Harry Y McSween Jr. & Scott L Murchie. American Scientist. Volume 87, Issue 1. Jan/Feb 1999.
The metallic tetrahedron, with its protective cover of inflated airbags, hit the ground of Mars at 60 kilometers per hour, bounced 16 times across the red desert floor and rolled end over end, finally coming to a stop amid a field of boulders. There was, of course, no one there on July 4, 1997, to watch the ungainly (many thought improbable) landing of the Mars Pathfinder spacecraft, but more than a hundred million miles away earthlings paid a great deal of attention. Pathfinder, designed principally as an engineering demonstration, proved to be a public-relations bonanza for NASA. But the spacecraft also delivered scientific instruments that investigated the structure of the Martian atmosphere and interior, its weather, and the geologic processes that sculpted the landing site and created the pervasive red soil. And for the very first time, rocks on the Martian surface were analyzed using a robotic rover.
Rocks, christened with nicknames like Barnacle Bill, Casper and Yogi, were clearly the scientific centerpiece of the Pathfinder mission. Even the choice of the landing site, as described by project scientist Matt Golombek of the Jet Propulsion Laboratory, was predicated on its having an abundance and variety of rocks. The confluence of Ares Vallis and Tiu Vallis (two ancient river valleys) was once the largest known floodplain in the solar system, a location specifically selected because of the variety of rocks thought to have been carried by catastrophic floods down the outflow channels and dumped near their mouths. Images of the Pathfinder landing site revealed a Marsscape of ridges and troughs strewn with rounded pebbles and boulders, resembling huge depositional fans on the earth. These characteristics, along with nearby teardrop-shaped hills streamlined by floods, confirm the geologic setting inferred earlier from orbital photographs.
The rocks were ripped up by the raging Ares and Tiu floods, presumably eroded from the ridged plains unit through which the rivers coursed and perhaps from the bedrock underneath. In the stratigraphic nomenclature adopted for Mars, the ages of these two units as mapped by Sue Rotto and Ken Tanaka of the U. S. Geological Survey are Hesperian and Noachian, respectively. Both are thought to be billions of years old, dating roughly from the time of formation of the oldest preserved continental rocks on the earth. Near the Pathfinder landing site are a number of younger impact craters that excavated the sedimentary deposits and lofted fragments, or ejecta, across the area.
Sorting out the geologic processes and events that created the local martian rocks was the petrological mission of the Mars Pathfinder. Through petrological investigations of terrestrial rocks, geologists routinely unravel complicated histories, determine the timing of various events and glimpse processes such as melting of our planet’s deep interior or deposition of sediments on an ocean floor. Close examination of a rock’s physical appearance, identification of the minerals it comprises and measurement of its chemical composition allow scientists to determine how the rock formed and whether it was subsequently modified. In designing the instrument package for the Mars Pathfinder, it was expected that a combination of observations and analyses would be sufficient to specify the origins of the martian rocks. The mobility required for this task was provided by the Sojourner rover, which might be considered a geologist of sorts (the Geological Society of America has named Sojourner as its first non-human Society Fellow). Although all the instruments aboard the lander and rover functioned as designed, understanding how rocks at the Pathfinder site formed has proved to be a challenge.
One difficulty encountered in interpreting the Pathfinder data is that the rocks, like nearly everything on Mars, are coated with fine red dust. Even the martian atmosphere owes its peculiar salmon color to suspended dust particles. Here we describe ways that multispectral images by the Imager for Mars Pathfinder (IMP) and chemical analyses by the rover-mounted Alpha Proton X-ray Spectrometer (APXS) can be used to peer through this dust coating, and the rocks that are revealed suggest a planet more petrologically complex than previously supposed.
The texture of a rock-its size, shape and the packing and orientation of its constituent particles-is usually an excellent indicator of its origin. For example, volcanic rocks are fine-grained or glassy, sedimentary rocks tend to be layered, and rocks formed by an impact are commonly breccias composed of angular fragments. Rock shape is sometimes related to texture, because the way a rock breaks into smaller pieces usually reflects its internal features. Rocks at the landing site exhibit a variety of shapes–rounded, sharp and angular, flat and tabular, and even irregular and lobate– which hint at different textures.
Textures are best observed directly, however, by close examination of freshly broken surfaces. This is one reason why geologists arm themselves with rock hammers and hand lenses. Unfortunately, all images of the Pathfinder site are confined to unbroken rock surfaces that have been molded by wind, partly obscured by dust and perhaps weathered, making textural interpretations ambiguous. Another complication is that the image resolution (1 to 5 millimeters per pixel) afforded by the IMP, which was mounted on the stationary lander and thus could not be moved closer to rocks in the field of view, is not sufficient to identify small features such as individual mineral grains. Miniature cameras (with resolution of about 1 millimeter per pixel) were mounted on both the front and rear of the rover, and these could be positioned close to rocks. However, even images from these cameras are unable to resolve individual grains in the rocks.
The most abundant textures on many rock surfaces at the Pathfinder site are pits. Some may be holes formed by gas bubbles, called vesicles, that commonly decorate lavas. As magma ascends toward the planet’s surface and the pressure on it decreases, the solubility of dissolved gases is reduced so that the magma effervesces, much like the release of carbon dioxide bubbles when a can of soda pop is opened. The presence of vesicles would indicate that the rocks are volcanic. In this instance, the pits were probably eroded out of vesiculated rocks by windblown sand. Many of the rocks have elongated pits resembling features called flutes that form during sand abrasion. The flutes have a consistent orientation from rock to rock, suggesting a prevailing wind direction. Many rocks in Antarctica, the most Mars-like surface environment on earth, have pits that were initiated from corrosion by salt and enlarged by wind erosion.
A few rock surfaces show pits accompanied by rounded bumps of comparable size and shape. The bumps could be large crystals or fragments that are relatively resistant to erosion. The occurrence of rocks exhibiting both bumps and pits is suggestive of large particles having been plucked from a sedimentary rock. A conglomerate is a coarse sedimentary rock formed by rounded pebbles cemented together with finer materials; in a breccia, the large particles are angular. If the bumps on Pathfinder rocks are pebbles, the pits could be sockets formed by their removal. The occurrence of conglomerates would be important because they are formed during flooding like that which shaped the landing site. Breccias can form by a number of processes, including the welding of fragments produced by impacts. Linear features, typically appearing as repeated light and dark bands spaced a few millimeters apart, are also seen on some rocks. These could be stress fractures, sedimentary layering, or aligned vesicles or mineral grains.
Overall, though, the observed surface textures are sufficiently equivocal to prevent definitive statements about the origin of these rocks. Based on these observations the rocks at the Pathfinder site might have formed as lavas that flowed onto the martian surface, as coarse sediments carried and deposited by water or as materials that were pulverized and melted by meteor impacts.
Designed by Peter Smith of the University of Arizona, the IMP contains a rotatable wheel with 12 filters, through which the Pathfinder scene was imaged at different wavelengths of light. The filters range from 440 to 1,000 nanometers, covering parts of the visible and ultraviolet portions of the spectrum. To obtain color images of the martian surface, images at red, green and blue wavelengths were combined. However, the filters were also used to measure the spectra of sunlight reflected from the rock surfaces. When exposed to light, atoms in certain minerals absorb specific wavelengths, so the corresponding reflected light is partly missing those wavelengths, resulting in dark regions (absorption bands) in its spectrum. The wavelength positions of these absorptions are “fingerprints” that can be used to identify some of the minerals present. The distinctive coloration of the Red Planet results from minerals rich in oxidized (ferric) iron, and these commonly have strong absorption bands centered at wavelengths less than 440 nanometers, with an edge that extends beyond 750 nanometers. Some iron-bearing minerals also absorb at longer wavelengths. For instance, pyroxenes (which are ferrous iron magnesium-calcium silicates that are abundant in many igneous rocks) generally have an absorption band near 900 to 1,050 nanometers, with longer wavelengths corresponding to higher calcium and iron contents.
Spectra of large regions of the martian surface, measured using telescopes on earth, show that the occurrence of ferric minerals in bright red regions covered with dust give the planet its distinctive coloration. Many of the dark gray regions thought to be dominated by rocks show the spectral fingerprint of pyroxenes with moderate calcium and iron contents. A notable exception is Acidalia, the vast dark region adjacent to the Pathfinder site.
It was hoped that the spectra of Pathfinder rocks would allow determination of their mineralogies. Athough rocks at the Pathfinder site have surprisingly varied color properties, they are nearly lacking in mineralogically diagnostic absorption bands. Most rocks are relatively gray, and (like Acidalia) lack a pyroxene-like absorption band within the IMP’s wavelength range. There are three plausible options to explain the absence of this band. The rocks may be solidified glasses, of either impact or volcanic origin, and thus lack crystalline pyroxene. Glasses also have a diagnostic absorption band, but it lies at 1,100 nanometers, beyond the wavelength range of IMP. Alternatively, the rocks may contain an opaque mineral that does not transmit light and consequently hides pyroxene. In terrestrial rocks the iron oxide magnetite often disguises the presence of pyroxene. Or the pyroxenes may be so rich in iron and calcium that their characteristic absorption falls at longer wavelengths than can be detected by IMP. If IMP’s wavelength range extended just a few tens of nanometers farther, one of these three possibilities might have been indicated.
Large rounded boulders at the Pathfinder site, thought to be the oldest materials present, are partly covered by a distinct maroon rind. This rind is absent from more abundant angular rocks, suggesting that it formed only during the site’s earliest history. The spectra of the rind are different from other materials, with a distinct ferric absorption band near 900 nanometers. The spectral shape is most easily explained by one of two common ferric iron oxides, maghemite and ferrihydrite. To IMP, both minerals appear nearly the same, but the conditions under which they formed may not be alike. Ferrihydrite, a mineral which gives a rusty color to shallow lake bottoms on earth, forms in liquid water. Maghemite likewise can form in water, but it can also form by weathering of precursor minerals without significant water present. The maroon rinds constitute tantalizing but inconclusive evidence for the idea that long ago the Pathfinder site was submerged under water. If IMP had just one additional filter at a diagnostic red wavelength, the two minerals could have been distinguished, and this ambiguity in understanding the rocks would have been eliminated.
Erosion and transport of dust by prevailing winds have had an enormous effect on color properties of the rocks. Gray rocks have flat spectra, without any distinctive absorption bands. Surfaces of both gray and maroon rocks are partly coated by windblown dust. Rock faces oriented away from the prevailing northeast wind direction are most likely to be draped by dust and tend to be red. Upwind faces are scoured by the wind and usually are gray. On large rounded boulders, the maroon rind is preserved on downwind faces and mostly abraded off upwind faces. In fact, except for the maroon rinds, nearly all color variations in rocks can be explained simply as effects of a thin red coating of dust on gray rocks.
One group of rocks that excited the Pathfinder scientists soon after landing has a bright pink color and tabular shapes. These rocks raised hopes of identifying sedimentary rocks. When the APXS analyzed the pink rock Scooby Doo, its composition was found to be identical to the soil. The rover also drove onto the rock and turned a wheel in place to try to scratch its surface, but the rock was unaffected. Laboratory experiments show that compacting Mars-like soil rich in ferric iron causes the soil to appear bright pink. Rocks like Scooby Doo may simply be compacted soil, with sufficient strength to resist abrasion by a spinning studded wheel.
Color variations unfortunately do not provide evidence of the Pathfinder rocks’ origins. There is no clear indication of color banding due to sedimentary layering. Only a few small rocks exhibit mottled color variations that might indicate that they are compositionally heterogeneous. Even those rocks with “bump-and-socket” textures are not spectrally mottled, which may cast doubt on the identification of them as conglomerates or impact-produced breccias.
A rough chronology for rocks at the landing site has been proposed based on the observed spectral features. The oldest rocks at the site are the large, rounded boulders with maroon coatings, deposited by floods and likely stained by reactions with liquid water. Sometime later, smaller gray rocks and broken fragments of maroon boulders were added to the scene, probably as ejecta from nearby impact craters. Erosion by windblown particles then partially removed the maroon rinds from some boulders. At one time windblown dust apparently accumulated several inches deep, but more recently winds have sculpted and partly denuded the site.
The rover-mounted APXS contained tiny pellets of radioactive curium, which bombarded the analyzed rocks with alpha particles. Three kinds of interactions of the target materials with alpha particles (producing backscattered alpha particles, protons and x rays) allowed analysis of the elements that the rock comprises. The measurements only determined the chemistry of the outermost rock layer, no more than a few hundred micrometers in thickness. Because of difficulties encountered in analyzing samples in the presence of the martian carbon dioxide-rich atmosphere, only data from the x-ray mode are currently available. Rudi Rieder of the Max Planck Institut fur Chemie in Mainz, Germany, and his colleagues have so far reported preliminary chemical analyses of five Pathfinder rocks and six soils.
When element abundances measured for the rocks are plotted against each other, they tend to form a nearly linear array, with soil analyses clustering at one end of the array (Figure 7). The most straightfoward explanation of these linear chemical trends is that they are mixing lines, representing rocks contaminated with varying amounts of adhering dust. This idea is strongly supported by a relation between measured composition and color. The martain soils are rich in sulfur and are very red (which can be expressed as a high red/blue spectral ratio). Gray (spectrally bluer) rocks coated with increasing amounts of dust become progressively redder and richer in sulfur (Figure 8).
Mars is home to some of the solar system’s largest volcanoes. It is logical to assume that a significant portion of its surface is covered with volcanic rocks, or perhaps sedimentary materials derived from them. Erupting lavas contain relatively little sulfur because the solubility of sulfurous gases in magma is low under normal conditions. If we assume that the Pathfinder rocks (or the fragments they comprise) are volcanic in origin, we can estimate their composition by plotting various chemical components against sulfur and extrapolating the straight lines they define to “zero” sulfur. The composition of the rock can be estimated by graphically removing the sulfur-rich dust coating in this way.
The interpretation of linear chemical trends as mixing lines has a consequence that was not anticipated in planning the mission: All the analyzed rocks must have nearly the same composition. The Pathfinder location was originally touted as a grab-bag site containing samples carried by floods from the ancient martian crustal highlands to the south. However, the distance to the highlands boundary is some 800 kilometers, much farther than rocks have been carried by even the largest terrestrial floods. It seems more likely that the rocks are locally derived, eroded and tumbled along perhaps for kilometers rather than hundreds of kilometers. The rocks in the neighborhood are still old, to be sure, but they might not be derived from the ancient highlands. If the floods did not carry these rocks for great distances, they might not have been mixed with rocks from other sources. However, it is worth noting that, owing to the difficulty of positioning the APXS sensor head against rock faces, only a few large rocks at the site were analyzed. Some smaller rocks may well have different compositions and origins.
Andesites or Icelandites
The sulfur-free rock composition is surprisingly rich in silicon. (Geologists commonly describe elements in rocks as if they were oxides because they are usually combined with oxygen to form minerals; in this case silicon becomes silicon dioxide, or silica.) Employing a widely used chemical classification scheme for volcanic rocks (Figure 10), the high silica content of the sulfur-free rock corresponds to andesite. The existence of rocks having andesitic composition on Mars came as a surprise. By far, the most abundant terrestrial lavas are basalts, with less than 52 percent silica (by weight). Basalts are also thought to be very common on Mars, based on spectral similarities between its rocky regions and a few basaltic meteorites that are believed to be martian samples.
The chemical composition of a rock can be recast into mineralogy using a calculation scheme called a “norm.” Assuming that a rock consists of some mixture of common igneous minerals, the norm is the proportion of those minerals that most accurately explains the abundances of the elements measured. The norm may not necessarily correspond to the actual minerals the rock comprises, but the equivalence is usually good for igneous rocks. The calculated normative mineralogy of the sulfur-free rock suggests that it is dominated by pyroxenes, feldspars (sodium-potassium-calcium aluminosilicates) and quartz (silica), with minor amounts of magnetite and ilmenite (iron-titanium oxides). The relative proportions of these minerals are consistent with those of igneous rocks. Of course, the rocks could still be sedimentary or impact produced, but in those cases perhaps formed from fragments of andesitic lava.
The term “andesite” is a loaded word for geologists, because on earth it has important implications for plate tectonics. Collisions of plates cause the edge of one slab of rock to be thrust downward, or subducted, beneath another. Andesitic magmas erupt above subduction zones, such as the one that underlies their namesake, the Andes Mountains of South America. Deeply subducted slabs of oceanic rock are heated, and as they are metamorphosed they release water which rises into the overlying mantle. The addition of water lowers the melting point of the solid mantle rock, producing basaltic magma. As this magma ascends towards the earth’s surface it begins to solidify, and some crystals settle out of the liquid and are left behind. This process, called fractional crystallization, changes the composition of the remaining magma, driving it towards higher silica contents. These magmas may also incorporate some of the enclosing crust, which tends to contain silica in greater abundance, further increasing the magmas’ silica content. By a combination of fractional crystallization and assimilation of crustal rocks, andesitic magmas are produced. Water is apparently central to the process, as it initiates melting in the first place and is concentrated in the resulting magma, which in turn affects its fractional crystallization.
After the first analyzed Pathfinder rock (Barnacle Bill) was described at a press conference as having the composition of andesite, many geologists were asked by reporters to explain the implications of this finding. Given the geologists’ terrestrial experience with this rock, it is understandable that there appeared numerous media reports suggesting that martian andesite may imply a planet with a wet interior, and perhaps with plate tectonics-in short a world more like our own than had previously been supposed. But is that really a necessary conclusion from these results?
There are several alternative, perhaps more likely, explanations for the existence of andesitic rocks on Mars. One, offered at the Barnacle Bill press briefing, is that these rocks were formed by repeated melting events. Partial melting produces a liquid with a higher abundance of silica than the rock that was partly melted. If this magma crystallizes and then is partially melted a second time, the silica content of the resulting magma is increased further. Thus, andesitic magmas might be produced in a series of melting steps, with each event further concentrating silica. A protracted process of this sort is what is envisioned for the formation of the earth’s continents. The continental crust is much richer in silica than oceanic crust and, in fact, has roughly the composition of andesite. Perhaps parts of the ancient crust of Mars formed in a similar manner, as they were remelted and refined by protracted volcanism and by large impacts.
Another possibility is that the Pathfinder site is not petrologically representative of the martian crust at all, but instead contains some unusual rocks that, although interesting, constitute a minor part of the surface. Fractional crystallization of basaltic magmas on earth is not restricted to subduction zones. In fact, anywhere basaltic magma occurs, it may undergo partial crystallization with separation of the crystals and liquid. In other settings, however, fractional crystallization occurs under different conditions, usually nearer the planet’s surface (that is, at lower pressures) and with less water available. These conditions promote changes in the order in which minerals appear, so fractional crystallization results in slight differences in the chemistry of the resulting silica-rich magmas. Andesitic liquids are still generated, but they have higher iron and lower aluminum contents at any given silica content. Such volcanic rocks were first recognized in Iceland and are now referred to as icelandites. The Pathfinder sulfurfree rock is also rich in iron and depleted in aluminum, and, in fact, icelandite provides its closest terrestrial analogue. Therefore, these martian rocks may have formed simply by fractional crystallization of basaltic magma, perhaps the same magma that crystallized to form most of the ridged plains unit through which Ares Vallis has been cut.
Rocks at the Mars Pathfinder landing site are difficult to classify, principally because of the limited spatial resolution of the Pathfinder cameras and because of coatings of windblown dust that obscure the rocks’ textures and spectral features and contaminate analyses of the rocks’ chemical compositions. An obvious lesson for future Mars explorers is the need to carry devices that can expose fresh rock surfaces for observation and analysis. At the present time we do not know whether a whisk broom may be sufficient or if a chipping or coring device may be necessary Another lesson is that future imagers should have sufficient resolution to reveal individual mineral grains, with enough filters to distinguish minerals like maghemite and ferrihydrite.
We believe that the physical, mineralogical and chemical characteristics of the analyzed Pathfinder rocks, as best they can be determined from available data, are consistent with volcanic rocks of andesitic composition. Some rocks might consist of volcanic rock fragments that have been eroded and deposited by wind, water or impacts, but without much chemical modification. The maroon rinds on the oldest rocks record an ancient environment, perhaps the wet one suggested by landforms at the site. After the catastrophic floods deposited the oldest boulders, the only major changes have been the addition of small angular rocks to the site, perhaps as ejecta from nearby craters, and the redistribution of dust by the wind. The Pathfinder site is actually stunning for how little it has changed over billions of years.
If these rocks are samples of the ancient martian crust derived from local basement outcrops, the sulfur-free rock composition may suggest the existence of crustal materials similar to those that make up the earth’s continents. Such crustal rocks would presumably have formed in an analogous manner, by repeated partial melting and solidification that progressively increased the silica contents of magmas and the rocks that crystallized from them. The compositional similarity between the sulfur-free rock and terrestrial icelandite, however, suggests another origin: simple fractional crystallization of basaltic magma. Fractional crystallization is an inefficient process, because a large amount of basaltic magma must be processed to produce a relatively small fraction of andesitic liquid. If the Pathfinder rocks are icelandites, then it is likely that the bulk composition of the martian crust would be basaltic rather than andesitic. Because the existence of martian rocks having andesitic compositions can be readily explained by recurrent partial melting or fractional crystallization, speculations about possible roles for water and plate tectonics in martian petrogenesis are probably unwarranted.