Back in the 1970s, NASA's Mariner 9 orbiter sent back images showing water-sculpted surfaces on Mars. This discovery settled a major debate – water once flowed on the Red Planet.

Since then, scientists have found more evidence suggesting that water played a significant role in Mars’ history. Martian meteorites contain traces of water dating back 4.5 billion years, and recent impact craters reveal ice beneath the surface even today.

But the big question remains, Did Mars ever have oceans?

A new study published in PNAS has made waves in the scientific community. The research, led by Jianhui Li from Guangzhou University in China, involved Chinese and American scientists analysing data from China’s Mars rover, Zhurong.

Zhurong’s findings provide an unprecedented look into rocks buried near what some believe to be an ancient shoreline. The researchers claim they have found evidence of beach deposits from a long-lost Martian ocean.

Searching for water on Mars

Mars rovers play a crucial role in studying the planet’s geology, soil, and atmosphere. They search for signs of water because water is key to determining whether Mars could have ever supported life.

One important indicator of past water activity is sedimentary rock. On Earth, sedimentary rocks form in lakes, rivers, and oceans, trapping clues about past environments. NASA’s Perseverance rover, for instance, is currently searching for signs of ancient life in a river delta inside Jezero Crater, a location believed to have once held a lake.

Zhurong, on the other hand, has been investigating something much bigger – an ancient Martian ocean in the planet’s northern hemisphere.

The Zhurong Rover: Uncovering Mars’ secrets

Named after a mythical god of fire, Zhurong was launched by the China National Space Administration in 2020. It was active on Mars from 2021 to 2022, exploring Utopia Planitia, a vast impact basin stretching 3,300 km in diameter.

Zhurong was investigating a series of ridges, believed to be paleoshorelines, which extend for thousands of kilometres across Mars. Many scientists believe these ridges mark the remains of an ancient global ocean that covered the northern third of the planet. However, this idea remains debated, and more evidence is needed.

What lies beneath Mars' surface?

To determine if the rocks in Utopia Planitia were shaped by an ancient ocean, the Zhurong rover collected data along a 1.3 km transect perpendicular to the suspected shoreline. Scientists wanted to understand what types of rocks were present and what they could reveal about Mars’ history.

Zhurong used ground-penetrating radar, which scanned up to 100 metres below the surface. The radar data showed layers of rock, some visible down to at least 30 metres deep.

Significantly, these layers dip gently into the basin, away from the shoreline – exactly the way coastal sediments do when deposited by ocean waves on Earth. Further analysis showed that the buried rocks are sedimentary, not volcanic, strengthening the case for an ancient Martian ocean.

A match for Earth’s coastal sediments

Scientists compared Zhurong’s radar data with similar studies of Earth’s sedimentary environments. The results were clear – the Martian rocks matched coastal sediments on Earth. In other words, Zhurong found evidence of a beach.

What this means for Mars’ climate history

The Noachian period (4.1 to 3.7 billion years ago) is often considered the wettest phase in Mars’ history. Orbital images have shown valley networks and mineral maps that suggest the planet had flowing surface water.

However, evidence of water during the Hesperian period (3.7 to 3 billion years ago) is more scarce. Many of the large outflow channels from this time, such as Kasei Valles, are believed to have formed from catastrophic groundwater releases, rather than standing water.

This has led many scientists to believe that Mars cooled and dried up by the Hesperian period.

Did Mars stay wet for longer?

The new findings challenge this idea. If Zhurong’s coastal deposits were indeed formed by an ocean, this suggests that surface water remained stable on Mars much later than previously thought – possibly extending into the Late Hesperian period.

If true, this means that habitable environments may have persisted longer than scientists assumed, increasing the chances that life could have once existed near this ancient Martian ocean.

While this study provides strong evidence for an ancient ocean, the debate is far from settled. More observations, data, and future missions will be needed to confirm just how long water lasted on Mars – and whether life ever found a foothold on the Red Planet.

(with agency inputs)