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Credit: NASA
The lunar “sinuous rille”, Schroeter’s Valley.
 

Mar 15, 2006
The Moon and Its Rilles

Planetary scientists describe it as a stupendous channel cut by flowing lava. But on closer examination, Schroeter’s Valley and its many counterparts on the Moon refute all attempts to categorize them in such terms.

The long, winding channel pictured above is the most prominent “sinuous rille” on the lunar surface—160 kilometers long and up to 10 kilometers wide—large enough to be clearly visible in Earth-based telescopes. It is also up to 1300 meters deep—a profound contrast to any observed effect of flowing lava on Earth.

Long prior to the space age, Schroeter’s Valley was the subject of many speculations. But crucial details were unknown until the Apollo lunar exploration missions in the late 60s and early 70s, when orbiting craft enabled astronauts to take high-resolution pictures of the lunar surface. The photographs in the composite shown here were taken from the Endeavour Command Module of Apollo 15.

The seven frames look approximately south, revealing the crater called “Cobra Head” at the upper left, from which emerges a winding path that narrows until it disappears on the right. Only the edge of the crater Herodotus is seen at the top of the composite. (An image of Herodotus can be seen along with the famous crater Aristarchus in our March 10 Picture of the Day.

Sinuous rilles are defined as long, winding valleys, usually with steep walls and often emerging from a crater. Of these phenomena, the Moon presents countless examples at all scales. Two instances will be seen in the lower portion of our March 10 picture.

Early speculations based on telescopic observation envisioned “cracks” on the lunar surface. Then the astronomer William Pickering suggested flowing water. A series of other speculations followed, most of them excluded by the findings of the Apollo missions, until planetary scientists eventually settled on flowing lava as the agent. The “standard theory” today states that sinuous rilles were created by lava either flowing across the surface or beneath the ground to form a “lava tube”, portions of which eventually collapsed.

A considerably larger version of the above picture can be seen here, and unless you are already certain that such formations are well understood by planetary scientists, it is worth the look. The enigmas and contradictions of standard theory lie in details impossible to deny.

Both the width and length of the Schroeter’s Valley far exceed anything ever accomplished by lava on Earth. But the reverse should be expected. On the Earth, the atmosphere is insulating, allowing lava to retain its heat. In the vacuum of space, heat will be much more rapidly radiated away. On Earth, as lava flows for long distances (counted at most in a few tens of kilometers, not hundreds), the cooling at the surface causes a “roof” to form. It may then continue to flow as a “tube” beneath the surface.  That is the only way the lava tube can achieve these comparatively modest lengths.

In an earlier Picture of the Day we showed the longest terrestrial example of a lava tube on Earth, associated with Barker’s Cave in Australia. It is 35 kilometers long and only about 35 meters in height. The contrast to the much larger lunar rilles could not be more stark And the only reason the Barker’s Cave lava tube could achieve its length is that, when the insulating crust was formed, the lava was able to retain its heat and continue flowing beneath the surface. No such event occurred in the case of Schroeter’s Valley: It would be impossible to sustain a kilometers-wide roof of rock; and there is no evidence of either a roof or of rubble from a roof’s collapse.

The moon has only about one sixth the gravity of the Earth, and it is gravity that gives flowing liquid its velocity, its erosive force and (most emphatically in the case of heated and melted rock) its ability to cover distance.  Yet lunar rilles extend up to 300 kilometers—almost nine times the length of the “record breaker” on Earth.

The walls of Schröeter’s Valley are both steep and deep. But where did all of the lava go?  A short-lived channel of water might narrow to a termination point without any overflow or outflow—it could simply be absorbed into the ground or evaporate into space.  But flowing lava eating away surface material to cut a deep channel would have to show up somewhere. We should see either breeches in the deep walls or evidence of abundant outflow. But instead, the channel simply dwindles until it disappears. In considering the picture above, it is essential that one realize what planetary scientists themselves acknowledge: The rille did not create the maria in which it sits. It cuts through the pre-existing maria. It is as if the material that once occupied the channel simply disappeared.

The “flowing lava” seems to have possessed many remarkable features. Even as it cut so deep (nothing comparable will be seen in any lava flow on Earth—not even at the much smaller scale of terrestial lava flows), this rapidly moving, molten rock, could make turns up to 90 degrees without affecting the “bends in the river” in any way. Neither the extreme sinuosity nor the parallelism of the rille walls conforms to the behavior of lava erosion.

Consider, for example, the sharply pointed prominence in the most emphatic change of direction about a third of the way down the rille from Cobra head. If the lava had the power to create such vertical cliffs—up to 1300 meters deep—how did that sharp prominence survive?

Curiously, the "flow" of rilles on other worlds isn't limited to "downhill" like lava and water-carved channels on Earth. All fluid-erosion theories have chosen to ignore that the apparent mouth of the “stream” is on high ground, and the narrowest part of the channel is on lower ground. The situation should be exactly reversed. As an erosion channel lengthens, more and more spoil must be carried by the eroding fluid, and the channel must grow wider to accommodate the load. The cross-sectional area of any fluid stream must remain constant. Where it is deep it must be narrow, where it is shallow it must be wide. However, rilles do not conform to this rule. The famous Hadley's Rille, amongst others, simply disappears for a short interval, then reappears.  Other rilles travel both up and down across considerable distances. The most extraordinary example is the Baltis Vallis on Venus, which rises and falls dozens of times, with some two kilometers separating its high and low points along its 6,800 kilometer length.

Once again, it is the things barely noticed, or forgotten, that provide the most telling clues. Within the meandering channel of Schroeter’s Valley is a much more narrow secondary rille. While planetary scientists are well aware of this rille-within-a-rille, almost nothing is said about its defining feature—a chain of small craters running virtually the entire length of the rille. Yet this feature is not uncommon. A nearby rille, Rima Prinz I reveals the same “preposterous” characteristic.

As a rule, the lunar rilles are much more heavily cratered than the surrounding maria, yet by their very presence on the maria they must be younger. Standard dating by “crater count” becomes preposterous. But what is the meaning of this non-random concentrations of craters along the rille’s paths?

The inseparable link between crater formation and rille formation—though substantiated on planets and moons throughout the solar system—becomes highly confused in standard treatments of the subject. Nevertheless, a unified answer has been available for decades, and the credibility of science may, in fact, depend on it.

Coming March 17: The Rilles Are Electric
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