In this diagram, preliminary Pathfinder APXS analyses of soils (yellow dots) extend the range of Viking soil analyses. The analysis of Yogi appears to be contaminated by dust adhering to the rock's surface. The rock composition can be estimated by subtracting a portion of dust; the resulting Yogi composition is very similar to that of Barnacle Bill (we assumed 50% dust having the composition of drift analysis A-5 and used a linear mixing model to subtract the dust which is only strictly valid if the dust, where present, is thicker than the APXS penetration depth). Barnacle Bill is also contaminated by dust, but to a lesser extent.

 

APXS analyses of Martian soils are compared with Viking soil analyses. Each element is normalized to silicon in this diagram. The yellow boxes representing Viking data include all analyses and their analytical uncertainties reported by B.C. Clark and others (1982) Journal of Geophysical Research, vol. 87, p. 10,064. Although the first APXS soil analysis (A-2) was reported to be almost identical to Viking soils, ssubsequent analyses demonstrate some variability and a few significant differences from Viking analyses. Specifically, soils at the Pathfinder site generally have higher aluminum and magnesium, and lower iron, chlorine, and sulfur. Scooby Doo, which appears to be a sedimentary rock composed primarily of compacted soil, also exhibits a few chemical differences form the surrounding soils. Analysis A-5 represents a deposit of windblown dust (called drift), whereas the other soil analyses may be cemented materials.

 

This diagram (preliminary X-ray data) illustrates chemical differences between terrestrial rocks and meteorites inferred to have been derived from Mars. The Martian meteorites (as well as Viking soil analyses) all plot to the left of the fields for Earth rocks. Pathfinder APXS analyses of rocks (stars) and soils (yellow dots) appear to plot in the gap between these previously defined fields, although they are similar to at least one basaltic meteorite. The other two stars represent the compositions of Barnacle Bill and Yogi. The analysis of Yogi appears to be contaminated by dust adhering to the rock's surface. The rock composition can be estimated by subtracting a portion of dust; the resulting Yogi composition is very similar to that of Barnacle Bill (we have assumed 50% dust having the composition of drift analysis A-5 and used a linear mixing model to subtract the dust which is only strictly valid if the dust, where present, is thicker than the APXS penetration depth). Barnacle Bill is also contaminated by dust, but to a lesser extent.

 

The Pathfinder APXS chemical analyses of Barnacle Bill and Yogi (corrected for adhering dust) have been recast into plausible minerals using the CIPW norm calculation. If they are fully crystalline igneous rocks, both possibly consist of orthopyroxene (magnesium-iron silicate), feldspars (aluminum silicates of potassium, sodium, and calcium), quartz (silicon dioxide), and other minerals that include magnetite, ilmenite, iron sulfide, and calcium phosphate.

 

This commonly used chemical classification for lavas shows that Barnacle Bill and Yogi (corrected for adhering dust) are distinct from basaltic Martian meteorites (shown as red squares). The Pathfinder APXS analyses have been corrected fo the presence of a small amount of salt, and sulfur is assumed to be present as sulfide. These rocks plot in or near the field of andesites, a type of lava common at continental margins on the Earth. The preliminary data for alkalis are likely to represent upper limits, so refinement of these analyses could shift them to slightly lower Na2O + K2O and higher SiO2. We do not presently know whether these are igneous (crystallized from a melt), sedimentary (grains/fragments deposited by wind or water or precipitates), or metamorphic rocks (deformed).

81940_full.jpg (264K)

81942_full.jpg (264K)

81943_full.jpg (450K)

81944_full.jpg (31K)

The surface near the rover's egress from the lander contains bright red drift (#1), dark gray rocks such as Cradle (#3), soil intermediate in color to the rocks and drift (#2), and dark red soil on and around the rock Lamb (#4). Globally, Mars is characterized by similar color variations. The spectra of these sites have been ratioed to the drift to highlight their differences. The rocks are less red and have less of a bend in the spectrum at visible wavelengths, indicating less ferric minerals and a more unweathered composition than drift. The intermediate colored soils appear intermediate in the spectral properties as well. The dark red soil at Lamb is darker than drift by about equally as red; the curvature of sppectrum at visible wavelengths indicates either more ferric minerals or a larger particle size.

The surface near the rover's egress from the lander contains mainly bright red drift (#1), dark gray rocks such as Cradle (#3), soil intermediate in color to the rocks and drift (#2), and dark red soil on and around the rock Lamb (#4). Globally, Mars is characterized by similar color variations. The spectra, measured using the full 13- color capability of IMP, provide evidence for the mineralogy of the unweathered rocks and highly weathered red soils.

The first color panorama returned by IMP after Mars Pathfinder's landing included several larger, gray rocks, bright red dust on a flat- topped rock and the ground between the rocks, and darker red soil exposed where Pathfinder's landing dislodged a small rock. The less red color and low reflectance of the rocks is consistent with the iron minerals found in igneous rocks, whereas the fine, bright drift has a spectrum indicative of a weathering product. The strength of the bend, or "kink", in the spectrum is related to the abundance and particle size of specific crustalline, ferric weathering products. In the false color image, the blue areas have a weak kink and are relatively unweathered, whereas the red areas' strong kink indicates an abundance of ferric iron minerals.

Rocks and soils on the surface are thought to be composed of minerals similar to those found on earth's surface. One of the most important tools for recognizing these minerals is the spectrum of sunlight reflected by them. At the visible and near-infrared light wavelengths measured by the Imager for Mars Pathfinder (IMP), the most important coloring materials in the martian surface are iron minerals. There are two broad classes of iron minerals. Minerals which occur in igneous rocks (such as pyroxene) have a relatively flat spectrum and they reflect only a small amount of light; they are said to have a low reflectance. Ferric iron minerals, which occur as weathering products, reflect longer-wavelength light and absorb short-wavelength light, hence their very red color. The relative brightnesses of Martian surface materials in IMP's different wavelength filter is a powerful tool for recognizing the iron minerals present.

81945_full.jpg (93K)

81946_full.jpg (155K)

82016_full.jpg (78K)

82018_full.jpg (217K)

The shapes of the spectra of surface materials can easily be measured from multispectral images. Measures of surface spectral properties can also be shown as false color overlain on an image to summarize spectral variations near the lander at a glance. The top image showns the region southeast of the lander in true color. In the bottom image of the same region, the strength of the kink in the spectrum at visible wavelengths (related to the abundance and particle size of weathered ferric iron minerals) is shown in false color. Blue rocks are the least weathered, red soils are most weathered, and green soils and rock faces show an intermediate state of weathering.

The earliest survey of spectral properties of the rocks and soils surrounding Pathfinder was acquired as a narrow strip covereing the region just beond the where the rover made its egress from the lander. The wavelength filters used, all in the binocular camera's right eye, cover mainly visible wavelengths. These data reveal at least five kinds of rocks and soil in the immediate vicinity of the lander. All of the spectra are ratioed to the mean spectrum of bright red drift to highlight the differences. Different occurrences of drift (pink spectra) are closely similar. Most of the rocks (black spectra) have a dark gray color, and are both darker and less red than the drift, suggesting less weathering. Typical soils (green spectra) are intermeidate in properties to the rocks and drift. Both these data and subsequent higher resolution images show that the typical soil consists of a mixture of drift and small dark gray particles resembling the rock. However two other kinds of materials are significantly different from the rocks and drift. Pinkish or whitish pebbles and crusts on some of the rocks (blue spectra) are brighter in blue light and darker in near-infrared light than is the drift, and they lack the spectral characteristics closely associated with iron minerals. Dark red soils in the lee of several rocks are about as red as the drift, bust consistently darker. The curvature in the spectrum at visible wavelengths suggests either more ferric iron minerals than in the drift or a larger particle size.

One of the more unusual rocks at the site is Ginger, located southeast of the lander. Parts of it have the reddest color of any material in view, whereas its rounded lobes are gray and relatively unweathered. These color differences are brought out in the inset, enhaced at the upper right. In the false color image at the lower right, the shape of the visible- wavelength spectrum (related to the abundance of weathered ferric iron minerals) is indicated by the hue of the rocks. Blue indicates relatively unweathered rocks. Typical soils and drift, which are heavily weathered, are shown in green and flesh tones. The very red color in the creases in the rock surface correspond to a crust of ferric minerals. The origin of the rock is uncertain; the ferric crust may have grown underneath the rock, or it may cement pebbles together into a conglomerate. Ginger will be a target of future super- resolution studies to better constrain its origin.

In this scene showing the rover deployed at Yogi, the colors have similarly been enhanced to bring out differences. The same three kinds of rocks are recognized as in the distance. Yogi (red arrow), one of the large rocks with a weathered coating, exhibits a fresh face to the northeast, resulting perhaps from eolian scouring or from fracturing off of pieces to expose a fresher surface. Barnacle Bill and Cradle (blue arrows) are typical of the unweathered smaller rocks. During its traverse to Yogi the rover stirred the soil and exposed material from several cm in depth. During one of the turns to deploy the APXS (inset and white arrow), the wheels dug particularly deeply and exposed white material. Spectra of this white material show it is virtually identical to Scooby Doo, and such white material may underly much of the site.

One of the first "multispectral spots" obtained by the IMP camera was of the Stripe Rock on Sol 4. A multispectral spot measurement obtains small images of a region of interest in all geology filters with no image compression. Stripe rock is of interest to Mars Pathfinder scientists because of a bright vertical stripe that appears on the center of the rock face. It was thought that this stripe might be an intruded vein of material of different composition than the surrounding rock.

The color image of this rock shows that the stripe is of similar color to the surrounding soils (see arrow). A detailed examination of the rock was conducted to extract preliminary reflectance spectra (that is, the variation of brightness with color) from nearby bright and dark soils, the stripe, and the surrounding rock. Although these data require further calibration (e.g., the lower reflectance at 965 nm is not reliable at this time), they do show that the general spectral characteristic of the stripe is quite similar to the nearby dark soil. This suggests that the "stripe" is actually an accumulation of soil deposited in a crack in the rock face.

Mars Pathfinder Mission
Mineralogy and Geochemistry Science Operations Group

Barnacle Bill Rock

Hypothesis: APXS data show composition of rock is consistent with volcanic andesite, but rough texture of surface suggests it may be a "breccia."

Could it be composed of many different rock fragments that combine to give a similar overall composition?

Method: Target Barnacle Bill with "multispectral spot" (all geology filters at full spatial resolution of about 1-2 cm per picture element)

Goal: Determine variability of reflectance spectra (mineralogy) across the face of the rock

If all spectra are similar: rock is "homogeneous" (composed of the same material)

If spectra vary: rock may be "heterogeneous" (such as an impact melt breccia or sedimentary conglomerate)

Result: Spectra taken from many different locations show only two basic kinds of spectra:

1. Soil-like deposits
2. Dark rock face

Implication: At spatial resolution of 1-2 cm, rock composition is homogeneous. However, rock may be composed of fine-grained materials (< 1-2 cm) that cannot be seen with this method.

Preliminary data acquired from the "multispectral spot" image sequence for Barnacle Bill rock. Images were acquired with no compression in all geology filters. Reflectance spectra (that is, the variation of brightness with wavelength, or color) are shown for background soil (green), soil-like deposits found on and within small holes in the rock (red), and dark portions of the rock face (blue). Comparison of the spectra of these three types of materials demonstrates that the rock has relatively homogeneous composition at the spatial resolution of the patches sampled (about 1-3 cm). That is, all soil-like deposit and rock face spectra cluster in both their overall brightness (reflectance) and shape of their reflectance curves. A more heterogeneous rock would show variable spectral characteristics across its face. Note that the spectra of the soil-like deposit is intermediate to that of the background soil and rock face spectra. This is consistent with the interpretation that the soil-like deposit is a relatively thin layer in which portions of the rock are also sampled within the patches selected.

Also shown are laboratory spectra of oxidized and unoxidized volcanic rocks from Earth. Scientists will compare spectra of terrestrial materials such as these to help determine the composition of the rocks observed at the landing site in combination with data returned by other instruments such as the APXS.