The primary scientific purpose of Pathfinder was to release the small Sojourner Rover, guided from Earth through the lander's communication system. The Rover moved independently (after departing the lander down a ramp) up to 20 m away on its six, flexible, rocker wheels, to take pictures and bring its spectrometer against rock surfaces for analysis.
This microrover, which weighed 11.5 kg (25.4 lbs), measured the following: length = 630 mm (24.8 in), width = 480 mm (18.9 in), and height = 280 mm (11.0 in). Using power from a solar panel, Sojourner moved at a maximum speed of 40 mm/min (0.13 ft/min) during the martian day. Its front and back wheels moved independently to control its steering. The Rover used a stereo camera, mounted at its front, which imaged objects in its path. Each excursion was intermittent, because operators on Earth determined direction changes to reach target rocks and avoid obstacles. This took extended time periods because of the long distances radio and video signals traveled back and forth. When it encountered small rocks, the wheels raised up to glide over the obstruction.
The stereo camera system on the Pathfinder took images of the surface around the lander, obtaining black and white views and colored ones with color filters. Sojourner's camera also collected images (mainly rock close-ups). In the three images below, the first shows part of the scene viewed by the camera, the second is a montage of part of the surrounding site, in which we identify (using friendly names that speak a personal touch) some of the rocks that it visited, and the third is a panorama of the 360° view around the Pathfinder (after Sojourner had departed) made by the camera mounted on the mast as it looked downward.
Many of the rocks are grayish on fresher surfaces. These surfaces appear weathered and pitted, and often are, at least partly, dust-covered. A typical large rock is Yogi (shown below), which appears to have a reddish dust blanket on one side and a sharp boundary with fresher rock (right side).
The primary instrument on the rover is the alpha-proton-x-ray spectrometer (APXS). This instrument uses high speed alpha particles to bombard soil or rock surfaces. These particles generate a back scattering of alpha particles, alpha-proton particles, and x-rays, whose energies as determined at the detectors are indicative of a wide number of elements. It thus detects, quantitatively, elements such as those in rocks, i.e., Si, Al, Ca, Fe, Mg, K, Na, Ti, Mn, Cl, S, P, O, and C. It can report these elements in their elemental form or as oxides. The diagram below is a histogram of many of these elements from Barnacle Bill, Scooby Doo, Yogi, and some soil sites. Barnacle Bill's composition is close to that determined earlier at the Viking site but the other rocks and soils are different.
The next graph (top), is a plot of values of elemental ratios of Fe/Si and Ca/Si from the Pathfinder site, compared with certain Earth rocks. The Mars rocks at this site are definitely not basalt. They plot closer to but not within the compositional field of the terrestrial volcanic rocks known as Andesites. When we recalculate the elemental composition of the rocks into what petrologists term normative rock compositions, we see that the mineralogy is similar to andesites (bottom diagram).
But the presence of normative quartz (leftover silica, after we assign all other SiO2 to the other minerals), technically makes this rock type a dacite. Andesites and dacites are the common volcanic rocks emitted from continental volcanoes, such as those along the Ring of Fire (e.g., the Pacific Coast Cascades). The presence of these rock types at this martian site, if typical of other sites, suggests that the Red Planet had melted and differentiated, so that at least part of its crust is andesitic. Paul Lowman (author of Section 12 of this Tutorial) had postulated more than a decade ago that an andesitic primordial crust probably developed early in Earth history and may be the norm for other inner planets.
Most of the rocks plot in the basaltic and andesitic fields of a diagram which shows the classification of basic to silicic igneous (volcanic) rocks found on Earth in terms of NasO + K2O versus Sio2:
Interpretation of MGS and Pathfinder data, supplemented by that acquired later by Mars Odyssey (see below), has led to this general map of the surface rocks and sediment cover on Mars. Basalts, overall the most common of the martian surface volcanic rocks, are shown in green and yellow; andesites in blue, and regions where dust cover is so thick that data on underlying bedrock could not be recorded appear in medium tan:
Some individual volcanoes appear to produce lava outpourings of different compositions from time to time, suggesting differentiation in the magma chamber. A good example is Nili Patera, in Syrtis Major, which produces both basaltic (blue) and andesitic (red) lavas as determined from MGS data; some of the latter may actually be dacites (andesitic but with quartz); one or more small peaks within the scene may have been further enriched in silica to gain a granitic composition:
The occurrence of andesites leads to several postulates: 1) these rocks are evidence of some differentiation in the martian mantle; 2) they may represent rocks involved in subcrustal release of water; or 3) they are anomalous local thin veneers on the dominant basalts caused by surface altering processes. No definitive conclusion has been agreed upon.
So far, martian Landers have gathered compositional data at five widely-separated martian sites (Pathfinder; the the two Vikings; the two MERS rovers.). In general, bedrock at the sites are similar in their chemical makeup. This may simply mean that the martian crust tends to be homogeneous, but even more likely is the explanation that martian wind circulation has scattered and homogenized compositional differences across the planet. The distribution of rocks and boulders at the Pathfinder site further supports earlier ideas that at one time in the Mars' past there was considerable water cover and resultant flow currents, causing a pattern sometimes observed on Earth as flood-related.
The last Pathfinder image we show below is a classic sunset picture, taken through the thin, dusty, martian atmosphere that produced a fan-shaped glow around the distant Sun.
The martian exploration program since the opening of the next millenium (we accept that time start as January 1, 2000) is presented on the next pages (19-13a and 19-13b).