Mars' great canyon complex, Valles Marineris, dwarfs the
size and splendor of Earth's own Grand Canyon. But while geologists have a
formed a fairly complete picture of how the Grand Canyon formed, the mechanisms
that carved out Valles Marineris and its component canyons have been a longstanding
mystery, with explanations ranging from massive
floods to tectonic processes like those that cause earthquakes and build
mountains on Earth.
"How did these gigantic canyons really form? Were they
all formed by floods, or were other things going on?" asks John Adams of
the University of Washington in Seattle and lead author of a new study that
seeks to answer the questions. "These have been controversial questions
going back to the very first Mariner pictures of Mars. And they're still
controversial questions, which means we don't really fully understand what's
going on yet."
The answer for how at least parts of the canyon complex
formed may lie in Hebes Chasma, a 190-mile-long (310-kilometer) scar cut into
the Martian surface and connected to the main body of Valles Marineris.
Adams and his colleagues, Alan Gillespie and David
Montgomery, think that Hebes, and other chasms, might have formed after salts
in the surface layers were heated up, causing water to melt out below the
surface. This water then rushed out through underground plumbing, causing the
layers of dirt and rock above to collapse, creating the canyon scar in the
surface.
Heating salts
Montgomery and Gillespie began with the observation of
sulfate salts in the area of the canyons. (These compounds have been found in
numerous spots around the Martian surface by orbiting
spacecraft and robotic rovers.)
These salts are hydrated, which means that molecules of
water are attached to the sulfate molecules. Montgomery and Gillespie thought
about what would happen if those hydrated salts were heated up. The physics and
chemistry involved in the process indicated that the water would separate out
from the salts, taking with it some dissolved materials. This muddy, briny
water would pool up underground, causing an increase in volume under the
surface.
The heating of these salts on Mars isn't a stretch, "because
it gets hotter as you go down with depth of course," Adams explained. "But
more importantly, the huge canyon complex of Mars is right next to the huge
volcanic complex, the Tharsis plateau." The heat involved with volcanic
processes could have warmed up the salts in the Martian regolith.
Once the muddy water was separated, it would have to escape
somehow from its subterranean well because there wasn't enough room under the
dirt and rocks to hold all that newly formed water. The two possibilities for
the water's escape were: bursting out over the surface; or funneling out below
it, like a plug pulled from a bathtub drain.
The scientists needed a place to test out the drain part of
the theory, which is where Hebes Chasma came in.
No way in or out
Hebes Chasma was of interest because it is "the very
best example of a fairly large canyon in the Valles Marineris complex that has
absolutely no inlet or outlet on the surface," Adams told
SPACE.com. "There's no way that water could have gone out and over the
edge there." So Hebes Chasma could not have formed by erosion from giant floods,
like the carving of the Grand Canyon but on a larger scale.
Montgomery and Gillespie and other researchers looked to see
if the landforms found in Hebes Chasma were what would be expected of a
situation in which the muddy brine drained out of it underground pool through a
series of subsurface channels, like pipes. Adams and his colleagues made
detailed geologic maps of the region to see if the features were consistent.
They also enlisted the help of geologists at the University of Texas at Austin, who built a desktop-sized scale model of Hebes Chasma and
filled it with silicon gel and sand mixtures, then drained it to see what would
happen.
"The results of that were absolutely astounding to
us," Adams said — they matched the formations found in Hebes Chasma to a
T.
The match of the two approaches led Adams and his colleagues
to conclude that Montgomery and Gillespie's process likely explained the
formation of Hebes Chasma.
There are, of course, still plenty of unanswered questions.
More drained chasms?
For one thing, the scientists don't know exactly when Hebes
formed or how long the process took.
Adams said the chasm likely formed early in Mars' history,
with the process stopping several billion years ago.
"It may have taken a few million years, or a hundred
million years, we just don't know the answer to that," Adams said.
Another mystery is where all that muddy brine went to.
"This is still the big mystery," Adams said.
It's possible that the briny mixture traveled
underground and came up and out elsewhere — a possible source of the
outflow floods thought to have formed other parts of the canyon.
Adams and his colleagues plan to look closely at more chasms
that also show evidence of material sliding down into depressions in the
surface to see if they can also be explained by this mechanism.
How much of Valles Marineris might have formed from the
drainage of muddy brines isn't known, and it's likely that the whole system
formed from a "mixed bag" of mechanisms, including floods, drainage
and tectonic forces, Adams said.
The new study was detailed in the August issue of the
journal Geology.
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