Curiosity Drives Over a Rock, Cracking it Open and Revealing an Amazing Yellow Crystal

On May 30th, the Mars Curiosity rover was just minding its own business exploring Gediz Vallis when it ran over a rock. Its wheel cracked the rock and voila! Pure elemental sulfur spilled out. The rover took a picture of the broken rock about a week later, marking the first time sulfur has been found in a pure form on Mars.

After Curiosity’s encounter with the broken rock and its pure sulfur innards, the rover trundled over to another rock, called “Mammoth Lakes” for a little drilling session. Before it left to explore other rocks, the rover managed to cut into that rock and take samples for further study to find out its chemical composition.

It’s not that sulfur isn’t prevalent on Mars. It is, but in different forms. The stuff is highly abundant in the Solar System, so this find isn’t as surprising as you’d think. However, Curiosity finding pure sulfur in the middle of broken rocks is a new experience in Mars exploration. So, of course, that’s raising questions about how it got there and its implications for habitable environments in Mars’s long history.

Curiosity’s Peregrinations

At the moment, the Curiosity rover is making its way through the Gediz Vallis. That’s a flow channel winding its way down a section of Mount Sharp (aka Aeolis Mons). That’s the central peak of Gale Crater. The rover has been heading up since 2014, charting different surface layers as it goes. Each layer was put down during a different era of Mars’s history. They could contain clues to the planet’s habitability in the past.

Fast-moving liquid water raged over the surface and carved Gediz. The floods carried a lot of rocks and sand and deposited them all along the way. Other piles of flood debris lie around the region, bearing witness to other ancient floods and landslides. “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 the Mast Camera, or Mastcam on Curiosity. “There was an exciting amount of activity here. We’re looking at multiple flows down the channel, including energetic floods and boulder-rich flows.”

Understanding Sulfur’s Presence

The surface materials in Gediz contain high amounts of sulfates. Those are sulfur-bearing salts that appear as water evaporates. They are a chemical clue that water existed in the region. Judging by some parts of the surface, it also appears the water ponded at some times, in addition to the floods that scoured the landscape and then deposited debris.

Now the planetary science team has to explain how a pure form of elemental sulfur got stuck in the middle of rocks, according to project scientist Ashwin Vasavada. “Finding a field of stones made of pure sulfur is like finding an oasis in the desert,” said Vasavada. “It shouldn’t be there, so now we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting.”

Putting Sulfur in Context

Sulfur, of course, exists on Earth, which helps scientists understand its behavior and the environments where it’s found. The presence of sulfur can be a result of various geological processes. The sulfur “cycle” includes the flow of sulfur from the core to the surface through volcanism. That’s not unusual. Sulfur commonly appears around volcanic vents. Mt Ijen in Indonesia is a good example. It sports extensive elemental sulfur deposits that are mined.

The volcanic moon Io in the Jupiter system features patches of different allotropes of sulfur. They’re also volcanic in origin, spewed out along with widespread lava flows. This moon has more than 400 volcanic features, making it the most volcanically active (and sulfurous) place in the Solar System.

The pure sulfur in the Mars rock most likely came from volcanic processes. They occurred sometime in the past, but that doesn’t answer how the crystals got inside the rock it crushed. Scientists have known for years that Mars was extremely volcanically active in the past. For a long time, they also thought it was dead, or at least dormant. The planet has no plate tectonics like we see on Earth, either. However, the Mars InSight mission found evidence of some seismic activity on the planet in 2021.

In 2023, planetary scientists at the University of Arizona offered up evidence of a giant mantle plume under Elysium Planitia that drove some kinds of activity in the more recent past. Gale Crater lies in this region and could well have experienced related volcanic and seismic activity during the recent geologic past. If so, that could help explain the presence not only of pure sulfur but also the flood-related sulfates deposited on the surface.

For More Information

NASA’s Curiosity Rover Discovers a Surprise in a Martian Rock
Recent Volcanism on Mars Reveals a Planet More Active than Previously Thought
Sulfur on Mars from the Atmosphere to the Core