Some of Mars' lava fields may actually be massive mud flows, say planetary scientists after taking a detailed look at the distinctive Cerberus Fossae, indicating that the ancient feature may not have been caused by volcanic activity at all.

If this is the case, then many other Mars lava flows need to be reexamined, argue Lionel Wilson of the University of Hawai'i, Manoa, and Peter Mouginis-Mark of Lancaster University in new research published in the journal Icarus.

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Using models and the latest elevation maps of Ceberus Fossae, the researchers concentrated on the velocity and depth of the flow textures seen as the material cut around boulders and washed up on slopes.

If it was lava, the behavior would be pretty similar to lava on Earth, which often has a broken, platy crust on top that shows how it flowed before cooling and solidifying.

But would mud flows have a platy crust too? The authors propose that the mud, after it erupted from the ground, oozed down the slope in a way unlike any mudflow on Earth. For one thing, the lower atmospheric pressure of Mars would cause the water inside the mud flow to boil. But because Mars' atmosphere is very cold, the mud on top of the flow would be in direct contact with the air and soon freeze to create the platy crust.

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"It probably sounds odd that you can boil and freeze water at the same time, but at Mars atmospheric pressure that can and does happen," said Wilson. The viscosity of the mudflow would have been about that of SAE 40 motor oil, he said.

The speed and manner that the lava flowed was important as well. The researchers believe it had very little turbulence within the flow to create the features seen today.

"The key issue is that flows with surface textures like this one that are seen elsewhere on Mars have been assumed to be lava flows," said Wilson. "But if our estimates of the flow speed and depth are correct, then any lava moving with this depth and speed would have a very low viscosity and would be turbulent."

But the surface textures of Cerberus Fossae, and especially the way the textures are preserved when the flow splits and recombines around obstacles, suggest that this flow was very smooth -- something that's called laminar flow. "So how can you get a laminar flow that has a low viscosity?"

Location of flow (shown in gray). Yardang material seen at lower left. Source area of flow is arrowed, and direction of flow was from west to east.L. Wilson, P.J. Mouginis-Mark

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.

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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.

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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."