The Royal Society of Edinburgh

Cormack Lecture: Further Adventures of the Curiosity Rover in Gale Crater, Mars

Professor Sanjeev Gupta Professor of Earth Science, Imperial College London

Thursday 27 November 2014

Report by Peter Barr

From deep time to Mars time

Glenelg in the Highlands of Scotland may be twinned with its namesake on Mars, but according to Professor Sanjeev Gupta, the whole Earth could be twinned with Mars, in terms of its geology – and the scientists guiding the mission now exploring the faraway planet are like 18th-century geologists with 21st-century tools...

“I'm just a data gatherer for future discoveries,” Gupta said at the end of his talk. “One day, a graduate student will use the data for a new interpretation.”

Future missions, he continued, may “doggy-bag” samples of rocks, send them up into orbit, and bring them back to Earth to study them, but the current mission (codenamed Curiosity) has already achieved its objective by proving that there were “habitable environments” on Mars that one day may reveal there was life on the planet.

The headline news from Curiosity – so far – is evidence of water. As the rover moves around on the surface, it sends back images of grains of sand as well as giant mountains, and one day it took a photo of a pebble which appeared to be rounded. This may have been a “brilliant stroke of luck,” as Gupta described it, but it took the team a few days to realise just how significant this finding was. On Earth, a rounded pebble would be treated as evidence of weathering by water or wind, but the scientists concluded that the shape was the result of the pebble tumbling and rotating in water – the first clear evidence of water on Mars. The scientists were shy at first to make a lot of noise about their discovery but the editors at Science magazine encouraged them to tell the world exactly what they'd found. The search for life continues but this was an important milestone on the way.

Deep timeGupta started his talk by discussing the origins of modern geology, showing a picture of Hutton's unconformity on the coast near St Abbs in the southeast of Scotland – “the most famous geological site on our planet” which first revealed the true age of Earth, or what scientists now call “deep time,” by showing evidence of ancient erosion side by side with rocks formed in completely different eras.

The 400 scientists and 200 engineers involved in Curiosity are not so very different to their counterparts three centuries ago. The major difference is their geological experience of Earth and new technology which they've been able to transfer to Mars. “The journey has moved to another planet,” said Gupta.

The Curiosity rover was sent to Mars three years ago, in a mission costing $2.7 billion. It is not a life-discovery mission, said Gupta, but a stepping stone to future missions, first trying to identify locations with the necessary conditions for life – i.e., energy and water, plus biogenic building blocks and chemical food. We now know Mars was “warm and wet” then started to dry out 3.6 billion years ago, roughly the same time that life first appeared here on Earth, and the mission set out to find signs of life inside the rocks on the surface as well as drilling down to depths of 7cm. Radiation, said Gupta, may have destroyed evidence of life so close to the surface, but future missions will drill down to depths of two metres, where the evidence may have survived.

Curiosity targeted two key environments – ancient lake and delta deposits – and landed on a flat gravelly plain inside the Gale Crater, which is 150km across and has a mountain in the centre called Mt Sharp, 5km high, formed of hundreds of layers of what looks like sedimentary rocks. After exploring the area at the base of the mountain, the rover will move up the mountain to reconstruct a geological record of Mars, with each layer representing tens of thousands of years – just like James Hutton “reading” the history of the rocks at St Abbs. The landing site was chosen based on the chances of discovering a habitable environment, rather than simply a safe spot to land, but Gupta also said the scientists did not know what rocks would be there, or what to look for once the rover was ready for action.

The mission moves “from MegaMars to MicroMars,” seeking to understand all sorts of features from craters and mountains to small grains of sand, using a range of equipment including cameras (taking lots of selfies), lasers and spectrometers to analyse the chemistry of rocks, x-ray diffraction to establish true mineralogy, and crucibles in which rocks can be heated to analyse the complex gases released, all powered by a nuclear plutonium battery.

The images collected can be autocorrected to focus on very small details, so geologists can tell – for example – if they are volcanic or sedimentary. “The process is extremely time- consuming and very complex, but the mission is really an engineering feat,” said Gupta. “The geologists are really only playing.”

Mars time

The mission team works to a punishing schedule and everyone has to adjust to each other's time zones around the world, as well as work to Mars time – much longer days. Every day, after the data comes in (including images), they have to decide what to do next – where the rover should move and what tasks it should perform. The scientists will get ideas and “horse- trade” their priorities, then fly them past the engineers to test them (assessing the complexity as well as the bandwidth and power required), while Gupta and five of his colleagues decide the long-term strategy. Even the tiniest movement of an arm on the rover to position a camera or drill a hole may take many hours of programming by dozens of people, followed by testing, to ensure the task is possible and avoid problems.

At first, the geologists could not be sure if they were looking at a giant lava lake or sedimentary layers, but gradually the evidence has started to build, confirming there was once flowing water, with rivers feeding into the crater. Remote sensing has indicated the presence of clay and sulphates, which could contain evidence of organic life, but the initial challenge was simply to establish if this was an environment which could support life. The rover has discovered fine-grained mudstones (sedimentary rocks) of the type which could contain organic life, but so far the analysis is negative. Drilling holes 6–7cm down has also revealed evidence of gypsum (a hydrated mineral), calcium and sulphate veins, providing evidence of water flow, but no signs of life as we know it.

Gupta also said the images of grains of sand on Mars look exactly the same as those found on Earth, and compared the landscape to the Mojave Desert. Gradually, the story of Mars can begin to be told, and as one scientist put it, the mission has changed our view “from Lego planet to real planet,” forcing some geologists to tear up their models. The scientists will continue to argue as they try to piece the whole story together, but we do know that the rocks are signs of river deposits and changes in the environment. No fossils have been found yet, added Gupta, but we have found some interesting chemistry and can already reconstruct the history of some of the rocks – very similar to samples photographed by Gupta in the Stoer peninsula in northwest Scotland in the mid 1980s.

“We have already discovered ancient habitable environments,” said Gupta, and the next frontier is to return there and study the site in more detail, focusing on ancient river channels and fine-grained mudstones which may contain evidence of organic life. Future missions face a big challenge, but Gupta mused that one day his grand-children may be able to camp out on Mars, just like he did in Scotland 30 years ago – and Hutton in his time.

Q&A

Q: It's inevitable we will colonise Mars in the future, so how long will it take to terraform the planet – using synthetic biology to make it habitable? (It was also pointed out that it would take thousands of years to create a habitable atmosphere.) A: Someone will want to colonise Mars – commercial or a nation race. NASA focuses on human exploration, but what's holding it back are the medical issues, including large bursts of gamma radiation. Despite the challenges, we could do more in ten minutes than Curiosity does in weeks.

Q: How much do we know about the composition of the crust of Mars? A: We know a lot about the surface, and have detected extensive deposits of clay and sulphates, which suggest the presence of water. One aim of the current mission is to find clay, but radiation may destroy traces of life and we need to drill down to two metres to get a better understanding of the chemistry of Mars. Rocks have particles in them and we can look at them and get a good idea of surface variability. One of the most exciting things about Glenelg (on Mars) was finding rock which has a geochemistry similar to rocks found on Skye. Rocks are like ‘dustbins’ which contain materials which tell you where they came from. We are like 18th-century geologists with 21st-century tools to help us build a more cohesive and more comprehensive picture of the way that Mars evolved.

Q: Can you estimate the age of the rounded pebble found in Gale Crater? A: I can hazard a guess, based on radiometric dating (argon gas released and potassium content), but all we can establish is that the minerals were formed more than two billion years ago – not the date of deposition. Ash beds would help establish dates, and you can also count the craters – the older the surface, the more craters there are and, based on calculations for the Moon, we can work out that the surface of Mars is about 3.6 billion years old. New instruments are needed to confirm this – e.g. we could send an argon lab to Mars. The next mission will cost about half of the first one, and we could collect samples, doggy- bag the rocks and send them up into orbit then return them to Earth. We are still making fresh discoveries with Moon rocks many years later, thanks to new technologies.

Q: The surface veins show water flow, but what about striations on the rocks – could that be evidence of glacial action? A: We don't know yet. We work by using multiple working hypotheses – for example, are these sand dunes; are the fractures tectonic; and are these glacial striations? We must explore all possibilities and crazy ideas. And it can take a long time to work through the arguments. Any conclusions we reach about Mars attract more attention than theories concerning the Earth. That's why there's a six-month delay before we release geochemical data – unlike the images, which we release after 24 hours.

The science is interesting, but making it happen and having all the arguments is a great privilege. What's going to happen is that a post-graduate student will use the data for a new interpretation. I am just a data gatherer, but we've already collected enough data to build a geological model. Q: What is the temperature range on Mars? A: Temperate ranges from about 20 degrees centigrade to very low. Early Mars was warm and wet, but climate models make this hard to prove, even though the geology ‘screams’ warm and wet. You would also expect large amounts of carbonates, but this is not the case. The surface was awash with water at one time, but this period could have been brief – e.g., a flash flood. To discover life has been the central goal, but even if we did find life, the arguments would continue to rage – just as our theories about the Earth are still disputed and debated. We are starting to realise that we need to understand the geological context, so next we'll go to Old Mars (more than 3.6 billion years old) to study the clay, which on the surface or deep down would preserve organic matter.

The Vote of Thanks was offered by Professor Martin Lee, Professor of Earth and Planetary Sciences, University of Glasgow.

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