On Friday, NASA’s Curiosity rover will celebrate its 10th anniversaryth birthday on Mars, and counting. The small car-sized rover made a soft landing on the floor of Gale Crater on the evening of August 5, on a mission to find evidence of past habitability on the Red Planet.
A decade later, Nasa can call Curiosity’s mission a resounding success. It established the long-term presence of liquid water on the surface of Mars in the planet’s ancient past, a prerequisite for the existence of life. It also established with certainty the presence of organic chemicals that are the building blocks of potential life.
And what’s more, Curiosity’s mission is still ongoing, with the intrepid rover still climbing a mountain of sedimentary rock in what was once an ancient Martian lake bed.
“About the condition of the rover, overall we’re just thrilled,” Ashwin Vasavada, project scientist for the Curiosity mission, told The Independent. “The rover is still capable of doing all the science that matters to the mission, even in 10 years.”
Curiosity and the science it enabled serves as an important bridge in NASA’s Mars science program, connecting past missions like the Spirit and Opportunity rovers — and even the Viking landings of the 1970s — with the Perseverance rover and the Ingenuity helicopter that also explore Red. Planet. Endurance may represent the cutting edge, drilling samples of Martian soil that will be returned to Earth for study in the 2030s, but Curiosity still has much to teach scientists, according to Dr. Vasavada.
“There’s still more to come,” he said, “that’s the bottom line.”
Curiosity, and Dr Vasavada, really take their inspiration from the same place – Mars in the 1970s.
“The images that really grabbed me when I was like 10 years old were the Viking lander taking pictures of this endless desert landscape going off into the distance,” he said.
Viking 1 landed on Mars on July 20, 1976, and in addition to capturing stunning images from the surface, we conducted the first ever astrobiological experiments on the Red Planet’s surface in the hope of finding evidence of alien life. Mars, after all, combined two very attractive features for scientists looking for signs of life, according to Dr. Vasavada: It had a decently high chance of having hosted life and was close enough to study.
“There might be places that have a better chance for life, you know, maybe now like the oceans of Europa, but they’re too far away to explore effectively,” he said. “Mars has always had that attraction, and the way I like to describe it is as if Viking was the attempt to hit a home.”
A failed attempt, according to most researchers. Although some scientists still maintain that the Viking 1 and 2 landers found evidence of life on Mars, the scientific consensus remains that the Viking results were inconclusive.
“I think if you were to ask planetary scientists in 1977, they’d just say we’re excited because there’s liquid water [on Mars], but we don’t really know much beyond that,” Dr. Vasavada said. “After that, NASA really had to regroup for a couple of decades.”
It wasn’t until the 1990s, when the space agency made the search for life in the universe a main focus, that Mars research picked up again.
“And they laid out a strategy where they wouldn’t go for the home run anymore,” Dr Vasavada said. Nasa would send orbiters, such as the Mars Global Surveyor launched in 1996, to Mars to study the surface and find the best places to land future missions. Next, they would send down rovers to confirm evidence that Mars once held large amounts of liquid water.
“If liquid water was never there, there is no chance that life was there,” said Dr Vasavada. “That’s where [the rovers] Spirit and Opportunity fit in. They were really sent there to look for signs of liquid water.”
“The next part of NASA’s strategy was then to extend the search beyond water to habitability, and that is Curiosity’s main goal,” Dr. Vasavada said. Curiosity would look not just for signs of liquid water, but for persistent liquid water, lakes and rivers that flow long enough for life to have a shot. And it would need to search for the chemical building blocks of life.
“That’s why the rover is a little big,” Dr. Vasavada said. “It has a number of different instruments that look at all these different aspects of what would make a habitable planet. And then we needed to be able to drill into rock samples, because we’re looking into the past.”
Of course, what past you look at depends on where you drill, “So we sent it to Gale Crater, which is a 100-mile diameter crater that has a big pile of sediment, sedimentary rock in it,” Dr Vasavada said. “And this was super exciting, because sedimentary rocks record a time history: sediment is laid down gradually over millions of years.”
The hope was that Curiosity could climb a three-mile-high pile of sedimentary rocks that had built up in what Nasa believed to be an ancient crater lake, drill into the mudstones and find evidence of ancient water and organic chemistry.
“So that’s where we were on August 4, 2012. The day before we landed,” Dr Vasavada said. “We had a great rover, and we had a landing site that offered the best chance we could think of anywhere on the planet to find a habitable environment, but there were definitely no guarantees.”
Proof of habitability
In the years since it landed 10 years ago, Curiosity has climbed 600 meters up the sedimentary rock pile in Gale Crater, according to Dr. Vasavada.
“What we found is that almost all of the 600 vertical meters are made of rock layers that have abundant evidence of the interaction with liquid water,” he said. “They are mud formed on the bottom.”
It takes time to build up 600 meters of muddy sediments, and so these measurements lead to one of the most important discoveries of the Curiosity mission, according to Dr. Vasavada: Liquid water was present in Gale Crater, on Mars, for a long time.
“It’s like hundreds of thousands of years, but probably more likely millions or tens of millions of years,” he said.
“Tied up with this discovery is the discovery of all the chemical requirements for life,” Dr Vasavada added. “We were able to drill into these mudstones, and within these mudstones we were able to find these organic molecules.”
So three out of four billion years ago, in Gale Crater, conditions were favorable for life on Mars.
“And what’s kind of fascinating is that a lot of the history that Curiosity has explored predates the time of Earth,” Dr. Vasavada said. “Earth became the habitable planet it is today mainly after the time period we are exploring in Gale Crater on Mars.”
Four billion years ago, alien explorers passing through our solar system would have chosen Mars as the most likely place to find life, not Earth.
Signs of life
But did Mars host life all those billions of years ago? That’s not a question Curiosity was designed to answer, but it’s one that can be answered with persistence. Curiosity’s successor will land in a dry river bed on Mars on February 18, 2021, and so far has spent its nearly 18 months on the Red Planet drilling rock and soil samples and storing them for later retrieval as part of Nasa’s Sample Return Mission. When the samples are returned to Earth in the 2030s, scientists will be able to bring full laboratories to the question of whether these samples contain signs of Martian life, extant or ancient.
It’s a mission that takes the baton from Curiosity, and one that would never have happened with Curiosity’s successes.
“If we found Gale Crater to be a barren wasteland,” said Dr Vasavada, “If we found a large pile of dust and we were completely wrong in our pre-landing hypotheses about it; it would be more difficult for NASA to proceed with a mission like Perseverance and put all that focus on looking for signs of life.”
But as he mentioned, Curiosity isn’t done yet.
The rover will celebrate its 10th anniversaryth birthday by continuing its slow climb up the sedimentary rock pile in Gale Crater, where the latest samples are beginning to suggest major changes on ancient Mars. Samples that once showed abundant clay minerals now show sulfate-rich rocks, according to Dr. Vasavada.
“We don’t know what that means, but one of our leading hypotheses is that these sulfate minerals were enriched through something like what happens when lakes and waters dry up and they leave behind salty minerals,” he said. “It could be a sign that the planet as a whole is doing that.”
So as Curiosity wanders, it may not only turn out to have found evidence of an earlier period of habitation, it may provide clues as to when and how that period ended.
Assuming the tough rover can keep trucking. Dr Vasavada believes the odds are good.
– We have had some quite significant challenges. We’ve had to redesign how we drill twice in the mission now because of some anomalies that have occurred and the loss of a couple of mechanical parts of the rover,” he said. “We’ve had a lot of problems in 10 years that aging machines do, but we has always been able to recover us.”