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.
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.
"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?"