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      Mars is appearing more and more like a planet that was very Earth-like in its infancy, with weathering processes and flowing water that created a variety of rock types and a warmer atmosphere that generated clouds, winds and seasonal cycles.

      Those observations, along with new images taken by the Mars Pathfinder rover and lander, and an update on the condition of the spacecraft, were presented at an Oct. 8 press briefing originating from NASA's Jet Propulsion Laboratory.

      "What the data are telling us is that the planet appears to have water-worn rock conglomerates, sand and surface features that were created by liquid water," said Dr. Matthew Golombek, Mars Pathfinder project scientist at JPL. "If, with more study, these rocks turn out to be made of composite materials, that would have required liquid water flowing on the surface to round the edges in pebbles we see on the surface or explain how they were embedded in larger rocks. That would be a very important finding."

      Golombek also stressed the amount of differentiation -- or heating, cooling and recycling of crustal materials -- that appears to have taken place on Mars. "We're seeing a much greater degree of differentiation -- the process by which heavier elements sink to the center of the planet while lighter elements rise to the surface -- than we previously thought, and very clear evidence that liquid water was stable at one time in Mars' past.

      "Water, of course, is the very ingredient that is necessary to support life," he added, "and that leads to the $64,000 question: Are we alone in the universe? Did life ever develop on Mars? If so, what happened to it and, if not, why not?"

      Despite recent communications problems with Earth, the Mars Pathfinder lander and rover are continuing to operate during the Martian days, when they can receive enough energy to power up spacecraft systems via their solar panels. The mission is now into Sol 94, or the 94th Martian day of operations, since landing on July 4.

      "Everything that we have seen over the last 10 days (with respect to communications) is like a twisty little maze with passages all alike," said Jennifer Harris, acting mission manager. "I am happy to report that we have made contact with the spacecraft using its main transmitter. We were able to confirm that we could send a command to the spacecraft to turn its transmitter on and then turn it off.

      "We don't know yet whether we are receiving that signal over the low-gain or high-gain antenna," she added, "but we should be able to determine this over the next few days."

      The Mars Pathfinder team began having communications problems with the spacecraft on Saturday, Sept. 27. After three days of attempting to reestablish contact, they were able to lock on to a beacon signal from the spacecraft's auxiliary transmitter on Oct. 1, which meant that the spacecraft was still operational.

      At that time they surmised that the communications problems were most likely related to depletion of the spacecraft's battery and uncertainties in the onboard clock. The last successful data transmission cycle from Pathfinder was completed at 3:23 a.m. Pacific Daylight Time on Sept. 27, which was Sol 83 of the mission.

      Since then, efforts have been made during each Martian day to reestablish contact with both the primary and auxiliary transmitter and obtain engineering telemetry that would tell the team more about the health of the lander and rover. On Oct. 7, the team was able to lock on to Pathfinder's signal, via NASA's Deep Space Network 34-meter-diameter (112-foot) dish antenna in Madrid, Spain, for about 15 minutes, using the main transmitter. However, in repeating the process on Oct. 8, they did not receive a signal.

      The rover, which receives its instructions from Earth via the lander, is currently running a contingency software program that was preprogrammed to start up if the vehicle did not hear from the lander after five Martian days. That program was powered on Oct. 6, on Sol 92 of the mission. In this contingency mode, the rover is instructed to return to the lander and begin circling it. This precaution is designed to keep Sojourner close to the lander in the event that the spacecraft was able to begin communicating with it again.

      If normal communications are reestablished, the rover team will send new commands to Sojourner to halt the contingency circling and begin a traverse to a specific location.

      Dr. William Folkner, an interdisciplinary scientist at JPL, presented data on the rotation and orbital dynamics of Mars, which are being obtained from two-way ranging and Doppler tracking of the lander as Mars rotates. Measurements of the rate of change in Mars' spin axis have important implications for learning more about the density and mass of the planet's interior. Eventually, scientists may be able to determine whether Mars' core is presently molten or fluid. The size of the core also can be used to characterize the thickness, or radius, of Mars' mantle.

      "By measuring the spin axis of Mars, we can learn something about the interior of the planet, because the speed of the change in its orientation is related to how the mass is distributed inside," Folkner said. "If the core is fluid, its spin and the way in which the planet wobbles slightly will be different from the spin and wobble of a planet with a solid core.

      "If Mars' core is solid, then it can't be less than about 1,300 kilometers (807 miles) in radius, out of the planet's total radius of 3,400 kilometers (2,112 miles)," Folkner added. "If the core is made up of something less dense than iron, if it's a mixture of, say, iron and sulfur, then the core would be bigger, but it couldn't be bigger than about 2,000 kilometers (1,242 miles) in radius."

      New close-up images of dunes around the landing site are showing some scientists clear evidence that there is sand on the surface of Mars. Identification of sand, as opposed to dust or pebbles, is a significant factor in establishing that weathering processes such as erosion, winds and flowing water all contributed to Mars' present landscape.

      "We've made significant progress in establishing that water was a dominant agent in forming the surface, and now we can say that there is another agent at work, and that is the wind, that has created and modified some of the landforms on a smaller and medium scale," said Dr. Wes Ward of the U.S. Geological Survey, Flagstaff, AZ, a member of the Imager for Mars Pathfinder team. "And because the water is no longer there, wind probably is the dominant agent shaping the Martian surface at this moment."

      Ward showed images of Ares Vallis, taken by the rover and Viking 1 orbiter images to point out the structural difference in these surface features. While Viking 1 surface features around a rock nicknamed "Big Joe" showed drifts, the dune-like surfaces in the Ares Vallis flood basin resemble sand that has been blown southwest over the landing site. The presence of sand also points to the likely presence of liquid water, needed to create these small, 1-millimeter-diameter granules, and weathering agents such as wind to blow them into small ridges and moats present around the Ares Vallis rocks.

      "The wind is quite an active agent," Ward said. "Sand is the smoking gun, and as far as I'm concerned, the gun is smoking and has Colonel Mustard's prints all over it. We are seeing sand at the landing site."

      Dr. Greg Wilson, of Arizona State University, who is on the Pathfinder atmospheric experiment team, reported increases in the pressure of the Martian atmosphere and a drop in surface temperatures.

      "We expect to see a continued increase in pressure and decrease in temperatures as the dust season approaches and winds begin to lift more dust into the Martian atmosphere," he said. "The dust season on Mars usually begins in the next few weeks."

      Additional information, images and rover movies from the Mars Pathfinder mission are available on JPL's Mars news media web site at or on the Mars Pathfinder project's home page at . Images from Mars Pathfinder and other planetary missions are available at NASA's Planetary Photojournal web site at

      The Mars Pathfinder mission is managed by the Jet Propulsion Laboratory for NASA's Office of Space Science, Washington, DC. The mission is the second in the Discovery Program of fast-track, low-cost spacecraft with highly focused science goals. JPL is a division of the California Institute of Technology, Pasadena, CA.

10/8/97 DEA