ARES VALLIS, MARS—With temperatures around -9°C in the mid-afternoon and down to -75°C at night, northern summer at the Mars Pathfinder landing site is nearing its close. Autumn will begin in about 2 weeks, right about the same time that Pathfinder's orbiting companion, Mars Global Surveyor, will reach the Red Planet.
As autumn approaches, the average daily temperatures will continue to decrease, and there is some potential for the winds to pick up. In the 1970s, dust storm season was seen to extend from northern autumn right through winter. In the late 1970s and early 1980s, the Viking 1 lander (which is located about 800 km from Pathfinder) detected changes in atmospheric opacity (that is, how much dust is suspended in the atmosphere) from about the start of autumn and on into the winter. Will Pathfinder see the same? Will strong winds blow through the site and rub out the tire tracks left by the 62-cm long rover, Sojourner?
Mars Pathfinder was designed to last for 30 Martian days (each day is about 24 hours, 37 minutes). The rover, Sojourner, was to last only 7 Martian days. As of August 30th—about 57 days after landing, both robots remained operational and "healthy." Sojourner, with its silica aerogel insulation, has managed to stay warm through the cold summer nights and is continuing to explore the geochemistry of rocks and soils near the lander.
The lander continues to send back images to document the rover's progress, to complete the "Super Pan" high-resolution mineralogy mosaic, and to monitor for changes that might be caused by wind. To extend the lander's life, the solar-powered spacecraft is turned off four out of every five nights to conserve power stored on batteries. The nights that the spacecraft is active are used for monitoring weather. On Sol 60 the rover's battery died unexpectedly (although the battery was designed with a mission lifetime of only 7 days, it was expected to last at least another month or so based on actual flight performance), ending the possibility of additional night-time (lowest noise) rock geochemistry measurements. However, daytime alpha proton X-ray spectra (APXS) measurements will continue for as long as the rover is able to operate on the surface.
Thus far, the extended mission has focused on obtaining more APXS of rocks and observing the local weather. On August 8th, meteorology team leader Tim Schofield of the Jet Propulsion Laboratory (JPL) reported that the group had detected several "dust devils" pass over the lander. The wind vortices were detected as abrupt pressure changes; the pressure drops, as the vortex reaches the lander, then begins to rise sharply, and returns to normal. At the same time, the wind shifts rapidly between east and west directions. By August 27th, Schofield said they had detected 12 such vortices passing over the lander, although more might have occurred because they did not have complete data coverage throughout the past month.
There has not been a visual (that is, image) confirmation of dust devils, thus it is unclear whether these vortices actually were lifting dust from the surface. Similar vortices were detected by meteorology instruments at the two Viking lander sites [Ryan and Lucich, 1983] , and dust devils have been seen from orbiting spacecraft [Thomas and Gierasch, 1985] , but none has ever been imaged from the ground level. Dust devils are considered to play a key role in raising and redistributing dust in the otherwise thin (~6.5 millibar) atmosphere.
The presence of dust devils might help explain the fact that the rocks at the Pathfinder site appear to be less dusty than previously thought. In July, the science team had noted that some of the rocks, such as Yogi (see Eos, August 5, 1997, p. 317–318), had a two-toned surface. One surface was reddish, the other bluish. The "blue" surfaces were interpreted as dust-free, the "red" surfaces as having a coating of micrometer-sized iron oxide-rich dust. However, further analysis shows that the two-toned nature of Yogi was a lighting effect. Seen under different light conditions, the "red" side of Yogi can appear "blue." Dust in the atmosphere scatters light in ways that can confuse the casual observer. The good news for the Pathfinder team, however, is that many more of the rocks at the landing site now appear to be relatively dust-free.
Dust-free rocks are good for Sojourner. The little rover's mission is to continue to place the APXS on rocks and find out their chemistry. At the August 27 press conference, Project Scientist Matt Golombek of the Jet Propulsion Laboratory said that they now had APXS data for four rocks—five if the white rock, Scooby Doo, is counted. Sojourner spent most of the month of August driving around in the "Rock Garden," an area with many dust-free, 0.2 to 0.8 meter-high, tilted and flat-topped rocks (Figure 1). The rover's cameras revealed rough and pitted textures on the surfaces of rocks like Wedge, Shark, and Flat Top (Figure 1). Despite earlier hopes that Flat Top would turn out to be a layered sedimentary rock, when seen up close it appeared to resemble a volcanic rock with fine vesicles that are somewhat ovoid and give the appearance of layering when observed from a distance.
Fig. 1. The Rock Garden. This area of tilted rocks was the main focus of Sojourner's activities in August. The rover obtained alpha proton X-ray spectra of the rocks Shark and Wedge, plus provided a close-up view of Flat Top and many other rocks in the area. In general, these rocks have elemental compositions and surface textures that are suggestive of a volcanic origin. However, their exact genesis and mineralogy will probably be the subject of debate for many years to come. The Twin Peaks on the horizon are only about 0.8 km away.
The interpretation of rock composition from APXS data is a slow and careful process. At the August 27 press conference, only the X-ray portion of the APXS data were discussed. APXS investigator Tom Economou said that so far the data hint at two rock types—one that is relatively silica-rich (that is, ~54% Si) and one that has less silica. Rocks named Wedge and Yogi were seen as having less silica, suggesting that if they are volcanic rocks, they would be classified as basalts. Rocks named Barnacle Bill and Shark, if they are volcanic rocks, might be classified as andesites. These results are still subject to debate and potential revision. For example, the team found that if the APXS signature of Martian dust is subtracted from the Yogi spectrum, then Yogi looks more silica-rich like Barnacle Bill. The team also found that the two possibly basaltic rocks appear to be enriched in sulfur relative to the basaltic meteorites from Mars (SNC meteorites) and relative to terrestrial basalts.
Sojourner is continuing its foray into the Rock Garden (Figure 1). Eventually it will return to "Mermaid Dune," a wind-blown drift that could be composed of sand. The rover examined this drift once before, but the team was not satisfied that it had completed its work there. After Mermaid, the rover will drive counterclockwise, back around the lander, to obtain an APXS measurement on a magnet at the base of the rover's ramp near Barnacle Bill rock. The magnet has been collecting magnetic dust since July 4th. While headed back to the magnet, a trip that might take another month, the rover will examine additional rocks and wind-blown drifts along the way.
After examining the dust on the ramp magnet, Golombek says, Sojourner will "head for the hills!" The surface north of the lander slopes uphill. On the other side of this hill is another mountain similar to Twin Peaks (Figure 1), except that it is singular, not twinned. This "North Knob" cannot be seen well from the Pathfinder lander. The team hopes that Sojourner will drive to the top of the nearby hill and provide a vista of North Knob and the surrounding hidden terrain.
Golombek says the team is impressed by how well the Mars science community predicted—prior to landing—what the landing site would be like. This establishes greater confidence in the model-dependent remote sensing results (thermal infrared and radar) that were used to estimate landing conditions. Also, he says, the discovery of a "high-silica" rock (Barnacle Bill and possibly Shark) could alter our perceptions of Martian volcanic and geologic history. And the wildly fluctuating temperatures (on timescales of seconds, minutes), the predawn clouds, and other meteorological phenomena are exciting, new, and were not observed by the Viking landers.
Mars Pathfinder's mission is continuing. Golombek says he "has a job" as Project Scientist through July 1998, thus he hopes the spacecraft will be monitoring the Ares Vallis environment for at least that long. Meanwhile, Mars Global Surveyor, which launched about a month before Pathfinder in late 1996, is scheduled to enter orbit in mid-September. The spacecraft will use aerobraking to alter its orbit over the next 4 months, and Pathfinder's daily observations of atmospheric opacity will play a role in watching for dust storms that might affect the aerobraking activities.
A special session on the Pathfinder mission at the Fall AGU Meeting in December will present many science results for the first time.—Ken Edgett, Arizona State University, Tempe
Ryan, J. A., and R. D. Lucich, Possible dust devils, vortices on Mars, J. Geophys. Res., 88, 11,005–11,001, 1983.
Thomas, P., and P. J. Gierasch, Dust devils on Mars, Science, 230, 175–177, 1985.
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