Hypothesis of an ancient northern ocean on Mars and insights from the Zhurong rover

Various landforms suggest the past presence of liquid water on the surface of Mars. The putative coastal landforms, outflow channels and the hemisphere-wide Vastitas Borealis Formation sediments indicate that the northern lowlands may have housed an ancient ocean. Challenges to this hypothesis are from topography analysis, mineral formation environment and climate modelling. Determining whether there was a northern ocean on Mars is crucial for understanding its climate history, geological processes and potential for ancient life, and for guiding future explorations. Recently, China’s Zhurong rover has identified marine sedimentary structures and multiple subsurface sedimentary layers. The unique in situ perspective of the Zhurong rover, along with previous orbital observations, provides strong support for an episodic northern ocean during the early Hesperian and early Amazonian (about 3.6–2.5 billion years ago). The ground truth from future sample-return missions, such as China’s Tianwen-3 or the Mars sample-return programmes by NASA, ESA and other agencies, will be required for a more unambiguous confirmation. This Perspective presents the evidence in favour of and against the existence of an ancient ocean covering the Martian northern plains (Vastitas Borealis), discussing in particular the contribution of the Chinese Zhurong rover observations in supporting the presence of an ocean and the key role of the upcoming sample return.