Once upon a time it seems that Mars had oceans. But the exact appearance of these bodies of surface water, their sizes and distribution is a matter of intense debate. Most evidence points towards the deep past, some 4 billion years ago as the age when Mars could have held marine environments. Since then the red planet may have been largely cold and arid, with only the occasional shift of climate conditions.

For many geoscientists, some of the most compelling evidence of those old, old watery bodies comes from the topographic and mineralogical signs of ancient shorelines. These so-called ‘contacts’ are seen as geological boundaries, especially in the northern plains of Mars. Crossing thousands of kilometers they are associated with past oceanic basins whose rather romantic names, like Arabia and Deuteronilus, belie their current state of extinction.

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