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 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 July 2000

Planetary Geology

James W. Head III

Exciting results continue to pour in about Mars, asteroids and the Galilean satellites of Jupiter, but failures of the Mars Climate Orbiter and the Mars Polar Lander in 1999 caused reexamination of Mars exploration. On the positive side, planetary exploration produced some fundamental results this year. These new results could be shared virtually immediately through the increasing sophistication and accessibility of the Internet.

A new moon emerges

The Lunar Prospector spacecraft was de-orbited and made to crash into the south polar region of the moon to test for the presence of polar ice (none was found). Prospector and earlier datasets allow complete analysis of the Moon’s gravity, topography, mineralogy, chemistry and other crustal and interior properties. These data suggest new ideas about the Moon’s evolution. The European Space Agency has begun the SMART-1 mission ( to be launched in 2002 to follow up on some of these ideas and develop new technology, and Japan will send Lunar-A to the Moon in 2002 to deploy penetrators with heat flow and seismic instruments on the nearside and farside (

Mercury on the horizon

The least known of the terrestrial planets, Mercury, will finally be explored by NASA’s  Discovery Mission (Messenger; Missions during 2008-2009 will investigate unusual polar deposits of possible water ice, planetary surface features, a very large core, a magnetic field and enigmatic crustal mineralogy and geochemistry.
Water on Mars

The Mars Global Surveyor Mission continues to produce spectacular results (  Mars Orbiter Laser Altimeter data are providing evidence for large drainage basins, ancient polar deposits, ocean-scale bodies of water in the past history of Mars, extensive volcanism and unusual crater characteristics. The Mars Orbiter Camera put Mars under a microscope at several-meter-scale resolution.

The thermal emission spectrometer recorded regional deposits of hematite and two types of igneous rock (basalts and basaltic andesites) apparently typical of the southern uplands and northern lowlands, respectively. Gravity and topography data suggest that heat loss in earlier Mars history may have been greater in the northern lowlands. The details of future Mars exploration missions are uncertain, but the goal is return of samples. The European Space Agency will send the Mars Express mission in 2003 with an imaging spectrometer, very high resolution stereo cameras and ground-penetrating radar (

      Portion of the south polar residual cap of Mars. The upper surface 
      is dotted with a combination of polygonal patterns created by shallow
      troughs and large, almost circular pits formed by collapse. It appears 
      the large arcuate and circular pits formed from collapse, which means 
      that something underneath them has been removed. Alternatively, the
      ice that makes up much of the polar material has somehow become
      compacted, allowing the surface to sag and create pits. This Mars Global
      Surveyor Mars Orbiter Camera view of the south polar cap surface was 
      obtained Nov. 3, 1999. This view covers 3-by-3 kilometers at 1.5 meters 
      at 1.5 meters per pixel. The pits are only a few meters deep, at most, as 
      determined by measuring shadows cast in them.
      NASA/JPL/Malin Space Science Systems. 

The building blocks of planets

Exploration of comets and asteroids is revealing the nature of the basic building blocks of the solar system. The Near Earth Asteroid Rendezvous spacecraft (recently renamed in honor of Eugene Shoemaker, the pioneering planetary geologist) went into orbit around the asteroid Eros ( Stardust is on its quest to return comet debris to Earth ( A Japanese mission, MUSES-C, will be launched in 2002 to return samples from an asteroid (

Volcanoes and oceans

The Galileo spacecraft returned new images of active eruptions on Io, catching a “curtain of fire” in the act. New results from the icy satellites suggest that there may be liquid water layers (“oceans”) beneath the surface of Europa, Ganymede and Callisto. More new data on Europa support a very young age and a relatively thin (a few kilometers) icy crust (;

And if that’s not enough ...

Cassini is well on its way to Saturn (passing Jupiter soon) to study Saturn's atmosphere, rings, magnetosphere, and icy satellites ( In 2004, the Cassini Huygens probe will descend to Titan's surface to assess processes that might have led to the development of life. Missions are being planned to orbit and land on Europa and to visit Pluto in the next decade or so. Most newly reported  systems of planets around other stars look little like our solar system. New knowledge of our own and other planetary systems will enable us to better understand the formative years and context of Earth.

Head teaches planetary geosciences at Brown University and is an investigator on the Mars Global Surveyor, Galileo Europa, Messenger and Mars Express missions. E-mail:

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