March 14, 2009

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After And More After

image by Jocelyn Sérot, France

Before and after images make a nice comparison to infer how a crater must have changed through time. Here we have two versions of after, illustrating two ways craters age. Egede is the 37 km wide crater at center left with a flooded floor. To its right is giant Aristoteles, whose formation sliced through the western third of pre-existing Mitchell (30 km in diameter). The rim of Egede is continuous (just barely on the west) so lava did not flood it from the outside, but must have risen up fractures under the crater and leaked onto its floor. (The fractures were created by the impact itself - fractured terrain extends down about a crater radius, far deeper the the normal floor.) We can tell a little bit about when the lava flooding occurred because there is a small cluster of secondary craters on Egede's floor. The secondaries came from Aristoteles so the floor lava, liked the similarly cratered surrounding lava, formed before the large crater. And the apparently similar height of Egede's rim above the inner lavas and the surrounding is consistent with both lavas being supplied from the same magma source that erupted each lava to the same height. We don't know how much time elapsed between the formation of the Egede crater and its lava flooring, but we know that the crater is older than at least the top level of the outside lava, which surrounds the crater. We have similar uncertainty about the exact timing of Mitchell, except that it formed prior to Aristoteles. The amazing thing about Mitchell is how relatively unbattered it is considering the gigantic explosion that occurred just a short distance away. Even the interior seems relatively undamaged, with a Lunar Orbiter image clearly showing its two small central peaks and the eastern part of its floor and rim. Mitchell aged instantaneously due to its unfortunate luck of being nearly at ground zero for a much larger crater. Egede degradation was also due to infilling, but of a different type of material, from a different source and by a different process. It may have taken weeks for the Egede's lava pond to form (and years to cool), versus minutes for Mitchell's transformation.

Chuck Wood

Technical Details
2009-03-03, 18h59 UT. Mewlon 210 @ F/23 (Barlow 2x) + DMK 31AF03, 15 FPS, Avistack, 390 frames stacked, 720 points

Related Links
Rükl plate 5
A stereo view of Mitchell.

Yesterday's LPOD: Finding the Past

Tomorrow's LPOD: Not Moonraker, Moonwake


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