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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 passed over a rock and cracked it open, revealing yellowish-green crystals never before seen on the red planet.
“I think this is the strangest and most unexpected finding of the entire mission,” said Ashwin Vasavada, Curiosity project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “I have to say, there’s a lot of luck involved here. Not every rock has something interesting in it.”
The Curiosity team wanted the rover to investigate the Gediz Vallis channel, a winding channel that appears to have been formed by a mixture of flowing water and debris 3 billion years ago. The channel is carved into part of Mount Sharp, which is 3 miles (5 kilometers) high. The rover has been climbing the mountain since 2014.
White rocks were visible in the distance, and mission scientists wanted to take a closer look. Sending instructions to Curiosity, rover drivers at JPL made a 90-degree turn to put the robotic 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 through Curiosity’s mosaic and saw a crushed rock lying in the rover’s tire tracks. A closer photo of the rock revealed the “mind-blowing” find, he said.
Some of Curiosity’s discoveries, such as lakes that have persisted for millions of years and the presence of organic matter, played a role in the rover’s ultimate mission goal: determining whether Mars could have hosted life.
Now scientists are on a mission to understand what it means to find pure sulfur on Mars and what it says about the red planet’s history.
A great invention
Curiosity has discovered sulfates, or sulfur-containing salts that form when water evaporates, on Mars. The team saw evidence of bright white calcium sulfate, or gypsum, in cracks on the Martian surface that are essentially hard water deposits left behind by ancient groundwater flows.
“Nobody’s bingo card said pure sulfur,” Vasavada said.
Sulfur rocks generally have what Vasavada calls a “beautiful, translucent, crystalline texture,” but erosion on Mars has essentially sandblasted the rocks’ exterior, causing them to blend in with the rest of the planet, which is largely composed of orange hues.
The team members were surprised twice, once when they saw the “amazing texture and color” of the rock, and once when they analyzed the rock using Curiosity’s instruments and got data showing it was pure sulfur, Vasavada said.
Previously, NASA’s Spirit rover broke one of its wheels while exploring Mars and had to drag it while using the other five to back up. The dragging of the wheel revealed bright white soil that turned out to be almost pure silica. The presence of silica suggests that Mars may have once had hot springs or steam vents, creating conditions favorable for microbial life if it had existed on the planet.
The silica discovery remains one of the most important findings of the Spirit rover, which operated on Mars from 2004 to 2011. Vasavada says it was what inspired the team to “look behind” the Curiosity rover; otherwise, they wouldn’t have seen the crushed sulfur.
“When I saw the image of sulfur, my jaw dropped,” said Briony Horgan, a Perseverance rover mission co-investigator and professor of planetary science at Purdue University in West Lafayette, Indiana. “Pure element sulfur is such a strange find because on Earth, we mostly find it in places like hydrothermal vents. Think of Yellowstone! So how this rock formed on Mt. Sharp is a big mystery to me.”
A field of ‘strange rocks’
As it approached the Gediz Valley, Curiosity sent back photos of an unusual landscape: a flat area about half the size of a football field, filled with bright-white, hand-sized rocks.
Vasavada said the team initially thought the “strange rocks” were part of debris from the channel, perhaps a layer carried by water from higher up the mountain.
But upon closer examination, including the accidental crushing of sulfur rock, the team now thinks a flat, uniform area of rock formed where they were found, he said.
The team was eager to take a sample of the rocks to study, but Curiosity couldn’t drill into them because they were too small and brittle. To determine the process that formed the sulfur rocks, the team looked to nearby bedrock instead.
Pure sulfur forms on Earth only under certain conditions, such as volcanic processes or hot or cold springs. Depending on the process, different minerals are formed simultaneously with the sulfur.
The team collected a large rock sample from the channel nicknamed “Mammoth Lakes” on June 18. Analysis of the rock dust, performed by instruments on the rover’s belly, revealed a variety of minerals never before seen during the mission, Vasavada said.
“The running joke for us was that we saw almost every mineral we’ve seen throughout the mission, but we saw them all in this rock,” he said. “It’s almost a cornucopia of riches.”
Layers of Mars history
Since landing on Mars on Aug. 5, 2012, the Curiosity rover has climbed 2,600 feet (800 meters) from the floor of Gale Crater to the base of Mount Sharp, the crater’s central peak, which is a large, dry ancient lake bed.
Each layer of Mount Sharp tells a different story about Mars’ history, including periods when the planet was wet and dry.
Recently, Curiosity has been systematically exploring different features of the mountain, such as the Gediz Vallis channel. Vasavada said the channel formed long after the mountain was formed because it carved through different layers of Mount Sharp.
After the water and debris left a trail, they left a 2-mile (3.2-kilometer)-long ridge of rock and sediment beneath the channel. Curiosity reached the channel in March and will likely stay there for another month or two, but it has been steadily climbing alongside the debris trail for some time.
Scientists have wondered whether the debris was created by floodwaters or landslides, and Curiosity’s research suggests that both torrential downpours 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, suggesting they were likely brought in by dry avalanches.
Water then permeated the debris, and chemical reactions created the “halo” shapes visible in some of the rocks Curiosity examined.
“This was not a quiet period on Mars,” Becky Williams, a scientist at the Planetary Science Institute in Tucson, Arizona, and co-principal investigator of Curiosity’s Mast Camera, said in a statement. “There was an exciting amount of activity here.
We see multiple flows along the channel, including energetic floods and rock-filled flows.”
Scientists are eager to uncover more details, including how much water was involved in the formation of the channel.
The Gediz Vallis channel has long been of interest to scientists, including Vasavada, who recalls looking at orbital images of the feature long before Curiosity landed on Mars.
“It’s always been a really interesting thing,” he said. “I remember the last hill the rover came over before the channel and suddenly you could see the landscape and the curved channel. Now, we’re actually here, seeing it with our own eyes, so to speak.”
The continuing journey of curiosity
There is no definitive evidence for how sulfur is formed, but the team is continuing to analyze data collected by Curiosity to determine how and when each mineral was formed.
“Maybe this rock slab has experienced many different types of environments,” Vasavada said, “and they’re somehow pressing on each other, and now we need to figure that out.”
Curiosity is continuing to explore the channel for more surprises, and once underway, it will head west along the mountain rather than heading straight up to search for more interesting geological features.
Despite 12 years of wear and tear, including some “close encounters” such as wheel problems and mechanical issues, Curiosity is still in very good shape, Vasavada said.
“I feel very lucky, but we’re also all cautious that the next one might not just be a close call, so we’re trying to make the most of it and we have this great landing site,” he said. “I’m happy we picked something worth 12 years of science.”
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