Mineral mysteries & planetary paradoxes: oxidized rocks show that conditions on Mars were substantially different in the distant past. But other pieces of evidence suggest that the surface hasn't changed much over time. Which is right? .
ask a group of experts to describe Mars in one word and you'll likely hear a diverse range of responses: hostile, Earth-like, dynamic, ancient, enigmatic. These descriptions are all true to some extent. Mars is a hostile world with a thin atmosphere, extremely cold temperatures, and very little water vapor. But the red planet is also Earth-like --it has an atmosphere, a colorful sky frequented by clouds, and familiar desertlike landscapes. It's dynamic, with volcanism, tectonism, and sedimentary geologic features. But much of the surface is ancient, preserving evidence for heavily cratered, lunarlike terrains that may date back to the earliest epoch of solar-system history. From the perspective of scientists who study the latest results from orbiting missions such as Mars Global Surveyor and 2001 Mars Odyssey, the observations are often baffling. They paint a picture of an enigmatic world of planetary-scale paradoxes.
From afar Mars can be divided into three kinds of terrain. Most of the surface consists of reflective, reddish orange areas known as the classical bright regions. Examples visible from backyard telescopes include Tharsis and Amazonis. There are also murky, reddish brown areas called the classical dark regions. The most famous of these is Syrtis Major, but other spots such as Acidalia and Sinus Meridiani are identifiable in telescopes. Finally, there are the polar regions, which consist of the bright and slightly pinkish polar caps themselves, or the surrounding bright but more diffuse and slightly bluish clouds and haze.
These terrains differ because of their range of compositions and physical properties. The polar caps and clouds, for example, are covered in fall and winter with dry ice (frozen carbon dioxide), which recedes to reveal residual caps of water ice (especially in the north) during the spring and summer. Both water and carbon dioxide (C[O.sub.2]) ice are white at visible wavelengths, but the actual caps are stained pinkish by fine-grained dust deposits that fall out of the atmosphere where they were lifted by the planet's frequent dust devils and occasional dust storms.
The polar clouds are thought to consist primarily of water ice. But C[O.sub.2] clouds probably also form at night over the polar regions, where the temperatures are low enough. These cold cloud particles can also intermingle with airborne dust, causing their normal, bluish hue to be slightly reddened.
The classical bright regions appear to consist in large part of this same fine-grained (micron-size) reddish dust. In these locales the particles are draped over the topography as deposits that range from only a few microns deep in some places to perhaps a few meters thick in others. In bright regions where the dust is thinnest, winds can easily "clean" the surface, revealing intrinsically bright and reddish materials whose composition is not yet well understood.
Both airborne dust and the classical bright regions are thought to include a substantial component of oxidized iron-bearing minerals. Telescopes and spacecraft measurements have directly detected the mineral hematite ([Fe.sub.2][O.sub.3]). It mainly exists in extremely fine-grained form, similar to the hematite deposits found in places like Mauna Kea, Hawaii, where very fine-grained ash and other volcanic deposits were oxidized by heat and occasional rain or snow. Mars Global Surveyor has identified a few small places where coarser-grained hematite appears to exist on the surface. One possible source for such deposits is precipitation from long-standing bodies of water. This is partly why one of these places, Meridiani Planum, has been targeted as the landing site for Opportunity, the second of NASA's Mars Exploration Rovers. Much interest is now focused on understanding these places in more detail.
The dark regions are much less dusty than the bright regions. They also can change dramatically with time. If the surface beneath the dusty veneer is intrinsically darker than the particulates themselves, wind stripping can result in very different reflectivity (albedo) patterns. The changes in dark regions like Syrtis Major and Solis Lacus over centuries of telescopic observations are likely caused by the comings and goings of such thin dust layers.
Surface materials in these areas are coarser grained (consisting of silt- to sand-size grains or larger), which partly explains why these places have lower albedos. But the rocks and minerals themselves must be intrinsically darker, similar in composition to the kinds of materials that erupt from typical lunar or terrestrial volcanoes. Interestingly, despite the fact that they are less dusty and more granular, the dark regions are still quite red.
This raises an important question: Why is Mars red after all? On Earth, fresh volcanic rocks generally erupt unaltered--that is, they contain unoxidized iron-bearing silicate minerals like [(Mg, Fe).sub.2]Si[O.sub.4] (olivine) and (Ca, Mg)[Si.sub.2][O.sub.6] (pyroxene). When these minerals are exposed to Earth's environment--our atmosphere consists of about 20 percent free oxygen ([O.sub.2])--the iron is quickly oxidized and the rocks take on the distinctive reddish color of rusted and weathered materials. Volcanic rocks consisting of similar unaltered minerals are thought to have erupted on Mars, especially early in the planet's history when its interior was hotter and its geologic activity more intense. However, the present C[O.sub.2]-rich Martian atmosphere contains little free [O.sub.2], and the average daytime surface temperature is around -20[degrees]C (-4[degrees]F), making oxidation and weathering reactions much slower and less efficient than on Earth.
To further confuse the issue, telescopic observations and more recent spacecraft measurements find evidence for unaltered pyroxenes and olivines on Mars coexisting in many places with the heavily oxidized reddish dust and dirt. This presents somewhat of a paradox: we know oxidation and alteration have taken place, but apparently not all of the original unaltered materials have been oxidized. If this were Earth, we might conclude that the unchanged rocks are simply younger and just haven't had time to become weathered. However, most of the least-altered Martian places are still ancient by geologic standards, showing a long, complex history of impact cratering dating back at least a few billion years and perhaps more.
There are several hypotheses to explain the apparent surface-weathering paradox. One is that the unaltered materials are being oxidized, but that these reactions proceed extremely slowly in the current Martian environment. Another idea is that Mars's surface conditions occasionally change, resulting in higher temperatures and perhaps wetter (and more oxidizing) conditions associated with volcanic eruptions, impacts, or periodic changes in the planet's polar tilt or eccentricity. During such times the weathering of rocks and soils could occur at a more rapid rate, but the ephemeral nature of these periods would mean that the alteration was "choked off" after only a short time, as surface conditions again became cold and dry. A third hypothesis is that conditions early in Mars's history made the surface much warmer and wetter for substantial periods of geologic time. During these more clement intervals, the atmospheric pressure and temperature were likely much higher, to the point at which processes like rainfall, snow, and glaciation may have been extensive. In this model, the planet's interior gradually cooled and the solar wind eroded the atmosphere (because unlike Earth, Mars's atmosphere isn't protected by a strong global magnetic field); consequently, surface conditions slowly changed to the cold, dry, geologically stagnant state that exists now.
The mingling of unaltered and weathered rocks and minerals on modern-day Mars is difficult but not impossible to explain if the planet's current climate has been unchanged over time. But more likely are the scenarios that invoke more rapid weathering environments, at least occasionally. In those cases the unaltered materials can be thought of as vestiges from the last episodes of volcanism, still freshly preserved because the climate hasn't yet changed back to more Earth-like conditions and perhaps never will.
Long-term clement climate conditions are less likely, based on the existing compositional evidence. Except for a few small coarse-grained hematite deposits, we haven't seen the minerals thought to be ubiquitous in such a climate regime. For example, during a protracted warm and wet spell on Mars, we would expect any C[O.sub.2] dissolved in rivers, lakes, or even oceans to eventually precipitate and/or react with rocks to form carbonate deposits like limestones. This is what happens on Earth: C[O.sub.2] participates in a long, slow, geologic-recycling program involving the atmosphere, the oceans (where it dissolves and precipitates out), plate tectonism (which carries the C[O.sub.2]-bearing rocks deep into the interior and melts them), and volcanoes (which release the C[O.sub.2] back to the atmosphere). Despite many searches, however, there is currently no convincing evidence for large deposits of carbonate minerals on Mars. Rather, telescopic and spacecraft studies find that they make up only 2 to 5 percent of Martian dust and are distributed rather homogeneously on the planet. Thus, the location of the putative carbonate reservoirs remains a mystery. Perhaps they are deeply buried, and the lack of crustal overturn on Mars prevents their recycling. Or perhaps they are simply not there.
Reading the Rocks for Clues
There are other pieces of geologic evidence to this Mars climate mystery besides the composition of the surface and atmosphere. For example, spacecraft imaging from orbit reveals large numbers of heavily eroded impact craters, exhumed layers of sedimentary deposits, and other features, all of which argue that conditions have changed over time. But erosion couldn't be the cause unless the atmosphere was thicker and thus more energetic in the past. Stacked deposits of ice and dust and other layered terrains in and near the polar caps also likely indicate changing atmospheric dustiness and/or surface temperatures over long time periods. Meanwhile, intricate valley networks attest to the action of liquid water, either on or below the surface, which has sculpted the landscape over even-longer time periods. Numerous outflow channels--marking where large volumes of water have catastrophically flooded the landscape--have also been identified.
More perplexing evidence comes from high-resolution Mars Global Surveyor images, which reveal small gullies and other apparently water-carved features that are geologically young by Martian standards (August issue, page 30). These hint that liquid water trickled across the Martian surface not so long ago. Perhaps it is still there today, buried underground.
Substantial support for this view has also come from the apparent discovery by the Mars Odyssey mission of very large, very shallow water-ice deposits (S&T: September 2002, page 22). Most of the ice detected within 1 meter of the surface is in the polar regions, but surprisingly large abundances have also been inferred in some equatorial and midlatitude areas.
All these additional pieces of evidence implicate water as the primary agent of the climatic changes. But no clear story has emerged--no unique answer has been found for the past history of Mars. Some of these pieces of evidence conflict with one another; others are only inferential. In many ways we are still in the evidence-gathering phase of our planetary-scale investigation. Missions like Mars Global Surveyor and Mars Odyssey are returning new data daily, yet much of it still awaits interpretation (and comprehension).
Further Testing the Hypotheses
A small armada of spacecraft will be observing, orbiting, landing on, and roving on Mars during the next year. Nozomi, a Japanese orbiter, will study the interaction of the Martian atmosphere and the solar wind. The European Space Agency's Mars Express orbiter will obtain high-resolution images and spectra of the surface, thus complementing existing Mars Global Surveyor and Mars Odyssey measurements. Riding along with Mars Express is Beagle 2, a small stationary lander and science station that, much like Viking, is equipped with a camera and a miniature sample-analysis laboratory.
NASA's two Mars Exploration Rover field geologists, Spirit and Opportunity, have been specifically designed to respond to discoveries about Mars made by the orbital probes. The instruments they carry will analyze the composition of the materials at each landing site in order to test the different hypotheses about the planet's past climate history. For example, Opportunity will attempt to confirm the presence of coarse-grained hematite at Meridiani Planum and search for other accessory minerals that could provide more clues about the hematite's hypothesized water-related origin. Meanwhile, Spirit will perform an up-close geologic and compositional assessment of the ancient crater Gusev, hypothesized by many to have once been filled with liquid water.
At the same time, professional and amateur astronomers using ground- and space-based telescopes (including the Hubble Space Telescope) will continue to search for additional clues about the red planet's surface composition and about how dust storms, clouds, and albedo changes influence the present climate. Exactly what all these new observations will reveal is unknown. However, it seems almost certain that Mars will continue to surprise us.
JIM BELL is a planetary scientist in the astronomy department at Cornell University. He is a member of the Mars Pathfinder, 2001 Mars Odyssey, and 2003 Mars Exploration Rover science teams.
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|Publication:||Sky & Telescope|
|Date:||Dec 1, 2003|
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