NEW INSIGHTS INTO THE GEOLOGIC EVOLUTION OF MARS BY THE HRSC EXPERIMENT ON THE MARS EXPRESS MISSION

After almost 4 years of operation in orbit, the High Resolution Stereo Camera (HRSC) Experiment on ESA's Mars Express Orbiter has covered almost 40% of the surface of Mars at a resolution of 10-20 m/pixel in color and stereo. By now, the international team of Co-Investigators has investigated much of the major structures appearing to have been shaped by volcanic, fluvial, glacial, or hydrothermal activity. Contrary to early Viking-based attempts of understanding the time-stratigraphic relationships on the martian surface by crater-counting techniques and principles of stratigraphic superposition, where most of the geological units and constructs came out as being rather old, in the range of billions of years, the new HRSC-based data tell us that Mars had continued activity throughout its whole history from more than 4 Ga ago until very recently. There is a striking appearance of peaking of the geological activity or episodicity of resurfacing at certain times: approx. 3.5 Ga, 1 to 1.5 Ga, 300 to 600 m.y., approx. 200 m.y. ago, respectively. Even more striking is that within relatively narrow limits, the cratering ages of the different age groups fall together with the age groups of martian meteorites. The martian meteorite ages reflect both igneous events and aqueous alteration events. So do the cratering ages. There is a remarkable paucity of age occurrences in the 2-3 Ga age range in the cratering data. This corresponds to a paucity of meteorite ages in the same age range. This appears to be a hint to either lower geologic activity in this time frame, or, more likely, the covering up of more ancient activity by subsequent events <2 Ga ago, with the exception of the residues from the time >3 Ga ago (the peak at approx. 3.5 Ga) when the martian surface was thoroughly shaped at a very high level of activity by gigantic volcanic, fluvial, glacial, and hydrothermal events which could not be completely erased by later events. The episodic behavior of martian geologic activity over time can possibly be explained by responses to the evolution of the interior of the planet that has just not reached plate tectonics. At the time of the peak of volcanic and fluvial activity around 3.7 to 3.3 Ga ago Mars changed its environment from more basic to more acidic conditions, and the large sulfate deposits formed. Then, rapidly, Mars fell dry on a global scale.