A giant volcanic island in an early Martian Ocean?

Tracking early oceans on Mars has fundamental climatic and exobiological implications but requires unambiguous datable markers. Here we show that the Olympus Mons giant volcano shares morphological similarities with active volcanic islands on Earth where major constructional slope breaks systematically occur at the sea-air transition in response to sharp lava viscosity contrasts. We propose that the upper rim of the 6-km high concentric main escarpment surrounding Olympus Mons most likely formed by lava flowing into liquid water when the edifice was an active volcanic island during the late Noachian - early Hesperian. Similar features on the northern flank of the 1800 km-distant Alba Mons volcano further support an extended early Ocean initially occupying Martian lowlands. Abnormal current shoreline heights imply major surface uplift promoted by internal dynamics during the main phase of development of the Tharsis Bulge at some time during the Hesperian. After ocean retreat, the uplifted shorelines were partly blanketed by late sub-aerial activity during the Amazonian. The newly proposed volcanic shorelines can be dated by radiometric methods, constituting exceptional targets to track early liquid water at key temporal steps and provide valuable insights into the timing and fate of Martian Oceans.