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Space Topics: PhoenixThe Phoenix Landing SiteWhen Phoenix lands on Mars on May 25, 2008, it will land at a site above Mars' "Arctic Circle," in a place where the annual freezing and vaporizing of water and carbon dioxide ice and frost have heaved and sculpted the ground into polygonally cracked patterns. The Sun will circle overhead 24.7 hours per day, supplying the lander with round-the-clock power as it digs beneath the surface for underground ice. The lander is aiming toward an area that the mission informally refers to as "Green Valley" (because it satisfies the landing site constraints so perfectly in all respects that it appears wholly green on the team's color-coded hazard maps). Green Valley is located within a group of hills called Scandia Colles, which lie within the broad, flat northern plains named Vastitas Borealis, at approximately 68 degrees North, 234 degrees East. Phoenix could land anywhere within an ellipse that is about 100 kilometers (62 miles) long and 19 kilometers (12 miles) wide. Phoenix targeted this location with a rocket firing on April 10, 2008.
Images from the Landing Site AreaThe sharpest images of the landing site are from the HiRISE camera on Mars Reconnaissance Orbiter. Searching a box that ranges from 67.2 to 68.5 degrees north and 232 to 236.6 degrees east yields a list of images that cross the landing site region and reveal a wide variety of terrain that has been shaped by the seasonal motions of ground ice and water vapor. All the HiRISE images are oriented roughly north-south and are about 6.25 kilometers wide, so it would take about 16 individual, non-overlapping HiRISE swaths to cover the entire ellipse. Some representative images are: PSP_001893_2485 crosses
the western area of the ellipse (longitude 233.1) What the area looks like at HiRISE resolution depends in large part on the time of year. When Mars Reconnaissance Orbiter began its work at Mars in November, 2006, it was late summer in the northern hemisphere, and the orbiter had very little time to work to capture images of the landing area before polar night descended. Images of the landing region showed hummocky patterned ground:
During the winter, the region is covered to a depth of several tens of centimeters with a seasonal cap of carbon dioxide frost. When the Sun returned to the northern latitudes, it illuminated a landscape still dominated by frost, which fills the polygonal troughs in this image:
What are these polygons? Scientists agree that they form as a result of seasonal changes in temperature and humidity. However, the exact details are still poorly understood. On Earth, such polygonal terrain occurs as a result of actual melting of some of the water in the ground. The water runs through pre-existing cracks, and when it gets cold at night the water re-freezes. Because water expands when it cools, the new ice acts as a wedge, shoving the crack open. With each thaw and freeze cycle, the crack opens wider. On Mars, a drier process may be operating, in which dust acts as the wedging material. This is one question that Phoenix will seek to answer. The polygons seem to be slightly larger in scale than the Phoenix lander, but odds are good that Phoenix will land within arm's reach of a polygonal crack. Once Phoenix lands, HiRISE should be able to spot the lander:
Two other cameras on Mars Reconnaissance Orbiter provide views of the landing site at different resolutions. The Mars Color Imager (MARCI) is low-resolution but provides frequent global color views of Mars that permit monitoring of the weather; the Context Camera (CTX) takes medium-resolution views, intermediate between MARCI and HiRISE. The two images below were captured on April 20, 2008; the CTX image contains the first dust devils spotted at the Phoenix landing site this spring.
Selecting the SiteThe Phoenix team have had more data than any previous mission on which to base their choice of landing site. Their first priority is spacecraft safety, both for its landing and its operations. The landing site had to satisfy a number of constraints:
In addition, to get the most science out of the mission, they must land in an area where Mars Odyssey's Gamma Ray Spectrometer sees evidence for abundant hydrogen (meaning abundant water) just below the surface. These and other initial criteria resulted in the mission focusing their efforts on an area of Mars between 67 and 70 degrees north latitude and 126 to 135 degrees east. However, once Mars Reconnaissance Orbiter arrived at Mars in November 2006 and began taking photos with its HiRISE camera, it became clear that the ground in this region was covered everywhere with boulders that were roughly the same size as Phoenix -- a potentially fatal hazard. So the mission had to quickly shift its focus to other potential sites. They settled upon a new region centered at 68.35 degrees North, 233 degrees East during a landing site selection meeting held in Pasadena, California in late January, 2007. A final tweak to the selected site occurred after launch, when the team elected to shift the landing ellipse downrange by about 13 kilometers in order to avoid some boulder-rich terrain at the northwest reach of the landing ellipse. The map at the top of this page accounts for that 13-kilometer shift. (Last updated April 25, 2008) |
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