If you look at photographs of Martian impact craters and the ejecta around them, you would, on occasion, see craters ringed with mysterious bright streaks that extend outward from the point of impact. What makes these features odd is the fact that they extend much farther from the craters than normal ejecta patterns, and that they are only visible in thermal infrared images.

Researchers from Brown University have come up with an explanation for how these streaks formed. In a study published in the latest edition of the journal Icarus, they argue these structures were created by high-speed tornadoes generated by large impacts.

According to the authors of the study, who used geological observations made using the THEMIS instrument on board the Mars Odyssey spacecraft , laboratory impact experiments and computer modeling of impact processes, large crater-forming impacts would have created tornado-like wind vortices that reached speeds of up to 500 miles per hour. These tornadoes would then have blasted away dust and small rocks as they swept across the Martian surface.

“Where these vortices encounter the surface, they sweep away the small particles that sit loose on the surface, exposing the bigger blocky material underneath, and that’s what gives us these streaks,” Brown University geologist Peter Schultz explained in the statement. “We know these formed at the same time as these large craters, and we can date the age of the craters. So now we have a template for looking at erosion.”

Hale_streaks_composite Bright streaks on Mars are often associated with smaller craters that were in place when the larger crater was formed. The raised ridges of the preexisting craters disturb the flow of the vapor plume, which causes vortices to form and scour the ground. Photo: NASA/JPL-Caltech/Arizona State University

The researchers’ experiments showed that after an asteroid or another large body impacts the surface, plumes of vapor travel outward from an impact point, just above the impact surface, at incredible speeds. Although these plumes do not directly gouge out the streaks — since they travel just above the surface — when these swirling winds strike raised surface features, the perturbed flow causes powerful vortices to form and drop to the surface. 

This phenomenon may be responsible for furrowing out the narrow streaks.

“This would be like an F8 tornado sweeping across the surface. These are winds on Mars that will never be seen again unless another impact,” Schultz said. "Brightness in the infrared indicates blocky surfaces, which retain more heat than surfaces covered by powder and debris. That tells us that something came along and scoured those surfaces bare."

While this hypothesis does explain how the bright streaks around some of the Martian craters formed, it also raises a few questions. For instance, the researchers found that streaks are only seen around craters that are about 20 kilometers (12.4 miles) wide, and, even more intriguingly, not all craters in this ballpark exhibit such features — something that the researchers can’t yet explain.

“The next step is to really dig into the conditions that cause the streaks,” Schultz said. “They may have a lot to tell us.”