Mars Pathfinder Preliminary Results


The APXS analysis of "Barnacle Bill" shows its origin to be consistent with the martian meteorites. The rock is about 60% felsic, 40% mafic, roughly 1/3 quartz, 1/3 feldspar, and 1/3 orthopyroxene. This would classify it as an andesite if it is an igneous rock, a highly differentiated quartz-rich rock compared to the martian meteorites, which are classified as basalts. This would indicate that Mars has been more thermally active in its past than was previously thought, producing at least some highly remelted and differentiated rocks. Tables of elemental compositions giving preliminary results of the X-ray spectrometer for rocks and soils in weight percent of elemental abundance, oxides, and comparisons with Earth rocks are available.

"Barnacle Bill" could also be a mixture of basalt or granite mixed in a sedimentary rock or impact melt. This has been tested by taking spot reflectance spectra at one to two cm resolution. The spectra show the rock to be homogeneous at this scale, so it does not appear to be a mixture of fragments larger than one cm. It could still be a mixture of smaller fragments, but this result and its good spectral match with fresh volcanic earth rocks strengthen the case that it is a volcanic andesite.

Preliminary analysis of the APXS data returned for "Yogi" suggested it was very different from Barnacle Bill. If Yogi is of volcanic origin, it appeared to be basalt, a primitive, unprocessed rock type. However, a thin covering of dust on the rock indicates there is probably a soil component mixed in these measurements. Rough estimates have been made of the contribution of the soil component. Subtracting this out gives a composition of Yogi similar to that of Barnacle Bill.

The rock "Scooby-Doo" appears to be a sedimentary rock composed primarily of compacted soil. The APXS analysis of Scooby-Doo shows only minor differences from the local soils analyzed.

An image showing some of the different materials on the surface is available (140 K). The red arrows in the image show rounded rocks, possibly shaped by the action of water. The blue arrows show sharp-edged rocks, thought to be impact ejecta or volcanic. The white arrows show the light areas believed to be deposits left behind by evaporating water.

Images from Pathfinder are consistent with the earlier results from Viking Orbiter images that Ares Vallis was the site of a massive flood about one to three billion years ago, and with measurements by the Viking Landers showing large quantities of iron oxides in the soil.

The analysis of soil samples by the APXS shows a very close match to soils examined by the Viking Landers. There are some differences, however. Soils at the Mars Pathfinder site generally have higher aluminum and magnesium, and lower iron, chlorine, and sulfur than those studied by Viking. Tables of APXS results are also available for the soils.

Soil experiments using the rovers wheels to dig in the ground show a friction angle equal to the angle of repose of 38 degrees, and soil friction of 0.2 kilopascal. The grain size is very small, less than 50 microns (similar to fine silt) and the bulk density is estimated to be 1.55 grams per cubic cm. Other areas and soils may be different. This information will be used to estimate thermal inertia and radar reflectivity properties.

The magnetic materials experiment shows dust collecting through time on the three strongest magnets after 26 days. Preliminary analysis indicates the possibility that all martian dust is at least slightly magnetic. The dust is believed to contain maghemite, a strongly magnetic mineral formed in environments of scarce oxygen. Further information on maghemite is available.


A study which measures the Doppler shift in radio transmissions from the lander has shown that Mars may have a solid iron core. The Doppler shift of the received radio signals gives an accurate determination of the relative velocity of the lander and Earth due to the spin of the Earth, movement of Mars and Earth in their orbits, and spin of Mars. This in turn can give an accurate estimate of the martian rotation rate, spin-axis direction, precession, etc. This has allowed scientists to determine that Mars may have a solid iron core with a radius of 1300 km or greater, or an iron and sulfur core about 2000 km in radius. This may point to hot, active interior early in Mars' history, and may imply cycling of gases between the interior and early atmosphere.


It was mid-summer in the northern hemisphere of Mars when Pathfinder landed. The Pathfinder Lander is at 19.33 N, 33.55 W. The Viking 1 Lander touched down at 22 N, 50 W, 2 km below datum elevation on 20 July 1976, and is used for many of the comparisons below.

The pressures measured over the first three days average about 6.75 mb, 10% to 20% smaller than those recorded by the Viking 1 Lander during the same season 21 years ago (note that this result is consistent with the elevation difference of about 100 meters between the Mars Pathfinder and Viking 1 landing sites). The pressure showed a slight decline over the first few weeks but is now starting to rise slowly. This rise should continue through December, 1997. The pressure rise is concurrent with the slow shrinking of the southern polar cap, now at its maximum extent, as the southern winter ends.

Temperatures measured from the top of the 1 meter mast on Mars Pathfinder varied from daily highs of about 260 K (+8 F) to lows of 196 K (-107 F). This is about 10 K degrees (18 F degrees) warmer than Viking 1 Lander measurements made at 1.6 meters. The sol-to-sol temperatures have been very repeatable over the first 30 sols, and should continue until about 60 sols after landing, after which they will start to show more variation.

Preliminary wind speed estimates give late evening and early morning prevailing winds from the SSE, which shifted in the early afternoon to be from the N to NE. This is very similar to what Viking 1 found at this time of year. During the day, winds were light at only a few km or miles per hour. At night the wind speed increased to about 10 to 20 mph (16 - 32 kph) from the south.

The repeatable weather patterns of northern summer found by Viking 1 have been verified by Pathfinder so far. These include diurnal (day-night) pressure changes and semi-diurnal changes by as much as 4.5% due to atmospheric thermal tides.

Interruptions in the normal pattern of temperature drops observed on a few nights may indicate water in the atmosphere is condensing as fog. Humidity measurements are planned later in the mission.

On sol 25, temperature sampling was done at 4-second intervals for the whole day. Temperature fluctuations by 15 to 20 K (30 to 40 degrees F) were observed over minutes or seconds at some periods, suggesting turbulent boundary-layer mixing between the warmer near-surface region and cooler layers above that. A "dust devil" was also detected passing by the lander on sol 25, and later high resolution sampling has detected more dust devil signatures.

More detailed information and historical weather reports are available at the Mars Pathfinder project weather page. Raw and reduced data are available online at

Other Atmospheric Science

Imaging of the sky and the sun at different elevations above the horizon show the atmosphere to be moderately dusty, consistent with what was seen by the Viking Landers. The optical depth is about 0.4 (approximately 35% of direct sunlight at noon is scattered or absorbed by dust). Visibility tends to be about 30 km. The dust appears to be spread vertically high into the atmosphere and is globally distributed. The sky does not appear to have the clear skies postulated before the landing, but is hazy and salmon-colored, as it was for Viking.

Extensive water-ice clouds have been imaged in the pre-dawn hours by the Lander camera. The clouds are moving from the NE at about 7 meters per second (15 mph) and disappear right around sunrise. The clouds are thought to consist of frozen water condensed around dust particles.

The wind-sock experiment shows good agreement with the meteorology data in terms of wind speed and direction. Dune-like features and tails of aeolian material extending from rocks have been identified in Pathfinder images. The directions of these show good agreement with the prevailing wind directions shown by nearby large-scale wind streaks and tails in Viking Orbiter images.

The materials adherence experiment, consisting of a glass plate over a solar cell mounted on the rover, measures the amount of dust collecting on the glass plate. The dust has been settling on the glass plate at a consistent 0.30% optical coverage per sol. This preliminary result indicates that the dust settles out of the atmosphere everywhere, not preferentially at the poles as has been hypothesized.

The density profile derived from the deceleration on entry of the spacecraft shows atmospheric densities up to a factor of 5 lower than those found by the Viking Landers above 60 km altitude, and somewhat lower than Viking below 60 km. Temperatures derived from the density profile show very similar values to those measured by Viking below 55 km, indicating the climate has not changed. Temperatures above 55 km are much lower than those measured by Viking, but this was not surprising because Pathfinder entered the atmosphere at night while both Vikings descended during the day. A minimum value of about 103 K (-275 F) was found at 80 km altitude.

Hubble images of Mars show the dust storm in Valles Marineris died down between July 2 and 9, some of the dust may have diffused to the Mars Pathfinder landing site. The atmosphere in general is becoming more dust-laden, and clouds seen earlier are dissipating. The altitude to the cloud tops in the Tharsis region is estimated to be 15 to 16 km.

Mars Satellites

The first images of Deimos have been obtained through the Imager for Mars Pathfinder through all spectral filters. Preliminary results show the moon to be slightly reddish in color. Further analysis will be done to try and model the composition of Deimos. An imaging spectra of Phobos has also been obtained but has not yet been downlinked.

The APXS Instrument
The Imager for Mars Pathfinder
The Atmospheric Science/Meteorology Instrument
The Wind Sock Experiment
The Magnetic Materials Experiment

Mars Pathfinder mission summary - Science magazine

Mars Fact Sheet
Mars Pathfinder home page at NSSDCA

Dr. David R. Williams,
NSSDCA, Mail Code 690.1
NASA Goddard Space Flight Center
Greenbelt, MD 20771

NASA Official: Dave Williams,
Last Updated: 30 December 2004, DRW