Archive for the ‘OMEGA’ category

AGU 2009 – Day 1

December 16, 2009

For those not familiar with the conference, the fall meeting of the American Geophysical Union is a terrifyingly, overwhelmingly large conference. Each year, something like 16,000 geoscientists descend on San Francisco to share their work. It is also one of the major planetary science conferences, so a lot of new results are first presented here.

Moscone Center in San Francisco. This building is filled with science at every fall AGU meeting.

This year, the first talks that I checked out on Monday were about radar observations of Mars. By sending radar waves from spacecraft to the surface and then recieving the reflected waves, we can learn a lot about Mars. In particular, since radar penetrates tens to hundreds of meters below the visible surface, it can reveal otherwise hidden structures. This has been especially successful at mapping the structure of the polar caps, because radar penetrates through ice quite well.

Roger Phillips gave a talk summarizing some of the results from the SHARAD radar instrument on MRO. Among other thers, SHARAD has found evidence that the spiral troughs in the north polar ice cap have migrated over time, as predicted many years ago by theoretical models. SHARAD has also found ancient buried canyons in the polar ice, which menas the ice caps have been around for quite a while. There are also some exiting new results implying that the material filling valleys in the Deuteronilus Mensae area is quite transparent to radar waves, and might in fact be something like glacier ice.

Image credits: NASA/ESA/JPL-Caltech/ASI/University of Rome/University of Washington St. Louis

After the radar talks, there were a whole bunch of presentations about aqueous alteration on Mars. One of the main lessons that I took away from those talks was that Mars is still a very confusing place. For example, Hap McSween used data from the Mars rovers and characterized typical soils at both landing sites. He found that the compositions of soils are roughly 70% unaltered material and 30% alteration products. He also showed that the soil compositions are quite similar between the two landing sites, which are on opposite sides of the planet, and that the unaltered portion of the soil is similar to the rocks at both sites.

However, the next talk by Josh Bandfield used orbital data and found that in general rocks on Mars have more of the mineral olivine than the soils. This is a somewhat different result than the rover data, and it might imply that rocks on Mars actually have more magnesium and iron than previously thought.

Other talks related to Mars alteration focused on “clay” and sulfate minerals detected on Mars. One that I found particularly interesting was by Paul Niles, who pointed out that Mars is an “obliquity-driven” planet. In other words, its tilt varies widely, and the Mars we see now is not typical. Niles suggested that during more typical periods, ice might have formed large layers at Mawrth Vallis, a location known for its strong hydrated mineral features. Melting at the base of that ice could have leached the rocks, explaining the presence of specific Al-bearing clay minerals.

Map of water-bearing minerals at Mawrth Vallis. Image credit: ESA/OMEGA team

Another interesting talk was by Itav Halevy, who took a look at how the presence of SO2 gas influences the formation of carbonate minerals. It turns out, even a tiny amount of SO2 gas (which is often released by volcanoes) can prefent the formation of CaCO3. If there is iron around, FeCO3 (the mineral siderite) forms instead. The implication is that sulfur minerals should form in different locations than clay minerals and siderite.

Continuing with the sulfur theme, Albert Yen talked about some results from the Spirit rover. He said that basically, if the rover had to get stuck, it picked a really fascinating place to do it! Based on the compositions measured, it turns out that there is too much sulfur in the soil to balance it out by assuming it is combined with other elements like Fe and Mg. That means there might be pure elemental sulfur mixed in with the soil, which would be consistent with hydrothermal activity!

My officemate and occasional contributor here at the Martian Chronicles, Briony Horgan, also gave a nice talk summarizing some of her recent work. For a long time there has been a question about the so-called “surface type 2” on mars. This surface type has higher than usual Si, but that could be due to a different type of lava, or alteration of the more common basalt seen elsewhere on Mars. Briony presented new evidence, based on the overall shape of the spectra of surface type 2 regions, that these areas might be due to a silica glass coating! This sort of coating could form when thin films of water from thawing ground ice altered the surface of sand grains, and would imply relatively recent alteration processes on mars.

Finally, the day ended with the Whipple prize lecture, which was unfortunately full of some misleading information about the history and status of Mars science. But that’s the topic for a future post.

Potential MSL Site: Mawrth Vallis

September 17, 2008

The Mawrth Vallis landing site is actually a set of four possible landing ellipses in an area with huge clay mineral signatures that is cut by a meandering outflow channel. There was some grumbling in the past about the fact that Mawrth advocates proposed four ellipses when everyone else followed the rules and only submitted one, but in the end I think it hurt them. They ran way over time today and had to speed through several very good presentations.

Part of the reason they went over time is because their first presentation spent a long time trying to convince people that the mission must go to a location from early in Mars history. We already knew that! They also had a lot of overlap between the individual presentations. But anyway, on with the science.

Mawrth is a bonanza of clay minerals. We saw modeling evidence which indicates that in some places >60% of the minerals in the rocks have water in them. I don’t know how much I trust the models, and I am suspicious of such large numbers, but at any rate, if there are thick beds of clays anywhere on Mars, Mawrth is a great candidate for that. The great thing about Mawrth is that not only is there a very strong phyllosilicate detection, but there are both Aluminum and Iron/Magnesium bearing clays, and they have a definite stratigraphic relationship. Throughout the area, and even in locations hundreds of kilometers away, the same layers are seen: a bottom layer of iron/magnesium clays, a top layer of aluminum clays and hydrated silicates, and a capping layer of something else. In between the two clay layers there may be another layer that represents a chemical reaction between them.

There is also a lot of interesting morphology in the area, including polygons that seem to correlate with clays, fractures in the bedrock that have light or dark “haloes” around them that may indicate fluid flow, deformed layers of sediment, and filled craters. Also, some of the layers appear to follow the topography so that maybe they were draped over it. This would be possible if they were some sort of ash fall from a huge eruption, or some other sedimentary unit that was draped over and then altered in place.

As i mentioned briefly above, the same layering sequence is seen in areas many hundreds of kilometers away from Mawrth Vallis, meaning that if we figured out Mawrth, we could figure out a big area of Mars. One of the confusing things about Mawrht is that there is no clear basin that would collect sediment, so the question arises: how do we know that these are huge stacks of ash or dust or sand or something blown by the wind? This was discussed at great length, but the bottomĀ  line that I took away was that, yes, it could all be air-blown. John Grotzinger, the project scientist, pointed out that on Earth, we learn a lot from stuff that has been transported and that to him it’s still an interesting site whether the clays were transported or formed in place.

Some people expressed the thought that, after being bombarded by a series of presentations about the site, they couldn’t see how a story would come out of it. The response to this was that, yes it is complicated, but it is not a complete mess with no coherent relationship. There are a lot of observables at the site with MSL, and I found this complaint kind of odd for this site. The group presenting showed us lots of hypotheses even if they didn’t call them that, and I don’t think it would be incomprehensible from the ground with the rover.

One of the final points raised was that, if the layers appear to drape current topography, how do we know they aren’t much younger than they seem. The answer given to this was extremely unsatisfying to me. Basically they said that one current theory (which happens to be the favorite one of the proposers of this site, since they came up with it) says that phyllosilicates formed early in Mars history, and therefor this site with phyllosilicates must be from that period. That wasn’t how they said it, but that’s what it amounted to, and that’s dangerous thinking.

Mawrth is definitely a fascinating site that could tell us a lot about Mars, and it has the best evidence for clay minerals seen on the planet. But life can be well preserved without clays and clays don’t always preserve life. A lot of people at this workshop are equating clays with biomarker preservation but it is not that simple. I think the presenters shot themselves in their collective feet by not being organized, not following the same pattern that the other presentations did and running way over time. I like Mawrth, but I came away feeling like there is a lot that we don’t know regarding its habitability and potential to preserve organics, so I don’t know if it will be carried.

Mapping Meridiani: Part 2

February 27, 2008

Last time, I gave some of the background information about my research. Now, armed with that knowledge, we can press forward and talk about what I do.

I look for hydrated minerals. A hydrated mineral is a mineral with water trapped in its crystal structure. The crystal acts as a protective cage, keeping the water bound within it even when the atmospheric pressure is too low for liquid water to be stable on the surface. There are three main types of hydrated minerals that have been detected on Mars. These are clays, sulfates and oxides.

Clays are also known as phyllosilicates or sheet silicates. This is because their crystal form as flat sheets of molecules. When it comes to looking for evidence of past habitability on Mars, phyllosilicates are ideal. Their sheet-like crystals can trap organic molecules between them like flowers pressed and preserved between the pages of a book. Even better, clays form through the interaction of neutral pH water with volcanic rocks, implying that there was a life-friendly environment for an extended period of time. Many Mars scientists believe that clay minerals are found in the earliest rocks on Mars, and record a time when the planet was warmer, wetter, and more habitable.

Sulfates are formed when acidic volcanic water and vapor interacts with rocks, or when large bodies of water evaporate. They can preserve some evidence of organics, but are not as good as clays. It is harder, though by no means impossible, for life to live in acidic waters. Sulfates may represent an era of martian history when the world’s water was drying up and volcanoes were changing the climate.

Finally, oxides are the result of slow weathering of volcanic rocks under current martian conditions: very cold and very dry. Mars is red because it is covered in iron oxides (a.k.a. rust). It doesn’t take much water at all to form them, and it is likely that, for most of the history of the planet, oxides have been the dominant mineral formed when the surface rocks are weathered.

To make my maps of these different minerals, I use data from the OMEGA instrument on Mars Express. OMEGA is an infrared mapping spectrometer, which means that it takes pictures of the planet at hundreds of different wavelengths of visible and infrared light simultaneously. This means that for a given location on the surface, we get information about how bright it is at hundreds of different wavelengths. In other words, every pixel of an image taken by OMEGA contains a spectrum. It is that spectrum that I use to figure out which minerals are present on the surface.

Every mineral has its own unique spectrum, so I use a computer to map which pixels in the image have spectra that fit with clays, sulfates, or hydrated minerals in general. First, an example of the sort of images that OMEGA takes:

meridiani_mosaic_tot.jpg

This is actually a whole bunch of OMEGA images, stitched together into a mosaic. It’s not perfect, but it allows me to map a larger region than that covered by just one image. For example:

blog_hydration.jpg

This is a map of the amount of hydration in Meridiani. The yellow (and one black) circles show places that were being considered as possible landing sites for MSL. Each circle is about 32 km in diameter. Most of these were thrown out at or before the landing site selection meeting in October of 2007, but the center circle in the three-in-a-row diagonal on the left is being considered because it is a very flat, safe site. The black circle is also being kept as a back-up, though I thought it should be one of the top priority sites because it is pretty safe and is right smack on top of loads of hydration! As you will see below, there also may be sulfates and clays in or near the black-circle site.

Clay mapping is a little more complicated, because you have to look for hydration, plus another signature in the spectrum. Here is a map showing hydration in blue, and little, barely visible specks of green and pink where clays are present.

newclays_example_small.gif

And here is a map of the sulfates in Meridiani:

blog_sulfateparam_min0_max05_ct13.gif

Ok, so I have shown off my pretty pictures. But what do they mean? Well, one thing to notice is that the hydration map shows that there are hydrated minerals over a wide region of Meridiani, but clays and sulfates are not so widespread. The clays are only found in tiny little outcrops, and the sulfates are mostly focused in one location, with a rather weak signature elsewhere. This implies that a lot of the hydration that we are seeing in Meridiani is due to something other than clays or sulfates. By looking at the spectrum, we can tell that it is probably some sort of iron oxide.

Also, take a look at this topographic map of the Meridiani region. Black and purple are low elevation, and red is high.

blog_meridiani_mola_grid_shaded.jpg

It is interesting to note that the place where the sulfates are concentrated is in a low valley. It shows up near the center of the picture in blue, with a sort of blue “finger” sticking off to the west, and is surrounded by higher terrain in green. Could this have been a lake long ago that dried up, leaving behind sulfate-rich salts? Maybe. Or maybe erosion has exposed sulfates that were created by hydrothermal activity in the area. Who knows? I don’t…yet.

I will leave with a final observation, which is what I am working on right now. The sulfates are in a valley, and the meager detections of clay minerals are in the surrounding, higher terrain. If we really are to believe that clays formed early in Martian history and sulfates formed later, it is strange to find sulfates below clays. Did something happen at Meridiani to make it an exception to an otherwise globally-obeyed rule? Or is the prediction that clays come first, followed by sulfates, and finally iron oxides a little too simple to explain what happened on Mars? I don’t know yet…but I’m hoping to find out.