Nearly 12 years ago, on August 5, 2012, Curiosity landed on Mars in the Gale Crater in search of signs of life loaded with 17 cameras, and 10 science instruments including spectrometers, radiation detectors, and sensors to better explore Mars’ environment. Guided by the question “Was Mars ever habitable [for microbial life]”, the 1-ton rover unveiled the chemical and mineral evidence of Mars being previously livable for microbes for a vast period of time. For example, Curiosity discovered organic molecules located in a crater on Mars that resembles molecular building units of gas and oil on Earth, indicating that the crater housed living beings 3.5 billion years ago, around when life originated on Earth. Moreover, with its discovery of rock containing manganese oxide, Curiosity demonstrated that Mars’ atmosphere was once oxygen-rich just like Earth’s, proposing strong proof of Mars's past habitability. Still, to that day, all of the instruments of Curiosity function pretty well and the rover continues to allow scientists to probe the red planet, Mars.
In its 12th year of the mission, to be more specific, 4208th Martian day, Curiosity made a surprising discovery of pure sulfur crystals on Mars in Gediz Vallis.
The Curiosity mission's deputy project scientist, Abigail Fraeman remarked that the discovery was fully shocking and that "It's probably one of the most unusual things that [they] found the entire 12-year mission." Similarly, according to Ashwin Vasavada, Curiosity project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, “...it’s the strangest find of the whole mission and the most unexpected”. The mission's project scientist at the Jet Propulsion Laboratory in California, Ashwin Vasavada describes the discovery with a metaphor: "Finding a field of stones made of pure sulfur is like finding an oasis in the desert. It shouldn't be there, so now we have to explain it."
Even though Curiosity has encountered forms of sulfur on Mars’ surface before combined with other minerals such as magnesium and calcium, this is the first time finding pure, i.e. elemental sulfur on Mars. Sulfate, a kind of salt containing sulfur, was commonly observed by Curiosity, especially since October 2023, but the bright yellow odorless pure sulfur is found in specific conditions that scientists did not anticipate being present on Mars. As Fraeman mentions, sulfur is mostly “coupled with oxygen and other elements that make it into a salt or something similar, but here, what we found was just chunks of pure sulfur”, denoting the significance of the discovery that scientists didn’t expect at all.
The six-wheeled rover discovered the sample while traveling through the Gediz Vallis channel, descending from Mount Sharp, located in the center of the Gale Crater after scientists led it toward a colored rock field. In fact, scientists state that examining the channel has been among the most important aims of Curiosity from the beginning since it is believed that the channel was formed by water carrying debris, as shown by the smooth rocks in the channel that was likely shaped by the water flow, similar to river stones. Additionally, some rocks display white halo markings, further suggesting the presence of water. Therefore, understanding the channel’s formation may give scientists hints about the history of water on Mars. The channel is carved into part of Mount Sharp, which stands 3 miles (5 kilometers) tall with each layer of it symbolizing a different period of Mars’ history. Curiosity has been climbing the sulfate-rich mountain since 2014, for a decade now, seeking to find out where and when Mars hosted microbial life if it hosted any.
On 30th May 2024, while approaching the Gediz Vallis channel, a field scattered with bright white rocks came into sight, and mission scientists directed Curiosity near rocks where the rover accidentally cracked some rock, revealing yellowish-green pure sulfur crystals that had never been spotted on Mars before.
The $2.5 billion rover detected the composition of the rock thanks to its Alpha Particle X-ray Spectrometer, attached to the end of its robotic arm, and then took photos of the pure sulfur crystals via a camera on its head at human eye level called Mastcam whose color quality is similar to two digital cameras on June 7, 2024. Scientists named this 5-inch yellow sulfur crystal sample “Convict Lake”, referring to a lake in the Sierra Nevada mountains, in California.
On June 8, Curiosity also photographed another rock named “Snow Lake” made of sulfur. Considering, 9 days before, the rover captured a similar-looking rock with yellow elemental sulfur inside, scientists expect all similar-looking rocks in that field to comprise sulfur.
However, Curiosity could not collect samples from these yellow sulfur crystals which were too small and brittle for a drill to take samples until the rover came across a large rock nicknamed “Mammoth Lakes”. On July 18, luckily, the rover drilled a hole in Mammoth Lakes through its robotic arm, which displayed a huge variety of minerals that has been never witnessed before on Mars. “The running joke for us was we almost saw every mineral we’ve ever seen in the whole mission but all in this rock,” Vasavada says, adding “It’s almost an abundance of riches.” Curiosity keeps the sample for further analysis as Gediz Vallis becomes an intriguing and exciting place for scientists for further studies.
According to Fraeman, it will take some time to analyze the discovery and what it indicates about the past environment of Mars. However, as Vasavada puts it “Discovering strange and unexpected things is what makes planetary exploration so exciting” as Curiosity uncovered surprising crystals after a decade of search on Mars.
Importantly, the find has contributed to the increasing body of evidence of other essential elements for supporting life found on Mars, such as carbon, hydrogen, nitrogen, oxygen, and phosphorus, which may suggest that Mars once had flowing water, a crucial component for life.
One aspect to consider is how these crystals formed since “there’s no reason to expect pure sulfur in this specific region” or a piece of evidence suggesting the presence of hot springs, that might formed the crystals as JPL spokesperson Andrew Good notes. Plus, scientists are not sure about the role of pure sulfur in Mars’ history or whether pure sulfur crystals are linked to other sulfur-based compounds found in the area. Hence, still, lots of questions remain to be answered and investigated.
Recently, NASA tweeted “We don't know much about these yellow crystals yet, but our team is excited to investigate.” Thus, crystals made up of pure sulfur sparked interest and curiosity among scientists since this unexpected discovery seems to be likely to convey new information about Mars’ geology, history, and the “potentially habitable environment it’s sustained” long ago, helping scientists to delve deeper into Mars as Curiosity carries on heavily contributing to space science with the new information it transmits about our neighboring red planet.
