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      Mars Global Surveyor's first look at Mars is showing scientists a world devoid of an active core and anything more than the relic of an ancient magnetic field.

      "Mars no longer has a global magnetic field generated by an internal energy source, like Earth and the other planets," said Dr. Jack Connerney, co-investigator of the magnetometer/electron spectrometer team, at an Oct. 2 Mars Global Surveyor press briefing at NASA's Jet Propulsion Laboratory. "It appears that the crust of Mars is strewn with multiple magnetic anomalies, which may represent the solidification of magma as it was coming up through the crust and cooling very early in Mars' evolution, but this is only the memory of a magnetic field."

      Mars Global Surveyor went into orbit around Mars on Sept. 11 after a 10-month journey to the planet, and detected the presence of a weak magnetic field within a week of its arrival. Evidence of this faint magnetic field confirmed long-standing theories that the red planet had, at one time in its history, a liquid core able to support a dynamo. Scientists believe this core probably froze and solidified early in the planet's evolution.

      The magnetometer data, acquired during one of the spacecraft's highly elliptical orbits around Mars during the week of Sept. 15-18, indicates that the planet's magnetic field is not globally generated in the planet's core, but is localized in particular areas of the crust, said Dr. Daniel Winterhalter, magnetometer experiment representative at JPL. Scientists plan to correlate these strong magnetic anomalies with topographical data obtained by Global Surveyor's camera and laser altimeter. That information may lead to the identification of particular topographic features in the crust.

      "The identification of these magnetic anomalies and their correlation with surface features may enable us to trace the history of the planet's interior, just as we are able to trace the history of Earth's interior using the magnetic anomalies that have been imprinted on the ocean floors," Winterhalter said.

      Mars' very localized field also creates a new paradigm for the way in which it interacts with the solar wind, one that is not found with other planets. While Earth, Jupiter and other planets have large magnetospheres, and planets like Venus have strong ionospheres, Mars' small, localized magnetic fields are likely to produce a much more complicated interaction process as these fields move with the planet's rotation.

      These observations and many more came just as the spacecraft finished the walk-in phase of aerobraking and was about to begin the main phase, which will last three months. All six of the spacecraft's science instruments had been turned on midway through the elliptical walk-in phase for calibration and engineering adjustments. Since its capture, the spacecraft's orbit has been reduced from 45 hours to 40 hours, 20 minutes. Through January 1998, the aerobraking and navigation teams will gradually circularize Surveyor's orbit into the final two-hour, 378-kilometer (234-mile) mapping orbit.

      "The spacecraft and science instruments are operating magnificently," reported Dr. Arden Albee, of the California Institute of Technology, Pasadena, CA, who is the Mars Global Surveyor project scientist. "The initial science data we've obtained from the walk-in phase of aerobraking are remarkable in their clarity, and the combined measurements from all of the instruments over the next two years are going to provide us with a fascinating new global view of the planet."

      Mars Global Surveyor carries six science instruments -- a camera, laser altimeter, magnetometer/electron reflectometer, thermal emission spectrometer and ultra-stable oscillator -- that will paint a global portrait of Mars, gathering data on the planet's atmosphere, surface and interior. The mission will enable scientists to determine Mars' current state and some of the major turning points in its evolution. Among a myriad of science objectives, Global Surveyor will study Mars' climate and its resources, and attempt to determine if life ever existed on the planet.

      During the past three weeks, the spacecraft has been aerobraking through the upper atmosphere of Mars each time it passes closest to the surface. Aerobraking operations are continuing to proceed smoothly. The spacecraft has completed 12 revolutions around Mars, including nine aerobraking passes through the upper Martian atmosphere, said Dr. Richard Zurek, an investigator at JPL who is leader of the Mars Global Surveyor atmospheric advisory team. Each of these atmospheric passes takes place at the start and low point of the orbit, known as the periapsis, as Global Surveyor orbits at current altitudes of about 110 kilometers (70 miles).

      So far, the upper atmospheric density has varied according to daytime and nighttime measurements by as much as 70 percent, said Dr. Gerald Keating, on the atmospheric advisory team from George Washington University, Washington, DC, and densities are five times higher than they were when the Mars Pathfinder spacecraft entered the upper atmosphere on July 4. Density profiles are being acquired on a daily basis and used to help guide the aerobraking team's work to shrink and circularize the spacecraft's orbit. Although the thickness of the Martian atmosphere continues to run slightly higher than predicted, no major changes to the aerobraking strategy are being considered because the spacecraft was designed to tolerate up to a 70 percent increase in atmospheric thickness.

      The first orbital images of the Martian surface in more than 20 years are showing geologic features that would dwarf some of the most spectacular features known to Earth. Initial science data show a canyon far deeper than Arizona's 1-mile-deep Grand Canyon and mountains standing much taller than Nepal's Mt. Everest. Vast expanses of smooth crustal flatlands in the northern hemisphere hint at a geologically younger portion of Mars, while new measurements of the planet's southern polar cap indicate drastically frigid temperatures of about minus 129 degrees Celsius (minus 200 degrees Fahrenheit).

      Mars Global Surveyor's camera revealed two regions of interest to geologists: a view of a highland valley network called Nirgal Vallis and an image of Labyrinthus Noctis, an area west of the Valles Marineris near the crest of a large updoming in the Martian crust. The images were presented by Dr. Michael Malin, of Malin Space Science Systems Inc., San Diego, CA, who is the principal investigator of the Mars Global Surveyor camera.

      Nirgal Vallis is about 15 kilometers (9 miles) across by about 45 kilometers (27 miles) in length, with many small sand dunes and different aged craters in the vicinity, Malin said. The valley is located at 28.5 degrees south latitude, 41.6 degrees west longitude. Of interest to scientists are the processes that helped shape this canyon.

      "The origin of this and many other canyons on Mars has been debated ever since the Mariner 9 mission," Malin said. "There are two leading theories: the first suggests that water flowing over the surface accumulated, as it does on Earth, then formed a drainage basin that allowed the water to flow further down into a larger channel. The alternative explanation was that ground water processes dissolved part of the subterranean materials on Mars, causing collapse and progressive deterioration of this particular region."

      Labyrinthus Noctis, the second image presented Oct. 2, is near the crest of a large updoming of the Martian crust that is probably thousand of kilometers in diameter, and near very large, 2,000-meter-deep (6,500-foot) canyons bounded by faults. Debris shed from the steep slopes has moved down into the region after the canyons opened. Small dunes are seen in the lower portion of this area, beneath the high cliffs.

      Global Surveyor's camera has acquired about a dozen high resolution images of Mars to date, which are being used to fine- tune the instrument in preparation for the start of mapping operations in March 1998. These first images were not the highest resolution expected during mapping because the spacecraft is not yet in the proper mapping orbit and the correct sunlight conditions have not yet been reached, Malin said. As the spacecraft moves into its Sun-synchronous orbit, in which it will cross the Martian equator at 2 p.m. local Mars time during each revolution, the Sun will be at a standard angle above the horizon in each image.

      The spacecraft's thermal emission spectrometer recorded sub- freezing temperatures at the southern polar cap, said principal investigator Dr. Philip Christensen of Arizona State University, Tempe. The instrument, which takes infrared measurements on the surface, also recorded temperature highs of about -7 C (20 F) at the warmest parts of the planet and a very clear, dust-free atmosphere.

      The laser altimeter, which fires 10 laser pulses a second at the surface, is also performing well, reported Dr. David Smith, principal investigator of the instrument and based at the NASA Goddard Space Flight Center, Greenbelt, MD. This experiment will measure the height of Martian surface features and provide elevation maps that will be precise to within 30 meters (98 feet) of surface features. From the 12,000 measurements already taken, Smith reported a notable inaccuracy in the location of some Martian features as shown on current maps based on Viking data. Global Surveyor will provide a much more accurate global map which will be used to guide future missions to the surface.

      Additional information about the Mars Global Surveyor mission is available on the World Wide Web by accessing JPL's Mars news site at or the Global Surveyor project home page at .

      Mars Global Surveyor is the first in 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.

10/3/97 DEA