The Curiosity rover has made its most unusual discovery yet on Mars: rocks made of pure sulfur. And it all started when the 1-ton rover happened to drive over a rock and crack it open, revealing yellow-green crystals never before seen on the Red Planet.
"I think it's the strangest find of the entire mission and the most unexpected," said Ashwin Vasavada, Curiosity project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "I have to say, there's a lot of luck involved. Not every rock has something interesting inside."
The Curiosity team was eager for the rover to explore Gediz Vallis Channel, a winding slit that appears to have been created by a mix of flowing water and debris 3 billion years ago. The channel is carved into a section of the 3-mile (5-kilometer) tall Mount Sharp, which the rover has been climbing since 2014.
White rocks were visible in the distance, and mission scientists wanted to get a closer look. Rover operators at JPL, sending instructions to Curiosity, made a 90-degree turn to position the robotic explorer so its cameras could capture a mosaic of the surrounding landscape.
On the morning of May 30, Vasavada and his team were looking at Curiosity's mosaic and noticed a shattered rock lying in the rover's wheel tracks. A closer look at the rock revealed the "baffling" find, he said.
A number of Curiosity's discoveries, such as lakes that are millions of years old and the presence of organic material, played a role in the rover's ultimate goal: trying to determine whether a habitable environment existed on Mars.
Scientists are now on a mission to find out what the presence of pure sulfur on Mars means and what it says about the history of the Red Planet.
A stunning find
Curiosity had already discovered sulfates, or salts containing sulfur that form when water evaporates, on Mars. The team saw evidence of bright white calcium sulfate, also known as gypsum, in cracks on the Martian surface that are actually hard-water deposits left by ancient groundwater flows.
"Nobody had pure sulfur on their bingo card," Vasavada said.
Sulfur rocks typically have what Vasavada describes as a "beautiful, translucent, crystalline texture," but weathering on Mars has essentially sandblasted the outside of the rocks, making them blend in with the rest of the planet, which is largely orange hues.
Team members were stunned twice: once when they saw the "beautiful texture and color inside" the rock, and once when they used Curiosity's instruments to analyze the rock and received data indicating it was pure sulfur, Vasavada said.
Earlier, while exploring Mars, NASA's Spirit rover broke one of its wheels and had to drag it along while the other five were used to drive backward. The drag force of the wheel revealed bright white soil, which turned out to be almost pure silica. The presence of silica suggests that Mars once had hot springs or steam vents, which could have created conditions favorable for microbial life if it ever existed on the planet.
The discovery of silica remains one of the most significant discoveries from the Spirit rover, which operated on Mars from 2004 to 2011. And Vasavada says it's the reason the team "looked back" at the Curiosity rover - otherwise they wouldn't have seen the powdered sulfur.
"My jaw dropped when I saw the image of the sulfur," said Briony Horgan, a co-investigator on the Perseverance rover and a professor of planetary science at Purdue University in West Lafayette, Indiana. "Pure elemental sulfur is a really strange find because on Earth we typically find it in places like hydrothermal vents. Think Yellowstone! So it's a big mystery to me how this rock formed in Mt. Sharp."
A field of 'strange rocks'
As we approached the Gediz Vallis Channel, Curiosity sent back photos of an unusual sight: a flat area about half the size of a football field, strewn with bright white rocks the size of your hand.
Initially, the team thought the "strange rocks" were part of the debris from the channel, possibly a layer that had carried water from higher up the mountain, Vasavada said.
But after closer inspection, which included the accidental shattering of the sulfuric rocks, the team now thinks the flat, uniform field of rocks formed where they were found, he said.
The team wanted to get a sample of the rocks to study, but Curiosity couldn't drill into the rocks because they were too small and brittle. To determine what process formed the sulfur rocks, the team looked at nearby bedrock instead.
Pure sulfur only occurs under certain conditions on Earth, such as volcanic processes or in hot or cold springs. Depending on the process, different minerals are created at the same time as the sulfur.
On June 18, the team took samples of a large rock from the channel nicknamed "Mammoth Lakes." An analysis of the rock's dust, performed by instruments in the rover's belly, revealed a greater variety of minerals than ever before during the mission, Vasavada said.
"The joke we always had was that we saw almost every mineral we'd ever seen on the entire mission, but all in this rock," he said. "It's almost a cornucopia of riches."
Layers of Mars' history
Since landing on Mars on August 5, 2012, the Curiosity rover has climbed 2,600 feet (800 meters) up the base of Mount Sharp from the floor of Gale Crater. The mountain is a central peak of the crater, which is a vast, dry ancient lakebed.
Each layer of Mount Sharp tells a different story about Mars' history, including periods when the planet was wet and periods when it became drier.
Curiosity has been systematically exploring several features of the mountain lately, including the Gediz Vallis Channel. The channel formed long after the mountain, as it cuts through several layers of Mount Sharp, Vasavada said.
After water and debris carved out a path, they left a 2-mile (3.2-kilometer) ridge of boulders and sediment beneath the channel. Although Curiosity arrived at the channel in March and will likely remain for another month or two, it has been steadily climbing alongside the debris path for some time.
Scientists have wondered whether flooding or landslides were the source of the debris, and Curiosity's research has shown that both violent water flows and landslides likely played a role. Some of the rocks are round, like river stones, suggesting they were carried by water, but others are more angular, meaning they were likely carried by dry avalanches.
Water then seeped into the debris, triggering chemical reactions that created the "halo" shapes seen on some of the rocks Curiosity has studied.
"This was not a quiet period on Mars," Becky Williams, a scientist at the Planetary Science Institute in Tucson, Arizona, and the deputy principal investigator of Curiosity's Mast Camera, said in a statement. "There was an exciting amount of activity here.
We are looking at multiple currents in the channel, including strong flooding and currents with many boulders."
Scientists are eager to reveal more details, such as how much water was present to create the canal.
The Gediz Vallis Channel has long interested scientists, including Vasavada. He recalls looking at orbital images of the object long before Curiosity landed on Mars.
"It's always been something that's just been really intriguing," he said. "I remember the rover driving over the last hill before we got to the canal, and all of a sudden you could see the landscape and the curving canal. Now we're actually here, seeing it with our own eyes, so to speak."
The continuing journey of Curiosity
There is no clear evidence for how the sulfur formed, but the team continues to analyze the data collected by Curiosity to determine how and when each mineral formed.
"Maybe this slab has experienced multiple different environments," Vasavada said, "and they overlap, and now we have to unravel that."
Curiosity will continue to explore the channel for more surprises. Next, the rover will drive west along the mountain, instead of straight up, to look for more intriguing geological features.
Despite 12 years of wear and tear, including some "near misses" such as wheel and mechanical problems, Curiosity is still in excellent health, Vasavada said.
"I feel very fortunate, but we're also all cautious that next time can't just be a dodgy situation, so we're trying to make the best of it and we have this landing site that's been so great," he said. "I'm glad we picked something that's worth 12 years of science."
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