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“Where Am I?”—High-Resolution Digital Topographic Maps Help Curiosity Navigate Mars
Mars rover Curiosity, which landed on Mars on August 5, 2012, is busy exploring Gale crater near Mars’ equator for evidence that life did or could exist there. (See “USGS Scientists Exploring Mars…,” this issue.) A key tool for scientists directing the rover is a set of high-resolution digital topographic maps prepared by geophysicist Randy Kirk (http://astrogeology.usgs.gov/people/randolph-kirk) and his team at the U.S. Geological Survey (USGS) Astrogeology Science Center in Flagstaff, Arizona.
Their mapmaking began in 2007 in support of landing-site selection for the Mars Science Laboratory (MSL), the mission using Curiosity to explore Mars. The ideal site must not only contain features of scientific interest but must also have terrain in which the rover can safely land and drive. How rough is the surface? How steep are the slopes? Are there reasonable routes the rover can traverse to reach the scientific targets? Topographic maps, which show not just features’ positions but also their elevations, are needed to answer such questions.
Kirk’s team used the HiRISE (High Resolution Imaging Science Experiment) camera on the Mars Reconnaissance Orbiter to map the landing-site candidates. HiRISE can take stereopairs—two photographs of the same area from slightly different angles—whose combination produces a three-dimensional image from which elevation data for every pixel can be derived. The digital topographic maps produced by Kirk and his team assign an elevation to each pixel, which represents 1 square meter on Mars’ surface. These maps provide much more elevation data than the paper topographic maps familiar to hikers, on which elevations are shown by contour lines and must be interpolated for areas between the lines. To distinguish their digital maps from traditional topographic maps, Kirk and his team call their products digital topographic models, or DTMs.
The group has created a total of 13 DTMs of Gale crater with a grid spacing of 1 meter, which means that any object larger than about a meter is visible. Such detailed views result in huge datasets. By the time Gale crater was chosen as the final landing site on June 11, 2012, Kirk’s team had delivered an average of half a dozen DTMs for each of the four finalist sites—about as much information for each site as is contained in the entire global topographic map of Mars (which comes from the Mars Orbital Laser Altimeter on the Mars Global Surveyor spacecraft; see http://pubs.usgs.gov/imap/i2782/). Thanks to rapidly advancing technology, this information was also about a million times the amount of data Kirk’s team produced by nondigital stereo mapping of the 1997 Mars Pathfinder landing site, the team’s first landing-site assessment. After Gale was selected, the team more than doubled the number of DTMs for it, filling in gaps in the landing zone and mapping the rugged science study area where the rover will drive.
Kirk and his group have provided key input in selecting landing sites for every successful U.S. Mars landing since 1997—including Mars Pathfinder, the first U.S. mission to put a rover (Sojourner) on Mars; the Mars Exploration Rover mission, which landed the rovers Spirit and Opportunity on opposite sides of Mars in 2004; and the 2008 Phoenix Mars mission. Current members of the group, in addition to Kirk, are:
Additional contributions to the Mars Science Laboratory mapping came from Robin Fergason, research geophysicist at the USGS Astrogeology Science Center, who provided the team with thermal (infrared) imagery of the four finalist landing sites. These images—taken by the Thermal Emission Imaging System (THEMIS) on the Mars Odyssey spacecraft—provide information beyond that revealed by visible light that can be used to discriminate solid rocks from loose sediment. (Read more about THEMIS at http://themis.asu.edu/about.)Now that Curiosity has landed, the highly detailed topographic models created by Kirk and his team are helping the rover navigate the terrain in Gale crater. Follow Curiosity’s progress at http://www.nasa.gov/mission_pages/msl/ and http://marsprogram.jpl.nasa.gov/msl/.
in this issue:
Topographic Maps Help Curiosity Navigate Mars
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