygen, nitrogen, that can help you see if there's fossil evidence of life in the rocks.
These two instruments inside the "stomach" of the rover accept rock samples that are cored through by a drill, and then ground up by a rock crusher. It also has a robotic arm to look up close and measure the chemistry. It also has a laser induced breakdown instrument that can tell us roughly what the composition of a rock is from far away, and it has color cameras on the mast. It also has a robotic arm with instruments. It will be built to last longer, and hopefully like Spirit and Opportunity, it will go beyond its expected lifetime.
Q: There's a lot of variety in the terrain. What inferences can you make about that?
A: With Spirit, we consistently see water having affected the rocks, but in different ways. If these layered rocks that we keep finding are volcanic, then it may be a sign that there were different magma compositions. We have that kind of situation in the Earth, where magma chambers at depth under the surface of the Earth change composition, or there could be different sources of magma, and that could result in different compositions. For the water that seems to have soaked the rocks and altered them, the chemistry of those waters could have varied. If the rocks are instead sedimentary, laid down by water, brought in by rivers and flowed into the crater, each layer could represent a different episode of a different river bringing different source material to the site. So we're not sure what the variety means with Spirit. We're still pondering it.
With Opportunity, there is variety in the texture, but it's been essentially the same kind of rock. It's layered rock very rich in sulfate, sedimentary, deposited either by wind or water, but it's very similar looking rock, with the exception of an iron meteorite that we encountered.
Q: Have you seen anything like desert varnish?
A: We did on one rock, clearly, near the rim of Bonneville crater. There, as we ground into the rock, we could see that we were removing an outer coating. We took measurements, and could see some indications of the chemistry of that coating. We don't know if that rock at some point was exposed to some surface water, or if it was buried in the soil where there was a little bit of water, forming this coating. But it had water signatures.
Q: Is Mars a very active planet?
A: Not that we know of. There's a desire among geophysicists to make measurements and set up a network to see what kind of activity there is. We have not seen any signs of volcanism, in looking at the current surface. We've been looking with Mars Odyssey in the infrared to see if there are any hot areas on the surface, and we haven't seen any. We have seen with Mars Express a lot of new assessments saying, "Volcanism was active not too long ago in the geologic past," so there isn't any reason why it couldn't still be active, and no reason you couldn't have a little bit of associated seismic activity. Mars has been predicted to have less seismic activity than the Earth and more than the Moon.
Q:Do you think future missions might revisit these same places later on down the line?
A: To me, it would make sense to go back to Meridiani sometime and look for an organic or biomarker signature in those rocks, or get some of those rocks and bring them back to the Earth to make careful measurements of them. They preserve evidence of an environment that could have been good for life, and these kinds of rocks would be good for possibly preserving fossil evidence of life.
Q: The Columbia Hills are in a big flat crater bottom. How did the Hills form?
A: I think that's one of our favorite things to argue about. That's a very basic question that we aren't able to answer right now. Were the rocks originally deposited with that 20 degree angle? Could we be on the side of some volcanic construct? Could it be that the rocks were originally deposited horizontally, and they've been tipped up by impact? We see signs from orbit that the Hills look strongly impacted, like they've been hit many times in the past.
Q: What do you expect as you go to a higher latitude?
A: Mars is not bland and homogenous, so you can't just say "to high latitudes." You have to specify where exactly on Mars. And then you would have ideas, based on what you see from orbit, as to what you'll see close-up when you get down on the surface. We didn't know exactly what we would find with Spirit and Opportunity. Look at Spirit - we didn't know it would land on volcanic rocks. We didn't know that the Hills would be a good place to go, until we got down there. So all I can make are very broad generalizations that, if you went to high latitudes, what we have seen from orbit is that there's a striking amount of ice in the soil. That is what Phoenix Lander wants to go look at for sure. But in terms of rocks at high latitudes, I don't know.
Q: You mentioned Star Wars in your introduction. Have you seen any footprints of droids on Mars?
A: (laughs) No, but that reminds me of a fun thing that we recently saw. Spirit is at the back of Larry's Lookout, and we come around, look at that rock ledge, and there's this big patch of soil that has slid down. We've noticed several places on Mars where this soil has a crusty top to it. And so this crusty soil had slid down, but how did that happen? Well, we used to be up on top on the other side, and we were grinding away, so you put two and two together. We may have been producing our own little seismic activity there, and causing a mini-landslide.
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