Archive for the ‘Curiosity’ category

The 4th MSL Landing Site Workshop: Day 3 – Engineering and Safety

September 30, 2010

With the details of all four landing sites on the table, we started day 3 of the meeting by hearing from the engineers and several scientists about the properties of the ellipses, the risks for landing and the capabilities of the landing system. First on the schedule was Mike Watkins, who explained why MSL is so unique in terms of assessing the risk for the landing site because the landing safety is essentially the same for the sites, so the tradeoff becomes more science-oriented and requires a lot more knowledge of the possible targets and traverse distances.

After Watkins, Ashwin Vasavada – the deputy project scientist – told us about the atmospheric simulations that his team has been doing to make sure the weather at the sites won’t mess up the landing system. He pointed out that we are one mars-year away from landing: “The next time Mars goes around the sun, we’ll be there to meet it.” That means that measurements being made right now will be really important in predicting the conditions when MSL arrives. It turns out that unlike previous missions, MSL will actually fly for about 100 km at a pretty constant altitude. This guided flight is what shrinks the ellipses down to nearly circular, but it means that we need to understand the weather for that whole distance. It sounds like the weather at the sites shouldn’t be a problem: Vasavada said that MSL should be able to land even if there is a dust storm occurring at the landing site.

Vasavada also showed this interesting plot of day and night temperatures at each landing site, along with an extreme "test case".

Next up, Ken Herkenhoff gave a summary of all the processing that goes into making the high-resolution elevation maps based on HiRISE stereo images. It is incredibly complicated to make these things, but they’re extremely important for planning the landing and traverses. Luckily, the folks at USGS have a lot of really clever techniques to make the products possible. The DTMs are available at the HiRISE website.

Matt Golombeck then gave his first of two presentations. This one was about counting rocks in the landing ellipses to make sure they’re safe. For the purposes of his talk, he defined a “rock” as anything that we don’t want to land on or get in the way. Rock counting has been done for all the previous landing sites on Mars, so we have some good “ground truth” to compare with the rock counts from orbit with HiRISE. They use an automated algorithm to count the rocks and then fit the size distribution to a model based on previous sites to predict the number of dangerous rocks that are too small to count. In the past it has been very successful, and they’re confident that all the MSL sites are safe enough. Golombeck said that Holden is especially safe in terms of rocks: “I think there’s maybe one rock in Holden.” There were a couple of very good questions after this talk. Rob Sullivan asked whether a small softball-sized rock would be a danger if we landed right on it with a wheel, and the answer was basically no. The engineers said that the biggest risk was getting a rock in the belly that would damage the rover and/or prevent it from moving. A second question from Steve Ruff was more generally, “how concerned should we be about landing with a hobbled rover?” The engineers said, again, don’t worry about it. They’ve been investigating some scenarios where the landing system might get stressed beyond the point where the materials have a linear behavior, but they aren’t worried about breaking the rover.

Next up, Robin Fergason presented about the thermal inertia of the sites. Thermal inertia is a measurement of how resistant a surface is to heating. Bedrock takes a long time to heat up and cool down so it has a high thermal inertia. Dust has a low thermal inertia. Fergason said that all the sites look safe in terms of thermal inertia and then discussed some of the details of each site. One thing that sort of bothered me was that she kept saying that lower thermal inertia might suggest alteration. It’s true, it might, but I would never bet a $2.3 billion rover on the fact that altered rocks have slightly lower thermal inertia. The values seen could just as well be
unaltered bedrock with a dusting of sand or dust on top. Of course, we see other evidence that the rocks at the landing sites are altered, but for the rocks with no clear spectral signatures, I don’t think it’s safe to assume they’re altered based on thermal inertia.

A thermal inertia map of the Gale Crater ellipse. I love how the alluvial fan pops right out in the thermal data!

After the thermal inertia discussion, Golombeck got up and gave a presentation with 83 slide in 30 minutes, flashing rapid-fire views of lots of different data-sets used to characterize the ellipses. He also told us that the orientation of the ellipses has changed a little bit so that now their long axis is due east-west. Taking a look at the 5 meter slopes he said that Mawrth and Eberswalde were significantly rougher than Gale and Holden (which makes sense, since Gale and Holden both land on top of nice flat alluvial fans). The topography at Mawrth and Eberswalde would be comparable to that seen in the Columbia hills, where Spirit has been exploring. Golombeck also showed that there are very few “inescapable hazards” such as craters in which you could land but then couldn’t escape. At this point he jokingly made fun of engineer Gentry Lee who had been worried about a big crater in the Mawrth ellipse, saying that the crater was not a target rather than an obstacle! There was also the amusing mention of a small mesa in Eberswalde where MSL would land but would have to go down some pretty steep slopes to escape. This possibility was later refferred to as the “Lion King” scenario by Rob Sullivan. It’s incredibly unlikely, but still hilarious to picture the rover greeting the sunrise from atop this viewpoint, which would, of course, be called Pride Rock.

MSL at Pride Rock. I spent too much time making this, but it had to be done.

At the end of this talk, Sullivan asked whether there is any concern that dust kicked up by the rockets could confuse the landing radar. The engineeres said that yes, there is some concern, but they are doing tests with the radar on helicopters to learn more. Basically, the engineer responding said “We could probably fly through the blowing stuff. We might not like it, but we could do it.”

Next, Devin Kipp, one of the engineers on the entry, descent and landing teams gave a presentation that took a look at all the things that could go wrong and basically said “we don’t think these things are going to happen”. He said that the chances of landing success in all of the sites are about 98-99%. Kipp also made the biggest understatement and the best example of NASA engineering-speak all day. When discussing the transition from wheel touchdown to actual roving he said: “hopefully the rover-surface interaction perpetuates for the rest of the mission.” I certainly agree with that!

Finally, Paolo Belutta, a rover driver, gave a presentation detailing how the traverse times for each site are going to be predicted. I suspect that these traverse times are going to be the deciding factor for which site is selected. Since landing on the sites is equally safe, the risk gets pushed into the traverse portion of the mission. Belutta is making detailed maps of the sites using HiRISE images and terrain models to estimate how long it will take to drive in any given location. Then, with input from the scientists, he will compute the traverse durations for several options in each of the landing sites. At that point we’ll come to the tough part where we decide whether the cost of a long traverse is worth the possible payoff.

Overall, it was an extremely positive morning and I think it laid to rest a lot of concerns that the science community had, based on rumors and partial information that had spread around the science teams. That said, I think it was also presented with full knowledge that this was a public meeting, and that word might get out if they admitted to any major problems. NASA has this mentality where it is afraid to acknowledge the difficulty of what it does until it has done it successfully. Personally, I think NASA should play up the risks and the difficulties ahead of time: they’re really interesting, and they make the final successes that much more exciting, and prepare people for the worst if it happens.

In any case, it was a fascinating morning. I’ll post about the big afternoon discussion of all the sites tomorrow. Stay tuned!


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

Big Sky Country

July 31, 2010

Well folks, I’m headed off to Big Sky Country tomorrow (aka Montana)! I’ll start the week at the MSL camera team meeting, where I will get all sorts of cool news about the MastCam, MAHLI and MARDI cameras which I will not be able to share with you.* After that, the lot of us will pack up and head to Glacier National Park to learn about the geology of the Belt-Purcell supergroup, and more generally, how to apply terrestrial geology to martian geology. I always enjoy field trips like this because I get to hike around on the rocks with a bunch of experts as well as many with less field experience, so there are lots of educational discussions. Also, did I mention the part where I get to drive and  hike around in spectacular scenery? Yeah. Times like this I’m reminded that my job Does Not Suck.

I’ll try to write a post or two about the trip once I actually understand the geology we’re going to see a little more. Hopefully the weather will cooperate and I’ll have some pretty pictures to share too!

*One of the difficulties with actually being involved in missions is that I can’t just write about all the cool stuff I hear about. I got scolded when this blog was just starting out for posting information before JPL or NASA had approved of it, so I tend to err on the side of caution now. It’s frustrating, but there’s nothing I can really do.