Cover: Regularly layered rocks such as these seen in western Candor
Chasma (6.5*S, 77.2*W) are among the best examples of sedimentary rocks
on Mars. This image was acquired in March 1999, shortly after the Mars
Global Surveyor spacecraft attained its final, mapping orbit. The area
shown is ~1300 m by 1600 m; layers are ~10 m thick. The spacing of ripples
at far right averages 8 m. Layering attests to a dynamic sedimentary
environment of global scale. [Credit Image: NASA/JPL/Malin Space Science
Systems] Note: The cover image can not be cropped or altered
in any way.

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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 years ago.

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.

Eight examples of very similar outcrops of light-toned, layered, cliff-forming
material exposed in locations separated by hundreds to thousands of
kilometers. [image: Science]

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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 on
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 sedimentary

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.

Fig.9. Gale Crater central mound stratigraphy based on MOC image MO3-01521
and the simultaneously acquired MOLA topograph profile. [image: Science]

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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 martian surface.

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 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.”


of layered unit outcrop expressions. [image: Science]

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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 Laboratory.

(A) Illustration of the relative brightness of light-, intermediate-,
and dark-toned outcrops. (B) Illustration of the relative thickness of
thin-versus thick-bedded outcrops. [image: Science]

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