with Simon Mansfield

Pathfinder Science Summary
JPL - December 2, 1997 - Based on the first direct measurements ever obtained of Martian rocks and terrain, scientists on NASA's Mars Pathfinder mission report in this week's Science magazine that the red planet may have once been much more like Earth, with liquid water streaming through channels and nourishing a much thicker atmosphere.

Among the more significant discoveries of the Mars Pathfinder mission was the identification of possible conglomerate rocks, which suggests the presence of running water to smooth and round the pebbles and cobbles, and deposit them in a sand or clay matrix, says Dr. Matthew Golombek, Mars Pathfinder project scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA. This scenario supports the theory that Mars was once warmer and wetter.

"If you consider all of the evidence we have at Ares Vallis -- the rounded pebbles and cobbles and the possible conglomerate, the abundant sand- and dust-sized particles and models for their origins, in addition to the high silica rocks," Golombek says, "it suggests a water-rich planet that may have been more Earth- like than previously recognized, with a warmer and wetter past in which liquid water was stable and the atmosphere was thicker."

A panoramic view of Pathfinder's Ares Vallis landing site, featured on the cover of the Dec. 5 issue of Science, reveals traces of this warmer, wetter past, showing a flood plain covered with a variety of rock types, boulders, rounded and semi-rounded cobbles and pebbles. These rocks and pebbles are thought to have been swept down and deposited by floods which occurred early in Mars' evolution in the Ares and Tiu regions near the Pathfinder landing site.

The cover image, which is a 75-frame, color-enhanced mosaic taken by the Imager for Mars Pathfinder, looks to the southwest toward the Rock Garden, a cluster of large, angular rocks tilted in a downstream direction from the floods. The image shows the Pathfinder rover, Sojourner, snuggled against a rock nicknamed Moe. The south peak of two hills, known as Twin Peaks, can be seen on the horizon, about 1 kilometer (6/10ths of a mile) from the lander. The rocky surface is comprised of materials washed down from the highlands and deposited in this ancient outflow channel by a catastrophic flood.

"Before the Pathfinder mission, knowledge of the kinds of rocks present on Mars was based mostly on the Martian meteorites found on Earth, which are all igneous rocks rich in magnesium and iron and relatively low in silica," Golombek and a team of Pathfinder scientists report in a paper entitled, "Overview of the Mars Pathfinder Mission and Assessment of Landing Site Predictions." The paper summarizes the scientific results of the mission, which are also detailed in six other papers in this issue. The scientists report that chemical analyses of more than 16 rocks and studies of different regions of soil -- along with spectral imaging of rock colors, textures and structures -- have confirmed that these rocks have compositions distinct from those of the Martian meteorites found on Earth.

"The rocks that were analyzed by the rover's alpha proton X-ray spectrometer were basaltic or volcanic rocks, with granite- like origins, known as andesitic rocks," Golombek reports. "The high silica or quartz content of some rocks suggests that they were formed as the crust of Mars was being recycled, or cooled and heated up, by the underlying mantle. Analyses of rocks with lower silica content appear to be rich in sulfur, implying that they are covered with dust or weathered. Rover images show that some rocks appear to have small air sacks or cavities, which would indicate that they may be volcanic. In addition, the soils are chemically distinct from the rocks measured at the landing site."

The remarkably successful Mars Pathfinder spacecraft, part of NASA's Discovery program of fast track, low-cost missions with highly focused science objectives, was the first spacecraft to explore Mars in more than 20 years. In all, during its three months of operations, the mission returned about 2.6 gigabits of data, which included more than 16,000 images of the Martian landscape from the lander camera, 550 images from the rover and about 8.5 million temperature, pressure and wind measurements.

The rover traveled a total of about 100 meters (328 feet) in 230 commanded maneuvers, performed more than 16 chemical analyses of rocks and soil, carried out soil mechanics and technology experiments, and explored about 250 square meters (820 square feet) of the Martian surface. The flight team lost communication with the lander on Sept. 27, after 83 days of daily commanding and data return. In all, the lander operated nearly three times its design lifetime of 30 days, and the small, 10.5 kilogram (23- pound) rover operated 12 times its design lifetime of seven days.

Now known as the Sagan Memorial Station, the Mars Pathfinder mission was designed primarily to demonstrate a low-cost way of delivering a set of science instruments and a free-ranging rover to the surface of the red planet. Landers and rovers of the future will share the heritage of spacecraft designs and technologies first tested in this "pathfinding" mission.

Golombek points out that the rocky surface and rock types found in Ares Vallis match the characteristics of a flood plain on Earth, created when a catastrophic flood washed rocks and surface materials from another region into the basin. Ares Vallis was formed in the same way that the 40-kilometer-long (25- mile) Ephrata Fan of the Channeled Scabland in Washington state was formed, says Golombek, adding that the Ephrata Fan was deposited when channels of water flowing down the Grand Coules filled the Quiney Basin.

Additional data from the Pathfinder landing site revealed that magnetic dust in the Martian atmosphere has been gradually blanketing most of the magnetic targets on the lander over time. "The dust is bright red, with magnetic properties that are similar to that of composite particles," Golombek states. "A small amount of the mineral maghemite has been deposited almost like a stain or cement. These results could be interpreted to mean that the iron was dissolved out of crustal materials in water, suggesting an active hydrologic cycle on Mars. The maghemite stain could be a freeze-dried precipitate."

Another team of scientists used daily radio Doppler tracking and less frequent two-way radio ranging techniques during communications sessions with the spacecraft to pinpoint the location of the Pathfinder lander in inertial space and the direction of Mars' rotational axis.

In his published paper, Dr. William Folkner, an interdisciplinary scientist at JPL, and co-authors present estimates of the Martian polar moment of inertia, which show that Mars has a dense core surrounded by a lighter mantle. The results imply that the radius of Mars' core is larger than about 1,300 kilometers (807 miles) and less than about 2,400 kilometers (1,490 miles). Mars' core and mantle are probably warmer than Earth's at comparable depths. Eventually, scientists may be able to determine whether Mars' core is presently molten or fluid.

"Variations in Mars' rotation around its own spin axis are thought to be dominated by mass exchange between the polar caps and the atmosphere," Folkner reports. "During winter, part of the atmosphere condenses at the poles. If the southern cap increased symmetrically as the northern cap decreased, then there would not be any change in moment of inertia or rotation rate. However, because of Mars' orbital eccentricity, difference in elevation and difference in albedo, the polar caps are not formed symmetrically.

"The unbalanced waxing and waning of the Martian polar ice caps results in seasonal changes in air pressure at the Pathfinder and Viking landing sites, " he says. "These changes in air pressure are correlated with changes in Mars' rotation rate, which have been observed in our radio tracking measurements."

The season and time of arrival of Mars Pathfinder in the late northern summer resulted in some variations in the temperature of the upper atmosphere compared to Viking data, reports Dr. Tim Schofield, JPL team leader of the atmospheric structure and meteorology instrument, and colleagues in their published report.

High in the atmosphere, at altitudes of 80 kilometers (50 miles) above the surface, temperatures were cold enough to make carbon dioxide condense and form carbon dioxide clouds. At altitudes of between 60 kilometers and 120 kilometers (37 miles and 75 miles), the Martian atmosphere was an average of 20 degrees colder than Viking measurements, Schofield reports. Seasonal variations and Pathfinder's entry at 3 a.m. local solar time, compared with Viking's entry at 4 p.m. local solar time, may account for these variations. On the surface, however, daytime temperatures were typically 10 to 12 degrees warmer than Viking surface temperatures.

Mars Pathfinder measured regular pressure fluctuations twice a day, which suggested that a moderate amount of dust is being uniformly mixed in a warm lower atmosphere, as was the case with Viking data. The daily average pressure reached a minimum on the 20th day of the mission (Sol 20), indicating the winter south polar cap had reached its maximum size.

Schofield reports that surface temperatures follow a regular daily cycle, with a maximum of 15 degrees Fahrenheit during the day and a minimum of minus 105 degrees Fahrenheit at night. The science team also observed rapid daytime temperature fluctuations of up to 30 degrees Fahrenheit in as little as 25 to 30 seconds. These observations suggest that cold air was warmed by the surface and convected upward in small eddies.

Pathfinder encountered winds that were light and variable compared to the Viking landers, Schofield reports. The winds blew steadily from the south during the Martian nights, but during the day they rotated in a clockwise direction from south to west to north to east. Whirlwinds or dust devils were detected repeatedly from mid-morning through the late afternoons.

Other scientific findings of the Mars Pathfinder mission, presented in this week's issue of Science, are:

  • Chemical analyses returned by Mars Pathfinder indicate some rocks appear to be high in silica, suggesting differentiated parent materials. These rocks are distinct from the meteorites found on Earth that are thought to be of Martian origin.
  • The identification of rounded pebbles and cobbles on the ground, and sockets and pebbles in some rocks, suggests conglomerates that formed in running water, during a warmer past in which liquid water was stable.
  • The measurement of the moment of inertia of Mars by tracking Pathfinder radio data indicates the radius of the central metallic core is greater than 1300 km but less than roughly 2000 km.
  • Airborne dust is magnetic with a mean size of about 1 micron. Interpretations suggest the magnetic mineral is maghemite, which may have been freeze- dried on the particles as a stain or cement, and that the iron may have been leached out of crustal materials by an active hydrologic cycle.
  • Remote-sensing data at a scale of generally greater than 1 kilometer and an Earth analog correctly predicted a rocky plain safe for landing and roving, with a variety of rocks deposited by catastrophic floods that are relatively dust free.
  • Imaging revealed early morning water ice clouds in the lower atmosphere, which sublimate away as the atmosphere warms.
  • Abrupt temperature fluctuations with time and height were recorded in the morning, which was consistent with warming of the atmosphere by the surface and convected upwards in small eddies into the atmosphere.
  • Dust devils were frequently measured by temperature, wind and pressure sensors, and at least one likely contained dust, suggesting that these gusts are a mechanism for mixing dust into the atmosphere.
  • The soil chemistry of Ares Vallis appears to be similar to that of the Viking 1 and 2 landing sites, suggesting that the soil may be a globally deposited unit.
  • Some rocks at the landing site appear grooved and fluted, suggesting abrasion by saltating sand-sized particles. Dune- shaped deposits were also found in a trough behind the Rock Garden, indicating the presence of sand.
  • The weather was similar to the weather encountered by Viking 1; there were rapid pressure and temperature variations, downslope winds at night and light winds in general. Temperatures at the surface were about 10 degrees Kelvin warmer than those measured by Viking 1.
  • The atmosphere has been a pale pink color due to fine dust mixed in the lower atmosphere, as was seen by Viking. Particle size and shape estimates and the amount of water vapor in the atmosphere are also similar to that measured by Viking.

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

    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 managed by the California Institute of Technology, Pasadena, CA.


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