While rovers visiting the surface of Mars are invaluable research tools for understanding surface chemistry and other questions, observing Mars from orbit can help us understand the planet’s history — as recent images of a martian impact crater show.
The Mars Express orbiter uses its High Resolution Stereo Camera (HRSC) instrument to take photographs of the Martian surface, including geographical features and locations of water ice. Recently, ESA has shared images created from HRSC data of deep gouges on the surface of the Tantalus Fossae region. These troughs are part of a fault system that stretches nearly 1,500 miles across and are located around the edges of a low-lying volcano called Alba Mons.
“The fossae were created as the summit of Alba Mons rose in elevation, causing the surrounding surface to become warped, extended, and broken,” ESA explains. “The Tantalus Fossae faults are a great example of a surface feature known as grabens; each trench formed as two parallel faults opened up, causing the rock between to drop down into the resulting void.”
Another image shows the same region but from a different angle, as generated from digital information collected by the HRSC instrument and others. By bringing together information about elevation and surface features from different instruments, digital terrain models can be generated that show what the terrain looks like in three dimensions.
The large crater shown in the images can be helpful in dating the grabens features. The crater would have been caused when a chunk of rock or ice came streaming through the thin atmosphere and hit the planet, throwing up debris and creating a shock wave that forms a circular shape in the surface rock. The force of this impact would obliterate any surface features which existed there previously.
As the grabens can be seen running across the impact crater, they must have been formed after the impact occurred. This is one way that geologists build up a picture of the history of distant bodies like Mars or the moon, by looking at the age of certain geological events relative to particular impact craters. This impact crater is an old one, and it was already present on the surface when the Alba Mons volcano started pushing up to the surface and forming the faults of Tantalus Fossae.
If you look carefully at the top image, you can see another impact crater, much smaller and located at the bottom left of the main crater, which cuts across the fault lines. That suggests it is much younger and was created by an impact that occurred after the fault system formed.
