Mars Rover Accidentally Reveals Pure Sulfur Crystals Never Found Before

Accidental Discovery Changes Mars Exploration

Scientists were stunned on May 30 when a rock that NASA's Curiosity Mars rover drove over cracked open to reveal something never seen before on the Red Planet: yellow sulfur crystals. When the rover rolled its 899-kilogram (1,982-pound) body over the fragile lump of mineral in May of last year, the deposit broke open, revealing yellow crystals of elemental sulfur, known as brimstone. This remarkable discovery happened purely by chance as Curiosity navigated the rugged terrain of Mars' Gediz Vallis Channel.

Using an instrument on the rover's arm, scientists later determined these crystals are elemental sulfur — and it's the first time this kind of sulfur has been found on the Red Planet. But where past detections have been of sulfur-based minerals — in other words, a mix of sulfur and other materials — the rock Curiosity recently cracked open is made of elemental, or pure, sulfur. The discovery marks a significant milestone in Martian geology, as while people associate sulfur with the odor from rotten eggs (the result of hydrogen sulfide gas), elemental sulfur is odorless. It forms in only a narrow range of conditions that scientists haven't associated with the history of this location.

Geological Mystery Deepens

The implications of finding pure sulfur on Mars extend far beyond a single rock sample. And Curiosity found a lot of it — an entire field of bright rocks that look similar to the one the rover crushed. This abundance suggests that there are a lot of things we don't know about the geological history of Mars, but the discovery of scads of pure sulfur just hanging about on the Martian surface suggests that there's something pretty big that we're not aware of.

Pure sulfur, on the other hand, only forms under a very narrow set of conditions, which are not known to have occurred in the region of Mars where Curiosity made its discovery. It isn't clear what relationship, if any, the elemental sulfur has to other sulfur-based minerals in the area. Scientists now face the challenge of understanding how these crystals formed in an environment that theoretically shouldn't support their creation.

Ancient Waterway Holds Secrets

The location of this discovery adds another layer of intrigue. The Gediz Vallis channel is an area rich in Martian history, an ancient waterway whose rocks now bear the imprint of the river that once flowed over them, billions of years ago. Scientists think that the channel was carved by flows of liquid water and debris that left a ridge of boulders and sediment extending 2 miles down the mountainside below the channel.

"This was not a quiet period on Mars," said Becky Williams, a scientist with the Planetary Science Institute in Tucson, Arizona, and the deputy principal investigator of Curiosity's Mast Camera, or Mastcam. "There was an exciting amount of activity here. We're looking at multiple flows down the channel, including energetic floods and boulder-rich flows." This dynamic ancient environment may hold clues to understanding how pure sulfur came to exist in such quantities.

Future Implications for Mars Science

While sulfur, it's important to understand, is an essential element for all life. It's usually taken up in the form of sulfates, and used to make two of the essential amino acids living organisms need to make proteins. However, since we've known about sulfates on Mars for some time, the discovery doesn't tell us anything new in that area. We're yet to find definite signs of life on Mars, anyway. Nevertheless, we do keep stumbling across the remains of bits and pieces that living organisms would find useful, including chemistry, water, and past habitable conditions.

The accidental nature of this discovery highlights how much remains unknown about Mars. Curiosity's instruments were able to analyze and identify the sulfurous rocks in the Gediz Vallis Channel, but if it hadn't taken a route that rolled over and cracked one open, it could have been some time until we found the sulfur. As Curiosity continues its journey through the ancient channel, scientists anticipate that this sulfur field may be just the beginning of revelations that could reshape our understanding of Martian geology and its potential for harboring the building blocks of life.