Planetary Geology
Martha S. Gilmore

Spacecraft exploring the solar system continued to make exciting new discoveries about Mars, Jupiter, and Saturn last year. Part of Mars' core was found to be liquid and 20 new Jovian moons were discovered, to name just a few.

Mars Global Surveyor (MGS) and Mars Odyssey continue to orbit Mars and return visible images at meter-scale resolution, including images of sedimentary rock layers. They are also returning data from temporal studies of wind dynamics, mass movements, and frost and dust deposition. Nighttime thermal images provided by the Thermal Emission Imaging System (THEMIS) aboard Odyssey show variations in the thermal properties of surface materials that correspond to grain size. THEMIS' multispectral channels are beginning to reveal variations on the surface due to mineralogy.

Data from both MGS and Mars Odyssey are providing a new and evolving view of the state of water on Mars. In 2002, neutron and gamma-ray detectors aboard Odyssey found abundant hydrogen, ~50% by volume-and, by proxy, water ice-in the upper meter of the martian regolith poleward of approximately 60° (see articles by Feldman et al., Science online, May 30, 2002; 10.1126/science.1073541; Boynton et al., Science online, May 30, 2002; 10.1126/science.1073722; and Mitrofanov et al., Science online, May 30, 2002; 10.1126/science.1073616). High-resolution images of surfaces at these latitudes show an abundance of smooth mantling material that appears to flow in some areas. This material has been interpreted to be a dust/ice mixture that may be deposited by condensation and/or precipitation during times of higher obliquity (see overview by Kerr, Science, v. 300, p. 234). Melting of such deposits is also suggested as a source of geologically young gullies (Christiansen, Nature, v. 422, p. 45).

A very fundamental measurement made by Mars Global Surveyor was not collected by its instruments. By tracking the precise orbital motion of the spacecraft over three years, the deformational response of the planet to solar tides-measured by its k2-Love number was determined. These data show that the core of Mars is not wholly solid and that part of it must be liquid (Yoder et al., Science, v. 300, p. 299). This information, reconciled with petrology of the martian meteorites and the planet's magnetic field, will constrain models for the martian interior.

Following several open meetings of the planetary community, two landing sites were selected for the Mars Exploration Rover missions, scheduled for launch June 5 and June 25 of this year with arrival in January 2004. The first site is a unique, Texas-sized deposit containing gray, crystalline hematite, which on Earth is often formed by precipitation from hydrothermal fluids. The second site is on the floor of Gusev crater, which contains geomorphic evidence of long-term sedimentation and is likely to have been the site of a lake. Both sites offer the two rovers potential access to rocks associated with aqueous processes on Mars. This mission is supported by field tests of similar rovers (Journal of Geophysical Research, v. 107, November 25, 2002).

In November 2002, the hardy Galileo spacecraft took images of the small moon Amalthea while measuring the intense radiation environment at its closest approach of Jupiter. Galileo has been maneuvered into a position from which it will impact Jupiter in September of 2003. This planned impact is designed to ensure that the spacecraft, in its death throes, will not hit Europa and biologically contaminate this water-rich world. Meanwhile, ground-based astronomers have discovered 20 more moons of Jupiter this year, bringing the total to 60 (Sheppard and Jewitt, Nature, v. 423, p. 261).

The Cassini spacecraft remains in good health en route to the Saturn system, where it will arrive in 2004. The spacecraft collected over 26,000 images of the Jovian system over six months when it flew by in late 2000 and early 2001. As reported by Porco et al. (Science, v. 299, p. 1541), these images show the enormous, centuries-old storm near the equator we call the Great Red Spot absorbing smaller, adjacent storms, while at Jupiter's pole, storms as large as the Great Red Spot form and dissipate in just weeks. Cassini also confirmed that two small, inner Jovian satellites contribute material to Jupiter's main ring.

These data pave the way for future missions. In early June 2003, the European Space Agency (ESA) hopes to launch its first interplanetary mission, Mars Express, with arrival in late December. The mission's orbiter carries cameras and spectrometers to measure the distribution of water vapor in the atmosphere and the first ground-penetrating radar instrument to go to Mars. The mission also carries a lander, the Beagle 2, which will land in an ancient impact basin that potentially contains rocks from some of the oldest surfaces on Mars. In the United States, the MESSENGER mission to Mercury is being assembled in preparation for a March 2004 launch. Preparations are also being made for potential missions to the Moon's South Pole Aitken Basin, a huge impact basin on the far side that exposes mantle materials. Also this year, NASA has commenced the groundwork for the Jupiter Icy Moons Orbiter Mission (JIMO).

With Cassini on its way, a fleet of Mars spacecraft soon to be launched, and new data being returned, a very exciting year lies ahead for planetary sciences.

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Gilmore is an assistant professor in the Department of Earth and Environmental Sciences at Wesleyan University. E-mail:

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