[meteorite-list] ASU Geologists Suggest Mars Features Are Result of Meteorite Strikes, Not of Evaporated Lakes

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Dec 22 12:50:55 2005
Message-ID: <200512221748.jBMHmnY00086_at_zagami.jpl.nasa.gov>

Public release date: 21-Dec-2005

Contact: Skip Derra
skip.derra_at_asu.edu
480-965-4823
Arizona State University

ASU geologists suggest Mars features are result of meteorite strikes,
not of evaporated lakes

Geologic features at the Opportunity landing site on Mars were formed
not by a lake that evaporated but by constant strikes from meteorites,
say two Arizona State University geologists.

The site where the Mars Exploration Rover Opportunity landed has
sediments and layered structures that are thought to be formed by the
evaporation of an acidic salty sea. The prevailing thought is that when
this Martian sea existed it may have supported life forms and thus would
be a prime site to explore for fossils.

However, ASU geologists L. Paul Knauth and Donald Burt, who along with
Kenneth Wohletz of Los Alamos National Laboratory, say that base surges
resulting from massive explosions caused by meteorite strikes offer a
simpler and more consistent explanation for the rock formations and
sediment layers found at the Opportunity site. The researchers published
their findings in the current issue of Nature.

The research could impact where and how scientists continue their
exploration of Mars in search for past life forms.

Impact surges "present a simple alternative explanation involving
deposition from a ground-hugging turbulent flow of rock fragments,
salts, sulfides, brines and ice produced by a meteorite impact," the
three state in their article "Impact Origin of Sediments at the
Opportunity Landing Site on Mars."

"Subsequent weathering by inter-granular water films can account for all
of the features observed without invoking shallow seas, lakes or near
surface aquifers," they added. "Layered sequences observed elsewhere on
heavily cratered Mars and attributed to wind, water or volcanism may
well have formed similarly."

When the Opportunity lander touched down on the Meridiani Planum in
January 2004, it began a very important period in planetary exploration.
The rover has operated for nearly two years -- when it was designed to
operate for 90 days -- and has returned many breathtaking images of the
Martian surface, as well as measurements of the surrounding geologic
features and chemistry.

Researchers on the Mars Exploration Rover team feel these observations
of this site point to an area once drenched in water, providing an
environment that could have supported life. The body of water gradually
evaporated away, due to the thin Mars atmosphere, leaving high
concentrations of salt behind and several telltale mineral deposits and
geologic formations. Because the observed signs point to an area that
once was a lake, or large body of water, it would be a good choice to
further explore the Meridiani Planum for fossils or other relics of
previous life forms.

But to Knauth, Burt and Wohletz the geologic features at the Opportunity
landing site can also be explained as being artifacts of a meteorite
strike rather than a one-time lakebed.

"When a meteorite hits there is a tremendous blast, like a nuclear
explosion," Knauth said. "On a planet with an atmosphere, around the
base you get a turbulent ground-hugging cloud of debris that goes out
and makes a sedimentary deposit. You get deposits that can go up to
almost 100 kilometers from big volcanoes. A big [meteorite] impact can
provide deposits over tens of thousands of square kilometers.

"Mars is cratered from one end to the other. All of these should have
made base surges," Knauth said.

Upon examining the evidence, the researchers believe the sediments and
structures at the Opportunity landing site are more likely caused by a
base surge than an evaporated lake. Some of the questions concerning the
observed sediments include a mixing of evaporative salts, textures of
the sediments and the existence of small spheroid concretions at the
landing site.

"The mixed chemistry of the salts is all wrong at the Opportunity site,"
Burt said. "If it were a large lake that slowly evaporated, then the
salt deposits would be more uniform going from least soluble (calcium
sulfate, jarosite) to most soluble (halides and Magnesium-sulfate).

"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.

One particularly interesting feature found at the Opportunity landing
site are little spheroid like concretions, or globules, of material.
Initially, scientists believed they were artifacts of water interacting
with the ground as it moves through the rocks. But in order to make as
many spheroids as they found, it would take large amounts of groundwater
to be present and "these things need to grow within rock. We didn't see
any evidence of this," said Knauth, who has been studying concretions on
Earth for 35 years.

"They were absolutely perfect little spheres," he said. "These turn out
to be abundant in base surges. They form like little hailstones. They
are the same shape, the same size and the same uniform distribution,
which concretions don't have."

Knauth explains that there is plenty of evidence of past water on Mars
and that there is a fair likelihood that some forms of life may have
existed on the Red Planet. But if the team's theory is correct, and the
surface features of Meridiani Planum were caused by meteorite strikes
and not a large lake, then the scientists need to be more creative in
where they focus the next steps of their exploration for evidence of
life forms, Knauth said.

Knauth said clues may lie in Martian rocks.

"If we know anything about Mars it is that it has been pounded
unmercifully by meteorites," he added. "We just need to live with it and
take advantage of it. Meteorites are excavators, they throw rock around
all over the planet and I think some of those are juicy astrobiological
targets."

He said most every rock on Mars is cracked and if there was microbial
life on the planet, microorganisms could be picked up by the wind and
driven to different parts of the landscape. They eventually could lodge
in the cracks of rocks and have calcium carbonate and other salts entomb
them."

He added that one of the meteorites from Mars has such carbonate in
cracks and possibly contains evidence of past life. Cracks filled with
the white material have been imaged repeatedly by the rovers on Mars,
but the current instrumentation cannot analyze them.

"If we want to find evidence of past life on Mars we need to look at
these cracks," Knauth explained. "If there were microbes blowing around
these rocks, they could settle and become entombed in these little fills.

"Forget trying to find fossils in situ in apparent lake or ocean beds,
that stuff probably is long gone, being pulverized by later impacts," he
added. "Instead we may have to look at these little films and fractures."

###

Source:
Paul Knauth, (480) 965-2867
 
Received on Thu 22 Dec 2005 12:48:49 PM PST


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