A major question in Mars history is whether the planet had an ocean in its northern hemisphere. New research provides evidence for a northern ocean or large sea based on stratigraphic analysis of sedimentary basin fill exposed at Aeolis Dorsa, a trough-like region of Mars about 500 km wide and 900 km long, surrounded east and west by 2 km-high plains and to the south by steep slopes up to the southern hemisphere.
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