For the first time in Mars exploration, a spacecraft has captured the full evolution of a Martian dust storm. NASA's Mars Global Surveyor mission also has returned new insights into the deeply layered terrain and mineral composition of the Martian surface, and to highly magnetized crustal features that provide important clues about the planet's interior.
These findings are among the early results from the Mars- orbiting mission being reported in today's issue of Science magazine.
This first set of formal results comes from data obtained in October and November 1997, while the spacecraft was just beginning to use the drag of Mars' upper atmosphere to lower and circularize its orbit in a process called aerobraking. At the time, a dust storm was brewing on Mars and had grown to about the size of the South Atlantic Ocean.
The Global Surveyor data suggest that it began as a set of small dust storms along the edge of the planet's southern polar cap, according to Dr. Arden Albee of the California Institute of Technology, Pasadena, CA, the Mars Global Surveyor mission scientist. By Thanksgiving, it had expanded into a large regional dust storm in Noachis Terra that covered almost 180 degrees longitude, while spanning 20 degrees south latitude to nearly the tip of the Martian equator.
"As this storm obscured the Martian landscape, we followed it in detail using several instruments onboard Mars Global Surveyor," Albee said. "The thermal emission spectrometer mapped the temperature and opacity of the atmosphere while the camera followed the visual effects. The effects of the storm extended to great heights of about 130 kilometers (80 miles) and resulted in great increases in both atmospheric density and variability from orbit to orbit. These atmospheric measurements have great significance for future Mars missions that will be using aerobraking techniques too."
Before the storm, atmospheric dust was generally distributed very uniformly, Albee said. Observations of the limb of the planet in the northern hemisphere revealed both low-lying dust hazes and detached water-ice clouds at altitudes of up to 55 kilometers (34 miles). Movement of these clouds was tracked by the spectrometer as the planet rotated. Atmospheric turbulence disrupted these cloud patterns as the small storms began to rise and kick more dust into the air. As the storm began to abate, small local storms began to crop up again along the edges of the south polar cap, and ice clouds formed in depressions as the carbon dioxide cap continued to retreat.
In addition to these unprecedented observations of a full- blown Martian dust storm, measurements from the spacecraft's magnetometer and electron reflectometer have yielded new findings about Mars' strong, localized magnetic fields. These patches of the crust, which register high levels of magnetism, are beginning to unlock some of the mysteries surrounding Mars' internal dynamo and when it died, said Dr. Mario Acuna of NASA's Goddard Space Flight Center, Greenbelt, MD.
"These locally magnetized areas on Mars could not form without the presence of an overall global magnetic field that was perhaps as strong as Earth's is today," says Acuna. "Since the internal dynamo that powered the global field is extinct, these local magnetic fields act as fossils, preserving a record of the geologic history and thermal evolution of Mars."
Magnetic fields are created by the movement of electrically conducting fluids, and a planet can generate a global magnetic field if its interior consists of molten metal hot enough to undergo convective motion, similar to the churning motion seen in boiling water.
"The small size and highly magnetic nature of these crustal features, which measure on the order of 50 kilometers (30 miles), are found within the ancient cratered terrain rather than within the younger volcanic terrain," Acuna said. "By correlating crustal age with magnetization, we have a perfect window on Mars' past, which will help us to determine when Mars' internal dynamo ceased operating."
High-resolution images of dunes, sandsheets and drifts also are helping reveal earlier chapters of Martian history. Landforms shaped by erosion are almost everywhere, according to Albee, and many bear a striking resemblance to Colorado's Rocky Mountains. Rocky ridges poke through the Martian dust just as the jagged edges of cliffs pierce through a blanket of snow in the Rockies. Martian dust appears to have spilled down the sides of ridges just as fresh snow slides down a ski slope.
"One almost expects to see ski tracks crisscrossing the area," Albee added. "These images present a sharp contrast to the images of boulder-strewn deserts found at the Viking and Pathfinder landing sites."
Newly released images from the Mars Global Surveyor camera, developed by principal investigator Dr. Michael Malin of Malin Space Science Systems, Inc., San Diego, can be viewed on the Internet at:
The Martian crust also exhibits much more layering at great depth than was expected. The steep walls of canyons, valleys and craters show the Martian crust to be stratified at scales of a few tens of yards, which is an exciting discovery, Albee noted. "At this point we simply do not know whether these layers represent piles of volcanic flows or sedimentary rocks that might have formed in a standing body of water," he said.
The thermal emission spectrometer, led by principal investigator Dr. Philip Christensen of Arizona State University, is beginning to obtain a few infrared emission spectra of the surface, although it is still too cold on the surface for the best results. The best spectra clearly indicate the presence of pyroxene and plagioclase, minerals which are common in volcanic rocks, with a variable amount of dust component. No evidence was found for carbonate minerals, clay minerals or quartz. If present in these rocks, their abundance must be less than about 10 percent.
Their absence indicates that carbonates are not ubiquitous over the surface of the planet, but they may still be found in specific locations that either favored their initial deposition or their subsequent preservation. This finding could have important implications for identifying areas that may preserve signs of ancient life on Mars, since carbonate minerals are commonly formed in biological processes, Albee said.
Striking results also have been obtained from Global Surveyor's laser altimeter over Mars' northern hemisphere, which is exceptionally flat with slopes and surface roughness increasing toward the equator, according to principal investigator Dr. David Smith of NASA Goddard Space Flight Center, Greenbelt, MD. The initial data for this region helps scientists interpret a variety of landforms, including the northern polar cap, gigantic canyons, ridges, craters of all sizes and shield volcanoes. Most surprising are views of extraordinarily mundane regions -- as flat as the Bonneville Salt Flats in Utah - that extend over vast northern regions of the planet.
Mars Global Surveyor will complete the first phase of its two-part aerobraking strategy at the end of March, at which time the science instruments will be turned on again for most of the next six months. Over this period, the spacecraft will stay in an 11 1/2-hour orbit and collect an additional bounty of data at a closest approach of about 170 kilometers (106 miles) above the surface, much closer than the spacecraft will pass over the planet once it has reached its formal mapping orbit in March 1999. This closer orbit will allow the science teams to take more detailed measurements of the Martian atmosphere and surface without magnetic interference from the solar wind.
"When we decided to slow the pace of aerobraking to reduce the force on the solar panel that was damaged after launch, we knew we would get a bonus - the ability to collect much more science data closer to the planet than will be possible during the prime mapping mission," said Glenn E. Cunningham, Mars Global Surveyor project manager at NASA's Jet Propulsion Laboratory, Pasadena, CA. "Additionally, the six-month period between the end of March and early September will yield an extraordinary opportunity as the lowest point of the orbit migrates over the northern polar cap. All of this information that is coming back now is really icing on the cake, a spectacular precursor to the global mapping data expected to start flowing next year."
Mars Global Surveyor is part of a sustained program of Mars exploration known as the Mars Surveyor Program. The mission is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, DC. JPL's industrial partner is Lockheed Martin Astronautics, Denver, CO, which developed and operates the spacecraft. JPL is a division of the California Institute of Technology, Pasadena, CA.
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