Mars beach discovered: China's rover finds traces of a primeval sea

Planetary researchers still like to laugh about the Martian canals. The Italian astronomer Giovanni Schiaparelli first described them as natural structures in 1877. Later, more and more of his colleagues saw them and soon interpreted them as civil engineering measures by Martians with irrigation problems. The writer HG Wells used this idea in his 1898 novel "War of the Worlds".

By 1909 at the latest, however, the existence of the Martian canals had been scientifically disproven. But the water issue has remained the leitmotif of Mars research ever since. After the Mars Global Surveyor probe collected laser data for a topographical map of the entire planet in the 1990s, the lower-lying areas, which dominate the northern hemisphere in particular, were naturally colored blue. The northern lowlands, known as the Vastitas Borealis, therefore look on a Mars globe as if an enormous sea were lapping there.

Now, billions of years ago, Mars did indeed have liquid water, as evidenced by huge river valleys, clay minerals and possibly even the planet's red color. Until now, it was assumed that the red Martian dust consisted of anhydrous hematite, but a study recently published in "Nature Communications" has provided evidence that it is ferrihydrite, a form of rust that requires a certain amount of moisture to form.

But it was controversial whether the water of primordial Mars was enough to fill an entire sea. New information on this question now comes from the Chinese Mars rover Zhurong. It landed on the edge of a bulge in the Vastitas Borealis in May 2021 and then, among other things, traveled a 1.3-kilometer-long stretch perpendicular to a possible coastline of the hypothetical North Sea. Zhurong's ground-penetrating radar, a Chinese-American team now reports in the "Proceedings of the National Academy of Sciences", came across a layer hidden beneath the surface rubble at a depth of about ten meters, which slopes gently towards the Vastitas. These could not be wind deposits, nor those of a river, nor solidified lava flows, write the authors. Rather, this must have been the beach of a Martian ocean that may have existed for millions of years.

The finding stimulates the imagination: Such masses of water cannot all have disappeared into space; rather, large parts of them must be slumbering in frozen form in the Martian soil, now waiting for Elon Musk's Mars drivers to drill for them. However, the technical restoration of a sufficiently dense atmosphere to allow surface water to return to Mars, a process known as terraforming, would take at least 900 years, an American botanist once estimated years ago, and the result would be landscapes somewhere between the Australian outback and Siberia. But canals could be built.