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Ancient ocean on Mars? Chinese rover finds marine sediments
Posted by
Paul Scott Anderson

May 30, 2023
Ancient ocean on Mars: Reddish planet with blue ocean areas, craters and
white clouds.
View larger. |

There has been growing evidence for an ancient ocean on Mars, in the
northern hemisphere, as seen in this artist’s illustration. Now, China’s
Zhurong rover has found what mission scientists say are marine
sedimentary rocks. Image via Ittiz/ Wikipedia (CC BY-SA 3.0).
We know that Mars had rivers and lakes in the distant past. But what
about oceans? There is indeed tentative evidence for an ancient ocean in
the northern hemisphere. However, scientists have been debating that
evidence for decades. But now, researchers in China and the U.S. say
they have found new clues in marine sedimentary rocks in Utopia Planitia
that may prove the existence of that ocean. China’s Zhurong rover
discovered the sedimentary formations. The researchers announced the
tantalizing findings in Science China Press on May 21, 2023.

Professor Long Xiao from the School of Earth Sciences at China
University of Geosciences led the research team. The team published its
accepted peer-reviewed paper in National Science Review on May 18, 2023.
The paper, still undergoing final editing, is available as a PDF.

New evidence for ancient ocean on Mars
China’s Zhurong rover has been exploring its landing site on the
southern edge of Utopia Planitia in the northern hemisphere of Mars.
This is within the northern lowlands that some scientists say was likely
once an ocean floor. The rover’s newest findings now bolster that
possibility. The rover has been studying the rocks in the area with its
multispectral camera (MSCam), and the science team says that Zhurong has
found marine sedimentary rocks. While most other data supporting the
ocean hypothesis has come from orbiting spacecraft, this new data is in
situ (on site).

It is the Vasitas Borealis Formation (VBF) that the mission scientists
interpret to be marine sedimentary rocks. As the paper explains:

Decades of research using remotely-sensed data have extracted evidence
for the presence of an ocean in the northern lowlands of Mars in the
Hesperian, but these claims have remained controversial due to the lack
of in situ analysis of the associated geologic unit, the Vastitas
Borealis Formation (VBF). The Tianwen-1/Zhurong rover was targeted to
land within the VBF near its southern margin and has traversed almost
1.2 miles (2 km) southward toward the interpreted shoreline. We report
here on the first in situ analysis of the VBF that reveals sedimentary
structures and features in surface rocks that suggest that the VBF was
deposited in a marine environment, providing direct support for the
existence of an ancient (Hesperian) ocean on Mars.

Multispectral images provide clues
Zhurong has been gradually moving south toward what is thought to be an
ancient coastline. The rover has taken 106 panoramic images so far
during its travels. Mission scientists have been studying the
multispectral images for clues about the rocks’ composition and origin.
They found bedding structures that are different from the usual volcanic
rock deposits on Mars.

In addition, they were different from rock formations created by blowing
sand.

Aerial view of track of rover, highlighted in yellow, with photos of
features from points along it.
View larger. | Traverse map showing the Zhurong rover’s route and key
observations so far. Image via Xiao et al./ National Science Review (CC
BY 4.0).
A shallow sea environment
Indeed, the images revealed bidirectional flow, a sign of possible
low-energy tidal currents. They are similar to formations created in
shallow marine environments on Earth. The mission team named this region
the Zhurong Member. The rocks feature small-scale cross-bedding,
lens-shaped flaser bedding and small channel structures.

The paper says:

Bidirectional current orientations are characteristically formed by
regular opposite directions of tidal currents in terrestrial shallow
marine environments, and uncommonly in fluvial environments. Although
aeolian deposits on Mars also contain some small-scale
cross-laminations, the lack of larger structures indicative of aeolian
environments supports the interpretation that these are of shallow
marine origin.

The images also showed that the layers in the cross-bedding overlap and
tilt in two opposite directions. The thickness of the strata, as well as
the size of sand grains, also differs. This is evidence for a
bidirectional water flow pattern. On Earth, this is common in
littoral-shallow sea environments.

Ancient shorelines of an ocean on Mars and a wild river
Last year, researchers at Penn State and Caltech said they discovered
definitive traces of an old shoreline in Mars’ northern hemisphere.

And just last week, scientists announced that the Perseverance rover has
found evidence for a rollicking river. It was the deepest and
fastest-flowing river yet observed – now all dried up of course – by any
rover or lander.

The new findings from the Zhurong rover would seem to support this,
adding another crucial piece to the puzzle. That ocean would have been
vast, covering most of the northern hemisphere of Mars. The next
question, of course, is whether it supported any kind of life. It will
be interesting to see what else Zhurong finds in its travels, as well as
any future missions to this now-dry ocean wonderland.

Bottom line: Scientists in China say that the Zhurong rover has found
new evidence for an ancient ocean on Mars, sedimentary rocks formed in a
shallow marine environment. The ocean would have covered hundreds of
thousands of square kilometers. They estimate the ancient Martian
shoreline to be 3.5 billion years old.

Source: Evidence for marine sedimentary rocks in Utopia Planitia:
zhurong rover observations

Via Science China Press/ Eurekalert!