The Curiosity rover has made its most unusual discovery yet on Mars: rocks made of pure sulfur. It all started when the 1-ton robot ran over a rock and broke it, revealing yellowish-green crystals never before seen on the red planet.
“I think it’s the strangest discovery of the entire mission and the most unexpected,” says Ashwin Vasavada, Curiosity project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “I have to say, there is a lot of luck involved here. Not every rock has something interesting inside.”
The Curiosity team was eager for the rover to investigate the Gediz Vallis channel, a winding groove that appears to have been created 3 billion years ago by a mix of flowing water and debris. The canal is carved into part of Mount Sharp, which is 5 kilometers high. The rover has been climbing the mountain since 2014.
White rocks were visible in the distance, and mission scientists wanted to get a closer look. Rover drivers at JPL, who send instructions to Curiosity, made a 90-degree turn to place the robotic explorer in just 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 rock crushed between the rover’s wheel tracks. A closer image of the rock made the “surprising” discovery clear, according to the scientist.
Some of Curiosity’s discoveries, such as lakes that lasted millions of years and the presence of organic materials, contributed to the rover’s mission’s ultimate goal: trying to determine whether Mars hosted habitable environments.
Now, scientists are on a mission to discover what the presence of pure sulfur on Mars means and what it reveals about the red planet’s history.
An impressive discovery
Curiosity had previously discovered sulfates on Mars, or salts that contain sulfur and are formed when water evaporates. The team has already seen evidence of bright white calcium sulfate, also known as gypsum, inside cracks on the Martian surface that are essentially hard water deposits left by ancient groundwater flows.
“No one had pure sulfur on their bingo card,” says Vasavada.
Sulfur rocks typically have what Vasavada describes as a “beautiful, translucent, crystalline texture,” but weathering on Mars has essentially weathered the outer portion of the rocks to blend in with the rest of the planet, which consists mostly of orange hues.
Team members were surprised twice — once when they saw the “beautiful texture and color inside” the rock and then when they used Curiosity’s instruments to analyze the rock and received data indicating it was pure sulfur, according to Vasavada.
Previously, while exploring Mars, the NASA’s Spirit rover broke down one of its wheels and had to drag it while using the other five to drive in reverse. Wheel drag revealed bright white soil, which turned out to be almost pure silica. The presence of silica suggests that hot springs or fumaroles may have existed on Mars, which could have created favorable conditions for microbial life, if it ever existed on the planet.
The discovery of silica remains one of the most important discoveries of the Spirit rover, which operated on Mars from 2004 to 2011. Vasavada says it’s what inspired the team to “look behind” the Curiosity rover — otherwise they wouldn’t have seen it. crushed sulfur.
“My jaw dropped when I saw the sulfur image,” reports Briony Horgan, co-investigator on 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 mainly find it in places like hydrothermal vents. Think Yellowstone! So it’s a big mystery to me how this rock formed on Mount Sharp.”
A field of “strange rocks”
As Curiosity approached the Gediz Vallis channel, it sent back photos of an unusual sight: a flat area, the size of half a football field, scattered with hand-sized white rocks.
At first, the team thought the “strange rocks” were part of the canal debris, perhaps a layer that the water carried from higher up the mountain, according to Vasavada.
But after closer inspection, including the fortuitous crushing of the sulfur rock, the team now thinks the flat, uniform field of rocks formed where they were found, it says.
The team was eager to collect a sample of the rocks for study, 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 considered 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 collected a sample from a large canal rock nicknamed “Mammoth Lakes.” An analysis of the rock dust, performed by instruments inside the rover, revealed a greater variety of minerals than ever before seen during the mission, according to Vasavada.
“The running joke for us was that we almost saw every mineral we’ve ever seen on the entire mission, but all of them in this rock,” he says. “It’s almost an abundance of riches.”
Layers of Martian history
Since landing on Mars on August 5, 2012, the Curiosity rover has climbed 800 meters to the base of Mount Sharp from the bottom 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.
Lately, Curiosity has been systematically investigating different features of the mountain, such as the Gediz Vallis channel. The channel was formed well past the mountain because it cuts through different layers of Mount Sharp, according to Vasavada.
After the water and debris carved a path, they left behind a 2-mile ridge of boulders and sediment beneath the channel. Although Curiosity arrived at the canal in March and will likely stay for another month or two, it has been steadily climbing alongside the debris trail for a while.
Scientists wondered whether floods or landslides caused the debris, and Curiosity’s investigations showed that both violent water flows and landslides likely played a role. Some of the rocks are rounded like river stones, suggesting they were carried by water, but others are more angular, meaning they were likely carried by dry avalanches.
Then, water seeped into the debris and chemical reactions created “halo” shapes that can be seen in some of the rocks Curiosity studied.
“This has not been a calm period on Mars,” said Becky Williams, a scientist at the Planetary Science Institute in Tucson, Arizona, and deputy principal investigator for Curiosity’s Mast Camera, in a statement. “There has been an exciting amount of activity here. We are looking at multiple flows through the channel, including energetic flooding and boulder-rich flows.”
Scientists are eager to discover more details, including how much water was present to help carve the channel 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 well before Curiosity landed on Mars.
“It was always something that was very intriguing,” he says. “I remember when the rover rolled over the final hill before we reached the canal, and suddenly you could see the landscape and the curved canal. Now we are actually here, seeing it with our own eyes, so to speak.”
Curiosity’s ongoing journey
There are no definitive clues pointing to how the sulfur was formed, but the team continues to analyze data collected by Curiosity to determine how and when each mineral formed.
“Maybe this slab of rock has been through several different types of environments,” says Vasavada, “and they’re kind of overlapping, and now we have to untangle that.”
Curiosity continues to explore the canal for more surprises, and after it moves, the rover will head west to drive along the mountain, rather than straight up, to look for more intriguing geological features.
Despite 12 years of wear and tear, including some “near misses” such as wheel problems and mechanical problems, Curiosity remains in excellent health, according to Vasavada.
“I feel very lucky, but we also all feel wary that the next one might not just be a near miss, so we’re trying to make the most of it, and we have this landing spot that’s been so wonderful,” he says. “I’m glad we chose something that was worth 12 years of science.”
NASA posts photo of “camouflaged” robot on Mars: “Can you find it?”
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