Buried Craters Found on Mars
A radargram presenting data collected by the Mars Advanced Radar for Subsurface and Ionospheric Sounding during the 1,886th orbit of the European Space Agency's Mars Express orbiter. It shows parabolic-shaped echoes from the rim walls of a buried impact basin.
Credit: NASA JPL

Astronomers have peered beneath the smooth surface of baby-face regions of Mars, and what they see isn't pretty.

Craggy lines and craters wrinkle the subsurface, revealing an impact-riddled past whose history has since been covered over in the low plains of Mars' northern hemisphere [image].

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.

The machine used in the investigation is the European Space Agency's Mars Express orbiter. It packs a radar instrument built by NASA and the Italian Space Agency.

"It's almost like having X-ray vision," said Thomas 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."

The technique uses echoes of waves that have penetrated below the surface, bouncing off features in the subsurface with electrical properties that contrast with those of materials that buried them.

Areas with fewer visible surface 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."

The study confirms previous work that had suggested buried craters based on barely detectable surface features.

The findings bring planetary scientists closer to understanding one of the most enduring mysteries about the geologic evolution of the red 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.

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