U.S. Geological Survey (USGS) scientists studying Mars have discovered minerals with profound implications for much of the the past history of the planet.
The mineral olivine, an iron-magnesium silicate that weathers easily by water, has been found in abundance on Mars. The presence of olivine implies that chemical erosion by water is low on the planet and that Mars has been cold and dry throughout most of its geologic history.
New surface maps of Mars, developed by USGS scientists through a monumental set of 500 trillion calculations, provide amazing clarity and allow for more detailed study of the planet's minerals.
"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 the 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."
Dr. Clark and Todd M. Hoefen at the USGS in Denver, Colo. found the plentiful olivine on Mars using data from the Mars Global Surveyor (MGS), Thermal Emission Spectrometer (TES). Olivine, a green mineral sometimes used in jewelry, is found on Mars in volcanic regions. Exposures include olivine-rimmed craters hundreds of miles across, and with different amounts of iron, giving lighter green and darker green rocks.
The Mars spacecraft has been in orbit in its surface mapping mission configuration for about 18 months, slowly building images of the surface. At this point, the TES instrument has measured about 3/4 of the surface, measuring a narrow six-mile wide stripe in each two-hour orbit. The USGS scientists just completed the 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. The more pole-ward regions have yet to be analyzed in this way. The calculations were done at the finest detail of the instrument and for the first time, maps of minerals could be made with clarity never before possible.
The TES instrument senses heat from the surface of Mars; it does so by splitting the heat spectrum, or "color" of the heat into either 143 or 286 wavelengths or colors of thermal infrared light. The variations in the amount of heat emitted by those colors allow scientists to determine what materials are at the surface.
The USGS scientists have identified details in the infrared spectrum that indicate what is probably a sulfate in the pervasive dust and many bright soils on Mars. Sulfates have been expected since the Viking Landers found sulfur in the Martian soils in 1976, but the host mineral was never identified. It could be a sulfide (sulfur attached to a metal) or a sulfate -- sulfur attached to three oxygen atoms. The latest study shows that sulfates are abundant, but there could still be small pockets of sulfides.
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 olivine occurs in darker basaltic volcanic rocks that cover a large portion of the planet. The sulfates occur in brighter rocks that are most likely mechanically weathered (meteor impacts, wind driven dust/sand erosion) with trace amounts of fine-grained hematite. The fine-grained hematite probably also comes from mechanical grinding of the coarser grained rocks.
The origin of the coarse grained hematite is still unknown. On earth, it is often associated with water, as in a hot spring or labeled. But such conditions would also produce other minerals that are not seen on Mars. The presence of water and chemical weathering would also produce clay minerals, which are not seen in the latest data nor in previous studies.
"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.
Clark concurs with other researchers that abundant water probably exists below the surface, but only in a frozen state and rarely, if ever, has it existed at the surface in a warm liquid form which makes searching for life much more difficult.
The Mars Global Surveyor, Thermal Emission Spectrometer instrument measures the reflected light from Mars (top image) as well as thermal infrared spectral colors (bottom). The reflected light view is similar to a black and white photo of Mars, which shows Mars is dominated by light and dark areas. In the upper left, several large volcanos are seen, labeled V1 to V4. The lower image is a color composite of reflected and thermal light, where red is reflected light (called albedo), a 7.27 micron spectral feature indicative of sulfates in green, and in blue are spectral features due to the green mineral olivine with some contribution from pyroxenes (both volcanic minerals). Small white areas are low iron olivine, for example at the area labeled N (an area called Nili Fossae), and the rim of the crater Argyre, labeled A. The small magenta spots near the center of the image, labeled H, are coarse-grained iron oxide. The other magenta area at upper left, labeled C, is due to clouds. Yellow and green areas probably contain more sulfate. Note the small round red spots on the left side of the image: these are giant volcanos (V1-V4), relatively sulfate free. Ancient lava flows from some of these volcanos also show as red and are relatively sulfate free. The vast areas of blue indicate abundant olivine which weathers so easily that it implies there has been little water and weathering reactions on Mars. Olivine occurs predominantly in the darker materials in the southern hemisphere. It also occurs in the giant 3000-mile long canyon system, Valles Marineris (labeled VM).
Notes added Nov 10, 2000:
The above image is a new version from the one released earlier. Both the first release and current release images are smaller versions of the full resolution image which is being submitted to a scientific journal for publication. In reducing the image, many of the small olivine zones were lost, being below the resolution of the press release image. This newer image was reprocessed so the many small olivine spots still show. As stated above, the olivine found so far covers only about 3% of the planet. The olivine spots on this map have been enlarged somewhat for easier viewing. The higher iron olivine in blue is more extensive so enlarging the spots gives the appearance of more solid coverage of olivine. The olivine is not this extensive; the blue regions indicate where the olivine outcrops and not necessarily the exact shape of the outcrops. The white spots are similar, but because they are more isolated, they show as more discrete outcrops. The white spots, representing lower iron olivine is are slightly smaller than represented here. The Nili Fossae area (labeled N on the map) is a large region of olivine outcrop and the solid white is approximately the correct size of the olivine zone.
From email questions we've received, there is some confusion over the concept of cold and dry and the geologic time period. The extensive olivine indicates cold and dry since the emplacement of the olivine, not the entire geologic history of Mars. However, the olivine occurs in many eroded areas, for example in canyons, so was emplaced when many of the erosional events occurred on Mars.
The olivine regions represent areas of higher concentrations of olivine and are not necessarily pure olivine. Laboratory spectroscopy studies of olivine are still underway to better constrain abundances. In some outcrops, we believe the olviine abundance may be in the 30% range, averaged over many square miles.
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