Planetary Geology
Martha S. Gilmore

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The past year provided a wealth of exciting data returned by several spacecraft exploring the solar system.

In October of 2001, the Mars Odyssey spacecraft joined the successful Mars Global Surveyor in orbit around Mars. Odyssey carries the first gamma-ray spectrometer and neutron detectors to map Mars, capable of detecting abundances of a number of elements in the martian regolith. Initial results show that the regolith poleward of 60 degrees south contains approximately 50 percent water within the upper meter, as do some areas in the northern plains of the planet (Boynton et al., Science online, May 30, 2002; 10.1126/science.1073722; Feldman et al., Science online, May 30, 2002; 10.1126/science.1073541; Mitrofanov, et al., Science online, May 30, 2002; 10.1126/ science.1073616). Odyssey also carries the Thermal Emission Imaging System (THEMIS) instrument, which will map the entire planet at infrared wavelengths with a resolution of 100 meters per pixel and with visible light images at 18 meters per pixel. THEMIS is returning the first nighttime thermal images of the surface, which show variations in the thermal inertia of materials. It is anticipated that the 100-meter-resolution multispectral data will reveal variations in mineralogy previously obscured by dust in images of poorer resolution. The result should be a complete mineralogical map of Mars.

Different mineral composition of the rocks, sediments and dust on the surface provide a colorful infrared image of geologic layering on Mars. This image shows a portion of Candor Chasma, a canyon within the great Valles Marineris system of canyons, at approximately 5 degrees south latitude, 285 degrees east (75 degrees west) longitude. The area shown is approximately 30 by 175 kilometers (19 by 110 miles). The image combines exposures taken by Odyssey’s thermal emission imaging system at three different wavelengths of infrared light: 6.3 microns, 7.4 microns and 8.7 microns. NASA/JPL/Arizona State University

The Mars Global Surveyor (MGS) craft is in the second year of its extended mission and continues to collect meter-resolution images and topography of the planet. From July to September 2001, dust storms obscured the majority of the planet and imaging efforts were concentrated on the South Pole, where the atmosphere was clear. A comparison of images of the carbon dioxide-rich ice cap showed very fast sublimation rates of ice over the last two years (Malin et al., Science v. 294, p. 2146). Topographic measurements showed that similar amounts of ice sublimated seasonally from the northern cap as well (Smith et al., Science v. 294, p. 2141). Such high sublimation rates may significantly alter atmospheric pressures on Mars over hundreds or thousands of years, with consequences for the stability of liquid water on the surface and the location and erosive power of winds.

Another study models the amount of liquid water that may be produced by melting of ground ice in the martian regolith when the planet is at high obliquity (105 year timescale), which results in increased surface temperatures and pressure. This work suggests that recent gullies on Mars may be relicts from the last era of high obliquity (Costard et al., ((Science)) v. 295, p. 110). In addition to studies of martian hydrology, continuing analyses of MGS data improve models of martian gravity, magnetics, atmosphere and geologic history. Several such studies are included in the Oct. 25, 2001, issue of the Journal of Geophysical Research--Planets (JGR-Planets).

Planning continues for selecting sites for the 2003 Mars Exploration Rovers. A top candidate is an unusual, Texas-sized area of the surface containing grey, crystalline hematite, which on Earth is often formed by precipitation from hydrothermal fluids.

The Galileo spacecraft performed its last close flyby of Io in January of this year, placing the craft on a trajectory to crash into Jupiter in 2003. Io continues to erupt furiously, and the study of Galileo visible and near-infrared data over several years reveals that Io's volcanoes erupt in varying styles: steady and long-lived vs. short-lived, intense eruptions and ultrabasic vs. basic lavas (evidence for sulfur flows remains ambiguous). The images also lend insight into the tectonics of ionian mountains and mass wasting. The Dec. 25, 2001, issue of JGR-Planets contains many of these studies. Analyses continue on the surface morphology and magnetic fields of the other Galilean moons, each of which may currently have or at one time had an underground ocean.

A recent study (Schenk, Nature v. 417, p. 419) uses impact craters to constrain the thickness of the icy crust of Europa to a minimum of 19 kilometers, which may preclude exchange between the ocean and the surface, and will make future exploration of the europan ocean more difficult.

On Sept. 22, 2001, the Deep Space 1 mission passed by the nucleus of the comet 19P/Borrelly, returning visible and infrared data at a 200-meter resolution. The young, morphologically variable surface contains no evidence of water or hydrated minerals and has active dust jets (Soderblom et al., Science v. 296, 1087). A comet is also the target of the Stardust mission, currently on its way to intersect with comet Wild 2 in 2004, where it will collect particles in the coma and return them to Earth in 2006. Sample return of solar wind particles is the goal of the Genesis spacecraft, which was launched in August of 2001 and will return samples in 2004. These missions will significantly improve our understanding of the chemical and isotopic composition of interstellar dust, the outer solar system and the solar nebula. The curation and handling of these samples will also pave the way for other sample return missions, particularly those planned for Mars.

We continue to analyze data from the Moon and Venus and of meteorite chemistry to improve our knowledge of the context of the solar system. Terrestrial analogs of martian surfaces were the topic of field trips in the Mojave desert in October of 2001, sponsored by NASA and the Lunar and Planetary Institute. The planetary community looks forward to the arrival of the Cassini spacecraft at the Saturn system in 2004, where it will drop a probe (with cameras and a gas chromatograph/mass spectrometer) into the atmosphere of Titan. The MESSENGER mission to Mercury remains on track for launch in 2004 to explore this Moon-like world. It is anticipated that the Pluto-Kuiper Belt Mission will launch in 2006 to explore what could be the ninth planet. It looks to be another exciting decade in planetary geosciences as we test our understanding of geologic processes on other worlds.

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Gilmore is an Assistant Professor of Earth and Environmental Sciences at Wesleyan University. E-mail.

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