Rocks in ancient Martian lake could hold remains of life

Evidence of a large lake system has been found in a Martian crater, with hopes that it could preserve the remains of life on the red planet. 

Images taken by NASA's Perseverance rover show layers of sedimentary deposits dating back 3.6 billion years. On Earth, these types of rock are known to be good at preserving fossils and organic material, with scientists hoping that the same may be true on Mars. 

Dr Keyron Hickman-Lewis is a Research Fellow in the Museum's Earth Sciences department and co-author of a new paper discussing these rocks.  

'When we consider the search for life on Mars, we expect that it would have occurred early in the planet's history, so we need ancient rocks,' explains Keyron. 

'To preserve fossils, we need rocks with a certain mineralogy. The kind of delta deposits seen in Jezero crater are rich in a group of minerals called clay minerals, which are exceptionally good at enhancing the preservation of organic materials.

'As such, Jezero crater meets these two fundamental requirements for searching for life on Mars.'

The research, conducted by an international team of scientists, was published in the journal Science.

Eye in the sky 

The Perseverance rover blasted off for Mars in July 2020 and spent seven months heading to the red planet. Its destination was Jezero crater, a 45-kilometre-wide region previously identified as possibly preserving evidence of Martian life. 

It is the site of an ancient river delta, with a large fan-like shape similar to those found at the mouths of the Nile and Mississippi rivers today. 

'We were expecting to find fine-grained sediments, as the spectral data from orbit indicated there were clay minerals in the delta,' Keyron says. 'Of course, it's one thing seeing something from orbit, but quite another having proof on the ground.

'Imagine yourself flying over a city centre: if you look only at the uppermost layer of the surface, you'll assume that planet Earth is made of concrete and cars, but on the surface you'll find soil, rocks, plants and other things that may not necessarily be detectable from the sky. Similarly, observing Mars from the ground gives us so much more information' 

Once it had landed, Perseverance examined the geology of its surroundings from a distance. One of these was a sheer-sided rocky hill, or butte, which was named Kodiak. It is believed to have once formed part of the river delta's fan before it became separated from it by erosion. 

'We've imaged Kodiak with two of the cameras on the rover,' Keyron says. 'The first is called Mastcam-Z, and the second is SuperCam. Mastcam-Z takes beautiful panoramas of the Martian landscape, while SuperCam has a powerful tool called the Remote Micro-Imager, which can take very high-resolution images at great distances.  

Keyron adds, 'We've taken panoramas of the delta and butte, and within these we have acquired a selection of high-resolution images that allow us to better understand the geological structures we've observed.' 

Between a rock and a hard place 

From the images, the researchers managed to identify a number of features that give clues about how Mars may have been in the distant past. Keyron says that the pattern of delta sedimentation at Kodiak is 'better than most textbook examples'. 

'You gain an unparalled insight when you can see the cross section through the delta, something we couldn't see from orbit,' explains Keyron. 'The rocks tell us about the past environment and the lake conditions during the early history of Mars.' 

While the fine sediments were expected from orbit, one aspect that couldn't be seen were boulders embedded within the layers of rock, giving evidence of ancient floods. 

'The boulders and the differences between the layers tell us something of the changing water conditions on Mars,' Keyron says. 'Since the transport of boulders requires a high energy water current, we know there was a fluctuating hydrological regime at the time of deposition.' 

This means that the boulders were likely washed into the crater, and they may also be able to tell Keyron and his colleagues something about the geology outside the crater.  

This is exciting because the terrain around the crater is even more ancient than the rocks within. It had not been planned to be studied for several years. 

With these images allowing them to identify targets of interest, the team are now planning what Perseverance should do when it reaches the delta in a few months. The team hope to take samples from the delta, and perhaps Kodiak itself, once the rover reaches its destination. 

'It's very much a voyage of discovery through the Jezero crater and it's an incredibly interesting locality,' Keyron says. 'I'm sure that there's much yet to be uncovered about the early history of Mars from this site.'