While off-roading within the Gediz Vallis channel on Mars in May, Curiosity accidentally cracked open a rock. This accident turned out to be a great find as the rock revealed bright yellow sulfur crystals. This is the first time that elemental sulfur has been found on Mars and it has scientists puzzled.

“Finding a field of stones made of pure sulfur is like finding an oasis in the desert,” said Curiosity’s project scientist, Ashwin Vasavada of NASA’s Jet Propulsion Laboratory in Southern California. “It shouldn’t be there, so now we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting.”

Curiosity, the gift that keeps on giving

Ever since 2014, the Curiosity Rover has been ascending Mount Sharp, finding clues about Martian geology. Each layer of this 3-mile-tall mountain represents a different period in Martian history. Curiosity’s mission is to study these layers to determine where and when Mars might have supported microbial life.

Since October 2023, Curiosity has been exploring an area rich in sulfates — a type of salt that forms as water evaporates. While sulfates are common, finding pure sulfur is unprecedented on Mars. And, based on the area’s geology, it’s not clear how the sulfur could have formed.

Gediz Vallis channel, identified from space years before Curiosity’s launch, has been a key exploration target. Scientists believe the channel was shaped by liquid water and debris, leaving a ridge of boulders and sediment extending down the mountain. Recent clues suggest both floods and landslides contributed to the landscape, with rounded stones indicating water flows and angular rocks hinting at dry avalanches.

Water has chemically altered the rocks within Gediz Vallis, creating white “halo” shapes through chemical reactions. These halos are similar to markings on Earth caused by groundwater leaking into rocks and causing chemical changes.

“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.”

All this Martian geology is very exciting, but it doesn’t tell us where the elemental sulfur came from.

Sulfur on Mars

Elemental sulfur is composed of pure sulfur atoms. Here on Earth (as in other bodies, like Jupiter’s moon Io), elemental sulfur occurs naturally in volcanic emissions or hydrothermal vents. This form of sulfur is less common than sulfur in mineral form.

While elemental sulfur is often found near volcanic or geothermal areas, it can be formed through other processes as well — for instance, through an evaporative process in ancient Martian lakes or seas, where sulfate minerals break down under specific conditions, leaving behind pure sulfur.

An additional analysis could provide some clues as to how the sulfur could have formed. Unfortunately, the rocks were too small and brittle to be directly sampled with Curiosity’s drill, so researchers are left guessing exactly how the sulfur on Mars was formed.

However, researchers have spotted another, larger rock they call “Mammoth Lakes”. Curiosity engineers found a part of the rock that would allow safe drilling and stable parking on the loose, sloping surface. The rover’s 7-foot robotic arm extracted powdered rock for analysis. This sample will help scientists better understand the materials that make up the Martian surface.

Now, Curiosity has departed from Mammoth Lakes and continues its journey, exploring new areas within Gediz Vallis channel.

While the precise origin of the elemental sulfur remains uncertain, it underscores the dynamic and complex nature of Martian geology. With each new find, we get one more piece of the Martian geology puzzle and we come closer to understanding whether Mars could have ever supported microbial life.