"With evaporated deposits you would not get what you are seeing chemically or mineralogically on Mars," Burt added. "At the Mars site, they have their most soluble salts mixed with the least soluble salts. On Earth, the least soluble evaporates first (like a bathtub ring) and the most soluble last, but in this deposit it is a complete mix."
The researchers explained that from orbital imagery it seems clear that Mars had a "warm-wet" interval very early in its history when there was water briefly on its surface. Most of the water escaped from the planet leaving behind brine that seeped into the rubble created from the early large meteorite impacts, the so-called "megaregolith."
When the planet froze, salts formed in the subsurface along with ice and residual brine. All of this would then be "excavated," basically thrown all together in subsequent impact events, the researchers said.
A specific sedimentary feature called festoon cross bedding, is one which scientists first examining the Mars evidence said was proof of flowing water in the area. Knauth, Burt and Wohletz said it also is a common structure resulting from base surges.
Knauth uses illustrations from structures found in Kilbourne Hole, N.Mex., and compares them to images taken of Martian strata. While these features can be explained as being caused by flowing water, the researchers write: "Cross bedding and other sedimentary structures form in base surges as they slow down and allow suspended particles to be pushed along the surface and worked into layers and cross beds."
"These features," Knauth said, "are quite common in base surges." In fact, such cross bedded sand deposits up to 1 meter think have been found at nuclear test sites in the Western U.S. and are common around volcanic blasts. Co-author Wohletz, an expert on base surges, proposed early on that cross bedded deposits should be common on Mars.
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Contact: Skip Derra
skip.derra@asu.edu
480-965-4823
Arizona State University
21-Dec-2005