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Sep 26, 2005
Martian Butte and Crater

Another feature on Mars that looks strange from the conventional view begins to look familiar when seen from a plasma point of view.

The caption accompanying this image (from the Mars Global Surveyor Mars Orbiter Camera) calls the light-colored flat-top rock with a moat around it a butte. The caption continues: “The dark landscape that surrounds the butte was once covered by the same rocks that make up this lonely remnant.” This is an understandable assumption in view of the butte’s position on the slope that runs from the 4-kilometer-high southern hemisphere of Mars to the 2-kilometer-low northern hemisphere. But it overlooks a lot of features and a lot of possibilities.

The most obvious feature is the moat. If the butte weren’t there, the depression would probably be called a crater. But it’s difficult to imagine an impact that would generate a central rebound in the shape of a layered butte. (It’s also difficult to imagine an erosion process that would strip away the light-colored layers for miles around and carve a uniformly wide moat—with a crater-like rim—around the steep cliffs of the butte. This may be why the moat is not mentioned.)

Another feature, more easily overlooked, is the system of ridges and cleanly cut grooves radiating away from the butte. Perhaps planetary scientists assumed these to be familiar erosion patterns. But this is not an unusual erosion pattern, except that many of the ridges and grooves appear to run continuously from the butte into the moat over the rim and onto the plain.

Also, the “dark landscape” is not a local feature: It’s part of an extensive dark region just to the east of Valles Marineris.

The awareness of plasma discharge as an erosion process enables one to imagine other possibilities. A “butte crater” is similar in many respects to rampart and pedestal craters. It’s reminiscent of domed craters—which have been reproduced in a plasma lab. It’s merely a variation on layered craters.

The rotating filaments in a discharge channel that carves a crater can be small enough to leave a large central area undisturbed. The secondary coronal discharge channels pulling charge (and surface material with it) into the strongest region of the electric field—around the edge of the primary channel at the rim—will tend to be arranged symmetrically inside and out. And if the arc that cut this crater was only a small component of the much larger thunderbolt that carved Valles Marineris and its extensions, it would have been situated in the more diffuse area that not only removed much of the surface electrically but scorched it as well.

David Talbott, Wallace Thornhill
Mel Acheson
  CONTRIBUTING EDITORS: Michael Armstrong, Dwardu Cardona, Ev Cochrane,
C.J. Ransom, Don Scott, Rens van der Sluijs, Ian Tresman
  WEBMASTER: Michael Armstrong

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