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Planetary News: Phoenix (2007)A Green Valley for PhoenixBy Emily LakdawallaJanuary 25, 2007
PASADENA, CALIFORNIA - On January 22 and 23, the scientists on the Phoenix Landing Site Working Group gathered here to determine whether a safe spot can be found to set down the polar lander. The meeting had some urgency, because the first images from the sharp-eyed Mars Reconnaissance Orbiter of a previously favored landing region within the northern plains contained a nasty surprise: thousands upon thousands of lander-sized boulders perched atop the otherwise smooth plains. These boulders represent an unacceptable hazard to the lander, so a new site needed to be found, and quickly. "We wanted to prove that in the data we've collected from Mars
Reconnaissance Orbiter and Odyssey and Mars
Express and Mars Global
Surveyor, we have sites
that we consider safe and that we can pursue with diligence," says Phoenix
Principal Investigator Peter Smith. "And I think we accomplished
the goal very nicely. We've got three sites. One is clearly the
safest, but since all the data hasn't been analyzed or even received yet,
we're a little hesitant to say that's the site. So we are
going to continue to pursue all three sites." The Phoenix landing site must satisfy a number of constraints:
More than a year ago, the Phoenix team had narrowed their considerations to sites within three regions, named "A," "B," and "C," within the 65-to-72-degree annulus around the north pole that seemed likely to contain smooth, rock-free sites. As of last spring, the team had narrowed it down to Region B, which stretched from 67 to 70 degrees north latitude and 126 to 135 degrees east longitude. But HiRISE images within Region B all seemed to contain those dangerous boulders. The three new landing sites are within Region A and an additional one, called Region D, that the landing site working group had to add to the mix. The safest of the new sites, referred to as "Box 1," is located at 68.35 degrees north, 233 degrees east, within a shallow valley, says Smith. "It's a valley about 50 kilometers [30 miles] wide, but it's shallow, about 250 meters deep. Either it was filled in or it never got any deeper -- nobody knows. If you were in the middle of it you wouldn't see the edges; you wouldn't even know you were in a valley." One tool that the landing site working group used to determine the safety of the landing regions painted each region in green, yellow, or red depending on whether the site was safe, contained potential problems, or was too hazardous. And the Box 1 valley "is so green that it's been labeled 'Green Valley,'" Smith said. In going to Region D, Phoenix would, in a way, be returning to its beginnings, Smith explained. The goal of the Phoenix mission is to dig a trench into the polar soil and retrieve samples of soil and ice from below the surface. Scientists expect to find ice close to the surface because of measurements performed by Mars Odyssey's Gamma Ray Spectrometer, which indicate huge reservoirs of hydrogen-rich material located very close to the surface in the north polar regions. Smith explained that the location now known as Region D "was the site we actually proposed [Phoenix] to go to, because this was the spot that had the most ice" within the latitude band accessible to the Phoenix lander. "In fact, there's almost as much ice as the polar cap." The landing site working group, headed by Washington University geologist Ray Arvidson, had initially eliminated Region D from consideration in part because it is so ice-rich, Smith said. "It doesn't satisfy all of the scientists, and that's why we went to Region B, because we think the ice is so close to the surface that there may not be much soil. And soil is an important part of our mission too. Because how do you understand the history of ice if you just get pure ice? It's the soil that tells you the history. So it's a combination of soil and ice that's the important thing, and we didn't want to land in a spot that's just an ice cap." The landing site working group is examining all data available to determine whether the site will have enough soil to allow Phoenix to meet all its science goals.
Boulders and PolygonsThe landing site working group is making full use of data from four Mars orbiters to try to find a safe site for the lander. The trickiest requirement is the one for low rock abundance. Determining whether Martian surfaces are covered with potentially hazardous rocks has been a particularly knotty problem for every Mars lander, because until recently the only way to determine for certain whether an otherwise safe-looking surface was actually covered with rocks was to put a lander on it. Orbiting instruments could make well-educated guesses as to whether surfaces were generally rocky or generally smooth, but no camera could actually see hazardous boulders sitting on the ground until the arrival of Mars Reconnaissance Orbiter. Fortunately for Phoenix, Mars Reconnaissance Orbiter quickly revealed that Region B, previously considered the safest, was in fact covered with large boulders, a fact that was a huge surprise to the Phoenix team. "Where the heck do the boulders come from?" Smith asked. "If they came out of the craters, you'd think they'd be buried with the craters. But yet right in the middle of the crater, the boulders are on the surface. How do they get up there? They didn't roll down a hill! They didn't get flooded up there by rivers! It's interesting. Something is pushing them up. And you've got entire craters hundreds of meters across that are as flat as a pancake. The surface has been shaped and geologically reworked." A clue to how the surface has been so reworked lies in the repetitive, polygonal fracture patterns found everywhere in the northern plains in the Mars Reconnaissance Orbiter images. "The polygons are formed by one of two processes," Smith said. "On Earth, they're ice-wedge polygons. When it gets cold and the ice contracts, you get cracks, and then when it gets warmer, water runs down in the cracks before the cracks can expand and reseal. So they start warping the surface -- they can't expand back where they were, so they buckle. On Mars, it may be that when cracks form, dust goes down. And that would be a sand-wedge polygon or a dust-wedge polygon, and it's a very different process because you don't need liquid water. So determining whether we have ice-wedge polygons or sand-wedge polygons would be a really great thing." Unfortunately, it's impossible to aim Phoenix to land directly on one of these polygonal cracks. But Smith is sanguine. "We never put this down as a mission goal because we never thought we could land right on a crack. But they're only 10 meters across. And there are polygons within polygons within polygons. They may even go down to smaller scales in some areas. It's looking pretty good, because if you just throw darts at one of those landing site pictures and put a picture of our lander on it, it's hard to find a place where you don't have a reach to one of those cracks." While the Mars Reconnaissance Orbiter images acquired to date have been invaluable to the Phoenix team, their areal coverage is limited; each detailed image covers an area only 24 kilometers in width. And with the season advancing toward autumn in the northern hemisphere, solar illumination of the Phoenix landing site is getting worse and worse, and seasonal clouds are beginning to form. Smith says that the last Mars Reconnaissance Orbiter images in Phoenix landing site regions will be taken over the next two weeks, after which no new images will be acquired until late in the Martian spring, a few months before Phoenix lands. So the landing site working group must attempt to use data from other orbiters in combination with the limited high-resolution HiRISE images to determine what areas contain those dangerous boulders.
To attempt to understand whether this is actually a problem, Smith says, "We've compared as many rock counts [from HiRISE images] as we can with the actual THEMIS observations to give us some correlation and make sure our process really does work. It's not perfect. So in the [three selected] boxes we won't depend on THEMIS data anymore; we'll blanket it with the HiRISE data and actually count the rocks." In order to perform the comparisons between THEMIS models of rock abundances with actual data from HiRISE, it has been necessary to count more than 10,000 rocks in HiRISE images, a task that has fallen to a long-suffering undergraduate student working under Ray Arvidson. The working group is now developing a computer program that can automatically count rocks in HiRISE images, so that when HiRISE can start imaging the landing site regions again, Smith says, they will be able to count all the rocks in all the pictures. The landing site working group meeting finished with a series of assignments given to different members to develop maps of rock abundances, surface slopes, and other data relevant to landing safety, and to deliver those products to Smith by early March. The final site selection will take place shortly afterward. However, the spacecraft actually has sufficient propellant to "go anywhere on the planet" with a trajectory correction maneuver following the August launch, Smith says. "We have some flexibility. Depending on when we launch [within the launch window], the angle of our landing ellipse changes, so we could certainly say, for example, that we'd target Box 1 if we launched early in the window, and Box 2 would be better if we launched late." In addition, the mission could even move the target to respond to data acquired shortly before landing. "We don't get much new information until three or four months before landing, and so if we find something that's just scary as heck and we find another [safer] place, we can still move to it even a few weeks before." Never before has so much information been available to guide the selection of a landing site for a Mars mission. And yet, for all of those pictures and measurements of Mars' surface, what's going on beneath the surface remains very poorly understood. When Phoenix lands in May 2008 and digs a trench into the polar soil, it will gain a view of Mars never before achieved by any orbiter or lander. |
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