Geologists Finding A Different Mars Underneath

Observations by the first project to explore a planet by sounding radar strongly suggest that ancient impact craters lie buried beneath the smooth, low plains of Mars' northern hemisphere. The technique uses echoes of waves that have penetrated below the surface.

"It's almost like having X-ray vision," said Dr. Thomas R. Watters of the National Air and Space Museum's Center for Earth and Planetary Studies, Washington. "Besides finding previously unknown impact basins, we've also confirmed that some of the subtle topographic depressions mapped previously in the lowlands are related to impact features."

Studies of how Mars evolved aid understanding of early Earth. Some signs of the forces at work a few billion years ago are more evident on Mars because, on Earth, many of them have been obliterated during Earth's more active resurfacing by tectonic activity.

Watters and nine co-authors report the findings in the Dec. 14, 2006 issue of the journal Nature.

The researchers used the orbiter's Mars Advanced Radar for Subsurface and Ionospheric Sounding, which was provided to the European Mars mission by NASA and the Italian Space Agency. The instrument transmits radio waves that pass through the Martian surface and bounce off features in the subsurface with electrical properties that contrast with those of materials that buried them.

The findings bring planetary scientists closer to understanding one of the most enduring mysteries about the geologic evolution of the planet. In contrast to Earth, Mars shows a striking difference between its northern and southern hemispheres. Almost the entire southern hemisphere has rough, heavily cratered highlands, while most of the northern hemisphere is smoother and lower in elevation.

Since the impacts that cause craters can happen anywhere on a planet, the areas with fewer craters are generally interpreted as younger surfaces where geological processes have erased the impact scars. The abundance of buried craters that the radar has detected beneath Mars' smooth northern plains means the underlying crust of the northern hemisphere is extremely old, "perhaps as ancient as the heavily cratered highland crust in the southern hemisphere."

Learning about the ancient lowland crust has been challenging because that crust was buried first by vast amounts of volcanic lava and then by sediments carried by episodic flood waters and wind.

Co-authors are Carl J. Leuschen, Johns Hopkins University Applied Physics Laboratory, Laurel, Md.; Jeffrey J. Plaut, Ali Safaeinili and Anton B. Ivanov of NASA's Jet Propulsion Laboratory, Pasadena, Calif.; Giovanni Picardi, "La Sapienza" University of Rome, Italy; Stephen M. Clifford, Lunar and Planetary Institute, Houston; William M. Farrell, NASA's Goddard Space Flight Center, Greenbelt, Md.; Roger J. Phillips, Washington University, St. Louis; and Ellen R. Stofan, Proxemy Research, Laytonsville, Md.

Additional information about the Mars Advanced Radar for Subsurface and Ionospheric Sounding is available at http://www.marsis.com . JPL, a division of the California Institute of Technology, Pasadena, manages NASA's roles in Mars Express for the NASA Science Mission Directorate, Washington.

The Center for Earth and Planetary Studies is the scientific research unit within the Collections and Research Department of the Smithsonian Institution's National Air and Space Museum. The Center's scientists perform original research and outreach activities on topics covering planetary science, terrestrial geophysics and the remote sensing of environmental change.