Below is the second part of my interview with planetary geologist Bethany Ehlmann. In the first part, she discussed two of her recent papers on Martian geology (see citations below). In this segment, she discusses water on Mars more generally.
Bethany Ehlmann: Part of being a graduate student is that I'm still learning what's been done before. So with that caveat...
NA: I understand. I'm writing my PhD thesis now, and I'm certainly discovering things now in the literature that I wish I had known about before I started my work!
BE: I really think it goes all the way back to Percival Lowell's canali: artificial straight channels that he thought he saw in his telescope. It really captured the public and scientific imagination even though later scientists couldn't replicate his find.
By the time NASA sent the first Mariner spacecraft, no one believed in the "canali" (or Martians that built them), but we weren't sure what we'd find. Mariner revealed a cratered Mars much like the Moon. Little evidence of water. But, when the Viking orbiters started mapping, the big channel systems like Kasei Vallis, Margaritifer Terra, and Vallis Marineris were obvious.
NA: So, this was in the 1970s.
BE: Right. Then we sent Viking to the surface. Some optimistic people thought we'd find life. But, the surface was cold and very dry. And, the life detection chemical experiments didn't provide any conclusive evidence for life. That was quite a damper. Perhaps we expected too much of Mars.
We = science community... I wasn't born yet!
NA: The ubiquitous scientific "we".
BE: Right. So, then the questions became ones of climate and water. Was Mars once really wet? If so, why did it change to the cold dry place it is today?
Mars exploration didn't really become the fast-advancing discipline it is today until the late 1990s. The Pathfinder rover and Mars Global Surveyor suite of instruments followed on the heels of an announcement of potential evidence for life in a Mars meteorite ALH84001.
NA: What did Pathfinder find?
BE: Pathfinder landed in Ares Vallis, in a field of rocks and sediments thought to be the end of a large catastrophic outflow channel.
NA: What does that mean: "catastrophic outflow channel"?
BE: An enormous, single (or just a few) event flood. We only have a few Earth analogs. The Channeled Scablands in Washington State are a catastrophic outflow caused by the breaking of an ice dam of glacial Lake Bonneville some 10000 years ago. Scoured terrain, very large scale ripple marks, huge transported boulders.
NA: So, this would just be a very obvious visible sign of past running water.
BE: Exactly. But what was interesting about Pathfinder was that the rover found boulders shaped like they might have been flood transported, but there was very little chemical evidence for alteration of rocks, suggesting the water hadn't hung around terribly long. Evidence from a thermal emission spectrometer, TES, also didn't find much evidence for water altered minerals, suggesting most of Mars was dry.
The exception was hematite deposits found in Terra Meridiani, where we ended up sending the Opportunity rover in 2004.
NA: So, I got the feeling that Opportunity's findings were a big deal. But, did we already know what to expect before Opportunity arrived?
BE: So there are several ways hematite can form. Many of which involve water. A few of which involve volcanic processes and not water or at least not a lot of it. "Following the water" story was the mantra that drove Mars exploration at the time. That's why we sent one rover to these enigmatic hematite deposits in Meridiani. And another to a crater with a channel leading into it that looked like it once held a lake.
Hematite is Fe2O3, an iron oxide.
NA: And then when Opportunity arrived, scientists were able to definitively say that this hematite was the result of a past lake?
BE: Exactly. We saw that the hematite was part of a rock unit with small scale ripple marks. Sedimentologists know from our experience on Earth that these form in shallow water. The rocks were basaltic sand and sulfate salt. Some of the hematite was in little round concretions indicating they "grew" in the sediments when groundwater flushed through. Our best guess was that Meridiani looked a lot like some of the Southwest U.S. Shallow ephermeral lakes: dry, and salty. And acidic. Sand dunes interspersed shallow lakes, and a groundwater driven water system.
NA: I see. So, I guess that Opportunity was the big "ah ha" moment, although really this was the culmination of decades of work.
BE: That's right. It confirmed standing water for a pretty long period of time on the surface of Mars. Plus more just beneath the surface. Then the story of the near infrared spectrometers like OMEGA and CRISM kicks in.
NA: Working out the details.
BE: Well, more than that. So, Opportunity's roving revealed this shallow, salty environment: hematite (plus other iron oxides) and sulfate salts. Shortly thereafter, also 2004, OMEGA mapped out the surface of Mars and found there were actually a lot of hydrated minerals, indicating alteration of rocks by water that we just hadn't seen before.
NA: So, water was once actually quite widespread on the surface of Mars.
BE: Exactly. One class was the sulphate salts like we saw in Meridiani. And, one was these phyllosilicates or clays which we see in Jezero crater, Nili Fossae, Mawrth Vallis, and which indicate a whole different kind of environment: a lot of water, a much longer period of time, and neutral to alkaline rather than acidic.
Jean-Pierre Bibring advanced a new paradigm to explain Mars history, based on this mineralogy. The Noachian (earliest period) where we see geomorphically well-developed valley systems was the equivalent to a "phyllosian" or clay forming period. Lots of water. Neutral to alkaline.
Then there was a global change, a little before the Hesperian catastrophic outflow channels. Perhaps a big pulse of volcanic activity? Perhaps a loss of atmosphere? In any event, Mars' liquid water started to go away. It was more desert-like, salty, and acidic. This led to sulphate salt deposits in the theiikian period.
NA: When did this happen?
BE: Around 2.5 or 3 billion years ago. Since then Mars has been very dry and cold. Little water, maybe some ice and glacial deposits, but not extensive interaction of water and rock.
NA: You pointed me to this figure from Bibring to illustrate these changes:
NA: Right, so there's no longer any liquid water on Mars, but there is frozen water, both in the ice caps and underground, I believe.
BE: Exactly. Lot's of it. So now we've got this dry but icy current Mars. Maybe when it tilts 60 deg on its axis every few 100,000s to millions of years there's some ephemeral water.
But we've got this ancient Mars that was quite wet indeed. With many different types of watery habitats. Some acidic, some alkaline. Some underground some above ground
NA: Why was there so much excitement recently when Phoenix discovered water ice just below the surface?
BE: It confirmed something researchers inferred should be there from both orbital data and climate modeling. And, it allowed us to conduct chemical measurements on the ice with Phoenix--light element, including carbon, analysis. Expect to hear results of those experiments later this year.
NA: Oh, OK. Do you have any previews for the readers now?
BE: I don't have any special access to those results. Working on one mission team--the CRISM instrument on Mars Recon. Orbiter--keeps me busy enough ;)
NA: I can imagine. OK, so, you've been kind enough to grace us with your presence, but, clearly, many other people were involved in these papers we've been discussing.
BE: Absolutely
NA: Could you briefly tell us who the key people are?
BE: Heading all the way back to Percival Lowell or just our current crop? ;)
NA: Let's just stick to the Nature Geoscience paper and the Nature paper. I imagine that both of us might need to get back to doing some science at some point.
BE: Exactly! Plus, it's thousands of names for the whole Mars water story.
NA: Your papers always have so many authors. I think it makes biologists suspicious.
BE: It takes a lot to get a spacecraft up in orbit and then the data into a form that can be analyzed on the ground. No more authors than some big bio lab studies or big physics projects, I think ;)
In any case, the Nature paper is led by Jack Mustard, a professor at Brown University, deputy PI of CRISM, and also my advisor. Scott Murchie is at the Applied Physics laboratory and is PI of the CRISM instrument. Stay tuned for his paper, focused on sulphates, which is in the publication process as we speak.
NA: So, in your papers do you have a "senior author"? This author would be listed last on a biological paper.
BE: Ah. Different system. Here the lead author is first. Usually then it's in order of contribution. For large teams, it's sometimes just alphabetical.
NA: With a name like Anthis, maybe I'm in the wrong field.
BE: I guess so, Nick. Well, you're welcome in planetary geology any time.
NA: I'll keep it in mind.
Now, before we each get back to doing some science, it's time for the lightning round.
BE: Uh-oh.
NA: It's just three questions, so it shouldn't be too bad.
BE: Shoot then.
NA: Question 1: Did you ever want to be an astronaut, and is that still a possibility?
BE: Haha. Well, I was one of those little kids who had bad eyes and wore glasses since the age of four. So, it was not really a possibility until PRK came along ;). I find the exploration and the science that we can do with spacecraft so exciting to be a part of, I'm not sure I'd trade it for astronaut training. But, if there were a chance to go to the Moon or Mars, I might reconsider. I am a geologist, after all, and it's always better to see the rocks in person! And, it would be a great personal challenge.
NA: I bet.
OK, question 2: Are we more likely to find life on Mars or somewhere outside of our solar system?
BE: That's an interesting one, especially since we're extrapolating from a single data point (Earth). On Mars, I think it comes down to: was there enough time for life to get started before things dried up? I think if we keep our exploration going at this pace, we'll get the answer in my lifetime. I'm sure there's life elsewhere in the galaxy. The question is when and how we might find it
NA: Finally, question 3: if you had NASA Administrator Michael Griffin's job, what would your priorities be?
BE: The NASA Administrator job is a tough one because you're leading the most capable agency in the world--really set up to figure out how to be able to accomplish anything--on a limited budget.
That being said, three things. I think we need to establish a reliable means of getting humans and cargo into space, post-shuttle. This may mean one method for both, or a mix of methods. Second, I think we need to reestablish, in concert with other agencies, NASA's preeminence in Earth observation: monitoring our planet for changes in climate, land use change. Third, we need to set up programs of robotic exploration that allow young scientists who are becoming experts in Mars, in the outer planets, in comets and asteroids clear future support to enable our rapidly evolving understanding of these bodies to continue apace, and to keep exciting the public with the new findings.
The last one is especially important. One reason I'm blogging with you is because I think what NASA does is often the most exciting stuff people don't know about.
NA: Is there too much focus on manned space exploration right now?
BE: Manned exploration has always been the biggest budget item. I think it should be a big part of our space program. I'd love to send humans to Mars. We'd find out so much. But we need to be careful that when budget overruns happen in the manned program that they don't swallow entire robotic missions and the scientific community that works on planetary missions.
We shouldn't be competing with each other -- human program vs robotic.
NA: I'm sure it's a very difficult balancing act.
Well, speaking of balancing acts, I suppose we should think about our scientific careers as well. So, on that note, let's wrap this up, since I don't want to keep you any longer.
I really appreciate you taking your time to talk with me, and hopefully we'll have the chance to do it again sometime.
BE: I had a great time chatting with you, too.
NA: Best of luck with your future work. It's certainly been productive so far.
BE: Thanks, Nick. Yours as well.
Ehlmann, B.L., Mustard, J.F., Fassett, C.I., Schon, S.C., Head III, J.W., Des Marais, D.J., Grant, J.A., Murchie, S.L. (2008). Clay minerals in delta deposits and organic preservation potential on Mars. Nature Geoscience, 1(6), 355-358. DOI: 10.1038/ngeo207
Mustard, J.F., Murchie, S.L., Pelkey, S.M., Ehlmann, B.L., Milliken, R.E., Grant, J.A., Bibring, J., Poulet, F., Bishop, J., Dobrea, E.N., Roach, L., Seelos, F., Arvidson, R.E., Wiseman, S., Green, R., Hash, C., Humm, D., Malaret, E., McGovern, J.A., Seelos, K., Clancy, T., Clark, R., Marais, D.D., Izenberg, N., Knudson, A., Langevin, Y., Martin, T., McGuire, P., Morris, R., Robinson, M., Roush, T., Smith, M., Swayze, G., Taylor, H., Titus, T., Wolff, M. (2008). Hydrated silicate minerals on Mars observed by the Mars Reconnaissance Orbiter CRISM instrument. Nature, 454(7202), 305-309. DOI: 10.1038/nature07097
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Wow great report and interview, just like I said in your first report, If all these things true so it means there's an opportunity for human to have a new source place of living :D