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Posted: August 26, 2008 Today, the 90th Martian sol and the end of the originally planned primary mission, Phoenix will perform the challenging task of scooping up soil from the deepest trench ever dug on Mars.
The deep ‘Stone Soup’ trench marks the border between two polygon-shaped hummocks that characterise the Martian plains where Phoenix landed. The trench is 25 centimetres wide and 18 centimetres deep. Image: NASA/JPL-Caltech/University of Arizona/Texas A&M University. The ‘Stone Soup’ trench is 18 centimetres deep and lies in a natural trough between two polygon shaped hummocks that characterise the arctic plains on Mars as well as in permafrost areas on Earth, where the ground goes through cycles of swelling and shrinking. In previous digging endeavours, Phoenix has struck ice as hard as concrete at a depth of about five centimetres, but so far, no such layer has been encountered in ‘Stone Soup’. "The trough between polygons is sort of a trap where things can accumulate," says Michael Hecht, lead scientist of the Microscopy, Electrochemistry and Conductivity Analyser (MECA) instrument. "Over a long timescale, there may even be circulation of material sinking at the margins and rising at the centre." The science team had considered two different sites as sources for the next sample to be delivered to the wet chemistry lab, but Stone Soup won out. "We had a shootout between Stone Soup and white stuff in a trench called 'Upper Cupboard,'" says Hecht. "If we had been able to confirm that the white material was a salt-rich deposit, we would have analysed that, but we were unable to confirm that with various methods." Both candidates for the sampling location offered a chance to gain more information about salt distribution in the Phoenix work area, which could be an indicator of whether or not liquid water has been present, since salts tend to concentrate in places that have been wet.
The phoenix workspace, informally named ‘Wonderland’, is located on the north side of the lander. Click for higher resolution. Image: NASA/JPL-Caltech/University of Arizona/Texas A&M University. While heading towards the goal of delivering a sample from ‘Stone Soup’ into the wet chemistry laboratory, Phoenix is also using its Thermal and Evolved-Gas Analyser (TEGA) to examine a soil sample nicknamed ‘Burning Coals’ collected last week from another trench – ‘Burn Alive 3’ – at a depth intermediate between the surface and the hard icy layer. Phoenix excavated down to the ice layer at a depth of about four centimetres on one side of the trench, leaving about one centimetre of soil above the ice on the other side, thus giving the science team a vertical profile of the soil that also includes samples of a surface material from the Rosy Red trench and ice-layer material from the Snow White trench. "We want to know the structure and composition of the soil at the surface, at the ice and in between to help answer questions about the movement of water, either as vapour or liquid, between the icy layer and the surface," says Ray Arvidson, a leader of Phoenix science team activities. Once the one-quarter to one-half teaspoon-sized sample of Martian soil enters TEGA it will first be heated to 35 degrees Celsius to look for ice, and then to 125 degrees Celsius to ensure that the sample is dry. The last heating cycle reaches 1,000 degrees Celsius where gases emitted helps the science team to determine the specific properties of the Martian soil.
A thin layer of water frost is visible on the ground near the Phoenix lander, and disappears shortly after sunrise. The Sun was about 22 degrees above the horizon when the image was taken, enhancing the detail of the polygons, troughs and rocks around the landing site. Image: NASA/JPL-Caltech/University of Arizona/Texas A&M University. "We are expecting the sample to look similar to previous samples," says William Boynton, lead scientist for TEGA. "One of the things we'll be looking for now is an oxygen release indicative of perchlorate." Perchlorate was found in a sample delivered to MECA in late June from the Dodo-Goldilocks trench and again in another sample taken from the Snow White trench on July 6. Seeing signs of perchlorate in TEGA would help confirm the previous results. "We expect to use the robotic arm heavily over the next several weeks, delivering samples to our instruments and examining trench floors and walls to continue to search for evidence of lateral and vertical variations in soil and ice structures," says Arvidson. The extended Phoenix mission phase will continue until late September.
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