Conversation with Mars geologist Matt Smith — Part 2 of 4
A week and a half ago I met up with Matt Smith who studies Mars at the University of Washington to talk about planetary science and Mars rovers over lunch. This is part 2 of our conversation. Part 1 is here.
TOAD: Tell me about what you do. Did you use any of the Spirit or Opportunity images or data in your research?
MATT SMITH: Nope. No, I take that back. I did. Because they were able to measure erosion rates at both locations and so I needed that for in situ measurements.
TOAD: What does that mean?
MATT SMITH: Just from the ground.
TOAD: As opposed to from the orbit?
MATT SMITH: Yeah. Or using some other method. There are craters on Mars called pedestal craters that sit above the surface. And they sit weirdly above the surface, they’re kind of hovering, they’re standing out from the rest of it… And it means that the surface was at some point way higher and it’s been eroded down and the craters are left behind. So we could figure out an erosion rate based on that. Or another way: at the Opportunity landing site there are things called blueberries which are these hematite spherules or concretions, these little balls of hematite.
TOAD: Hematite is a kind of rock?
MATT SMITH: It’s a mineral. It’s a really iron-rich mineral. So they found hematite on the surface of these little blueberries. And they can look at the rock and say: well, the density of these spherules in the rock is X. But when it’s eroded down and you see on the surface the lag of all that hematite of the spherules left behind, you can tell that this much rock must have been eroded to leave behind this density of spherules on the surface. So they can come up with an erosion rate based on that.
TOAD: So you worked with some of those erosion rates. What did you do with them?
MATT SMITH: I looked at each of the Spirit and Opportunity landing sites and I counted the craters that were — actually, someone else counted the craters — that were right around them. There’s a density of craters per unit area that you can measure. And I had a model that said: if you have a certain amount of erosion over time, your crater population should look like this. So I fit that model to the crater population to back out an erosion rate. It gave me a model-derived erosion rate. And then I matched that to what they actually calculated for the erosion rate on the ground and it was within the range that they calculated.
TOAD: And that was evidence that the model describes this erosion phenomenon well.
MATT SMITH: Yeah.
TOAD: Is this interesting for geology on Earth or only on Mars?
MATT SMITH: Only on Mars or on other planets. Because on Earth you have the capability to figure out dates of rocks other ways. But if you’re only looking at images and you’re not actually taking samples back, from Mars or the Moon or any planet, then you can’t actually measure the age of the rock any other way than by the craters on it. And so it’s only really useful for planetary surfaces.
TOAD: And why do you want to know the age of rocks on other planetary surfaces?
MATT SMITH: You’re piecing together the geologic history of the planet. In part because of the rovers, we know that on Mars pretty much most of the action, water-wise, was very early in its history, like, the first billion years of its history was when all the water was there.
TOAD: Which is how many billions of years ago?
MATT SMITH: 3 – 3.5.
TOAD: Same age, Mars and Earth?
MATT SMITH: Most of the Solar system formed at about the same time. So, there’s all this evidence of lakes and deltas and rivers and all kinds of stuff. And a lot of the evidence — all the minerals that you find that indicate presence of water are in these really old terrains. You could have mineral detections or geomorphic evidence like deltas, which means that there must have been a lake there. You want to be able to tie these features to a specific age. So finding the age of those surfaces is important. So I went back and reconsidered some of the surfaces with my model. Maybe the age of a glacier that someone gave before was wrong. They went and dated a glacial deposit, and they said: well, it must be really recent. But it’s a small deposit, and you’re looking at really little craters which are really susceptible to erosion, so maybe it’s a lot older. Or, there were these geologically significant events in its history and you want to know when they happened.
TOAD: Is this your main line of work or is this tangential to what you normally do? You were given an award for best geologist in the department recently, I hear. Was it for this work?
MATT SMITH: The work I do now is using orbital spectrometers. So I look both in the thermal wavelengths and the visible and the near-infrared wavelengths. Because different minerals have different absorptions they interact with light differently and you need those two wavelength ranges. I’m using a combination of those two different wavelengths to figure out the geologic history and the aqueous history of a specific region on Mars.
TOAD: Not the Gusev crater, some other region?
MATT SMITH: Very far from Gusev crater. In Syrtis Major.
TOAD: Let’s go back to the rovers. It was a huge thing, right? It was this huge NASA project, but from the geology of Mars point of view, how many people were working on this project? With the data that it generated?
MATT SMITH: There were people who were actually on the team, the participating scientists who got funded to work on this, led by Steve Squyres. And probably the number of people who were in that, I would say in the hundreds.
TOAD: But then once it started happening, they made the data public, right?
MATT SMITH: Right. Anyone can interpret the data.
TOAD: So are there conferences on Mars where everybody is using the dataset from Spirit and Opportunity?
MATT SMITH: Yeah. I kind of caught the tail end of it, because most of the results were coming out in 2004-2005. And when I was starting really to get into Mars science, it was 2006-2007, so it was after the really big discoveries. But I was around when they found silica.
TOAD: The silica that got unearthed by the stuck wheel being dragged through the surface. So Steve Squyres said that silica right under the surface means that there used to be a wet non-acidic environment in that particular space.
MATT SMITH: Actually, he argued that it was acidic. Because there’s two ways you can form silica in that high a concentration. It was 90% silica, which is a really high quantity, which was the interesting thing, and there’s two ways to do that. Number one: silica is really soluble in alkaline environments, so really high pH. If you run water through something that’s really high pH, the silica will dissolve out and then precipitate somewhere else when the pH changes. That’s one way to do it. The other way is to run a really hot, really acidic fluid through a rock and then everything else besides the silica dissolves and you’re left with silica. So they were trying to figure out: was it the one or the other? Because acidic environments aren’t very good for life, but alkaline is better. They argued that it was acidic because sulfate-rich rocks were found nearby, and those are also found in acidic environments. And also because you often get an acidic hydrothermal alteration in volcanic environments. Near volcanoes you often see ventings of sulfurous fumes. Those tend to be really acidic. And nearby they did see all these volcanic deposits. So they argued that it was an acidic alteration, not an alkaline alteration.
TOAD: How much of all this research is driven by trying to figure out whether there was ever life in those waters?
MATT SMITH: All of it.
TOAD: All of it? So this whole thing about the erosion of surfaces, or dating surfaces, it’s nice, but we really want to know if there was life on Mars 3.5 billion years ago, and maybe it traveled to Earth after that?
MATT SMITH: That’s possible. Or if it was there at all? If it was there at all, that’s revolutionary. So I think all Mars science these days is mostly fueled by this: “was there life there at some point?”. That’s the goal of all the science. Because Mars science was essentially dead before they found this meteorite in Antarctica. They found it in the 80s, and then it was reinterpreted in the 90s. They found in there something that kind of looked like a microbe. And there were a few pieces of evidence for life which could be explained without life, but the fact that they were all in one meteorite – then they were like, well, it could be evidence for life in a Martian meteorite, which reinvigorated the entire Mars program. So everything since then has been essentially: is there life there?
TOAD: Fair enough. So what would you say was the biggest contribution of Spirit and Opportunity in this?
MATT SMITH: I think, showing how much water there was on the surface. And ground water. Both Spirit and Opportunity found evidence for a lot of water. Opportunity determined three episodes of water. They found these water-lain minerals that had been broken up and transported. So these minerals were water-reworked. Which means, there was an initial episode of water to deposit these minerals, and then a second episode of water to erode them and re-deposit them. And then there was a third episode of water that went through and deposited the hematite concretions. So they discovered these three pulses of water. So now we know there was sustained water for a certain amount of time, but it was also coming and going. There was a complicated history of water on the planet. Which we had no idea about before.
TOAD: That’s pretty cool.
MATT SMITH: Uh-huh. And same with Spirit. Spirit found water pulsing through the surface and causing these deposits to form: the really high-silica deposits, the sulfate deposits, and carbonates recently too, which are a big deal. So, just how complicated is the history of water on the planet?
TOAD: What about when it finally got stuck in this strange position that didn’t let it get too much light? For a year after it got stuck it kept communicating and taking pictures. Was anything at that point — the data it was sending — was anything from that point onward at all interesting, or was it just pretty images, or not so pretty images?
MATT SMITH: I haven’t heard anything that was interesting from that period.
TOAD: So that was just unfortunate.
MATT SMITH: Yeah. And also power was so low at that point anyway, and a lot of instruments were aging. The alpha emitter for the APXS was severely diminished at that point. It took a really long time for each measurement, with the instrument sitting on the surface. So it took a really long time to do analyses and only in the surrounding areas where it got stuck, so I don’t think it did a lot of interesting science while it was camped out at the end.
TOAD: The instruments are aging, but Opportunity is still going strong, right? And talking and measuring. Are its instruments OK? Was it just lucky somehow?
MATT SMITH: It’s in a more favorable location. It was a lot closer to the equator (2˚S) than Spirit (15˚S).
TOAD: Opportunity is going strong, and it’s moving towards the Endeavour crater. Three miles away now.
MATT SMITH: Yeah. That’s what I heard.
TOAD: Something interesting in the Endeavour crater?
MATT SMITH: Endeavour Crater is the biggest of the craters visited by Opportunity, 22 km in diameter. And the bigger the crater, the deeper it excavates, so we’ll be seeing some of the deepest and oldest rocks yet observed by the rover.
But I guess, maybe I was a little bit misleading when I said that 100% of Mars research these days is about trying to find signs of life because a little bit is about thinking if humans could live on Mars.
TOAD: Oh. When we’re done destroying this planet?
MATT SMITH: Exactly. So, thinking about where the water is as a capability of sustaining humans if we ever went there. So, that’s something people think about. But not super-seriously.
Parts 3 and 4 coming soon.