4 July 1998
Special Report: A Year After Pathfinder
A year after the landing of Mars Pathfinder, mission scientists say that data from the spacecraft paint two strikingly different pictures of the role of water on the red planet, and yield surprising conclusions about the composition of rocks at the landing site.
"Many of the things that we said last summer during the excitement after the landing have held up well," said Dr Matthew Golombek, Pathfinder project scientist at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "But we have now had more time to study the data and are coming up with some new conclusions."
Similar to ongoing science results from NASA's Mars Global Surveyor spacecraft currently in orbit around Mars, Pathfinder data suggest that the planet may have been awash in water three billion to 4.5 billion years ago. The immediate vicinity of the Pathfinder landing site, however, appears to have been dry and unchanged for the past two billion years.
Several clues from Pathfinder data point to a wet and warm early history on Mars, according to Golombek. Magnetized dust particles and the possible presence of rocks that are conglomerates of smaller rocks, pebbles and soil suggest copious water in the distant past. In addition, the bulk of the landing site appears to have been deposited by large volumes of water, and the hills on the horizon known as Twin Peaks appear to be streamlined islands shaped by water.
But Pathfinder images also suggest that the landing site is essentially unchanged since catastrophic flooding sent rocks tumbling across the plain two billion years ago. "Since then this locale has been dry and static," he said.
While the area appears to have been untouched by water for aeons, wind appears to have been steadily eroding rocks at the landing site. Analysis of Pathfinder images shows that about one to two inches (three to five centimetres) of material has been stripped away from the surface by wind, Golombek noted.
"Overall, this site has experienced a net erosion in recent times," said Golombek. "There are other places on Mars that are net 'sinks,' or places where dust ends up being deposited. Amazonis Planitia, for example, probably has about three to six feet (one to two meters) of fine, powdery dust that you would sink into if you stepped on it."
Chemical analysis of a number of rocks by the alpha proton X-ray spectrometer (APXS) instrument on Pathfinder's mobile Sojourner rover, meanwhile, reveals an unexpected composition that scientists are still trying to explain.
The current assessment of data from this instrument suggests that all of the rocks studied by the rover resemble a type of volcanic rock with a high silicon content known on Earth as andesite, covered with a fine layer of dust. All of the rocks appear to be chemically far different from meteorites discovered on Earth that are believed to have come from Mars.
"The APXS tells us that all of these rocks are the same thing with different amounts of dust on them," said Golombek. "But images suggest that there are different types of rocks. We don't yet know how to reconcile this."
When molten magma oozes up from a planet's mantle onto the surface of the outer crust, it usually freezes into igneous rock of a type that geologists call a basalt. This is typical on the floors of Earth's oceans, as well as on the maria or "seas" of the Moon and in many regions of Mercury and Venus. By contrast, andesites typically form on Earth in tectonically active regions when magma rises into pockets within the crust, where some of its iron and magnesium-rich components are removed, leaving rock with a higher silicon content. "We don't believe that Mars has had plate tectonics, so these andesites must have formed by a different mechanism," Golombek said.
The rocks studied by Pathfinder most closely resemble andesites found in Iceland and the Galapagos Islands, tectonic spreading centres where plates are being pushed apart, said Dr Joy Crisp of JPL. Andesites from these areas have a different chemical signature from andesites formed at subduction zones (areas where one edge of a crustal plate descends below another), mostly because wet ocean sediments carry more water down into the mantle at the subduction zones. "On Mars, where the water content is probably lower and there is no evidence of subduction, we would expect a closer chemical similarity to Iceland andesites," said Crisp.
The Martian rocks may have other origins, however. They could be sedimentary and influenced by water processes; they could be formed by melting processes resulting from a meteor impact; or, a third alternative is that the rocks might be basaltic, but covered by a silicon-rich weathering coating. "In any event, the presence of andesites on Mars is a surprise, if it is borne out as we study the data further," said Crisp. "Most rocks on Mars are expected to be basalts lower in silicon. If these are in fact andesites, they are probably not very abundant."
Scientists are looking forward to more data from the Thermal Emission Spectrometer instrument on the Mars Global Surveyor to reveal more about the chemical composition of the planet's surface, especially once the orbiting spacecraft begins its prime circular mapping mission in spring 1999.
In other recent Pathfinder science findings, Dr Steven Metzger of the University of Nevada found direct evidence of gusting winds called "dust devils" in images from Pathfinder's lander. Such dust devils had been seen in some Viking orbiter images and inferred from measurements of atmospheric pressure and winds by other instruments on the Pathfinder lander, but were not spotted in actual surface images until Metzger's discovery.
JPL planetary scientist Dr Diana Blaney has been using data from Pathfinder, other spacecraft missions and ground-based observations to study weathering on Mars. Her work suggests that Mars is uniformly covered by a fine coating of dust formed by an unusual process involving meteor impacts and volcanic gases that add sulfur.
NASA's next Mars missions, the 1998 Mars Climate Orbiter and Mars Polar Lander, are in testing now for launch in December and January, respectively. Whereas Pathfinder's science focus was on exploring rocks with its mobile robotic geologist, the Mars Polar Lander will focus on a search for water under the planet's surface, equipped with a robot arm that will dig into the soil at the landing site near the planet's south pole.
Launched on December 4, 1996, Pathfinder reached Mars on July 4, 1997, directly entering the planet's atmosphere and bouncing on inflated airbags as a technology demonstration of a new way to deliver a lander and rover to Mars. The lander operated nearly three times its design lifetime of 30 days, while the rover operated 12 times its design lifetime of seven days.
During the mission, the spacecraft relayed 2.3 gigabits of data to Earth. This unexpectedly large volume of information included 16500 images from the lander's camera, 550 images from the rover camera, 16 chemical analyses of rocks and soil, and 8.5 million measurements of atmospheric pressure, temperature and wind.
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Picture courtesy of NASA/JPL