Saturday, September 4, 2010

Boulder saturation

Second look at the LROC Featured Image, from LROC Operations Center, Arizona State University, May 20, 2010 - close up of a 'house-sized' bolder and the end of its bouncing trail, angling back into a worn crater - coming to a halt millennia ago. The 200 meter wide field of view from the interior a large crater situated in still larger 42 kilometer-wide Henry Frères (23.39°S, 301.06°E). ("Hole in one, in a hole in one," Lillian Ostrach, from LROC NAC M122597190L - LRO orbit 3200, March 7, 2010; alt. 46.24 km; res. = 0.50 meters) [NASA/GSFC/Arizona State University].

The history of the Moon is a history of bombardment. From the global to the micro-scale the history of the Solar System has been etched in detail with little erosion other than subsequent bombardment. Other than the splash, relatively brief flow and inundation of melted material, most of the resulting debris has been broken rock, in sizes ranging almost as wide in scope as those of the Moon's craters.

Fortunately, most of this debris has been shattered and shattered again, gardened into pieces small enough to become packed down by micrometeorites, and the Moon turned out to be perfectly safe to walk on. Like the craters, Moon rock is another language time used to write the history of our home star and its system of orbiting bodies, especially Earth, all preserved from the dynamic, relentless erosion supplied by Earth's lithosphere and watery atmosphere.

Everywhere on the Moon where sufficient grades meet up with "flat lands," boulders and their bouncing trails can be found. This has been established and confirmed, as many expected, since the Apollo era. Now, LRO is providing us with our first comprehensive large scale view of these processes.

In March, one boulder along with its meandering trail downward into the interior of 42 kilometer-wide Henry Frères (23.39°S, 301.06°E) caught the trained eye of the planetary geologists at Arizona State University, who operate the wide and narrow angle cameras on-board the Lunar Reconnaissance Orbiter.

The size of the thumbnail image from LROC observation M122597190L did the scene justice, but not when reduced down within the limitations of this blog template (400 x 800). Even though we downloaded the 253 megabyte original, it's taken since May to sort though the fire hose of data being returned by LRO, allowing a second look. We offer it above rotated 90 degrees and in such a way allowing closer examination.

How long ago this boulder was shaken loose from the rim to begin and end it's bouncing journey back into the host crater's interior is not easy to determine. The effects of optical maturity hint at more than 900 million years ago, but that's not narrowing it down very much.

The shape of the boulder is recorded in the pattern it carved, alternately bouncing and then rolling its way to the point where it was briefly caught by the much older crater near the bottom of the slope, where it obviously hesitated and then rolled backward to a stop.

The imagination can almost see the event as it happened, in less than a wink of an eye in comparison to its age. And our experience can also easily imagine just how long such a feature would last, even in the lightest perennial rainfall, here on Earth.

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