The answer, suggest Wilson and Mouginis-Mark, is that you must have a very runny liquid, but with some internal strength to damp out the turbulence. In mud that would mean having a lot of fine solid particles, which is not a problem.
"No lava that anyone has ever encountered on Earth (or elsewhere) has both the low viscosity and the high strength that is implied for our flow," said Wilson.
That said, the issue is far from settled.
"It's very compelling and they're doing very good science," said planetary scientist Andrew Ryan of Arizona State University. Ryan studies the nearby, but much larger Athabasca Valles flow. "But I'm far from convinced that it's a mudflow. A lot of the features are identical to what we see in my study on a very nearby area."
However, Athabasca Valles is far less likely to be a mud flow because it would be difficult to explain where so much mud would have come from, Ryan explained.
Cerberus Fossae, on the other hand, could have been released from an underground reservoir, marked by a depression at the top of the flow. The upper 3 kilometers (1.9 miles) of Mars' crust is essentially permafrost. But if something like volcanic activity cracks that permafrost layer, the water, muddy water or even watery mud, would come boiling up to the surface.
"We think it is possible that our flow is similar -- but on a much smaller scale -- and that it is mud because both water and fine rock particles have been flushed to the surface instead of just clean water," Wilson said. "But what let the water escape on this occasion, and why this one had the fine particles and others didn't, are things we are still thinking about."