Mars Global Surveyor
Mars Orbiter Camera

Sketches Illustrating Proposed Formation of Layered Units,
Massive Units, and Thin Mesa Units in Martian Terrain

MGS MOC Release No. MOC2-265O, 4 December 2000


These sketches portray a cross-section through the upper few kilometers (upper few miles) of the martian crust and illustrate the steps of deposition and erosion of layered, massive, and thin mesa units observed in martian craters, chasms, and elsewhere as outcrops of sedimentary rock. These steps and processes were inferred from study of Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images. The illustrations represent a synthesis of ideas described in the December 8, 2000, paper, "Sedimentary Rocks of Early Mars," in Science.

(1) Starting Conditions:

This sketch shows the initial conditions. In this case, 
the upper crust of Mars---portrayed by the brown and orange 
materials---is already layered. The "dimples" in these layers 
are cross-sections through buried impact craters formed by 
meteor impacts. In the diagrams that follow the craters and 
uplands at the top of this section---note the atmosphere is 
portrayed as golden-beige in color---will have new layered, 
massive, and thin mesa sedimentary units deposited upon them.

(2) Thin layers form first:

The first sediments deposited in craters and perhaps upon 
the surrounding landscape are thin layers like those seen in
southwestern Candor Chasma. In this sketch, four thin orange 
layers have been deposited at the top of the section.

(3) Massive units form next:

Deposition of thin layered units was, in some cases, followed
by deposition of thick, massive units that show very little 
layering and can be up to 2 km (1.2 mi) thick in some places. 
Examples of "massive" units can be seen in Gale Crater's 
central mound.

(4) Layered and Massive units are eroded:

Some time after the thin layered units and thick massive units
were deposited, they were eroded. In some cases, such as in
Gale Crater's central mound, there were also periods of erosion
that occurred between the deposition of layered and massive units.
The erosion of these units is inferred to have occurred mostly at 
a time very early in martian history, but the fact that these 
materials usually have very few small meteorite impact craters 
on them today suggests that their erosion might be continuing 
today as well.

(5) Thin mesa units form and erode:

After the thin layered units and thick massive units were
deposited and eroded to about their present state of
degradation, thin, usually dark-toned material was deposited on 
top of them. This material, which we call "thin mesa units", might 
have been deposited much more recently in martian history than the 
other materials.  Subsequently, these too were eroded to leave 
small mesas and patches of severely ridged-and-grooved material 
behind. Examples of thin mesa units include the smooth dark 
material on the ridge in Valles Marineris in the release, 
"Light-toned Layered Outcrops in Valles Marineris Walls,"
MOC2-263, December 4, 2000; and the dark ridged material 
of Melas Chasma in "Ridged Terrain on the Floor of Melas 
Chasma," MOC2-232, May 22, 2000.

Artwork by Malin Space Science Systems;
Credit: NASA/JPL/Malin Space Science Systems

Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, CA. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, CA and Denver, CO.

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