Sunday, September 19, 2010

LOLA data improves the crater count

Updated September 20, 2010 0051 UT

Another Gap. Following up on a global crater count and mean elevation study of LRO laser altimetry (LOLA), spotlighted by NASA, Sept. 16, another conspicuous, surprisingly oblong gap in the distribution of >20 km craters appears in and around Mare Orientale [NASA/GSFC/LOLA/Brown/SVS].

A study of 5,185 lunar craters of similar size, their global distribution and how their interior elevations deviate from the Moon's global average appears to confirm work by Wilhelms, El Baz and others, published a half-century ago.

Amazingly, those earlier investigators, who improved existing maps of the near side and mapped what was still being learned about the wildly different far side, did not have the benefit of laser altimetry streaming down from the LOLA instrument on board the Lunar Reconnaissance Orbiter (LRO).

More amazing, the Moon's mean elevation, its average radius of 1737.5 kilometers, was far from accurately understood. The LAT package on board JAXA's Kaguya (SELENE-1) isolated the Moon's elusive center further, from within 2 km to within about 200 meters.

In comparison, the Brown University study authored James W. Head is rather like modern lunar instrumentation rated against Apollo guidance computers.

Originally, grid by grid, with slide rule geometry and calculus, plugging time and illumination angles into formula, crater counts of extraordinary accuracy were weaved together by patient investigators. Their published conclusions continue to be confirmed in the mining of laser data from LRO. But questions raised by them stubbornly remain unanswered by 21st century remote sensing. The Ground Truth is still irreplaceable

James W. Head of Brown University has performed a global census 5,185 lunar craters >20 km. in diameter. The study, published in Science, includes a global color-coded tally of the crater's interior elevations, showing deviation from the Moon's global mean "sea level" of 1737.5 km. Not surprisingly, a thinner population of such craters are found in and around familiar near side basins, reconfirming conclusions from long ago that the huge plains represent younger surfaces. (Of craters included in the Brown University census, green = mean global elevation; bluer = below, yellower = above.) [NASA/GSFC/LOLA/Brown/SVS]

So what are these data telling us, confirming theories and restating questions asked by the Light and Shadow slide rule guys of the Apollo era?

Broadly speaking, the Moon holds a reliable record of the history of the Solar System, a record largely lost to water, dynamic weather and plate tectonics on Earth. The Moon's obvious proximity shows this history is also the history of Earth, particularly the history of conditions in that part of the Solar System simultaneously occupied by both bodies.

And the Moon's surface tells a story writ large in bombardment, beginning a very long time ago with the large impactors, like the 4 billion year-old event that formed the 2,100 km-wide South Pole-Aitken basin or the 1290 km-wide Imbrium event that probably happened less than a few hundred million years later. All through the course of the past 4,500 million years, smaller but also respectable kinds of interlopers that punched out the craters in the Head census have continued to "encounter" the Moon with decreasing frequency.

If our dating of features on the Moon's surface is close to being correct, the fall-off in this more common kind of bombardment must have been fairly rapid. Otherwise, the 3.9 billion year-old near side basins, "only" a half-billion years or so after the Moon's magma ocean solidified, would be more punctuated with craters.

The evidence, particularly after studies of the Moon's far side literally entered the picture in 1959, hints that between the formation of SPA and the more familiar near side basins, a gradual decline in these "mid-sized" impacts may have reversed for a 150 to 200 million years before resuming its decline. This is the strongest evidence we have for what's become known as the Grand Bombardment, possibly a juggling of material perturbed as the outer planets, for some unknown reason, waltzed for several million years until stabilizing into their present orbits.

The presentation of the LOLA data study, prepared by the Science Visualization Studio (SVS) at NASA Goddard, was atypical in not including the classic near and far side panels; the two hemispheres shown side by side, centered on the 0° and 180° meridians. The SVS illustrations do include the separate panel above, centered on 90° and 900 frames from their animation. And the animation presents the 5,185 color-coded craters in a way that fancifully builds up gong from east to west as Moon rotates once around. This method does not allow for a view centered over any areas of interest other than the two equatorial slides. If you want to see the census results over Mare Orientale, for example, as at the head of this post, the area of interest falls behind before becoming fully populated [NASA/GSFC/LOLA/Brown/SVS].

Nevertheless, as the the Moon rotates, another second gap appears in the crater count, this time arguably in the the lunar highlands but hardly typical in composition, an oblong gap 2000 kilometers north to south and 1200 km wide centered on Mare Orientale.

Orientale was a late comer, slightly smaller and perhaps more energetic than the great basin-forming impacts of a billion years earlier. If dating methods are reliable, then the fall-off in >20 km-wide impact events had fallen to a trickle by the time of Orientale's formation, 3.1 billion years ago. Some studies hint the Orientale event was energetic enough to have caused an upwelling of molten material in the near side basins, the many ponds of mare material within South Pole-Aitken and elsewhere.

Additional Reading:
The Moon through LRO's eyes
Kelly Beatty
Sky & Telescope

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