Clumpy Soil
Published on Jun 10, 2008 at 1:45 pm.
4 Comments.
Filed under Mars.
The Phoenix lander is now well into its mission to study Mars. The first steps, of course, are to look around with the cameras. But, while the pretty pictures are what excite the public the most, scientists are much more excited about the other instruments on board the craft. These instruments will tell us the composition of the Martian soil. Despite many press reports that state Phoenix is looking for life on Mars, what it is really doing is looking for the chemistry necessary for life. If you don’t have the chemicals, then you don’t have life as we know it. And, if it is not life as we know it, is it really life? Well, that’s a question left to a future blog entry.
The next step, though, beyond looking around the surface is to reach out with the robotic arm and push stuff around. Phoenix did that last week. This gives us an idea of the consistency of the regolith (soil). Next, Phoenix scoops up some soil to study a bit more carefully. Then, what we’ve all been waiting for, Phoenix dumps a scoop of soil into one of the instruments to study. The instrument chosen is one that studies volatile materials in the soil. Volatile materials are those with low vaporization temperatures. So, the soil is dumped into a small oven that is then sealed. The oven heats up to about 1000° C. The volatiles are baked out of the soil and are studied using a mass spectrometer. That tells us the composition of the volatiles. There are several ovens on board, so soil samples from several locations can be studied. The ovens don’t have a provision to be emptied, so they are used one time. By the way, this sort of study of Martian soil dates all the way back to the Viking days.
A number of spacecraft have landed on Mars. The Vikings landed in 1976. Pathfinder landed in 1997. The Mars Exploration Rovers Spirit and Opportunity landed in early 2004. We’ve learned a bit about Martian soil from those missions, we thought. The Martian soil is loose, with some small rocks in it. There is no liquid water on Mars, and the soil is dessicated. On Earth, it is water in the soil that tends to make it clumpy and hold together. That isn’t the situation on Mars. But, it is the soil that needs to go into the oven, not small rocks. And, the looser the soil, the better it works. So, on top of the intake to the oven is a small screen. The space probe’s robotic arm dumps a load of soil onto the screen. The screen then shakes and the small grains fall inside, leaving the rocks and large clumps outside. At least, that is how it is supposed to work.
As you can see in this image, the soil is piled all over the intake. But, as things move, not much is sifting into the oven. A few days ago, I wrote about an image of the soil in the robotic arm’s scoop. I commented on the clumps in the soil at the time. But, the rather unexpected cohesiveness of the soil seems to be a problem. I didn’t realize that at the time, and apparently neither did the scientists on the Phoenix team.
You see, the soil is so clumpy that it is not sifting through the screen into the oven! If the soil can’t get into the oven, then it can’t be studied. Oops. We have not seen soil quite like this on Mars before. Close, perhaps, but not quite like this. So, this is a bit of a conundrum. Unfortunately, the screen does not appear to be retractable. Apparently, no one thought that such an option would be needed.
All is not lost, though. There are other ideas. Perhaps the scoop can be used to stir soil and crush the clumps before dumping onto the other intakes. Another option might be to sprinkle smaller amounts of material to try to get a handle on the situation. A far more drastic option might be to top the pile of material sitting on the screen with the arm, if it will reach (not recommended due to possible damage to either the arm or the craft itself). And, I am sure that they are thinking of all sorts of other options. Phoenix will be there for some months to come, so there is no real hurry to get everything done fast. It is best to take time, analyze the situation, and work out a solution. Then, of course, the solution can be tested out here on Earth with a mock-up before trying it for real on Mars.
Fortunately, this is the first real problem that Phoenix seems to have encountered. Sure, there have been a number of smaller problems, but you expect things to go wrong with any mission. The trick is to get past the problems and continue with a successful mission.
-Astroprof
Image courtesy NASA, JPL, UA
Ed Minchau on June 10, 2008 at 2:59 pm: 1
I keep seeing the contention that there is no moisture in the soil, and yet all the evidence points to exactly that. Hydrophilic salts abound in the soil (I created a Martian soil simulant for an elementary school based on the percentages of various salts determined by Opportunity and Spirit, with the remainder made of baked red Georgia sand. Within 24 hours it had absorbed considerable moisture from the atmosphere, enough to make it clumpy). Viking determined that the relative humidity in the atmosphere was 100%. Opportunity got stuck in Purgatory dune, and the treads were completely packed with clumpy soil (unlike the dry soil that they used in the duplicate robot on earth to try getting it unstuck; on that one the soil sloughed off easily). And finally, let’s bear in mind that Phoenix’s landing site was specifically chosen because of the presence of water as determined by the US Geological survey neutron map.
Astroprof on June 10, 2008 at 3:17 pm: 2
Ed, you make a very good point. This site was selected specifically because of the high probability of finding water near the surface. However, that was supposed to be ice, not liquid water. And, I don’t think anyone questions that there once was liquid water there, so the salts in the soil could be what it making it so clumpy. That would be my guess, but I am not a soil scientist. As for Viking measuring 100% relative humidity, remember the temperature and pressure there. Under those conditions, 100% humidity isn’t very much water content. But, as you also point out, Opportunity found clumpy soil. And, of course, we’ve only seen the soil in a very few select places on the planet, so we do not have a fair sample. This is clearly telling us that we do not yet understand Mars. We still have a lot to learn. Before it is all over, we might have to totally rethink what we’ve been thinking about conditions there.
Travis Hunley on June 11, 2008 at 11:08 pm: 3
I don’t suppose the landing thrusters could have produced surface moisture (from substrata ice?) that could have contributed to the observed soil cohesion??
CCPhysicist on June 13, 2008 at 9:16 pm: 4
Imagery shown today indicates that a seventh “shake” of TEGA appears to have gotten some soil into it.
http://www.jpl.nasa.gov/news/phoenix/images-all.php?fileID=13840
They also show the result of a “sprinkle ” test on June 9 that has now been used to deliver a suitable sample to the microscope and will also be used to deliver another sample to TEGA. Clever solution.