Mars Curiosity rover takes a last look at mysterious sulfur

NASA’s Curiosity rover is preparing for the next leg of its journey, a months-long trek to a formation called the boxwork, a set of weblike patterns on Mars’s surface that stretches for miles. It will soon leave behind Gediz Vallis channel, an area wrapped in mystery. How the channel formed so late during a transition to a drier climate is one big question for the science team. Another mystery is the field of white sulfur stones the rover discovered over the summer.

Curiosity imaged the stones, along with features from inside the channel, in a 360-degree panorama before driving up to the western edge of the channel at the end of September.

The rover is searching for evidence that ancient Mars had the right ingredients to support microbial life, if any formed billions of years ago, when the Red Planet held lakes and rivers. Located in the foothills of Mount Sharp, a 3-mile-tall (5-kilometer-tall) mountain, Gediz Vallis channel may help tell a related story: what the area was like as water was disappearing on Mars. Although older layers on the mountain had already formed in a dry climate, the channel suggests that water occasionally coursed through the area as the climate was changing.

Scientists are still piecing together the processes that formed various features within the channel, including the debris mound nicknamed “Pinnacle Ridge,” visible in the new 360-degree panorama. It appears that rivers, wet debris flows, and dry avalanches all left their mark. The science team is now constructing a timeline of events from Curiosity’s observations.

The science team is also trying to answer some big questions about the sprawling field of sulfur stones. Images of the area from NASA’s Mars Reconnaissance Orbiter (MRO) showed what looked like an unremarkable patch of light-colored terrain. It turns out that the sulfur stones were too small for MRO’s High-Resolution Imaging Science Experiment (HiRISE) to see, and Curiosity’s team was intrigued to find them when the rover reached the patch. They were even more surprised after Curiosity rolled over one of the stones, crushing it to reveal yellow crystals inside.

Science instruments on the rover confirmed the stone was pure sulfur—something no mission has seen before on Mars. The team doesn’t have a ready explanation for why the sulfur formed there; on Earth, it’s associated with volcanoes and hot springs, and no evidence exists on Mount Sharp pointing to either of those causes.

“We looked at the sulfur field from every angle—from the top and the side—and looked for anything mixed with the sulfur that might give us clues as to how it formed. We’ve gathered a ton of data, and now we have a fun puzzle to solve,” said Curiosity’s project scientist Ashwin Vasavada at NASA’s Jet Propulsion Laboratory in Southern California.

Spiderwebs on Mars

Curiosity, which has traveled about 20 miles (33 kilometers) since landing in 2012, is now driving along the western edge of Gediz Vallis channel, gathering a few more panoramas to document the region before making tracks to the boxwork.

Viewed by MRO, the boxwork looks like spiderwebs stretching across the surface. It’s believed to have formed when minerals carried by Mount Sharp’s last pulses of water settled into fractures in surface rock and then hardened. As portions of the rock eroded away, what remained were the minerals that had cemented themselves in the fractures, leaving the weblike boxwork.

On Earth, boxwork formations have been seen on cliffsides and in caves. But Mount Sharp’s boxwork structures stand apart from those both because they formed as water was disappearing from Mars and because they’re so extensive, spanning an area of 6 to 12 miles (10 to 20 kilometers).

“These ridges will include minerals that crystallized underground, where it would have been warmer, with salty liquid water flowing through,” said Kirsten Siebach of Rice University in Houston, a Curiosity scientist studying the region. “Early Earth microbes could have survived in a similar environment. That makes this an exciting place to explore.”