October 16, 2006
Mars: Time for a New Chronology?
The classic technique for assessing the history of a rocky planet's geology is to count craters. On average, areas with longer exposure to space have had more impacts, and therefore more craters. By counting craters, scientists have broken the geologic history of Mars into three eras: Noachian (warm and wet), Hesperian (volcanic), and the present-day Amazonian (cold and dry). Following Earthly practice, each era is named for the location where the characteristic terrain was first identified.
European scientists have now used a high-resolution imaging spectrometer aboard the orbiting spacecraft Mars Express to construct a new picture of the planet's geologic history. Although the new analysis does use crater-counting to confirm the age of various locations, the researchers propose to redefine Mars' geologic eras on the basis of the landscape's mineral content, not impact density or terrain features.The new analysis may foster a deeper understanding of the planet's history, and improve the odds of success in the upcoming search for traces of life. But by proposing to rename the geologic history of Mars, the researchers may have violated geologic precedent.
In the April 21 issue of the journal Science, a group headed by Jean-Pierre Bibring, of the Institut d'Astrophysique Spatiale (Institute of Space Astrophysics) in Orsay, France, revealed results from OMEGA, an instrument aboard Mars Express, which the European Space Agency launched in June 2003. OMEGA (the Visible and Infrared Mineralogical Mapping Spectrometer) measures 350 different frequencies of visible and infrared light reflected from the Martian surface, and can identify minerals by their spectral signatures. The article reported on one Martian year (687 days) of studies, covering about 90 percent of the planet's surface. Bibring is OMEGA's principal investigator.
The conventional ("time-stratigraphic") chronology of Mars includes three periods, with these very rough date estimates:
-- The Noachian Period (from the planet's formation to about 3.5 to 3.8 billion years ago) was named for Noachis Terra, a vast highland in the southern hemisphere. Heavy bombardment left many craters, but widespread evidence of water erosion was interpreted as an indication that the surface was warm and wet.
-- The Hesperian Period began as the bombardment eased, and may have lasted until 1.8 billion years ago. Named for Hesperia Planum, a high plain in the southern hemisphere, the Hesperian was marked by intense volcanic activity, which covered many craters from the Noachian Period.
-- The Amazonian Period (from the end of the Hesperian to the present era), was named for Amazonis Planitia, a low plain in the northern hemisphere. The Amazonian is too cold for liquid water, at least at the surface.
OMEGA detected two types of water-bearing minerals: phyllosilicates and sulfates, each characteristic of a different era, says Bibring. Phyllosilicates - clays - appeared during the early part of the Noachian era, when, as previous spacecraft have revealed, the surface of Mars was shaped by the unmistakable features of water erosion. Because clays on Earth are deposited when fine sediments settle from water, "it is very difficult to understand the formation of phyllosilicates without standing water," Bibring says.
Due to the presence of phyllosilicates, Bibring and colleagues named the period of their predominance the phyllosian period.
Sulfates appeared in the next era, which Bibring termed the theiikian era, as huge outpourings of lava altered the surface and delivered vast amounts of sulfur to the surface. This volcanism has long been noted, and Viking long ago detected sulfur in the upper level of soil, but the vast amounts of sulfur and wide distribution of sulfates that OMEGA found suggested that the eruptions lofted huge quantities of sulfur into the atmosphere, which reacted into sulfates in the few locations where water was made available by the tectonic activity that caused by the raise of Tharsis, before the Noachian period ended. Because of the thin atmosphere at the time, any standing water disappeared too quickly for theiikian terrain to be a good candidate for a fossil search, Bibring says.
Mars Express builds on ample preexisting data on Mars, says Kenneth Tanaka of U.S. Geological Survey in Flagstaff, Ariz., a pioneer in Mars chronology. With its increased number of spectral bands, "the OMEGA spectrometer on Mars Express has been able to detect with greater sensitivity the presence of hydrated minerals on the surface of Mars than the spectrometers onboard Mars Global Surveyor and Mars Odyssey."
OMEGA's identification of hydrated minerals gives depth to the conventional view of Mars's three geologic periods, says Bibring. "When you have environmental conditions for forming phyllosilicates, you could not form sulfates, and conversely, when you had sulfates forming, you could not form phyllosilicates." Clays require alkaline conditions, and sulfates require acidic conditions, he says. "We see from the pH, and many other indications, that the global environment has changed. Through mineralogy, we see that these long-term properties depend on environmental conditions."
Bibring and colleagues proposed an alternative chronology of Mars, with three periods that largely, but not completely, overlap the conventional chronology:
-- Phyllosian: marked by the presence of phyllosilicates, clays with a large proportion of iron.
-- Theiikian (Greek for sulfate): marked by the presence of occasional sulfates, an indication of intense volcanic activity, formed by a reaction with water.
-- Siderikian (Greek for ferric iron -- Fe+3): marked by anhydrous ferric oxides and the absence of liquid water.
The new analysis brings a lot of new information to the table, says Michael Carr, a veteran Mars geologist at USGS in Menlo Park, Calif. "Phyllosilicates were not known before. It was a puzzle: During the Noachian, there was erosion to form valley networks, which implied warm conditions because it looked like they were formed by surface runoff. But why, why could we find no evidence of weathering?"
OMEGA found that evidence, in the form of clays, which typically are created when water erodes rock and makes microscopic grains that later settle out to form clay, often in river deltas and lake beds.
But if the mineralogical information is welcome, the new names are a different matter. "There is a convention that has long been used in terrestrial geology," says Carr, "to name an era after the type area," the first well-described instance of that particular era. The Pennsylvanian Period of the Paleozoic era, for example, was named for features in that state that were deposited 323 to 290 million years ago. "In the early 1980s, we followed the same system with Mars, naming things for the type locations," Carr says.
So why upset geologists with a new system? "I think they are playing games. He [Bibring] is not suggesting a new chronology; he is suggesting a new set of names for the Noachian, Hesperian, and Amazonian. It seems nonsensical to me."
But Bibring responds that the new data shows a different picture of the planet's history, and that calls for new names. "The eras are not identical. We still have three eras, but the timing is decoupled from the previous three. We are not renaming eras, we are redescribing the history based on a mineralogical analysis of Mars."
In any case, the OMEGA results could improve the odds of finding fossil evidence of ancient life on Mars. Scientists have long viewed the planet's modern surface, which ranges from minus 5 to minus 87 degrees C (from plus 23 to minus 125 degrees F), as too cold for liquid water and thus inhospitable to life. Although conditions may be slightly warmer below the surface, it's the planet's past that intrigues astrobiologists, not its present.
Scientists agree that early Mars was warm enough for liquid water. "Probably the environmental conditions when life could have formed was in the phyllosian period," says Bibring. "We are convinced that if you want to have the answer to whether Mars could have hosted life, you have to go to the clay-rich areas."
And since the phyllosian ended before the Noachian, it is the ancient clays - the phyllosilicates - that should be the target of exploration.
Not quite, Carr says. To find fossils, "You want to go to the Noachian era, but you don't necessarily want to chase phyllosilicates. There are lots of places to look. It depends on your model of life." The decision, he says, should reflect "your best guess of the conditions under which life might have thrived and also been fossilized. You get up a list of criteria, and try to translate that into geology. God knows where that path would lead you."
In any case, by contributing a finer-grained view of Mars's mineralogy, the Mars Express data may become central to the study of the Red Planet. "If you have only geomorphology [landforms] to derive history, when you see a feature, there are different ways to explain it," says Bibring. "On Earth, we could not understand the history if we only had images. The fact that you can add compositional data to the images gives you a huge step forward. But we need the context." Geomorphology and mineralogy, he says, "complement each other."
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