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Planetary Geology
Geoffrey Collins

It has been another exciting year for planetary geology, as our robotic emissaries and groundbased instruments continue to push back the edges of our solar system, and we learn more about how geology works on other planets.

The star of the show this year has again been Mars, with another flotilla of spacecraft arriving at the red planet. Although the Japanese Nozomi orbiter and the British Beagle 2 lander were both lost, three spacecraft survived to return new information: the European Mars Express orbiter and the two U.S. Mars Exploration Rovers — Spirit and Opportunity. Mars Express is obtaining spectacular high resolution stereo images of the surface, and its spectrometers have found evidence for small amounts of methane in the atmosphere.

The current NASA mantra of Mars exploration is "follow the water," and evidence for the role of water on Mars has become more complex and intriguing this year. Images from the Mars Global Surveyor and Mars Odyssey spacecraft have turned up more evidence for long-lasting fluvial sedimentary environments in Mars' past (Malin and Edgett, Science, v. 302, p. 1931-1934). Orbital spectroscopy of the surface provides evidence for both small amounts of carbonates in the surface dust (Bandfield et al., Science, v. 301, p. 1084-1087), which could indicate thicker greenhouse gases in the past, and exposures of olivine (Hoefen et al., Science, v. 302, p. 627-630), which would have quickly weathered away if Mars had been recently warm and wet.

The Spirit rover landed in Gusev crater, where the geomorphology indicated an ancient lakebed. However, one of the first minerals the rover found on the surface was olivine. Later measurements from Gusev have shown minerals in rock fractures that could have been deposited by groundwater, but so far no clinching evidence for massive amounts of water on the surface has been found at this site. The Opportunity rover aimed for a flat region, which orbital spectroscopy indicated was full of gray crystalline hematite; it found a landscape unlike any other visited by our martian robots. The soil is covered with a deposit of BB-sized spheres, which appear to be made of hematite and which are weathering out of the local bedrock. The bedrock itself is finely layered, contains abundant jarositeand shows trough cross-bedding — all evidence that at least this part of Mars had a watery past.

While the rovers search for evidence of liquid water, analysis of the orbital imaging data is supporting a view of Mars that includes the recent action of frozen water. Ice-rich mantles and glacier-like flow features on Mars may be pointing to a model of Mars with orbitally-driven climate cycles that bring it in and out of ice ages, with Mars currently in an interglacial period (Head et al., Nature, v. 426, p. 797-802). If this ice age theory is true, Mars appears to have the opposite response to Milankovitch cycles than Earth; Mars may enter ice ages when the poles are warmer because of an acceleration of water transport from the poles to the rest of the planet (Mischna et al., Journal of Geophysical Research, v. 108, E002051).

At the same time that Spirit was landing on Mars, the Stardust spacecraft flew through the tail of comet Wild-2, collecting samples of newly ejected cometary material for return to Earth in 2006. Stardust's camera captured several detailed images of the 5-kilometer-diameter nucleus, which is covered with craters and irregular pits.

Not all planetary discoveries are made by spacecraft. Images from the Palomar observatory pushed the envelope of our solar system wider with the discovery of Sedna, an object slightly smaller than Pluto whose eccentric orbit takes it far beyond the Kuiper Belt of icy planetesimals and into the inner regions of the theorized Oort Cloud.

Radar observations from the Arecibo telescope weighed in on two longstanding planetary questions this year. High-resolution mapping of the Moon's poles with long wavelength radar designed to penetrate meters into the soil did not detect the strong radar echoes that would be expected if there were large amounts of ice in permanently shadowed areas (Campbell et al., Nature 426, p. 137-138). Meanwhile, radar pulses aimed at Saturn's largest moon Titan returned specular glints from parts of the surface (Campbell et al., Science, v. 302, p. 431-434). This is evidence for very smooth, probably liquid, surfaces on Titan, lending credibility to the decades-old idea that Titan may have seas of liquid hydrocarbons beneath its hazy atmosphere. This summer, the Cassini spacecraft will enter the Saturnian system, and we will soon have a much clearer picture of what interesting geological processes lurk below Titan's orange haze.

The Europeans have launched two new missions this year — the SMART-1 mission will arrive at the Moon late this year, and the Rosetta mission launched in March will investigate and land on a comet in 2014. The Japanese Hayabusa asteroid sample return mission was successfully launched last year, with a planned asteroid encounter next year. Upcoming U.S. missions include MESSENGER, which will be launched to Mercury this summer; Deep Impact, launching at the end of the year to a smashing rendezvous with a comet next summer; and New Horizons, to be launched in early 2006 toward Pluto and beyond.

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Collins is an assistant professor of geology at Wheaton College in Norton, Mass. E-mail:

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