In May 2021, China’s Zhurong rover, part of the Tianwen-1 mission, successfully landed on Mars, in a region called southern Utopia Planitia. This mission aimed to investigate one of the most significant mysteries of Mars’ past: Was there once a vast ocean in the planet’s northern lowlands?

Now, after examining the geomorphological features and surface composition of Utopia Planitia sent back by Zhurong, researchers conclude that a nearshore zone once existed in this region, hinting at a rich watery past.

For decades, scientists have speculated about the presence of a massive ocean that may have once covered Mars’ northern plains. Some estimates say that up to a third of Mars may have been covered by water. The theory mostly revolves around a large area that looks like an ancient shoreline. This ocean, if it existed, would have formed during Mars’ Noachian and Hesperian periods, approximately 3.7 to 3.5 billion years ago. Back then, the planet was warmer and wetter than it is today.

Utopia Planitia was likely a part of the ancient ocean bed.

Bo Wu at Hong Kong Polytechnic University spearheaded a comprehensive analysis of the topographic features in the rover’s landing area. Like several other researchers, they’ve been looking at the Martian surface using satellite data. But Mars’ harsh conditions and the thick layers of dust obscuring its surface limit the usefulness of the data. This is why Zhurong is so important.

The rover is equipped with a range of instruments, including high-resolution cameras, ground penetrating radar (RoPeR), and spectrometers, designed to analyze Mars’ geology and search for signs of past water activity.

Mars’ geology hints at water surfaces

One of the critical findings from Zhurong’s mission was the discovery of layered sedimentary rocks with water-related lamination patterns. These rocks, observed through the rover’s cameras, contain hydrated minerals like silica and sulfates, which typically form in the presence of water. The spectral data indicated significant absorption bands that align with the presence of opal-like silica, suggesting a past environment where water was abundant.

The identification of sedimentary structures, especially those with cross-bedding features, implies that these rocks were shaped by water currents, similar to those found in Earth’s coastal regions. This is a strong indicator that Mars may have once had a dynamic shoreline influenced by tides or waves.

Researchers then found numerous geological forms that seem to be carved by water. They found something that looks like a deep marine zone, exhibiting extensive water flow with deeper, turbulent water conditions. They also found a shallow marine zone, characterized by more fine-grained sediments that seemingly deposited in calmer and shallower marine areas. And, between these areas, the team found transition areas.

It’s not only the surface area, either. Zhurong is equipped with a ground-penetrating radar that allows researchers to peer underground.

Zhurong’s ground-penetrating radar provided additional insights into the subsurface structure of Utopia Planitia. This equipment sends a radar pulse underground then measures how this pulse is reflected. The readings will indicate what’s happening underground. These radar scans revealed multiple layers of sedimentary deposits beneath the Martian surface, some extending as deep as 80 meters.

These layers appear to be remnants of the Vastitas Borealis Formation (VBF), which is thought to have formed from sediment deposition in a standing body of water. The radar data also suggested the presence of ice-rich layers, hinting that water may have been trapped beneath the surface for extended periods, potentially into the Amazonian epoch (the most recent geological period on Mars, which started 2 billion years ago).

What does this mean for Mars’ habitability?

By now, there’s overwhelming evidence that Mars had rich bodies of water. Researchers even have a pretty good idea of how these ocean-type bodies evolved in geological time.

It started with a flooding period 3.68 billion years ago, when large areas were inundated with water, creating a vast shoreline that stretched for miles and miles. Then Mars’ climate cooled, and the water body gradually froze over. Sediments carried by water flow settled at the bottom, creating layered deposits.

Over time, as Mars’ atmosphere thinned and temperatures dropped, the remaining water either sublimated or seeped into the ground, leading to the barren surface we see today. Yet, the sediment layers observed by Zhurong’s radar suggest that water persisted beneath the surface well into the Amazonian period.

If Mars was once a planet with vast oceans, that could also hint at a climate suitable for life (particularly microbial life). The presence of hydrated minerals further suggests that these waters were not merely transient but may have existed long enough to influence the planet’s geological history.

Zhurong’s mission is far from over. As it continues to explore the Martian surface, it may uncover more evidence of ancient water systems. The rover’s data will be critical for future missions, including plans to return samples from Mars.

Journal reference: Scientific Reports DOI: 10.1038/s41598-024-75507-w