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Planetary News: Phoenix (2008)Martian Clumps Confound Team as Phoenix Does the TEGA ShakeBy A.J.S. Rayl
From the beginning, the Phoenix mission to Mars has been something of a dream. It took the framework of a cancelled spacecraft and rose a new mission from the "ashes." Once it got off the ground, the spacecraft garnered a reputation as one of the "best behaved" ever. It's retro-rocket landing May 25 looked like clockwork. Green Valley proved to be about as perfect a site as anyone could have imagined, and initial indicators are Phoenix came to rest on what it came to find, what Mars Odyssey revealed years ago was there -- ice. There have been a few minor glitches since Phoenix's picture perfect landing. Glitches, though, are expected with every planetary mission. All in all, everyone, from team members to media representatives and people in the street, seems to be excited about the prospects for the first mission to land in the northern arctic plains of the Red Planet. During the last several sols, however, Phoenix has dug up a Martian mystery that is hidden in the physical properties of its clumpy reddish-brown soil and that is presenting the team with its first signifcant scientific challenge. With so many missions at Mars now – three working orbiters, two rovers, and one lander – getting to our neighbor now some 170 million miles away and collecting scientific data may seem easy to the uninitiated. According to virtually every Earthling who's ever tried to reach out to the Red Planet, there's nothing easy about Mars and nothing on the harsh, desolately red planet is a matter of course. "Things never go perfectly as planned," said Doug Ming, Phoenix science team member, of NASA's Johnson Space Center, during a press conference earlier today. Last Thursday, June 5, Phoenix delivered a scoopful of clumpy Martian topsoil to the Thermal and Evolved Gas Analyer (TEGA), which was to be the first instrument to conduct analysis on the sample. The lander carried the delivery out with such finesse and precision, the achievement all but ended the checkout and characterization phase and initiated the mission's research agenda. All it had to do was dump it into the TEGA funnel on Friday, and it did that with swiftness and enough accuracy to place a pile of Martian topsoil on the instruments sifting screen. Then, Mars tossed one of its mysterious curve balls. "On Saturday we found out that we had a very large amount of soil that had been delivered to the screen, which is the opening into the funnel that leads the dirt into the oven itself," explained William Boynton, of the University of Arizona, TEGA lead scientist. Inside the oven, the sample's mineral make-up was to be sniffed and baked out. "Although we had an awful lot of dirt on that screen, virtually none of it made it down into the oven," he said. That is not what the science team was expecting.
"We're a little surprised at how much this material is clumping together when we dig into it," noted Ming. What exactly is binding these Martian clumps so tightly together is the mystery du jour. "We aren't sure what's causing this," said Boynton. It could be due to just a very tiny amount of absorbed moisture, perhaps even from the spacecraft's landing rockets, or the glue might be some kind of salts or even electrostatic forces or a combination of all of those, he said. The team is continuing to debate and discuss the possibilities. Phoenix fired up mechanical shakers inside TEGA and vibrated the sifting screen for 20 minutes, yesterday, on the mission's Sol 14. That successfully loosened a few particles from the tiny clods resting on the screen, but only a few. "We were expecting thousands to fall though," said Boynton. And the instrument needs thousands. So, what passed through the threshold was not enough to even come close to filling the tiny oven below; therefore, it didn't constitute enough of a sample to be analyzed. "We need somewhere on the order of 20 to 30 milligrams to get a good sample through," Boynton said. "When we just have one or two particles, we can't say with accuracy how much of a sample we got, but I would guess we have substantially less than 1 milligram." The TEGA team also tried a slightly different approach to lure Phoenix's prey, running the vibrator at a higher frequency. "We thought that might move the soil a little more effectively," Boynton explained. "We found out today that did not work. We had a very large particle pass thorough and into the oven, but it's not enough to do analysis. This soil is very cohesive and it's very hard for it to get through the sifting screen. We did get an indication that the vibrator is, in fact, working, however."
Science team members weren't really all that surprised by the clumpy topsoil Phoenix landed on, but they were surprised by its seemingly obstinate cohesiveness. "We actually knew from the Viking sites, as well as the[Mars Exploration Rovers] MER rovers, that the soil can tend to clump and clod," Boynton acknowledged. "We were thinking it would be easier to break up and that with the vibrator, which is pretty strong, it wouldn't have any problem moving the stuff through the screen." In order for the vibrator to do its job though, the soil actually has to move relative to the screen. "You have to see the little grains bouncing on the screen so they have a chance to bounce through the holes," Boynton elaborated. Those screen holes are just one millimeter wide, and now science team members are thinking there's just so much dirt on the screen it's weighing it down, he noted. As a result, they cannot get any relative motion with the soil particles. It's a heck of a dilemma. It means, essentially, that Phoenix did almost too good of a job in its dumping the Martian soil it scooped up. "Originally, we were concerned about getting enough soil into the oven and we didn't think about the riches of having too much," said Boynton. "We didn't think it would all come out [of the scoop] so rapidly. Now we realize we are much better off with small amounts of soil." This newfound realization about the clumpy, and indurated or hardened, soil properties, sent the TEGA team back into a huddle and today Phoenix was commanded to test a revised method for delivering samples from the stubborn clumps to its instruments. Basically, the new approach calls for the lander to sprinkle or "dribble," as Boynton puts it, the sample onto and into the instruments instead of dumping the whole scoopful. To accomplish that, Phoenix will hold its scoop at an angle above the delivery target and sprinkle out a small amount of the sample by vibrating the scoop, running a motorized rasp on the bottom of it.
"We're going to try and vibrate the motor at three different times of day at different temperatures, with the idea there may be different amounts of relative humidity, if moisture has anything to do with it," said Boynton. "If that doesn't work, it is likely we will use our new, revised delivery method on another thermal analyzer cell. We think it will be more effective if can transfer a small amount soil so it doesn't clog up the screen," he added by way of summary. "That's where we are now. It will be several more days before we know what path we're going to take." The next sample would most likely go to oven #5, right next to oven #4, he added. During the past weekend, Phoenix conducted its routine atmospheric measurements and weather monitoring, and built its photography portfolio, using its cameras to take pictures of nearby targets and the greater surroundings. These imageswill help expand the scientists’ knowledge base of the landing areaand the mission's work sites. Phoenix also stretched its "wing" on Saturday and used the robotic arm camera to snap pictures close to and under the lander that area unreachable by the larger Surface Stereo Imager (SSI), Ray Arvidson, of Washington University St. Louis, co-investigator for the robotic arm, said over the weekend. As part of its picture-taking assignments, Phoenix took more images of Snow Queen, the smooth patch of either ice or rock under the lander that was apparently exposed when the descent thrusters blew away the topsoil as the spacecraft touched down. “We are mapping with the robotic arm camera where the SSI can’t see to extend our knowledge of the site and to see details of the polygon structures of the near field, close to the lander,” Arvidson explained.
With the SSI, Phoenix continued to document the movements of the robotic arm, as well as take pictures of interesting targets in work area, materials on the spacecraft, and the Martian arctic landscape, Ming said. The lander snatched another sample from the Baby Bear site for the optical microscope yesterday, one of the three instruments that comprise the Microscopy, Electrochemistry, and Conductivity Analyzer (MECA). Today, Phoenix was to conduct a "pre-sprinkle test," Ming said, using a small amount of soil from that new baby Bear sample. "We'll try to get rid of some of larger clods, then move the robotic arm over MECA where we will conduct the sprinkle test and try to deliver a sample amount on top of MECA to show we can deliver to the optical microscope in two sols," said Ming. The science teams hopes to actually deliver a sprinkle sample to optical microscope in two sols or on Wednesday, said Ming. If that practice session goes well, the team assembled at the University of Arizona plans to sprinkle material from the same scoopful onto the optical microscope later this week, perhaps as early as Wednesday. The TEGA team, meanwhile, will continue to plan for its next delivery, still confidant their instrument will have its day it the Martian sol soon. "We are more optimistic about this new technique where we'll be dribbling a little bit of soil into the instrument and we're reasonably confident that's going to work," said Boynton. TEGA is designed to grind up the ice samples too "and before too long we expect to be down when the ice is," he added.
A decidedly Earthly spring that is clearly visible in one of the images of the landing pad was identified today as coming from the biobarrier that encased the robotic arm for its journey to Mars. Apparently during the unfurling process, the spring broke. If everything had gone perfect from landing to now, the Phoenix mission would be further along no doubt and the composition of the first sample would have been baked and sniffed by TEGA. But, in response to media inquiries about worry amongst the team members, the Phoenicians remain confidant and upbeat, enabling their dream to continue. "We're going down a decision tree and being methodical as we go," said Boynton of the work on getting a Martian topsoil sample into the TEGA oven. "We've got a long ways to go before we start worrying about worst-case scenarios." "Any day you can go to work on Mars," as Ming put it, "is a fantastic day."
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