AGU: Wrapping up my notes on Phoenix
Posted by Emily Lakdawalla
2008/12/19 02:08 CST
The Phoenix presentations at AGU opened with Peter Smith giving a sort of conference keynote speech about the mission, for which he was allotted a very generous hour. (Most of the talks at AGU are given much much shorter time slots, 15 or even only 12 minutes, of which some part has to be left for questions.) Most of Peter's talk was of an overview nature. One of the points that he and other Phoenix scientists made repeatedly is that while the previous Mars landers have gone to Mars with the purpose of studying ancient environments, as preserved in rocks, Phoenix went to Mars to study a modern environment. What they mean by that is they wanted to study the environment as it exists near the poles today, to answer questions about the present behavior of water on Mars. It's a point that they've made before, but it hadn't crystallized in my head until this meeting.
Peter covered many of the other presenters' work in his overview presentation, so I'll go on to the rest of my notes and wrap up the post with any loose ends from Peter's talk. He was followed by Ray Arvidson, who talked about the operational details of where and why they dug and sampled, and Mark Lemmon, who talked about the use of the cameras to document all of this activity. One of the interesting comments in Mark's presentation was the observation that of the rocks present at the Phoenix landing site, not many "ventifact" features were present. "Ventifact" describes a rock whose surface has been shaped over many, many years by the sandblasting action of wind. Many of the rocks near where Spirit landed in the basalt plains of Gusev are ventifacts; they occur in an environment where little disturbance has happened for a very long time. The lack of ventifacts at Phoenix' landing site means that the rocks are relatively recently exposed; Mark said that it was a "cryoturbation" dominated terrain, which means that seasonal effects on ice in the ground mobilize the rocks and soil, turning them over with time.
Next up was Bill Boynton, whose talk on the Thermal and Evolved Gas Analyzer I covered on Tuesday. He was followed by Mike Hecht, who spoke about the Wet Chemistry Laboratory (WCL) results, particularly the discovery of perchlorate. He showed some graphs illustrating the relative amounts of different ions detected in the WCL beakers. The most numerous ions in solution were doubly-charged magnesium cations, and the perchlorate anions, which have a single negative charge; there were roughly equal amounts of the two detected in the soil. So it seems that the perchlorate that they detected in the soil was most likely magnesium perchlorate, but that leaves some excess magnesium to account for, Hecht said (since, to balance the charges, there'd be two perchlorate anions bonded to each magnesium cation). In addition to the magnesium, they also positively detected sodium, potassium, and calcium. Measurements that he said were "still murky" were the quantity of soluble sulfate ion, whether there were any gradients in the soil composition from surface to the surface of the ice table a few centimeters below, and the question of how the cations they detected were associated with anions to make chemicals.
In summary, Hecht said, "We landed expecting to find lots of sulfates, and maybe halite. Instead, we found calcite-buffered solutions, like most water on Earth; we found perchlorate-laced soil; and we're still hunting for sulfates." He said that there's a long list of research disciplines affected by this unexpected soil composition; topping the list is astrobiology. As I mentioned in my previous Phoenix post, they now think that the abundant perchlorate in the soil may have oxidized any organics present before they could be detected by TEGA. "This calls into question any thermal method used to detect organics," he said; and he added that we may even need to go back and look at the Viking results again and think about how the presence of perchlorate could have affected them. The high concentration of perchlorate in the Phoenix soil is hard to account for; but Hecht pointed out that it's very soluble, so that if there's any liquid water present in the soil, the perchlorate would quickly go into solution, and then get left behind if the water disappeared either by freezing or evaporating.The next presentation was the one by Sam Kounaves that I summarized on Tuesday. After him came Uwe Keller, who talked about the strange behavior of the soil that Phoenix was trying to work with. As I am sure you all recall, the soil was incredibly clumpy and sticky. Of course, Keller's a scientist, and a German one at that, so instead of "clumpy and sticky" he said "cohesive and adhesive." But, he said, "the soil changes once it is isolated from the ground." When they got samples into the scoop, sometimes they'd upend the scoop only to find that the soil would cling to the walls. But the next morning, the scientists would find that the sample had fallen out. Keller said that this meant the soil was cemented together by a "volatile agent." He hypothesized that it was dessication -- drying out, losing water molecules -- that led to the soil's slow loss of cohesion after it was dug out of the ground. "I think it's very likely that water adsorption plays a role in this process," he said. Adsorption is not a phenomenon I learned much about in high school chemistry; all I knew is that it had something to do with water molecules sticking to mineral grains.
Fortunately, at the press briefing on Phoenix held earlier in the day, the panel (which consisted of Ray Arvidson, Aaron Zent, and Peter Smith), they talked about adsorption at length. Zent said that what exactly adsorption is and what it does to the behavior of water and other chemicals in the soil is "a matter of confusion even among the science team." He tried to explain how adsorbed water was different from the three "classical" physical states. "Water interacts strongly with surfaces of mineral grains. It is not gas, it is not liquid, it is not ice, it's a different phase....In a gas, water molecules are floating around. In ice they are locked in a rigid structure in which they do not move. In a liquid they form [short-lived] bonds and break up." But when water molecules are adsorbed onto the surface of a mineral grain, "the water molecules are actually influenced by the presence of the minerals that they are stuck on. It changes their energy; it changes the way they can rotate; it changes their mobility; it changes the way they can dissolve things. But they are water molecules, and they do have some mobility. At temperatures below the freezing point [of water] you can move things around."
Zent continued, "the higher the relative humidity is, the thicker the films [of adsorbed water] get. The thicker they get, the more liquid-like they become, because each layer of water molecules is less and less influenced by the mineral grain below. You don't have to have conditions that are suitable for liquid water to get processes that these films can assist with," like chemical reactions.
Peter Smith went on to explain what can raise the relative humidity to levels where adsorbed water starts doing interesting things, chemically speaking. As I've mentioned before, the tilt of Mars' rotation axis, which is referred to as Mars' obliquity, can vary a lot over a period of many thousands of years. Today, it's about 25 degrees, similar to Earth's tilt, but Mars' obliquity swings from 10 to 40 degrees and sometimes even more.
When the obliquity is very high, summers are long and intense, as the pole is pointed much farther toward the Sun at high summer than it is today. "Once the obliquity gets beyond 35 degrees, the polar cap becomes unstable. It releases water as fast as the atmosphere can accept it. Conditions get very, very humid, and water gets transported past our landing site to the equator. This changes climate dramatically."
With all the extra water in the air, the films of water adsorbed on the soil grains get much thicker, and even though the temperature may still not rise above the freezing point of water, you can get some chemistry happening in the soil that you would ordinarily think of as happening only in the presence of liquid water. "There's no clear evidence that at high obliquity you get liquid water," Zent said. "But you don't really need it to do a lot of things you associate with liquid water, like chemical processing. And there are microbes [on Earth] that live quite happily in that, because they can get nutrients and make waste and do other things in these thin films of water. So this is kind of a key way in which we think that the results from Phoenix might point towards a little bit more activity in the past 100,000 years or so than we see right now."
All of this was parenthetical to Uwe Keller's presentation on the properties of the soil. He (and several other presenters) mentioned that the soil was made of two very different components. There's a very fine component that is orange-ish, what Mark Lemmon called "Mars-colored." It is so fine that neither the high-resolution close-up mode of the robotic arm camera nor the optical microscope could resolve individual soil particles; only the Atomic Force Microscope (AFM) could actually see the soil particles. They made up about 80% of the soil by volume, and probably represent airborne dust, the sort that's blown all over Mars. The other component of the soil were rounded grains 50 to 200 microns in size that were strongly attracted to the optical microscope's magnets. That size range, 50 to 200 microns, is on the small end of the scale of what an Earth geologist would call "sand-sized grains," as opposed to "silt," which is made of smaller particles. These grains would be moved around by saltation, a process where wind pushes grains and occasionally lifts them into the air; when they fall to the ground they knock other grains into the air. Saltation is the process by which wind moves sand, building sand dunes and sandblasting landscapes.
Ray Arvidson also spoke about the strange properties of the soil that Phoenix sampled. "We think the cohesion [the clumpiness] is at least an order of magnitude higher than anything seen on the Moon. This is not simple sand and dust. The material has been processed and made coherent, or cohesive. Yet when we look at the material in the optical microscope, we see a bimodal size distribution -- very fine dust, airfall stuff, Mars dust. The things concentrated on the magnets [in the Optical Microscope] are rogue sand grains, bounced along in saltation mode. So the bulk of soil that we are looking at has come in from sky, but it has been processed, put into a coherent character." The processing has changed it, made it behave differently from the dust observed by other landers like Spirit and Opportunity, Ray said: "It is not simply windblown sand and dust that is the same as the loose material seen elsewhere on the planet."
So it sounds like they are beginning to develop some explanations that make some sense of the Phoenix results. Ray summarized, "We have a phenomenal data set about the modern water environment, and we have ice, and we have soil that doesn't look like any soil from elsewhere on Mars. I have a sense that the water molecules moving up and down through [the soil] have really processed it. But there's so much information that came in in the 152 sols of the mission, and we don't understand it yet. Critical will be TEGA and wet chemistry, which are both multivarite experiments, and lab work is critical to that."
Peter said that's what's next for the Phoenix team. "Near future plans include lab testing of candidate species" that might explain the ambiguous TEGA results. "The low-temperature releases of carbon dioxide and water are not yet constrained; there is a long list of possibilities, and perchlorate in soil is oxidzing" some of the chemicals that were present. "There is potential for other carbonates, and aqueous minerals; even oxidized organics are possiblities." For the WCL results, "the complex chemistry needs lab support to understand the interactions between sensors and solution. Models to explain ice-atmosphere interactions are just being started."
The Phoenix team doesn't have much time to come up with explanations before other scientists will get a chance to start work: the Phoenix data archiving schedule is a brisk one. The first release of data to NASA's Planetary Data System, covering the first 30 sols, will be on December 24.
And what's next for Phoenix itself? Peter, who has had fairy tales on his mind since Phoenix was first conceived, said that the Phoenix lander "entered Sleeping Beauty mode on November 2." What's going to happen when the Martian spring comes? "It's a question of whether her prince will come," he said. It's a lovely fairy tale, but like most such, it's probably not going to turn out to be a true story; once asleep, Phoenix is unlikely ever to wake up.
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