Archive for the ‘Water on Mars’ category

The 4th MSL Landing Site Workshop: Day 3 – Final Discussion

October 2, 2010

We wrapped up the landing site workshop on wednesday afternoon by revisiting each of the four sites and discussing them in turn. Unfortunately, the way that we did this was very disappointing, and made for a frustrating afternoon.

The discussion was centered around a word document that was projected up on the screen in the room. Over lunch, the meeting leaders had conferred and listed what they thought were the key points for each site that everyone agreed with, along with areas of future work. But they should have known that getting a room of 200 scientists to agree with something is nearly impossible, and it didn’t help that their list of points was a mish-mash of actual, irrefutable observations (e.g.  “there are strong clay mineral signatures at Mawrth”, or “there is a huge layered mound at Gale”) and complete speculations (e.g. “the delta at Eberswalde preserves organic molecules” or “the layered rocks at Holden are likely lake deposits”) and everything in between.

The result was that we spent most of the afternoon going through this document line by line and debating word choices and phrasing and grammar. It was incredibly frustrating, to the point where I almost had to leave the room. By the end, there were probably only five or so people actively participating, while everyone else in the room silently suffered through watching this document being written by committee.

After the discussion, we did end up with a central hypothesis for each of the sites, along with a list of future work.Gale was the first site we discussed, so a lot of the time was spent figuring out how we were going to do this document editing as a group. I also was on the edge of my seat because my adviser was off the the side telling me that I should chime in. I always get stressed out when I’m expected to comment or ask a question when I don’t really have a comment or question to make. In the end I asked about the wording of one of the bullets that said Gale has a “relative paucity of fluvial channels”. Because it actually has tons of them. But apparently “relative paucity” meant “there is more volume in the mound than could have come from those channels” so the sentence was not edited. Those two things mean different things to me, but I didn’t press the point.

Anyway, all of that meant that I didn’t take careful notes for Gale, but the final hypothesis was something along the lines of “how do the environmental and mineralogical signatures preserved in the layered rocks reflect the aqueous processes in the crater?” Although someone else suggested a more concise and to-the-point version: “Did the big-ass mound form in a lake?” Future work for Gale included getting a better handle on the age of the deposits and identifying specifically where MSL would go to look for organics.

For Mawrth the process was a little smoother, but still painful. I don’t know how professional scientists can have so much trouble stringing together a coherent set of words. in the end, the hypothesis they came up with was: “Mawrth records geologic processes during early martian history when  aqueous phyllosilicate-forming processes were pervasive and persistent and provides the opportunity to understand early habitability.”

Future work for Mawrth was to figure out the timing of the formation of the stratigraphy and the mineralogy, and just generally getting a better grasp on the possible depositional settings. There was also the question of what MSL would do on an extended mission, and Jean-Pierre Bibring called for astrobiologists to take a look at the site and try to fill in that part of the story.

By the time we got to Holden, the process for the discussion was in place, and the site advocates had had time to write down a hypothesis. unfortunately, it was wordy and awkward and immediately had to be edited, but in the end it boiled down to: “Holden preserves evidence of a fluvial-lacustrine system that provides the opportunity to apply a systems approach to evaluating a sustained, habitable environment.”

For Holden, the future work was to see if we can learn anything more about the light-toned layered deposits to see if they were formed in a lake. Also, looking at the orientation of the layers in the site was mentioned specifically.

Finally, Eberswalde benefitted from watching all of the other sites go first, and the fact that it has the clearest hypothesis to test of any of the sites. Sanjeev Gupta delivered this hypothesis: “Eberswalde crater’s stratigraphy, geomorphology and mineralogy records the evolution of a crater lake and associated fluvio-deltaic systems and additionally represents a habitable sedimentary environment that is favorable to the preservation of organics.” This one was clear enough and well worded enough that people actually applauded when they realized we wouldn’t have to agonize over the wording.

I don’t have notes for future work for Eberswalde, but I am pretty sure it included looking more carefully at the minerals seen from orbit and identifying specifically where to go to access the “bottomset” beds, which should have the best chance of preserving organics (the Eberswalde presenters repeatedly answered this question – you go the the bottom of any preserved stratigraphy – but people seemed determined not to hear this answer)

There was also a repeated call for all of the sites to identify specific targets for MSL to investigate, which can be fed into the rover drivers’ attempts at estimating how long it will take to get to the primary targets of each site.

So, that wraps up the 4th MSL landing site workshop. We didn’t vote any of the sites “off the island” this time, but I can tell you the impression that I got. I think Eberswalde is the clear leader among the four sites because, if you’re going to look for organics on Mars, it is the only place where we can be confident that there was a low-energy depositional environment that would concentrate and preserve organics. Based on the vibe in the room, I think Mawrth would come in second place if we had voted. Even though we have no idea how the rocks there formed, they have the undeniable advantage that you land on your primary target. Also, their team has always been a very cohesive and outspoken group at these meetings. They can be annoying in their fanaticism over their site, but I think their disciplined messaging may have had some influence.

I think Gale crater would come in just behind Mawrth. It has a lot of great qualities: including clear detections of both phyllosilicates and clays, and a very thick stratigraphic sequence that would tell you about a lot of environments. But Gale is a go-to site, and even though I showed a lot of cool stuff in the ellipse, people are a bit uncomfortable with the go-to sites. Gale particularly worries people because driving up a big mound sounds risky. We saw that based on slopes, there looks to be a clear path up the mound, but i wouldn’t be surprised it the rover drivers end up telling us it’s going to be extremely slow to climb it. The other issue is that we can’t say for sure what the mound is made of. It might have formed in a lake, but then again it might not have. This is true for all the sites except Eberswalde, which is pretty clearly a delta, but it seems to be emphasized most for Gale.

Finally, I think Holden would come in last. The Holden presenters did a great job highlighting all the good science that can be done on the alluvial fans in Holden, and with the go-to targets. But Gale has many similar features, only more-so. At both sites you land on an alluvial fan and then drive to layered rocks. Except the rocks at Holden are 100s of meters thick and the rocks at Gale are 1000s of meters thick. Holden does have nice examples of pre-impact breccia as well as the flood deposits, neither of which you would get to see at Gale, but those targets are toward the end of a long traverse at Holden, and may not tell us much about habitability.

So, that’s how I think the voting would go if we had voted at this meeting. My own ranked order shuffled around throughout the meeting: each site made a very convincing argument, so tended to jump higher on my list until the next site presented its case. Now, with a few days of perspective, I would rank the sites in this order: 1. Eberswalde, 2. Gale, 3. Mawrth, 4. Holden. I personally put Gale ahead of Mawrth because even though we don’t know the depositional setting for either site, at Gale there is enough stratigraphy that you get multiple chances to find a habitable environment.

Of course, there are caveats to this. If Eberswalde really did form from melted runoff caused by the Holden impact, that might knock it down on the list. If we can figure out the depositional environment at Mawrth and it’s not just impact ejecta, that might bump it up in the ranking.

The bottom line coming out of this meeting is that there is still a lot we can learn about all of  these sites. Some of it can only be learned by a rover, but some of it just requires a deeper look at the data we already have. MSL is a phenomenally powerful (and phenomenally expensive) machine, and we only get one. I hope this meeting gets people digging deeper to learn about these sites so that we send MSL to the best place possible, whatever that may be.


The 4th MSL Landing Site Workshop: Day 2 – Eberswalde Crater

September 29, 2010

The final site of the four that we discussed yesterday was Eberswalde, which of course is interesting because of the big delta that is preserved in the western part of the crater.The first presentation on Eberswalde was an impassioned and really interesting talk by terrestrial geomorphologist Bill Dietrich. Bill talked about how Eberswalde is an excellent site for going beyond just making qualitative statements about water on Mars and actually learning quantitative information about the fluvial system. By making surface measurements, Dietrich showed that you can learn things like how much water was actually flowing and what the individual flow events were like. He also talked about the meandering channels in the delta and those seen on the floor of Eberswalde and Gale.  Dietrich showed evidence from earth that to form meandering channels you need to have something that strengthens the banks of the river. Typically it is vegetation on earth, but ice, clay-sized particles or chemical cements can also do the job, and MSL would be able to tell which of these it was.

"Your pulse should quicken every time you see this picture!" - Bill Dietrich

After Dietrich’s talk, my officemate Melissa Rice gave a presentation highlighting the types of things in the landing ellipse. Her talk pointed out that there’s more to do in Eberswalde than just land and drive due west to the delta: there is a consistent stratigraphy in lots of the outcrops in the ellipse, along with chunks of ejecta from Holden, inverted channels, cemented fractures, and other smaller fans. I think her talk eased a lot of people’s minds who wanted to like Eberswalde but were afraid of a long go-to portion with not a lot to do until you get to the delta.

Next up, another terrestrial expert, Sanjeev Gupta gave a talk about the value of a source-to-sink relationship. He started his talk with an aerial photo of a desert landscape on Earth with really complicated geology and had this hilarious quote from when he was a postdoc: “I was ‘skycraned’ into this area, given a three month ‘battery life’ and told to worry about my career.” He went on to drive home how awesome Eberswalde is because you can see exactly where the sediment came from and where it ended up. “Field geologists on Earth would kill to have what we see at Eberswalde.” Gupta also emphasized that at Eberswalde we have a clear testable hypothesis: that the feature observed is a delta that formed in a lake.  he also said something that I didn’t know: the slope of a delta is determined by grain size, so the fact that Eberswalde has a very shallow slope suggests very small grain size, which is good for preserving organic molecules.

An oblique view of the Eberswalde delta topography. The entire delta is preserved in inverted relief, likely due to coarser grains in the channel beds making them more resistant to erosion.

Kevin Lewis gave a sort of summary presentation that also highlighted a couple of important points. The first is that many of the channels that cut the wall at Eberswalde end at the same elevation, and that elevation matches pretty well with a saddle point where water from the western crater would spill over into the eastern crater. This spillway might have acted like a dam to maintain the water level in Eberswalde at a specific elevation. Lewis also reminded everyone that the presence of a delta gives MSL a “built-in roadmap”: if you want to reach the fine-grained sediment that is most likely to preserve organics, you want to go to the lowest beds in the delta.

Finally, Nicolas Mangold gave a very interesting and controversial talk suggesting that all of the flowing water in Eberswalde may be due to the hot ejecta from Holden crater melting ice. This would mean that Eberswalde could have formed on a Mars that was otherwise not particularly friendly to life and that the lake could have been very transient.

That launched a really vigorous discussion about the site.  Ross Irwin claimed that the amount of water required to carve the channels in Holden and Eberswalde was way too high to have all come from ejecta-induced melting. And John Grant pointed to other nearby craters of a similar age that have alluvial fans but would not have been triggered by the Holden impact. There were cries that we need to look for evidence of a shoreline, followed by others responding that it’s unlikely you would see a shoreline.

A context topographic view, showing the ellipse in Eberswalde crater. Holden crater is the big crater in the south of this view.

There were also a number of questions that appeared to me to be deliberate and rather transparent attempts to make this site look worse than Mawrth. Coincidentally, these questions all came from Mawrth advocates! First, someone asked whether all we would really be learning about at Eberswalde is “what sorts of processes form deltas on Mars”. Kelin Whipple responded, saying that if you don’t care about that specific question that’s fine, but by being able to take the study of the hydrologic system to a detailed quatitative level, it teaches you a lot about the climate of early Mars. Sanjeev Gupta also echoed that, saying that the delta gives you information that lets you test the big questions about martian climate.

Another  question that I found really pretty absurd, was “how would MSL’s measurements supplement our understanding of Mars beyond “wow! There’s a delta!” I don’t know where this person was when Bill Dietrich and Sanjeev Gupta were giving their presentations. Dietrich very politely recapped his points, that we would go from a qualitative to a quantitative understanding of the system with lots more information about the environment. He countered this question by saying that you could just as well simplify Mawrth down to “Wow there are clays on mars!”

A third example of these rather leading questions was along the lines of: “We’ve been focusing on the structure of this delta, but what do we use all the compositional instruments on MSL for?” The room responded to this one with shocked disbelief before Scott McLennan, a geochemist, pointed out that understanding the context is just as important for geochemistry as it is for fluvial geomorphology.

There was a lot more discussion, but those were some of the more “interesting” questions. I found it rather annoying that the Mawrth advocates asked leading questions that they (I hope!) already knew the answer to in an attempt to make Eberswalde look bad. Still, at the end of the day, I think Eberswalde is the clear favorite landing site. The only troubling thing is the possibility that it could have formed rapidly due to melt from the Holden ejecta, and that is the number one thing to look into before we decide to send MSL there.

To me, Eberswalde is an incredibly strong site because it fits the MSL objectives almost perfectly. It is the only place on Mars where there is a clear place to look where the processes would concentrate and preserve any organic material that was present. It is the best evidence for lakes on Mars, and toward the end of the discussion of the site, Jim Bell had a good comment. He pointed out that there’s this apocryphal story that MER landed in Gusev and everyone was totally shocked that there was lava there instead of a lake bed. He said that actually there was a lot of debate beforehand. For Eberswalde, the community is much more in agreement that there was a lake. It would obviously be a big deal to visit a lake on Mars, but it would also be a huge result if we went to Eberswalde and it turned out not to be a lake!

If you want to read the rest of the discussion of Eberswalde, check my notes. It had, by far, the longest and most interesting discussion session. You can also read ahead to see what we heard about this morning in terms of the safety of the sites, and some sketchy notes of the afternoon discussion. And if you really want the nitty gritty details, the presentations from the landing site meeting are now available online.

The 4th MSL Landing Site Workshop: Day 2 – Holden Crater

September 29, 2010

The drainage system that Holden and Eberswalde interrupted. Image excerpted from Ross Irwin's LSWG presentation.

The second site that we discussed yesterday was Holden Crater. Ross Irwin gave the first, overview presentation. Holden is a 155 kilometer crater that formed right in the middle of a huge drainage system that spans from the Argyre basin to the northern plains, and at Holden you would land on a bunch of coalescing alluvial fans on the western crater floor and then drive southeast to access some nice phyllosilicate-bearing light-toned layers. Irwin emphasized that even though the light-toned layers are a key target, Holden is all about visiting a bunch of diverse water-related settings. He also made the interesting argument that MSL is likely our only chance, at least within the next decade or two, of sending a plutonium-powered rover to Mars that is capable of landing at a site as far south as Holden and Eberswalde, whereas Gale or Mawrth could be accessed by a later, solar-powered rover. Irwin also gave extremely detailed lists of what science MSL could do at each stop along the traverse. These sorts of lists are exactly what we need to make for all the sites because they really get us thinking in detail about MSL’s capabilities and what we would learn.

Next up was Kelin Whipple, who is an expert on alluvial fans here on Earth. Kelin described in more detail what fans can tell you about the climate when they were deposited (a lot!) and showed that the fans at Holden could be made by mud flows or water flows and that either way they would be really interesting.

After that, James Wray, a fellow Cornell grad student and Goddard ’06 NASA Academy alum, gave a presentation about the mineralogy at Holden. He particularly looked at some of the rocks in the walls of the crater, which would be present in the materials of the fans in the landing site. James found a pretty diverse set of minerals, including two types of olivine, two types of pyroxene, some phyllosilicates and some possible salts or high-silica minerals. James also pointed out that there are some mounds of material in the eastern ellipse which may be outcrops of pre-impact material.

A topo map of Holden crater. MSL would land on the gently sloping alluvial fans in the western portion of the crater and the drive southeast to study light-toned layers, deposits from the flood that breached the crater rim, and ancient bedrock.

Following James’ presentation, Debra Buczkowski discussed the results of work with Kim Seelos studying what looks like a really extensive layer of phyllosilicate-bearing rock just below the surface  in the region to the northwest of Holden and Eberswalde. They found that the clays in Holden that appeared to be in-place were different from those in this extensive layer, but that some of the Holden clays that looked like they had been transported might be the same stuff.

Finally Peter Buhler gave a presentation of his work on a small trough to the southeast of Holden which may also have been a system of lakes, and emphasized that Holden and Eberswalde could give us a “contextualized” hydrology of the broader region.

The discussion started off with a question from Dawn Sumner about whether the alluvial fans on the wall are interbedded with the light-toned layers, or whether the fans are just on top of the light toned stuff. This is a tough question to answer because as alluvial fans form, they advance outward into the basin, hiding everything underneath. Next my adviser, Jim Bell, asked whether all the great science described for the Holden fans could also be done on the fan at Gale Crater, to which Kelin Whipple said “Yes!” although he pointed out that it’s hard to tie the fans to the stratigraphy of the mound at Gale. Another question about Holden was what the compositional variation of the fan material would be: in other words,  how far would we have to drive to get samples of different stuff? The short answer was: we don’t know until we get there. But James showed that there was a variety of rock types up in the crater wall. Whipple explained that if the fan was made by mud flows, which tend to come from a very localized source, they each layer in the fan would be different, whereas if it was formed by a more watery runoff flow then it would have a more blended composition.

After hearing the presentations for Holden I felt a lot better about it as a site than I had before, but I still have the nagging thought that it’s very similar to Gale: you land on alluvial fans, then traverse to visit nice layered stratigraphy, except at Gale the stratigraphy is much more diverse and much thicker than at Holden. If I had to guess, based on the vibe in the room, I would guess that Holden and Gale are the bottom two sites in the ranking. Part of that may have been the timing of the presentations for Holden (after lunch lull) so I’ll see how it looks after the more detailed discussion today. As for my own personal ranking of the sites, it has been jumping all over the place as I hear the presentations from each group. I’m very interested to hear what the engineers tell us today, and to see how the giant discussion this afternoon plays out.

The 4th MSL Landing Site Workshop: Day 2 – Mawrth

September 29, 2010

Holy cow. Today was jam-packed with interesting stuff about Mawrth Vallis, Holden Crater and Eberswalde Crater! I took tons of notes, and I will try to use those to assemble a coherent picture of what was presented and discussed today. But if you’re too impatient to wait for me to work through those and post the more coherent summary, here are the notes in their raw and unedited form. Read them at your own risk, they’re full of jargon and typos and abbreviations! I’ll update that file tomorrow with tomorrow’s notes too.

In the meantime, I’ll start to translate those notes, starting with the first site on the table this morning: Mawrth Vallis. Joe Michalski started off the day with an overview of Mawrth emphasizing some of the key points about the landing site. Of course the obvious draw for Mawrth is that it is the best exposure of clay minerals on Mars. Clays form in wet environments and are good at trapping organics, so they are very desirable for MSL.  Joe (and subsequent speakers) also emphasized that Mawrth also has morphologic diversity and that it is an extremely old portion of the martian crust. Joe also pointed out that the fact that we see lots of clay minerals at Mawrth is not because Mawrth is unusual for early Mars. It’s much more likely that much of the early crust has similar minerals, but Mawrth is the best exposure.

The colors in this HiRISE image correspond to changes in the mineralogy. A similar stratigraphy is seen throughout Mawrth and the surrounding region.

The next talk was by Janice Bishop who summarized the mineral diversity in the site. She showed a bewildering number of spectra from Mawrth, and drove home the fact that the mineralogy observed occurs in the same stratigraphic order all over mawrth and all over much of the Arabia Terra region on Mars, supporting the idea that understanding Mawrth would teach us about a huge section of the planet. One of the interesting things that Janice and others showed is that these compositional layers are observed in some layered rock in the floor of Oyama crater, the huge crater to the west of the ellipse. This is interesting because it is thought that the rocks in the ellipse are older than Oyama, and obviously the rocks filling Oyama are younger. The fact that they show the same mineral stratigraphy suggests that the related alteration came after the physical deposition of the rocks.

After Janice’s talk, Eldar Noe Dobrea gave an overview of the morphology of the site. He showed a lot of nice HiRISE images of the site, and also gave examples of possible models for how the rocks at Mawrth were deposited, including impact ejecta, airfall, lakes and rivers, or an ocean. Eldar also showed a map of the ellipse that was completely peppered with markers indicating that he had observed layers. This was a contentious issue later in the day because Dawn Sumner, a sedimentary geologist who mostly studies the earth, gave a presentation saying that she didn’t see any convincing layering in the ellipse. Eldar also did something that John Grotzinger specifically said was very useful: he listed some of the thinks we don’t know about the site. In particular, we don’t know the origin of the clay-bearing rocks or the clays themselves. We also don’t know the amount of water that eroded the landscape or its pH, and we don’t know much about the dark unit that caps the stratigraphic sequence at Mawrth.

Another pretty HiRISE picture of Mawrth, showing some of the detailed surface textures.

Next up Damien Loizeau gave a nice talk about what we would do on the first day, month and year at Mawrth, and then Jean-Pierre Bibring gave a concluding presentation. Bibring emphasized the great age of the Mawrth Vallis rocks, pointing out that on earth rocks as old as Mawrth do not exist intact. All we have to work with are mineral grains preserved in younger rocks. So by going to mawrth we might actually learn a lot about Mars as well as about the conditions on very early earth.

Finally, as I mentioned above, Dawn Sumner gave a presentation based on her studies of the physical stratigraphy at Mawrth. She suggested that most of the features at Mawrth are due to cratering and that many of the “layers” are either fractures in the rock from impacts, or are not traceable for more than a few hundred meters at most. (Dawn also showed some very cool movies illustrating some of her points, using a new 3d visualization program that some of the other folks at UC Davis have developed called Crusta.)

After Dawn’s presentation there was a discussion period for Mawrth Vallis. One of the points that came up a few times is that the physical stratigraphy that Dawn was looking at is different from the mineral stratigraphy that seems to show more Al-rich clays above more Fe and Mg-bearing clays. There were also comments suggesting that it may actually be desirable to go to a complex ancient cratered surface precisely because it is confusing and there is no good terrestrial analog for us to study here  on Earth. Finally, there was a question about the abundance of clay minerals. In mixing models, Francois Poulet claims to see up to 60% clay minerals at Mawrth, based on OMEGA visible and near-IR data. But in thermal IR observations don’t see nearly that abundance. Steve Ruff, an expert in the thermal IR, said that TES does see some evidence of alteration but not at that sort of abundance. One expanation that he suggested is that the phyllosilicates might be poorly crystalline, so that VNIR observations see something but thermal IR doesn’t.

At the end of the discussion of Mawrth, I felt a lot better about the site than I did before going in. There is clearly a lot of good stuff to do there, and it has a couple of undeniable advantages: it is clearly the oldest site, and you get to land on your primary target. But I’m also concerned by what I hear from terrestrial geologists who are very concerned about how much Mawrth would actually tell us about the habitability of Mars. Yes, it has spectacular phyllosilicates, but it’s not clear that they would trap any organics since we don’t know what the depositional setting was. I think despite this uncertainty, if you polled the community, Mawrth would be one of the top two sites.

4th MSL Landing Site Workshop: Day 1

September 28, 2010

It has begun! Today was the first of a three day workshop in which the Mars science community (not just those directly involved in the MSL mission) gathers together and hashes out what we know and what we don’t know about the four finalist MSL landing sites.

For me the week actually started yesterday at the MSL team meeting, where we got lots of updates on the various aspects of the mission. Unfortunately, I don’t know how much of what I heard yesterday is safe to share with the world. Having been scolded before for sharing too much here I will just say that it’s really exciting to see all the pieces of the puzzle coming together.

Today, and for the rest of the week, everything is fair game to share with you, so I will do my best! Emily Lakdawalla will also be blogging parts of the meeting, so I’ll also point you to her posts about the meeting as they go up. In fact, you should go check out her introductory post right now. You might also want to refresh your memory about the four sites by reading this summary.

Today started off with some overview talks from the project manager Mike Watkins and project scientist John Grotzinger. Mike showed some awesome pictures of MSL (aka Curiosity), which is really starting to look like a rover. A jaw-droppingly huge rover. Watkins also had some nice analogies to go along with his pictures to convey just how huge Curiosity is. Remember the little Sojourner rover? It could just about use MSL’s wheels like hamster wheels. MSL is also really tall: its mast cameras could look Shaq straight in the eye, and its gigantic arm is so long it could almost dunk!

Curiosity is a huge machine. Note that the giant blob of instruments are not yet mounted on the arm in this photo.

I’m going to cheat and share a little of what I heard yesterday because this arm is just amazing and they used another good analogy. The instrument package at the end of the arm is huge: about 35 kg. You can picture this as something the size and weight of a lawn mower mounted on the end of a seven foot arm. The arm itself weighs 70 kg, and it is strong enough to dead lift that lawnmower even when the arm is fully extended. The MER rovers weigh 172 kg, the MSL arm weighs 105 kg. It. Is. Huge.

It finally sank in when I saw the presentations yesterday and today just how massive this rover is.

Anyway, back to today’s presentations. After the introduction, there were a series of talks about the preservation of biosignatures. One of the main take home points from these was that context is critically important for detecting and understanding biosignatures. If you understand the geology, you can try to look in places that maximize your chances of finding something. And then if you do find something, you can draw much better conclusions if you know its environment. This sort of argument makes a site like Eberswalde, where the story of a delta deposit in a crater lake is pretty well understood much more appealing than a site like Mawrth, where there are beautiful minerals, but we don’t really know the geologic story. Gale and Holden are both somewhere in between.

After biosignatures, there was a set of presentations about the mineralogy of the landing sites. This started with Frank Seelos unveiling the new and improved website containing a set of CRISM spectral images of the landing sites. You should go check them out: he specifically said that he would love it if we all melted his servers!

An example CRISM map of part of the Mawrth Vallis landing site, showing off the spectacular hydrated minerals in the site.

We also heard from Selby Cull, a graduate student who is attempting to use detailed models to figure out how much of a given mineral is in a sample based on its spectrum. Although it sounds straightforward, this is actually a phenomenally complex problem that planetary scientists have been grappling with for decades. It would be awesome if we could do this, but I’m skeptical.

After Selby, Ray Arvidson gave a talk about tying orbital to surface data at the MER Opportunity landing site and lessons learned that MSL should keep in mind. Bottom line (paraphrased) is that wherever you go, Mars is going to be more surprising and more interesting than we can imagine!

Finally Ralph Milliken shared his results on the compositions of the landing sites based on CRISM observations. It turns out that, using a subtle spectral parameter, you can estimate whether a given clay mineral detection is more or less “evolved” (where evolution could be due to burial or impacts among other things). Ralph also showed some exciting new stuff at Gale crater, including an example of a set of minerals visible in the southeastern portion of the Gale crater mound that looks very similar to the stack of mineral signatures seen near the landing ellipse.

A colorful map of the mineralogy at Gale Crater from Ralph Milliken. Greens are phyllosilicates, blue and magenta are sulfates, red is olivine, and orange is mixed sulfates and clays. The landing site is at the top center of this image, and the rover would ideally drive to the northwestern part of the mound, where there is lots of CRISM coverage.

Finally, after the mineralogy talks were over, we started hearing presentations about the specific landing sites. Gale was up first, and after Ken Edgett summarized the crater’s global and regional context along with some of what you can see within the crater, I gave my presentation showing off the interesting stuff that can be accessed in the ellipse and on a notional traverse up the lower portion of the mound. After me, my adviser Jim Bell presented about the composition of the stuff at Gale, and then finally Dawn Sumner gave a really interesting presentation about how to test some of our hypotheses at Gale Crater.

In the discussion of Gale the inevitable questions came up. One was: how old is the mound? The “problem” with answering that is that we think most of the terrain at Gale was buried and has since been exhumed again. That can totally throw off your estimated ages based on crater counting. The properties of the surface matter can too: some surfaces are hard and erosion resistant, so they tend to have more small craters than adjacent surfaces even if they are younger.The best estimate is that Gale crater formed at the end of the “noachian” period of martian history and the mound formed sometime in the late noachian to early hesperian. The exact timing of these epochs isn’t all that well constrained, but the noachian ended around 3.8 to 3.5 billion years ago, and it’s fair to say that Gale and its mound are about that old.

Three views of Gale crater. Top: An HRSC topographic map of the crater. Middle: A THEMIS thermal inertia map of the crater. Brighter areas are rockier, darker areas are dustier. Bottom: A THEMIS "decorrelation stretch" map, showing variations in color that can be roughly tied to composition. Pink is olivine, blue is dusty. Source: Anderson and Bell (2010)

We also talked about what it might mean if the whole mound is wind-blown grains. It’s certainly possible, but very difficult to tell from orbit. If the grains just filled up a dry crater, that might not be so good for habitability, but the mound is carved by canyons so we know there was flowing water well after the mound material had been deposited and turned to rock. On the other end of the spectrum, the mound might all be wind-blown grains, but those could have been trapped in the crater because it was full of water. As we heard earlier in the day, fresh basaltic sand plus water would provide the chemical energy necessary to support a pretty impressive chemolithoautotrophic (aka rock-eating) biomass.

There was also a lot of discussion of the evidence that Gale was actually buried. Someone asked whether there are any patches of layered material on the crater wall that match the mound layers. There aren’t, but I pointed out that there is a nice outlier of layered material hidden in the dune field 20km west of the mound itself. Ken Edgett reminded people (as he has been doing for 10+ years) that “this isn’t the mars you’re used to thinking about”, that entire regions have been buried and exhumed, and that this can happen without leaving a trace on the underlying surface. He also pointed to Henry crater as a Gale-sized example that does have a mound that matches with material on the walls. The question of why Gale seems to have preserved its huge mound of sediment, but all the other craters nearby did not keep theirs came up, and John Grant suggested that we do crater counting on the nearby craters which we know are older than Gale to see if their surfaces are anomalously young – as if they had been buried for a few billion years.

I also got the question of whether we should try to visit all of the cool stuff that I showed in the landing ellipse if the true goal of the mission is the mound. This is going to be the core question for this mission. At all the sites but Mawrth, you land on cool stuff but your main goal is outside the landing ellipse. The tradeoff between doing the “safe science” right away in the ellipse and sacrificing science early on to be able to make it to potentially more rewarding targets is going to be a huge part of the landing site discussion over the next few months. On Wednesday we are going to hear from some of the engineers and rover drivers about the engineering constraints on the mission, and then the Landing Site Working Group (a small subset of scientists that have been looking very closely at the landing site) will work with the rover drivers to identify the key targets at a given site and the key observations we want to make at those targets. From there, the engineers will come up with a set of potential traverses with detailed estimates of how long each will take. Since all of the sites are safe to land in, I think these estimates will play a major role in finally narrowing down to the final selection next spring.

The 4th MSL Landing Site Workshop

September 24, 2010

Well folks, I’m off to Pasadena to help the Mars community decide where to send its next rover. Long-time readers will recall that i’ve been to a couple of these things before and they’re always fascinating. I was going to post a reminder of what the four finalist sites are, with pros and cons and all that, but it turns out I don’t have to! My friend Lisa Grossman, a former Cornell astronomy major, is now a science writer for Wired! She interviewed me and my adviser earlier this week and put together a nice article summarizing the sites. I’m quoted in it quite a bit, so rather than repeat the same stuff, I’ll just point you over to her piece.

There are a few points of clarification that I should mention. First, the article says that MSL is searching for life, and that’s not really true. MSL is searching for signs of habitability. Obviously finding life would be a pretty good sign! But habitability is broader than just the search for life or even the search for organic molecules. Evidence for habitability could come from the texture of a certain rock telling us that it was deposited in water, or from the detailed chemistry revealing that the minerals in the rock could only form in benign liquid water.

Also, she’s right that some of the clays at Mawrth are kaolinites, which tend to form on earth in tropical soils. But to clarify, I don’t think anyone is saying that the huge amount of kaolinite clays at Mawrth are the result of tropical conditions. They do suggest that there was a lot of water involved though, which is why Mawrth is so interesting.

A final clarification: in the article, it mentions that it will take “several days of hard driving” to get to some of the go-to sites, where the really interesting stuff is outside the ellipse. If it were several days, that would be no big deal. It is going to be more like a year or two. A lot of people are really nervous about landing, only to have to buckle down and drive drive drive to get to the main target of the mission. Of course, all of the sites have good science to do in the landing ellipse, but that is a blessing and a curse for a go-to site. On the one hand, you get some results early on in the mission, but if you don’t have a lot of discipline, you can spend all your time staring at the rocks at your feet and never get to go climb the mountains in the distance.

With that, I’ll let you go read the article. You can also check out my old blog posts about the sites from the last time one of these workshops was held (Gale, Holden, Mawrth and Eberswalde). I’m going to do my best to take notes and blog about the meeting, and Emily Lakdawalla of the Planetary Society will be there for part of the meeting as well. We’ll do our best to keep you informed!

PS – You should totally check out the comments on the Wired article, where someone calls me out for saying that there was water on Mars and says that we Mars scientists are either stupid or have ulterior motives. Someone hasn’t been paying attention to every press release about mars for the past decade or so.

Gale Crater Geomorphology Paper – Published!

September 16, 2010

Big news folks! The huge paper that I’ve been working on for the last couple years is finally, unbelievably, published! Even better for you, it is published at the Mars journal, which is an open-access journal. Just head on over and you can download all 53 pages of pure, distilled Science!

In case you don’t want to wade through that many pages (and almost as many figures) of Mars geomorphology jargon, I’ll summarize here.

Gale Crater is a large (155 km diameter) crater that sits just south of the martian equator on the boundary between the rugged, cratered highlands to the south and the smooth plains of the northern hemisphere. Gale is special because it’s not just a big hole in the ground: in the middle of the crater is a vast mountain of layered rocks that towers nearly 6 kilometers above the crater floor. As I’ve mentioned before on this blog, geologists love layers, because they are formed in the rock record when something changes. So the Gale Crater mound is a giant record of the changes that have taken place on Mars since the crater formed3 or 4 billion years ago. This fascinating stratigraphic section has evidence of water-bearing minerals like clays and sulfates, and the detections of these minerals seem to follow certain layers, so the hope is that those layers were deposited during a time when Mars was more habitable. That’s why Gale Crater is one of the four finalist landing sites for Mars Science Laboratory!

Even though a lot of people knew that Gale had this interesting mound, not a lot of work had been done on the crater, so in an effort to help the Mars community learn all we can about the geology of this possible landing site before making the decision whether or not to land there, I dove into the image data for Gale crater. I started with the Context Camera (CTX), making a huge mosaic covering the whole crater. I used that mosaic to map out easy-to-map features like sand dunes and branching channel networks.This was scientifically useful because it shows that there are lots of channels formed by water flowing into the crater, but it was also a good way for me to learn to use the mapping software ArcGIS.

After that, I spent a while working on determining the orientation of the layers in the mound using a digital elevation model based on CTX images. This work was also educational, but ultimately after showing it to several colleagues I found it was drawing criticism and wasn’t telling us a lot about the mound, so that was cut from the final paper.

I moved on and pored over the various HiRISE images of the potential landing site and the mound, identifying “units” based on their appearance in HiRISE, supplemented with CTX, thermal inertia data from THEMIS, and spectral data from CRISM. In doing so, I was able to put together a more detailed picture of what a rover might encounter if it landed on the crater floor and drove up the mound.

Three examples of inverted channels in the proposed MSL landing ellipse.

The landing site is centered on a fan of material that extends from the northwestern wall of the crater and ends a few kilometers from the base of the mound. Looking closely at the fan, it turns out that about two thirds of it is mantled with what looks like dust or soil, but the final third, closest to the mound, has been stripped bare, exposing a bunch of fractured layered rock. That’s interesting in an of itself, but the fan isn’t the only thing in the landing ellipse. I also found some examples of inverted channels: riverbeds that were resistant to erosion and ended up as mesas when the surrounding land was stripped away. There are also patches of a unit that I called the “mound-skirting unit” within the ellipse.

This mound-skirting unit is found all around the crater and tends to be erosion resistant, forming mesas. It also looks like it might be related to flowing water in some places: chains of mesas made of this unit extend across the floor from channels and fans on the crater wall. But elsewhere, there are parallel ridges in the mound-skirting unit that might be the remnants of ancient dunes that were petrified. One notable patch of the mound-skirting unit is partway up on the northwest side of the mound, right at the end of a channel that was carved into the layers of the mound and later filled with debris. This has been referred to as a fan, and if I had just looked at it without context I would have called it a fan too! But it has almost the exact same texture as the mound skirting unit nearby. My interpretation for this was that the original fan is mostly eroded away, but because there was a fan sitting on top of the mound skirting unit in this spot, it was protected from erosion so it remains as a mesa at the end of the filled channel.

Examples of erosion-resistant ridges on the Gale mound that might be due to water percolating through the rocks.

Another interesting thing that I noted about the surface of the lower mound is that there are cracks all over the place that look like they are resisting erosion, becoming lattices of ridges. This could happen if water was flowing through the rocks, cementing the material near the fractures and making them last longer than the un-cemented surrounding rock. These fractures occur in a part of the mound that shows hydrated sulfate signatures in CRISM, so there is some supporting evidence that water was involved.

I also looked at some of the material near the top of the mound, far beyond where MSL is likely to reach even with multiple extended missions, and I’m really glad I did. One of the big questions about the Gale mound is what type of rocks it is made of. It’s a lot nicer for habitability if you can say for sure that rocks formed in a lakebed instead of a desert, but it’s frustratingly difficult to tell the difference with orbital data. The upper part of the Gale mound might be an exception though: If you look really carefully, in some of the HiRISE images there is a strange “swirly” looking pattern that I interpreted as a cross-section through ancient sand dunes. If true (and there are certainly other possible explanations), this tells us that the upper mound was once a location where sand collected to form dunes, meaning it was probably a low point rather than a high point.

Examples of the weird texture in the upper mound that might be evidence that these rocks were once sand dunes.

The idea that Gale has been buried isn’t a new one, and after looking at the geology of the mound, I think it has actually been buried and unburied several times. The lower layers of the mound have deep gorges carved into them and craters on the surface, suggesting that they’re old, and that they were already eroded into a mound back when water was eroding things on mars. Above the lower mound is the upper mound, which looks like it was deposited separately, possibly much later. There is a channel in the lower mound that disappears beneath the upper mound to mark the “unconformity” between these two units. This means that just because I found what I think are petrified dunes in the upper mound, the lower mound isn’t necessarily the same stuff.

There’s lots more in the paper, but those are some of the key points. In a couple weeks I’ll be presenting at the MSL Landing Site workshop, where people who have been studying the four potential landing sites will share their results and the whole Mars community will argue and try to decide which site we should land at. These meetings are always exciting, and I’ll do my best to blog about the meeting here. The final decision won’t happen at the upcoming meeting, but it will be interesting to see if any sort of consensus starts to form. (don’t hold your breath!)

Is Eberswalde Really a Smoking Gun?

August 3, 2010

The other day in Mars journal club, we took a look at a paper about the “fan” in Eberswalde crater. You may recognize this name: it is one of the four finalist landing sites for MSL. The site was chosen because at the western end of the crater, there is a feature that most Mars scientists consider to be a delta, formed when sediment transported by rivers  encountered standing water and settled out.

The paper we looked at considered an alternative to the delta hypothesis. Instead, Jerolmack et al. proposed that the fan could be an alluvial fan, formed by river channels that “avulsed” back and forth to form a gradually sloping fan. “Avulsed” means that the river abruptly changes its course. When you average out over lots of avulsions, you get a broad, shallow cone of material deposited: an alluvial fan.

Jerolmack used a computer model of this style of fan formation and fit it to the slope of the Eberswalde feature’s surface. They found that, if the Eberswalde fan is actually alluvial, it would have formerly extended about 40 km out into the crater, and that it could have formed extremely rapidly: in tens to hundreds of years, and with no need for a standing body of water.

That’s a much different story than the more popular one: that the Eberswalde fan is a delta that formed over many thousands of years in a lake. Which scenario is correct? I don’t know. It may not be possible to know from orbit. That’s part of why Eberswalde is a candidate MSL landing site: if our “smoking gun” evidence of a lake on Mars turns out to be an alluvial fan deposited in 50 years, then that certainly has an influence on the question of the habitability of Mars as a whole! Of course, everyone hopes that it is truly a delta, in which case it would have been favorable for preserving organic biomarkers and would record a lot of information about the martian environment.

There’s only one way to know for sure: land there!

ResearchBlogging.orgJerolmack, D. (2004). A minimum time for the formation of Holden Northeast fan, Mars Geophysical Research Letters, 31 (21) DOI: 10.1029/2004GL021326

New Evidence for an Ocean on Mars?

July 19, 2010

There’s a new Nature Geoscience paper that has made a big splash in the Mars community, reviving interest in the possibility of a northern ocean. This news was making the rounds a couple weeks ago, but I decided to hold off because at last week’s Mars Journal Club we discussed the paper.

The idea behind the paper is deceptively simple. The authors, Gaetano DiAchille and Brian Hynek, searched all over Mars for landforms that looked like deltas: fan-shaped features that form when flowing water encounters standing water and drops its load of sediment. By finding the elevation of the deltas, they could see if they all formed in the same big body of water.

A topographic map of Mars (red is high, blue is low). The white line marks the location of the putative shoreline. The red squares mark deltas that empty into the northern lowlands. Green triangles and blue diamonds mark deltas that are in smaller, closed basins. This image is Figure 2a in the paper.

Many of the delta-like features on Mars lead into closed basins, such as craters. These basins might have formed isolated lakes, and so their deltas were not counted when trying to infer the shoreline of the ancient ocean. On the other hand, DiAchille and Hynek found 17 deltas the did not empty into closed basins. These ones instead formed at the edge of the northern lowlands. Amazingly, when the authors measured their elevation, these 17 deltas -which were spread all around the planet – turned out to have the same elevation to within a few hundred meters! This suggests quite strongly that they all formed when flowing water encountered a large northern ocean. The inferred shoreline of the ocean matches pretty well with some stretches of the highland-lowland boundary that had previously been suggested as possible shorelines. The possible shore also is consistent with where the fluvial channels on Mars are observed. On a map of channels in ancient martian terrain, only 1% of the channels are at elevations below the elevation of the shoreline!

It’s also worth noting that some of the deltas that are in closed bases fall within the error bars on the possible shoreline. That might suggest that there was a global groundwater system, so that lakes near the ocean had the same water level.

There’s no doubt about it, this is a cool paper. I did have some questions about their methods. In particular, how do they know the “deltas” are actually deltas and not alluvial fans? Especially if these features formed three billion years ago, erosion could have significantly changed their shapes. It would be pretty easy to erode the edges of an alluvial fan to make something that looks more like a delta. Considering that the very best example of a delta on Mars – the Ebeswalde Delta – has been the subject of intense scrutiny and debate about whether it actually is a delta, I was surprised to see how nonchalantly the authors claimed that the fan-shaped features were deltas.

Still, this paper goes a long way toward resurrecting the idea of a northern ocean on Mars. It will be interesting to see how the community reacts to this in the coming years.  An ocean on Mars would be a huge discovery, but as the saying goes: “extraordinary claims require extraordinary evidence.” The extremely narrow error bars on the delta elevations are pretty extraordinary, but I’m sure many people will remain unconvinced.
Di Achille, G., & Hynek, B. (2010). Ancient ocean on Mars supported by global distribution of deltas and valleys Nature Geoscience, 3 (7), 459-463 DOI: 10.1038/ngeo891

Ice Caves on Mars!

June 9, 2010

Hey, guess what? There might be caves with ice in them on Mars! You should go check out my post about this cool new possibility over at Universe Today!