ARES VALLIS, MARS—In keeping with the theme of its Fourth of July landing, Mars Pathfinder seems to have found itself in a red, white, and "blue" landscape. While Mars is known as the "Red Planet," it is the white and somewhat-blue stuff that is grabbing the attention of the science team.
At a press briefing on July 22, the team showed an enhanced color image of the region in front of the rock, Yogi. (A gray-scale version of this image is seen in Figure 1). The most exciting feature in the image is a white patch of subsurface material revealed by a deep scour in the soil that was caused by the rover wheels as it turned to examine Yogi. The white patch has a visible reflectance spectrum similar to the white, crusty material exposed by wind about 2.5 m away.
Fig. 1. Center stage of Sojourner's activities during the first 3 weeks on Mars. In this image, obtained about 1 1/2 weeks into the mission, Sojourner can be seen with its alpha proton X ray spectrometer up against the rock, Yogi. On its way to Yogi, the rover wheels exposed white material (box) that occurs beneath the smooth patch of soil in front of Yogi. This white material is probably the same as exposed by wind further to the right, labeled "white crust." Sojourner examined the dark soil next to rock, Lamb, on its 20th Martian day, then traveled 3 m to Soufflé. Week 3 ended with the APXS placed on the rock, Souffle'. Yogi is about 1 m high and located west of the lander.
Team member Mike Malin talked about these crusts during the first week of the mission. He interpreted them as evidence for the past presence of water that left behind dried mud and/or evaporite material. Also revealed by the new color perspective on the landing site: Yogi has a two-toned surface. About 1/3 of the rock as seen from the lander is blue-gray, the other 2/3 has a red, rust-colored surface. The red material on Yogi and everywhere else at the landing site contains oxidized iron. The blue-gray surface on this rock is thought to contain unoxidized iron.
The science team provided two possible interpretations for the sharp contact between the "red" and "blue" portions of Yogi: either it is a geologic contact between two layered rock units; or it is the result of preferential wind deposition of bright, red dust on one portion of the rock but not the other. The wind interpretation seems more likely, with either the blue face being swept clean of red dust, or with the red face being the site of wind "plastering" red dust onto its surface.
It is the bluish surfaces on rocks like Yogi that the science team is most interested in—the assumption is that these are dust-free and weathering-free surfaces upon which the alpha proton X ray spectrometer (APXS) could best determine the composition of the rocks. Unfortunately, in the case of Yogi it appears that the APXS was placed on the dusty, red side (Figure 1); thus the team is working on subtracting the dust spectrum from the APXS results for Yogi that were described earlier in the mission.
Bluish rocks are not the only ones grabbing the team's attention. White rocks are intriguing, as well. A "white" rock, Scooby Doo, was the center of attention early in the week. The Sojourner rover took an APXS spectrum, then drove its wheels on the rock to see if it would crumble or flake. The rock was unaffected by Sojourner's scraping; and by the end of the week the team announced that Scooby Doo has a chemistry similar to that of the soils at the landing site, but initial analysis shows that it contains slightly higher amounts of calcium and silicon.
Despite hopes that Scooby Doo would prove to be a carbonate or evaporite material, it appears that this hard, flat rock still provides a curious mystery yet to be solved. After Scooby Doo, Sojourner's APXS looked at some dark soil near the rock Lamb, then trucked a full 3 m across the flat area in front of Yogi, to end the week with its APXS placed on a rock that looks like a Soufflé (Figure 1).
The number of press briefings has dropped to only 1 per week, as the Pathfinder team shifted emphasis toward collecting maximum amounts of data as quickly as possible. After some glitches in the communication stream between Earth and Mars early in Week 3, data began to pour in as if from a firehose. By the end of the week, the team was reading back portions of the "Super Pan"—a full-color, stereo panoramic view of the landing site taken using the 12 visible/near-infrared filters of the IMP imager built by a team at the University of Arizona.
"This Super Pan will be the ultimate data set that we will all work with for years and years," said team member Jim Bell of Cornell University. The IMP has 12 filters in each of its two camera "eyes"—These bands range between 443 nm (nanometers) and 1003 nm, ideal for identification of some iron- and OH-bearing minerals.
Pictures and data placed on the Pathfinder web site (http://www.mpf.jpl.nasa.gov/) during Week 3 began to include lists of the chemical elements detected at various locations by the APXS. The Pathfinder "Webmasters" reported that the most "hits" received in one day—46 million—occurred on July 8th. This number is double the previous record, which occurred on the official web site for the Olympic Games in Atlanta, Ga., last year.—Ken Edgett, Arizona State University, Tempe
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