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Credit: NASA

 

May 24, 2007
One Crater, Many "Puzzles"

Elevated craters, called "pedestal craters," exist in considerable numbers on the surface of Mars. But the explanations given grow increasingly implausible.

The innocent looking crater in the picture above throws an uncomfortable light on one of innumerable Martian challenges to planetary science. Though NASA scientists identify it as an impact crater, its floor is higher than its surroundings. That is the first mystery of pedestal craters. How does an impact create a cavity raised above the surrounding terrain?

NASA scientists have proposed explanations that are not only speculative, they are often contradicted by the images themselves. In analyzing the picture above, the Mars Global Surveyor team begins with the assumption that the site of an impact crater “has been modified by wind erosion.” The process supposedly occurred in this way: the impactor struck a region of relatively loose material susceptible to erosion. The force of the impact created ejecta that hardened into an “armor” against subsequent erosion. According to the Mars Global Surveyor website, “This caused the crater and ejecta to appear as if standing upon a raised platform—a feature that Mars geologists call a pedestal crater.”

But the remarkable complex of ridges required something more: “Next, the pedestal crater was buried beneath several meters of new sediment, and then this material was eroded away by wind to form the array of sharp ridges that run across the pedestal crater's surface.”

So apparently, the crater and its immediate surroundings have two different explanations: first a hardening of ejecta followed by erosion of surrounding material; and, second, a deposition of fresh material followed by the sculpting of “yardang” ridges by further wind erosion.

Of course, nothing would be more easily blown away than the supposed loose material scattered by the impact. As numerous experiments with impacts and explosions have shown, the scattered debris is not melted. So there is no foundation for the claim that ejecta created “armor” against erosion. Though the authors are ambivalent as to the relationship of the ridge patterns to the “impact” ejecta, it is quite apparent that the crater lies in a layer of ridges that do not stand in a radial relationship to the crater. Many of them do not intersect with the crater at all, but run as parallel ridges to the north and south of the crater.

This is surely why they have superimposed an improbable “yardang” topography upon an equally improbable “pedestal” form of an impact crater. If the two overlying ridge systems were created by two radically different formative events, how did it happen that the crater is centered within both. Appeals to coincidence are one of the first signs that something is wrong at the level of foundational assumptions.

Theoretical assumptions have a way of removing anomalies from one’s field of view. In the electrical interpretation, anomalous ridge patterns are one of the keys to the geologic history of Mars. Consider, for example, the ridges running in a northerly direction. More than a half-dozen of these ridges terminate in bifurcation. No similar examples occur in the southerly direction. Another coincidence?

Bifurcation at the initiation point of an electric discharge is one of the most common features observed in laboratory experiments. Bifurcation involves entwining current filaments under the influence of magnetic fields produced by the currents themselves. It is these magnetic fields that “pinch” the filaments into rope-like configurations. The magnetic force is also responsible for the flaring out of the filaments at the point of discharge initiation. This feature of high-energy discharge draws our attention to the rope-like qualities of the ridges, while the flaring in the northerly direction offers a strong clue as to the primary direction of current flow.

In this instance and thousands of others on Mars, it is clear that the red planet will not give up its secrets under the demands of standard theory. Supposed “yardangs,” “dunes,” and “wind erosion” must be counted among the primary failures of theory to account for the patterns of discovery. To show that these patterns are in fact predictable under the electric view of Martian history will be a primary purpose of our Picture of the Day in coming weeks.

 

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