The Olivine Martian Odyssey
The presence of a common green mineral on Mars suggests that the red planet could have been cold and dry since the mineral has been exposed, which may be more than a billion years according to new research appearing in the Oct. 24 edition of Science. This transparent-green mineral called olivine is sometimes used in jewelry on Earth, and has been found in abundance on Mars as an iron-magnesium silicate that weathers easily by water. If water was abundant recently on Mars, olivine would not be abundant.
Todd Hoefen, a U.S. Geological Survey (USGS) geophysicist, led a team of researchers from USGS, Arizona State University and NASA, that found abundant quantities of olivine on Mars. "Not seeing minerals that indicate chemical weathering is also consistent with the abundant olivine and that implies the chemical weathering is very low. Thus, a consistent picture is forming that says Mars' surface has remained cold and dry for a long time," said Hoefen.
Olivine is significant because it decomposes rapidly in the presence of water. Finding olivine on the surface is therefore a good indicator of a dry Martian surface. Olivine, a transparent green-colored mineral found in many volcanic regions, is susceptible to chemical weathering and readily alters to other minerals such as iddingsite, goethite, serpentine, chlorite, smectite, maghemite and hematite in the presence of water. Except for trace amounts of hematite, which gives Mars its red color, none of these other weathering products have been found. The olivine occurs in many eroded areas, for example in canyons, so was emplaced when many of the erosional events occurred on Mars.
"The large expanses of olivine, about one-million square miles, means chemical weathering on Mars was very low and has been low for most of its geologic history. This information contradicts a popular view of a past warm, wet period in Mars' geologic history," said USGS scientist Dr. Roger N. Clark at an annual meeting of the American Astronomical Society Division of Planetary Sciences in Pasadena, Calif. "If the warm period never occurred, other explanations for Mars' large canyons are warranted, and some have been proposed by other researchers."
|Mars as seen near opposition late August 2003, by the Hubble Space Telescope. This Isidis region is closest to the dark region, shown as Syrtis Major in the middle right. The Ganges Chasma is closest to the the region shown on the image middle left, by Shiaparelli Crater.
The team detected a 30,000 square kilometer area rich in olivine, in the Nili Fossae region of Mars. Nili Fossae has been interpreted as a complex of grabens and fractures related to the formation of the Isidis impact basin, where post-impact faulting exposed the abundant olivine. They have also found smaller deposits of olivine all over the planet, all indicating a surface dominated by volcanic processes. The Mars Express lander, Beagle 2, will touch down on Isidis Planitia, a large plain just north of the equator, on Christmas Day, 2003.
About three percent of the surface mapped so far contains abundant olivine, and another three percent contains coarse-grained hematite, consistent with Mars' red color. The fact that so much olivine is exposed at the surface indicates that there has been little to no weathering due to water, thus no liquid water-mineral chemical reactions. The age of the surface is somewhat uncertain but is probably over 3 billion years old.
They based their conclusions on data obtained from a Thermal Emission Spectrometer (TES) carried by the Mars Global Surveyor (MGS). It took approximately three years for the MGS spacecraft and the TES instrument to gather the data for the analysis, and scientists another year to analyze the results. The USGS scientists just completed a set of 500 trillion calculations to combine all the data obtained so far into surface maps covering the area from 45 degrees north latitude to 45 degrees south latitude.
From such a vantage point, features as detailed as the size of a bus can be imaged. The MGS spacecraft is healthy and continues to map Mars. A new batch of high resolution photos from its Mars Orbital Camera, taken between February and July 2002, were added online in April and they bring the total number of images in the online gallery to more than 123,800. The images are available from the Mars Orbiter Camera Gallery.
In addition to the Mars Global Surveyor, another spacecraft is also providing a different view of Mars. NASA's Mars Odyssey spacecraft also looks for thermal emissions in the infrared region of the light spectra. The first overview analysis of a year's worth of high-resolution infrared data gathered by its Thermal Emission Imaging System (THEMIS) was also published in a June issue of Science. THEMIS is providing planetary geologists with detailed temperature and infrared radiation images of the martian surface. The images reveal geological details that were impossible to detect even with the high-resolution Mars Orbital Camera on NASA's Mars Global Surveyor and that have 300 times higher resolution than MGS's Thermal Emission Spectrometer.
|Evidence for recent underground water seepage?
"With a visible light camera, I can take a picture of a lava flow, but even with the highest resolution cameras that we have today the smallest thing we can see is the size of a bus and in order to do geology I need to have more detail," said THEMIS Principal Investigator Philip Christensen, Korrick Professor of Geological Sciences at Arizona State University.
"The camera on Mars Global Surveyor," continued Christensen, "takes exquisite images that show layers, but it doesn't tell me anything about composition - is it a layer of boulders with a layer of sand on top? I have no way of knowing. With the THEMIS temperature data, I can actually get an idea because the layers vary - and each layer has remarkably different physical properties."
Analyzing the spectra from the ten different bands of infrared light the instrument can detect, the THEMIS team has begun to identify specific mineral deposits, including a significant layer of the mineral olivine near the bottom of a four-and-a-half kilometer deep canyon known as Ganges Chasma near the east end of the vast Valles Marineris trough system.
"This gives us an interesting perspective of water on Mars," he said. "There can't have been much water - ever -- in this place. If there was groundwater present when it was deep within the surface, the olivine would have disappeared. And since the canyon has opened up, if there had ever been water at the surface it would be gone too. This is a very dry place, because it's been exposed for hundreds of millions of years. We know that some places on Mars have water, but here we see that some really don't."
Overall, Christensen notes that the emerging diversity and complexity of the planet point to the likelihood of future surprises and keep enlarging the possibilities for discovery on Mars.
"With Odyssey, we are looking at Mars in its entirety, in context. It's remarkable how much this has already changed our view of the complexity and richness of the planet. We discovered that it has a really dynamic geologic history. It has far more ice and water than we thought -- we're seeing snow and gullies, layers - and there are also processes involving volcanoes, impact craters and wind. It's a fascinating place."
Twin NASA rovers and one European Space Agency probe are planned for Mars landing in late 2003 and early 2004. The first Mars Exploration Rover, or MER, will arrive at Mars on Jan. 4, 2004, the second, Jan. 25.
Plans call for each to operate for at least three months. These missions continue NASA's quest to understand the role of water on Mars.
Related Web Pages
Evidence for Snow on Mars - and Perhaps an Abode for Life?
Mars Odyssey web site (with new images)
Mars by Stories
Impact Crater Landing Sites for the 2003 Mars Exploration Rovers
Mars Exploration Rover Homepage
2003 Mars Exploration Rover Mission