ARES VALLIS, MARS—Earthbound astronomers know the problem. You finally get your telescope time. You go up to the mountain. And it's socked-in by clouds—you can't get the data.
Now Mars Pathfinder knows the same problem. Pathfinder, also known as the Sagan Memorial Station, was attempting to observe the bright morning "star," Earth. It was Pathfinder's sixteenth day on the planet, and Earth had risen several hours before the Sun. The IMP (Imager for Mars Pathfinder) team from the University of Arizona was eager to get this image of our home world—in part because it might inspire the public, and in part because Earth is one of a variety of astronomical targets that are used to help determine how much dust is suspended in the atmosphere.
While looking for the "Pale Blue Dot," the Sagan Station instead made a discovery that its namesake would have found fascinating—clouds (Figure 1). "This is the first time that an astronomical image from another planet was canceled because of bad weather," quipped IMP scientist Mark Lemmon at a press conference held July 31, 1997. Lemmon showed several images of water-ice clouds in the predawn skies over the Ares Vallis site. These images were mostly taken 15 and 16 days after the July 4, 1997, landing. The clouds were seen to "burn off" shortly after sunrise. Lemmon also showed sequences of sunrise and sunset images. These data and other daily observations of the Sun are also being used to characterize the amount of dust—and now, clouds—in the atmosphere, and how these vary over time.
Fig. 1. Thwarted astronomical observation of Earth. "You are here" marks the location where Earth should have been in the predawn skies to the east of the Mars Pathfinder lander on Sol 16 (the sixteenth day on the surface). However, the Earth observation was obscured by clouds. The clouds were at an altitude of about 16 km. They appeared pink-hued, and are thought to consist of water-ice nucleated on fine, red dust. The clouds rapidly dissipated after sunrise.
Pressure and temperature at the landing site are also of great interest to the Pathfinder team. Pressures measured at the site are of the order of 6.7 millibars, whereas Earth at sea level is about 1 bar. Robert ("Bob") Haberle of the NASA Ames Research Center said that the team is intrigued by the fact that the atmospheric pressure pattern varies from day to day. On some days, there will be 2 or 3 high-pressure "peaks" and "valleys," on some days there will be four. The pressure variations can change as much as 0.3 millibars in a single day; this is a 4.5% pressure change. Haberle said that if a similar effect were to occur on Earth, it would be like having a Class 5 hurricane every day.
Atmospheric pressure at the landing site reached a minimum around mid-July and has been rising ever since. Haberle explained that this minimum was an indication that the south polar ice cap (composed of water and carbon dioxide) had reached its maximum extent and is now beginning to sublimate and retreat. Pathfinder, which is located in the northern hemisphere at about the same latitude as Mexico City on Earth (19°N), is in the midst of summer. However, Pathfinder's location is proving to be a weather station capable of providing information about conditions in the far southern hemisphere, which is now experiencing a bitter, cold winter.
During its fourth week on Mars, the microrover Sojourner explored more of the landing site. At the end of Week 3, Sojourner had parked at a rock named Soufflé and attempted to obtain an alpha proton X ray spectrometer (APXS) measurement of its composition. However, the APXS was not placed on the rock properly, and no data were obtained. The team carried out a series of rover engineering tests that provided considerable information about the nature of the soils that are driven through. They also provide engineering data that will be useful in designing the next NASA rover, which will land in January 2002.
Sojourner thus spent Week 4 traveling clockwise from Soufflé (west of the lander) to Mermaid "Dune" (southeast of the lander). The rover took a variety of images with its forward cameras that revealed textures in rocks and a plethora of evidence for past wind action at the site. Some rocks have been scoured and fluted by sediment entrained in the wind, and fine-grained materials display a variety of ripples, tails, and scour marks (Figure 2).
Fig. 2. Sojourner was about 5 m southeast of the lander when it snapped this spectacular picture that shows a variety of details regarding the manner in which fine-grained materials (&#;4 mm) have been organized by wind action. Note various ripples in the bright area from center to lower right. Note debris tails behind cobbles and pebbles in the lower left. The dark, smooth-surfaced object labeled "Mermaid" is a drift of low-reflectivity sediment that was investigated by Sojourner's APXS at the end of Week 4. This image was taken on Pathfinder's 26th Martian day. The crater rim on the horizon is indicated for reference (compare to Figure 2 in "Week 1" article, Eos, July 22, 1997).
Geologist Robert Sullivan of Arizona State University said that all of the wind tails behind the rocks line up nicely with wind streaks in the lee of craters seen in orbiter images taken by Viking 21 years ago. The combined wind streak analysis provides evidence that the strongest winds, those that actually move sediment, come consistently from the northeast. These northeasterly winds usually occur during late northern autumn and into northern winter, according to previously published predictions (see end of article for further reading).
As of August 5, 1997, Mars Pathfinder concluded its primary mission. Project Scientist Matt Golombek said on July 31st that the station and its rover had completed all of the preplanned science, technology, and engineering goals. As the project moves into its extended mission phase, the rover APXS will determine the composition of more rocks at the site. The cameras will continue to shoot the "Super Pan" (a 12-filter and 3-color stereo panorama), which takes many days to obtain because lighting conditions must be similar in each portion of the 360° scan.
Pathfinder's Extended Mission will include continued monitoring of the weather conditions and the IMP will periodically take pictures of some places around the lander to look for changes caused by wind. In addition, the two-way tracking of the lander via radio over the course of one Earth year will provide data needed to determine Mars' polar moment of inertia to 1% or better. The moment of inertia result will allow for improved determination of the iron content of the Martian core and mantle and variations in obliquity, which influences Martian climate over time.
Mars Pathfinder and Sojourner are expected to continue collecting data for several months. There is much hope that the lander will continue to send data through at least July 1998, and there is hope that the rover can be sent to the tops of nearby hills (that is, &#;500 m away) to see what is on the other side. Because it is largely solar-powered, Golombek said, the lander and rover will be able to keep functioning, "until something breaks;" but when this might occur cannot be predicted. The first scientific results are expected to be published in Science in December. A thorough review of results will also be presented at a special session at the AGU Fall meeting in December. Of course, the data relayed to Earth will keep some scientists busy for many years to come.—Ken Edgett, Arizona State University, Tempe
Edgett, K. S., and P. R. Christensen, Rocks and aeolian features in the Mars Pathfinder landing site region: Viking infrared thermal mapper observations, J. Geophys. Res., 102, 4107–4116, 1997.
Greeley, R., A. Skypeck, and J. B. Pollack, Martian aeolian features and deposits: Comparison with general circulation model results, J. Geophys. Res., 98, 3183–3196, 1993.
Sagan, C., Pale Blue Dot , 429 pp., Random House, New York, 1994.
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