A black and white image of the landscape of Oxia Planum based on orbital studies

Fluvial sinuous ridges, which stick up above the surface of Oxia Planum, represent what once were Martian rivers. Image © NASA/JPL/MSSS/Peter Grindrod

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Ancient Martian rivers discovered at planned ExoMars rover landing site

Rivers up to 600 metres wide once meandered across the surface of the red planet.

A new paper reveals that long ridges on the surface of Mars track the course of ancient waterways and may contain sediments that could preserve evidence of life. 

The next rover to land on Mars could discover signs of a once mighty river ecosystem.

The Rosalind Franklin rover, part of the European Space Agency's (ESA) ExoMars mission, is currently scheduled to launch in 2028 after suffering a series of delays. The ESA is meeting at the end of this month to finalise revised plans for the mission.

Ahead of that meeting, a new study, published in Earth and Planetary Science Letters, suggests that if the mission goes ahead, the rover could explore the remains of large meandering rivers that once spread across the planet's surface.

This landing site, known as Oxia Planum, is rich in minerals known as phyllosilicates, which are sought after by scientists because of their potential to preserve the organic remnants of Martian life. 

Dr Joel Davis, a Scientific Associate at the Museum who led this paper, says, 'This landing site is unique because it will be the oldest part of Mars that has been explored with any lander or rover.'

'That's important, because to look for evidence of ancient life, we have to go somewhere we think the conditions would have been suitable for life. This work shows that Oxia Planum could have had several different periods when conditions at the surface could have been habitable.' 

A black and white image of the landscape of Oxia Planum based on orbital studies

Some of the rivers would have been hundreds of metres wide. Image © NASA/JPL/MSSS/Peter Grindrod

How does Oxia Planum compare to other parts of Mars?

Mars is, on average, around 225 million kilometres away from Earth, which is roughly 600 times further than the distance between the Earth and the Moon. Despite this distance, humans have been sending scientific missions to the red planet for more than 50 years.

The first spacecraft to orbit another planet, NASA's Mariner 9, arrived on 14 November 1971, narrowly ahead of two spacecraft sent by the Soviet Union. While these probes arrived later, they did carry the first rover to land on Mars, even if contact was lost with them shortly after landing.

Since then, a variety of spacecraft have successfully touched down on Mars, including six rovers. These probes have significantly improved our knowledge of the planet, particularly what it was like over three billion years ago when it is possible that it could have supported life.

Compared to the areas previously visited by spacecraft, Oxia Planum is much older. It is on average around 500 million years older than the deposits in the Jezero crater which is currently being explored by NASA's Perseverance rover.

Dr Peter Grindrod, a Research Leader at the Museum and co-author on the paper, explains, 'Parts of Oxia Planum have been buried in the past, possibly by volcanic activity, debris from huge meteorite impacts, or wind-deposited sediments, before being exposed again by wind erosion.' 

'What we're left with in this case is the opposite of what would normally be expected, with the path of the rivers sticking out above the landscape in positive relief.'

'We've described these formations as fluvial sinuous ridges (FSRs), and they've survived because the old river systems are more resistant to erosion than the surrounding landscape.'

The FSRs point to a past Mars that is very different from the one we know today. Large rivers, stretching at least 70 kilometres long and as much as 600 metres wide, formed from four distinct catchment areas and would have meandered through the flat Oxia Planum.

While scientists are uncertain how these rivers were fed, it is possible that rainfall or snow and ice melt could have been responsible. The presence of water could have then led to the formation of the phyllosilicates, which have until now driven scientific interest in the site.

'We knew the site was rich in phyllosilicates, but we didn't know how they ended up there,' Peter says. 'There have been many different hypotheses, but as the majority of the river systems we've mapped contain phyllosilicates it could well be the case that these clay minerals formed in river environments.'

The findings are part of a multi-year effort by UK and international researchers to investigate Oxia Planum prior to the rover landing. This work was funded by the UK Space Agency through the Aurora programme. 

A digital illustration of the Rosalind Franklin rover

The Rosalind Franklin rover is named after the eponymous British scientist whose work was vital in uncovering the structure of molecules such as DNA. Image © ESA/Mlabspace, licensed under CC BY-SA 3.0 IGO via Wikimedia Commons.

Decision time for ExoMars

The discovery of phyllosilicates at Oxia Planum was one of the main reasons it was selected as one of four possible landing sites for an ESA rover on Mars. A successful landing would be a milestone for the agency, whose previous attempts to land on the red planet have both met with failure.

In the end, Oxia Planum won out because of its combination of scientifically interesting features and relative safety for landing a rover in. It is also similar to an area of Utah, USA, surrounding the Green River which is used to simulate Mars missions and test rovers.

Landing Rosalind Franklin in Oxia Planum would allow the rover to test out some of the paper's theories, including those about FSRs. If the rover discovers pebbles at the site, these could be used to help estimate how water would have flowed across the landscape.

If confirmed, the sediment laid down by this water could have trapped organic molecules from Martian life, and which may still be preserved. 

'Rosalind Franklin will be looking for signs of life on Mars by drilling two metres below the planet's surface,' Peter explains. 'This should be below the depth that radiation can penetrate to, so if there is any organic material it should be protected.'

However, whether or not the rover gets that far is yet to be decided. The fate of the mission is set to be decided by decision makers attending the ESA's council meeting, which will take place in Paris, France, on 22 and 23 November 2022.  

11 programmes are on the agenda for discussion, taking in everything from the development of Earth observation programmes to new rockets and space science.

While the fate of the rover hangs in the balance, researchers are making use of the ExoMars Trace Gas Orbiter, which is already in operation, to study the planet from space. 

It is hoped that the discoveries made by the orbiter, including papers such as this, will allow Rosalind Franklin to hit the ground running if, and when, it arrives on Mars.