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Organic molecules in Martian crater help to reconstruct planet’s history
NASA’s Perseverance rover may have discovered organic molecules on the surface of Mars.
While they don’t prove the existence of alien life, they add further evidence that Mars has a much more complex past than previously realised.
Mars’ Jezero crater may have revealed its most intriguing finding yet, following the discovery of organic molecules in its rocks.
Analyses of rock samples collected by the Perseverance rover suggest the presence of aromatic molecules, a group of compounds which all contain a distinctive ring of carbon atoms. While it’s not the first time organic compounds have been found on the surface of the red planet, it suggests a greater diversity of these molecules than was previously known.
Dr Joby Razzell Hollis, a Natural History Museum researcher who co-authored the new paper, says, ‘We weren’t sure what to expect, but we’ve been surprised by the sheer variety of molecules we saw in our analyses.’
‘Different geological units of Jezero show different signatures, which suggests there are a wide variety of potential organics on the planet. This is exciting, because it can tell us so much about the planet, and we’re really looking forward to getting them back to Earth.’
While organic molecules are the building blocks of life on Earth, it’s not yet known what their origins are. Though they could have been made by ancient lifeforms, it’s currently just as likely that they were made by inorganic geological or chemical processes at the surface of Mars.
The rover is currently depositing promising samples across the surface of Mars in preparation for a future mission that will return them to Earth, where laboratory testing can confirm their presence or absence and explore their origins.
The findings of the study were published in the journal Nature.
Found in Oman in 1999, part of Sayh al Uhaymir 008 is now used by the Perseverance rover to check its sensors. Image © The Trustees of the Natural History Museum, London (All Rights Reserved).
Organic molecules on Mars
For more than 50 years robotic probes and landers have been sent to Mars to find out more about Earth’s neighbouring planet. Missions such as Viking, which touched down in the 1970s, were crucial for revealing what conditions on Mars are like.
These missions analysed samples of the planet’s rocks and atmosphere, recording its distinct chemical signature. Scientists subsequently realised that an unusual group of meteorites, known collectively as the SNC meteorites, shared this signature and therefore came from Mars.
The first clues that Mars could have organic molecules came from these meteorites in the 1990s, and a piece of a Martian meteorite from the Museum has even been taken by Perseverance back to Mars to help calibrate and test its sensors. But while these meteorites are important, they’re not as scientifically useful as returned samples will be.
Dr Keyron Hickman-Lewis, a Research Fellow at the Natural History Museum who works on the Perseverance mission, says, ‘Martian meteorites are a valuable source of evidence for geological processes on Mars, but their exact origins at the surface of the planet are typically unknown.’
‘By returning a suite of samples from Mars, we will constrain their relationships in space and time. This will allow us to answer a range of questions about Mars, both at present and in the distant past.’
For now, the NASA team are making use of Perseverance’s sensors to get a preliminary understanding of the samples.
The rover is equipped with the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals instrument, or SHERLOC for short, which shines a laser of ultraviolet light at rocks of interest. Depending on which light is absorbed and which is emitted, this can be used to determine the chemical composition of the compounds in the rock.
This information is then combined with images from the Wide-angle Topographic Sensor for Operations and Engineering (WATSON) to reveal the distribution of different compounds across the surface of the rock.
Differences in the distribution and composition of organic molecules across the samples suggest that there may have been a variety of processes which led to the formation of these rocks back when Mars had water. This raises the prospect that Mars may once have been habitable.
‘These are potential organics, but not proof of life,’ Joby explains. ‘These kinds of molecules can include the building blocks of life, but we don’t yet have enough information to prove that.’
‘However, the diversity of these molecules suggests that if organics were in the lake filling Jezero crater, they may have been preserved in these rocks. We’ll only know this for sure when the samples return to Earth.’
The presence of these molecules within what appear to be sulphate crystals complicates matters. While sulphates are known to be able to preserve organic material from the environment, such minerals can also be involved in abiotic chemical processes that form organic molecules without life needing to be present.
Perseverance does not have all the required equipment to be able to confirm where these organic molecules came from. Instead, it’s sealing the evidence into sample tubes in the hope that they can be brought back to Earth as soon as the mid-2030s.
The SHERLOC instrument allows the preliminary identification of organic molecules, ahead of laboratory testing when samples are returned to Earth. Public domain image © NASA/JPL-Caltech via NASA image library.
Bringing Mars samples back to Earth
Once the samples are back on Earth, it will likely be several years before they’ll be generally available for study. At first, they will be kept in a highly sterile environment, where techniques such as CT scanning will allow researchers to investigate the samples without opening the tubes.
‘When we return these samples to Earth, our research will follow principles known as “planetary protection”,’ Keyron explains. ‘Just as we try to protect Mars by minimising any contamination of the planet with Earth’s biosphere, we have to ensure our planet is also protected.’
‘This preliminary research will allow us to characterise the internal structure of the sample and its mineralogy, which will help to identify sections that might be good to focus on in the future.’
When the samples are ready to be opened, the team will then focus on conducting a range of tests that will provide fundamental information about the rocks such as their organic content, which will provide the foundations for future research.
Over time, these analyses will narrow down the possible reasons for how the organic materials came about.
‘Evidence of life is what we call the hypothesis of last resort,’ Joby says. ‘It means we have to eliminate all other explanations until it is the only one left.’
In addition to life, the samples will also be used to answer other fundamental questions about Mars. They can offer evidence of how Mars formed, the timings of key events, and even what its magnetic field may have once been like.
‘Together, these questions will allow us to reconstruct an ancient environment at the surface of Mars,’ Keyron says. ‘We will be able to reach greater depths of understanding with these analyses than ever before, and we should be very excited about that.’
Perseverance is currently en route to the edge of the Jezero crater, where it hopes to find carbonate rocks that might also be rich in organics. In just over a decade’s time, these rocks, along with the other samples, will help to answer some of the biggest and most fundamental questions about Mars.
