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The Curiosity rover made the most unusual discovery to date on Mars: rocks made of pure sulfur. And it all started when the 1 tonne rover happened to drive over a rock and break it open, revealing yellow-green crystals never before seen on the red planet.
“I think it’s the weirdest moment of the whole 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 here. Every rock has something interesting in it.”
The Curiosity team was keen on the rover to explore the Gediz Vallis channel, a winding groove that appears to have been created 3 billion years ago by a mixture of flowing water and debris. The channel is carved into a section of the 3 mile high (5 kilometer high) Sharp Mountain. The rover has been scaling the mountain since 2014.
White stones could be seen in the distance, and the mission scientists wanted to take a closer look. The rover drivers at JPL, which send directions to Curiosity, made a 90-degree turn to position the robotic probe 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 crushed rock lying among the rover’s wheel tracks. A closer picture of the rock revealed the “mind blowing” thing, he said.
Some of Curiosity’s discoveries, such as lakes that lasted millions of years and the presence of organic matter, contributed to the goal of the rover’s final mission: to try to determine whether there were habitable environments on Mars.
Now, scientists are on a mission to figure out what the presence of pure sulfur on Mars means and what it says about the history of the red planet.
Great find
Curiosity had already found sulfates on Mars, or salts containing sulfur that are formed when water evaporates. The team has seen evidence of bright white calcium sulfate, also known as gypsum, within cracks on the Martian surface that are essentially hard-water deposits left behind by ancient groundwater flows.
“No one had pure sulfur on their bingo card,” Vasavada said.
Sulfur rocks typically have what Vasavada describes as a “beautiful, translucent and crystalline texture,” but weathering on Mars essentially sanded the outside of the rocks to blend in with the rest of the planet, which is composed of mostly shades of orange.
The team members were surprised twice – once when they saw the rock’s “ghostly texture and color inside” and then when they used Curiosity’s instruments to analyze the rock and found data that indicated that it was pure sulphur, said Vasavada.
Previously, while exploring Mars, NASA’s Spirit rover broke one of its wheels and had to tow it while using the other five to propel itself back. The pull of the wheel revealed a bright bright soil, which turned out to be almost pure silica. The presence of silica suggests that Mars may have once had hot springs or steam vents, which could have created conditions that would have been favorable for microbial life if it had ever existed on the planet.
The discovery of silica remains one of the most important findings of the Spirit rover, which operated on Mars from 2004 to 2011. And Vasavada says it’s what prompted the team to “look behind” the Curiosity rover – otherwise so the sulfur would not be crushed.
“My jaw dropped when I saw the image of sulfur,” said Briony Horgan, co-investigator of the Perseverance rover mission and professor of planetary science at Purdue University in West Lafayette, Indiana. “Pure elemental sulfur is a very strange discovery because on Earth we mostly find it in places like hydrothermal vents. Think Yellowstone! So it is a great mystery to me how this rock was formed in Mt. Sharp.”
‘strange rocks’ area
As it approached the Gediz Vallis channel, Curiosity sent back pictures of an unusual sight: a flat area, about half the size of a football field, scattered with bright white hand-sized rocks.
At first, the team thought the “strange rocks” were part of the debris from the channel, perhaps a layer that carried water from higher up the mountain, Vasavada said.
But after closer inspection, including fortuitously pressing the sulfur rock, the team now thinks that a flat, uniform field of rocks formed where they were found, he said.
The team wanted to take a sample of the rocks to study, but Curiosity was unable to drill into the rocks because they were too small and brittle. To find out what process created the sulfur rocks, the team looked at a nearby bedrock instead.
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 formed at the same time as the sulphur.
On June 18, the team sampled a large rock from the channel known as “Mammoth Lakes.” Analysis of the rock dust, by instruments inside the rover’s belly, revealed a greater variety of minerals than had ever been seen during the mission, Vasavada said.
“The running joke for us was that we saw almost every mineral we ever saw in the whole mission but all in this rock,” he said. “It is almost an abundance of wealth.”
Series of Martian 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 the central peak of the crater, which is a huge, 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 been systematically investigating various aspects of the mountain, such as the Gediz Vallis channel. The channel was formed well after the mountain because it cuts through different layers of Mount Sharp, Vasavada said.
After carving out a trail of water and debris, they left behind a 2-mile (3.2-kilometer) ridge of boulders and sediment below the channel. Although Curiosity entered the channel in March and will likely remain for another month or two, it has been climbing steadily near the debris trail for some time.
Scientists have debated whether the debris was caused by floods or landslides, and Curiosity’s investigations have shown that both violent water flows and landslides likely played a role. Some of the rocks are rounded like river rocks, suggesting they were transported by water, but others are more angular, meaning they were most likely delivered by dry land.
Then, water seeped into the debris, and chemical reactions created the “halo” shapes seen on some of the rocks studied by Curiosity.
“This has not been a quiet period on Mars,” Becky Williams, a scientist with the Planetary Science Institute in Tucson, Arizona, and 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 flows down the channel, including energetic floods and boulder-rich flows.”
Scientists are keen to find out more details, including how much water was present to help carve the channel in the first place.
The Gediz Vallis channel has long fascinated scientists, including Vasavada, who recall viewing orbital images of the feature long before Curiosity landed on Mars.
“It was always something that was really interesting,” he said. “I remember when the rover type went over the last hill before we got to the channel, and suddenly you could see the landscape and the curved channel. Now, we are here for real, to see it with our own eyes, so to speak.”
Curiosity’s ongoing journey
There is no smoking gun focused on how the sulfur formed, but the team continues to analyze the Curiosity data collected to determine how and when each mineral formed.
“Maybe this slab of rock has experienced many different types of environments,” Vasavada said, “and they’re sort of overprinting each other, and now we have to sort that out.”
Curiosity continues to explore the channel to look for more surprises, and after moving on, the rover will turn back to drive along the mountain, rather than straight up, to look for more interesting geological features.
Despite 12 years of wear and tear, including some “close calls” like wheel issues and mechanical problems, Curiosity remains in good health, Vasavada said.
“I feel really lucky, but we all also feel careful that the next one is just a close call, so we’re trying to make the most of it, and we have this landing site that was so amazing,” he said. . “I’m glad we chose something that was worth 12 years of science.”
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