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The Curiosity rover has made its most unusual find yet on Mars: rocks made of pure sulfur. And it all started when the 1-ton rover happened to pass through a rock and split it open, revealing yellowish-green crystals that had never been seen on the red planet.

“I think this is 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 must say there is a lot of luck here. Not every rock has something interesting inside.

The Curiosity team was eager for the rover to explore the Gediz Vallis channel, a meandering groove that appears to have been created 3 billion years ago by a mixture of flowing water and debris. The canal is carved into part of the 3-mile (5-kilometer) high Mount Sharp. The rover has scaled the mountain since 2014.

White rocks could be seen in the distance and the mission scientists wanted to take a closer look. The rover drivers at JPL, which send instructions to Curiosity, made a 90-degree turn to put the rover in the right position for its cameras to capture a mosaic of the surrounding landscape.

On the morning of May 30, Vasavada and his team looked at Curiosity’s mosaic and saw a broken rock lying among the rover’s tracks. A closer photo of the rock clarified the “mind-blowing” find, he said.

Some of Curiosity’s findings, such as lakes that have existed for millions of years and the presence of organic materials, play a role in the rover’s ultimate mission goal: trying to determine whether Mars hosts a habitable environment.

Now scientists are on a mission to find out what the presence of pure sulfur on Mars means and what it says about the red planet’s history.

Curiosity had already detected sulfates on Mars, or salts that contain sulfur, which form when water evaporates. The team saw evidence of bright white calcium sulfate, also known as gypsum, in cracks on the Martian surface, which are essentially hard water deposits left by ancient underground water flows.

“No one had pure sulfur in their bingo card,” Vasavada said.

Sulfur rocks typically have what Vasavada describes as “a beautiful, translucent, crystalline texture,” but Martian weathering has essentially sandblasted the outside of the rocks to blend in with the rest of the planet, which is largely of shades of orange.

Team members were stunned twice — once when they saw the rock’s “gorgeous texture and color inside” and then when they used Curiosity’s instruments to analyze the rock and got data showing it was pure sulfur, Vasavada said.

Previously, while exploring Mars, NASA’s Spirit rover broke one of its wheels and had to drag it while using the other five to propel itself backwards. The skidding of the wheel revealed a bright white soil that turned out to be almost pure silica. The presence of silica suggests that hot springs or steam vents may once have been on Mars, which could have created conditions favorable for microbial life if it had ever existed on the planet.

The discovery of silica is still one of the most important discoveries of the Spirit rover, which operated on Mars from 2004 to 2011. And Vasavada says it inspired the team to “look behind” the Curiosity rover — otherwise they wouldn’t have seen crushed sulfur .

“My jaw dropped when I saw the sulfur image,” said Bryony Horgan, co-investigator of the Perseverance rover mission and professor of planetary sciences at Purdue University in West Lafayette, Indiana. “Pure elemental sulfur is a very strange find because on Earth we mostly find it in places like hydrothermal vents. Think Yellowstone! So it’s a big mystery to me how this rock formed in Mount Sharp.

As it neared the Gediz Vallis channel, Curiosity sent back pictures of an unusual sight: a flat area about half the size of a football field, dotted with bright white hand-sized boulders.

At first, the team thought the “strange rocks” were part of the channel’s debris, perhaps a layer that water had transported from higher up the mountain, Vasavada said.

But upon closer inspection, including the accidental fracturing of the sulfur rock, the team now believes that the flat, uniform field of rocks formed where they were found, he said.

The team was eager to sample the rocks for research, but Curiosity was unable to drill into the rocks because they were too small and fragile. To determine what process formed the sulfur rocks, the team instead looked at nearby bedrock.

Pure sulfur only forms 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 a sample of a large boulder from the channel, called “Mammoth Lakes.” Analysis of rock dust by instruments in the rover’s belly revealed a greater variety of minerals than had ever been seen during the mission, Vasavada said.

“The big joke for us was that we pretty much saw every mineral we’ve ever seen in the entire mission, but all in this rock,” he said. “It’s almost an abundance of riches.”

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 the central peak of the crater, which is a vast dry ancient lake bed.

Each layer of Mount Sharp tells a different story about the history of Mars, including periods when the planet was wet and when it became drier.

Recently, Curiosity has systematically explored various features of the mountain, such as the Gediz Vallis channel. The channel formed well after the mountain because it cuts through different layers of Mount Sharp, Vasavada said.

After water and debris carved a path, they left behind a 2-mile (3.2-kilometer) ridge of rocks and sediment below the channel. Although Curiosity arrived in the canal in March and will likely remain for another month or two, it has been steadily climbing up the debris path for some time.

Scientists wonder whether floods or landslides caused the debris, and Curiosity’s investigations show that both turbulent water flows and landslides likely played a role. Some of the rocks are rounded like river rocks, suggesting they were carried by water, but others are more angular, meaning they were probably delivered by dry avalanches.

The water then soaks into the debris, and chemical reactions create the “halo” shapes that can be seen on some of the rocks that Curiosity has examined.

“This has not been a quiet period on Mars,” Becky Williams, a scientist at the Planetary Science Institute in Tucson, Arizona, and deputy principal investigator for Curiosity’s mast camera, said in a statement. “There was exciting activity here.
We are looking at multiple flows down the channel, including vigorous floods and boulder-rich flows.”

Scientists are eager to uncover more details, including how much water was present to help carve the canal in the first place.

The Gediz Vallis channel has long been of interest to scientists, including Vasavada, who remembers looking at orbital images of the feature long before Curiosity landed on Mars.

“It’s always been something that’s been really intriguing,” he said. “I remember when the rover kind of rolled over the last hill before we got to the canal and suddenly you could see the landscape and the curved canal. Now we’re actually here and we’re seeing it with our own eyes, so to speak.”

There is no evidence of 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 rock slab has experienced multiple different kinds of environments,” Vasavada said, “and they’re kind of imprinting each other, and now we have to figure that out.”

Curiosity continues to explore the channel to look for more surprises, and after moving on, the rover will head west to drive along the mountain instead of straight up to look for more intriguing geological features.

Despite 12 years of wear and tear, including some “close calls” such as wheel problems and mechanical problems, Curiosity remains in excellent health, Vasavada said.

“I feel very lucky, but we also all feel cautious that the next one might not be just a close call, so we’re trying to make the most of it and we have this landing spot, which was so wonderful,” he said. “I’m glad we chose something that was a science for 12 years.”