Scientists are closing in on an explanation for how mysterious pure sulfur rocks ended up on the surface of Mars.

NASA's Curiosity rover accidentally discovered them at Gale Crater in 2024 by driving over them, crushing the material, and exposing a bed of crystals the color of Mello Yello. The mission team later realized this wasn't a small patch of rare rocks but a vast field sprawling 50 yards.

When sulfur is made naturally on Earth, it is usually linked to superheated volcanic gases and hot springs. Another way it can form is through interactions with bacteria, said Abigail Fraeman, deputy project scientist on the Curiosity mission, two years ago.

"We don't think we're anywhere near a volcano where the rover is, so that is a puzzling feature to find in this particular location," she told Mashable then. 

But a team of researchers now has an alternative idea that wouldn't require hot springs or Martian life. Instead, a space rock may have slammed into an area that already had a cache of sulfur buried underground. The extraordinary heat from the collision could have melted the sulfur into a liquid, thus allowing it to cascade downhill for a few miles before hardening into solid chunks like the ones the rover found. 

Curiosity is surrounded by plenty of sulfates — rocks that contain sulfur mixed with other materials — at Gale Crater. But unadulterated sulfur is a whole different story. No other Mars mission has found pure elemental sulfur just lying around.

Scientists recently presented their study of the pure sulfur on Mars at the European Geosciences Union General Assembly in Vienna, Austria. 

The smoking gun for the scientists' asteroid hypothesis is a small, damaged crater about 1,280 feet wide uphill from the sulfur deposits. Part of that crater's lip is missing; it could have been a natural channel for molten sulfur to spill out and flow about 2.5 miles downhill.

The sulfur may have pooled behind piles of rock that had fallen into the valley. A handful of clues support this, but one of the most compelling comes from the round holes spotted in the rocks. Researchers think they may be bubbles that foamed as gas escaped from the cooling liquid sulfur, according to one of two papers presented at the Lunar and Planetary Science Conference in Houston in March. Rover images showed more holes at the higher points of the deposit, which is what scientists would expect from a pool gradually cooling from the bottom up.

To test the asteroid impact idea, researchers modeled space rocks striking the Martian surface at speeds between about 11,000 and 22,000 mph with computers. The simulations showed that faster impacts melt more sulfur, according to another LPSC paper. 

But there was a catch. Only about 20 to 25 percent of the melted sulfur stayed inside the crater. The rest was blasted out or vaporized. 

When the scientists assumed sulfur made up only a small portion of the ground — as is common on Mars — the simulated crash didn't produce nearly enough melted sulfur. When the ground around the crater was composed of at least half sulfur, the impact could have produced the right amount of liquid sulfur to explain the rover's discovery.

The team suggests the sulfur itself could have come from ancient volcanic eruptions that already existed at the site of the asteroid collision. The impact could have melted those rocks rather than created them. 

The simulations are still rough, the researchers say, as they don't have a specialized physics model yet for how sulfur behaves under the extreme pressures of an asteroid impact.

Curiosity is approaching the region where scientists think this sulfur-rich material may be located. Future observations could reveal whether those rocks truly contain a lot of sulfur, helping to confirm whether an asteroid impact could have created the strange sulfur deposits.