The Zhurong rover provides evidence that this region of Mars was covered by an ocean!

Red Mars could have been blue in the distant past. This is at least what the observations reported by the Chinese rover Zhurong suggest. They came to support a hypothesis put forward several decades ago about the former existence of an Arctic Ocean on the planet. A hypothesis that was, thus far, sorely lacking from the data collected On site.

And that was one of Zhurong’s tasks. Collect evidence to confirm or disprove the existence of an ancient ocean on Mars. The rover’s landing site has already been carefully chosen to achieve this goal. In the year 2021, Zhurong has landed at the southern end of the Utopia Plain, in the northern hemisphere of the planet, at what has been interpreted as an ancient coastline.

The sediments are strangely labeled but bear signs of Mars’ past

During its nearly two-kilometer journey, the rover has collected all kinds of information, in particular thanks to its MSCam multispectral camera, allowing for a better understanding of the terrain in this region. Thus Zhurong studied in detail the named geological formation Facitas borealis. Its cameras produced 106 panoramic images that allowed scientists to study the surface morphology and structural properties of rocks exposed along the probe’s path.

The results are presented in an article published in the journal National Science Review. And there is a surprise! Images transmitted by the rover show that the rocks are neatly arranged and record a well-known pattern on Earth. These are not igneous rocks, so common on Mars, but sedimentary rocks with inclined plates. The studied outcrops are actually built by different sedimentary units consisting of oblique lines. However, the direction of these lines (we are talking about lamination) is opposite between each unit. This organization indicates a certain dynamic in sediment deposition: it indicates a bidirectional flow that is not associated with aeolian formations, like most dunes currently observed on Mars. On Earth, these inclined plates whose orientation regularly alternates are in fact typical … of a shallow marine environment subject to the action of tides!

A stream going one way, then the other

When the tide rises, the current arranges the sediment as small ripples that are built up as they advance towards the coast. In section, we will thus notice plates sloping towards the periphery. During tides, on the contrary, new ripples of sediment will be deposited on the previous ones, but this time they advance towards the ocean. Therefore, we will notice in the section, for this sedimentary unit, inclined plates sloping towards the coast. Thus the entire stratigraphic column shows fine sedimentary units with alternating directions of oblique lamination. This is exactly the formation observed on Mars in formation Facitas borealis ! But Zhurong’s notes make it possible to move on to the analysis of the ancient environment.

At the base of the stratigraphic column (the oldest part), there are already sediments without lamination, indicating a calm sediment environment, not subject to tidal influence. In the upper part of the column (the smaller part), on the contrary, we notice the non-planar units, whose small cross-shaped shape is usually associated with the development of small channels that cross each other over time. Thus the superposition of these units (flat, laminated, then trough) attests to the evolution of the coastal environment of this ancient ocean, and more specifically to marine gradient (a relative decrease in sea level).

Therefore, these separate sedimentary structures are extremely informative and provide evidence that an ocean did indeed occupy the northern region of Mars 3.6 billion years ago. A primary consequence of retracing the planet’s past and searching for possible traces of life.