Fwd = Sedimentary Rocks On Mars May Suggest An Ancient Land Of Lakes
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Original Subject: Sedimentary Rocks On Mars May Suggest An Ancient Land Of Lakes
Original Date: Mon, 4 Dec 2000 09:56:04 -0800 (PST)
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American Association for the Advancement of Science
Ginger Pinholster, gpinhols@..., 202-326-6421
FOR IMMEDIATE RELEASE: 4 DECEMBER 2000
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Science report: sedimentary rocks on Mars may suggest an ancient land of
8 December 2000 -- Layered geologic outcrops on Mars, described in today's
issue of the journal Science -- may be composed of sedimentary rock that
dates from the earliest span of martian history, between 4.3. and 3.5 billion
Images of these sedimentary rock exposures, captured by the Mars Orbiter
Camera (MOC), suggest that parts of ancient Mars may have resembled a
land of lakes, and that the geology of early Mars was much more dynamic
than previously suspected.
If life existed on Mars during this time period, researchers believe that the
fossil remnants of that past life may be sandwiched within the sedimentary
rock layers, just as they are on Earth.
The martian outcrops, in some cases a few kilometers thick, appear to be
made of fine-grained materials deposited in horizontal layers, the hallmark
of sedimentary rock. These outcrops are found inside craters, between
craters, and within chasms, said Michael C. Malin and Kenneth S. Edgett of
Malin Space Science Systems in San Diego, California.
The Science researchers identified three main outcrop types from the MOC
images: layered units, massive units, and thin mesa units.
Layered units, as their name suggests, consist of relatively thin rock beds --
some only a few meters thick -- stacked on top of one another in distinct
groups. Massive units appear as one bulky rock layer with no clearly defined
horizontal bedding. In a few cases, these types appear together, with the
massive unit always perched on top of the bedded unit like a thick, indistinct
coat of frosting on a layer cake.
Thin mesa units, with surfaces ranging from smooth to pitted to ridged and
grooved, are almost always found on top of eroded massive or layered
While sediments can be deposited in a variety of ways -- including wind,
water, volcanic activity, and even cosmic impact -- the prevalence of the
martian sedimentary outcrops within basin-like features suggests that they
were deposited by water, perhaps in lakes that formed within the craters
and chasms, said Malin and Edgett.
Under this scenario, sediments may have been transported into the lakes in
regular, swift pulses, building up thin layer units. Massive units may have
been deposited when the lake became stagnant or deep enough to cause
sediments to sift down through the water over longer intervals.
"Some of the MOC images of these outcrops show hundreds and hundreds
of identically thick layers, which is almost impossible to have without
water," said Malin.
The sedimentary units show no telltale signs of wind deposition, and the
researchers concluded that explosive volcanic eruptions and impact
cratering probably could not have produced enough sediment to create
the large-scale and geographically widespread outcrops seen on the
Although Malin and Edgett favor water as the sedimentary suspect, they
also offer an alternative model that involves changes in atmospheric
pressure on early Mars. They suggest that periods of relatively high
atmospheric pressure -- caused by fluctuations in the amount of solid
carbon dioxide on the planet's surface -- could have increased the
atmosphere's ability to carry dust produced by heavy cratering.
To confuse matters, the Science researchers don't know where the original
sediments came from, or how they were transported to their final resting
places, since there are no traces of gullies or streams or other channels
associated with the outcrops. They think that erosion may have wiped out
both the source of the sediments and their travel routes.
In some cases, sedimentary rock has eroded out of the crater in which it
formed, also vanishing without a geologic clue.
To Malin, the history of martian geology looks like a jigsaw puzzle.
"In the center of the puzzle, we have these layered rocks, which are good
evidence of an extremely dynamic environment. On either side of this
well-developed puzzle piece, we have mysteries."
In any case, Mars' sedimentary rocks suggest a very active early history
for the planet.
"This makes Mars more complicated and more exciting. This record is going
to tell us a lot about what early Mars was like, and maybe the early Earth
as well, since we don't have a lot of rocks on our own planet from this time
period," said Edgett.
This research was supported by the National Aeronautics and Space
Administration through Contract No. 959060 from the Jet Propulsion
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