Lander targets Mars' water
BY STEPHEN CLARK
Posted: August 5, 2007
When NASA's Phoenix lander arrives at Mars next May, it will join a flotilla of spacecraft already at the red planet working as a team of robotic scientists to probe the existence of water.
Two NASA orbiters and a European craft are currently circling Mars. NASA's two Mars rovers are also operating in their fourth year since landing in January 2004. All of the missions are working to map the planet and assist in the search for past or present water.
"The Mars program is constructed of interrelated orbiters and landers," said Doug McCuistion, NASA's Mars exploration program director. "We do reconnaissance from orbit, followed up with ground truth and greater understanding from systems actually on the surface."
Phoenix blasted off from the Cape Canaveral Air Force Station at 5:26 a.m. EDT Saturday, but it will take 10 months for the craft to traverse the solar system to reach Mars. The 772-pound lander will streak into the Martian atmosphere and fire braking rockets to gently touch down on a sweeping plain near the planet's north pole. Landing is scheduled for May 25, 2008.
The landing site was chosen because it is believed to harbor water ice near the surface, and an eight-foot robot arm will dig up to 20 inches into the soil to snag ice deposits. The soil will be put into a miniature laboratory inside Phoenix to analyze its composition.
NASA's Mars Odyssey and Mars Reconnaissance Orbiter probes played a key role in selecting the Phoenix landing site. Instruments on those spacecraft provided high resolution imagery and estimates of ice depth for potential Phoenix touchdown points.
"I am a very strong proponent of using orbiters to find the most interesting places to send the rovers and landers," said Phil Christensen, lead scientist for the Thermal Emission Imaging System instrument on Mars Odyssey.
The Odyssey sensor, called THEMIS, can detect changes in surface temperature in the infrared wavelength. Scientists can draw conclusions from the temperature changes to determine the presence of ice and large rocks.
"Surfaces with deep ice cool off through the late summer and fall differently than those that have a buried ice layer," Christensen said.
The Gamma Ray Spectrometer, another instrument aboard Mars Odyssey, has also detected plentiful water ice in the subsurface of the planet's polar regions.
"From an historical point of view, what Phoenix does is it's the first real response to a significant to a significant scientific discovery. Odyssey found the water and said there is a significant amount of water right there," said Barry Goldstein, the Phoenix project manager at NASA's Jet Propulsion Laboratory.
Powerful radars on Mars Reconnaissance Orbiter and Europe's Mars Express have also found reservoirs of frozen water deeper underground.
The initial survey of the Martian polar regions for Phoenix yielded a multitude of possible landing sites. Scientists thought they had chosen a front-runner in mid-2006, but detailed images from the newly arrived Mars Reconnaissance Orbiter showed the area was littered with boulders that could pose serious danger to Phoenix.
The High Resolution Imaging Science Experiment, or HiRISE, is the most capable camera ever deployed into the solar system. The instrument can resolve objects as small as one meter.
HiRISE arrived aboard the Mars Reconnaissance Orbiter in March 2006, and imagery from the camera caused scientists to consider alternate landing sites.
"HiRISE images were used to certify the initial landing site, which was rejected based on the boulder abundances," said Alfred McEwen, the instrument's principal investigator.
Phoenix must land at a site with no rocks larger than 14 to 18 inches, according to mission officials.
When HiRISE determined the primary landing site was unsafe, THEMIS data was used to search for other favorable regions. The instrument can detect large rocks during nighttime passes because they retain heat, which increases temperature returns in the infrared wavelength, Christensen said.
"The advantage of THEMIS was that we had complete coverage of the entire northern plains, whereas HiRISE could only image a very small fraction of the plains. What we did was to find the coldest regions in the THEMIS temperature maps, and then verify that these indeed have very few rocks using HiRISE," Christensen said.
After extensive analysis, the Phoenix science team selected a new landing site with fewer hazardous rocks. The preferred region lies in a large shallow valley 30 miles wide and about 800 feet deep.
The location lies at about the same latitude as central Greenland or northern Alaska on Earth, according to scientists.
Data from THEMIS also confirmed the new region has water ice within two to six inches of the surface.
"All areas at these high latitudes probably have shallow ice, so the main consideration was landing safety," McEwen said.
Mars orbiters will continue to guide scientists to interesting and safe landing sites in the future. Mars Odyssey instruments helped the selection process for NASA's twin rovers in 2003, and data from Mars Reconnaissance Orbiter will be factored into choosing a touchdown point for the agency's Mars Science Laboratory, a rover the size of a small car that will arrive at the red planet in 2010.
"These examples demonstrate the incredible utility for having ‘eyes' in orbit to find landing sites," Christensen said.
Phoenix is designed to complete a mission lasting about 90 Martian days. It will land in the spring season on Mars, when the ice caps have retreated to regions further north. But scientists expect the carbon dioxide ice to eventually encroach on Phoenix as winter approaches, likely encasing the lander in a debilitating deep freeze.
Spirit and Opportunity, the two NASA rovers that found proof of vast amounts of water on ancient Mars, are still operating more than three years after reaching their designed lifetimes.
"Our experience has been that luck is a good thing to have when you're exploring Mars," said Steve Squyres, chief scientist for the rovers.
"One thing that three-and-a-half years on Mars has taught me is humility. I wouldn't even begin to guess what Phoenix is going to experience."
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