Scientists have dug into the history of these geological features ahead of a new mission to the red planet, revealing a time when water flowed across its surface.

Large parts of Mars' highlands may have been washed away four billion years ago.

While the red planet might today be cold, dry, and dusty, geological evidence shows that water once flowed across its surface. Evidence of rivers, crater lakes and drainage basins have been found across Mars by planetary scientists using orbiting satellites and rovers on the surface.

New research has focused its attention on a group of thousands of mounds in Chryse Planitia on the border between Mars' northern and southern hemispheres. It reveals that they are the last remnants of ancient highlands which retreated by hundreds of kilometres after water wore much of them away billions of years ago.

Dr Joe McNeil, a scientist at the Natural History Museum who began this research during his PhD at the Open University, says that the mounds are "a prime location for future Mars missions."

"This research shows us that Mars' climate was dramatically different in the distant past," Joe says. "The mounds in Chryse Planitia are rich in clay minerals, meaning liquid water must have been present at the surface in large quantities nearly four billion years ago."

"As a result, the mounds preserve a near-complete history of water in this region within accessible, continuous rocky outcrops. The European Space Agency's upcoming Rosalind Franklin rover will explore nearby, and could allow us to answer whether Mars ever had an ocean, and if it did, whether life could have existed there."

The findings of the study were published in the journal Nature Geoscience.

What is the Martian dichotomy?

"Plate tectonics give the Earth its continents and ocean basins, but Mars has no plate tectonics of its own," Joe explains. "This means it's difficult to explain why the southern highlands of Mars have a very thick and ancient crust, while the northern lowlands are very young and thin."

"Some models have suggested that Mars' north was struck by one or more giant impacts early in its history, effectively creating the northern lowlands as a large depression. Alternatively, it could be the result of mantle-driven processes similar to plate tectonics, but which ended quite early on in Mars' history."

However it formed, the Martian dichotomy stretches around the entire planet. But its appearance varies around the planet. In some areas it is an imperceptibly shallow slope linking the highlands and lowlands, in others a sheer cliff as much as half a kilometre high.

Joe has been focused on this region as part of his research. He's been particularly interested in a region of mounds, which are up to half a kilometre high and a couple of kilometres wide, as part of his work with the UK Space Agency.

"No one has really looked at the mounds before, but as the Rosalind Franklin rover is due to start exploring nearby in four years' time, it's important to understand them so the rover's observations can be understood in the regional context."

What are Mars' mounds made of?

"Each mound is made up of a series of layers, each of which is a record of a past event," Joe says. "The oldest are at the bottom and are made up of rock that is around four billion years old. For a geologist, looking at these layers is like looking at pages of a book - each one tells a story!"

"The mounds contain up to 350 metres of clay-rich rock, which shows that there must have been a lot of water present on the surface for a long time. It's possible that this might have come from an ancient northern ocean on Mars, but this is an idea that's still controversial."

Water present on Mars' surface caused chemical reactions with the rocks that formed clay-rich sediment in the highlands. Over time, these highland areas eroded away at the edges, leaving the mounds behind. Exactly how long this process took is currently difficult to say.

"It's difficult to precisely date regions of the planet without samples to test in the lab," Joe explains. "Instead, we count craters. We can assume that places with more craters are older, so areas with relatively less craters are probably younger."

"I've estimated that this process took between 100-200 million years. This could mean that the sediment built up quickly and then eroded slowly, or vice versa."

Narrowing down these dates further will mean bringing samples of Mars to Earth, something that is currently being planned but is not expected to happen until the 2030s. By finding out more about Mars, Joe explains, we can find out more about Earth too.

"Mars is a model for what the early Earth could have looked like, as its lack of plate tectonics means that Mars' ancient geology is still in place," he says. "Looking at early Mars helps us to understand the early Earth, and as more missions visit the red planet, the more we'll be able to dig into our own planet's history."

Research Report:Dichotomy retreat and aqueous alteration on Noachian Mars recorded in highland remnants

Related Links
The Natural History Museum
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