December 15, 2008

Main Lunar Crater Sequence Animated

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Left animation: Kurt A. Fisher, Salt Lake City, Utah from Lunar Orbiter and Clementine Right video: UltraSlo.com via YouTube
Click to download left frame as animated gif (2.5mb)

The main lunar sequence consists of changes in crater morphology resulting from increasing energy levels from larger, denser and faster impacters. Small, porous and slow impacters have relatively less energy and make small bowl-shaped craters, such as ALC class Albategnius C (5km dia.). Larger energy levels from larger, denser and faster impacter causes the lunar surface to rebound, creating a central uplift in TRI class craters (Triesnecker - 20km dia.) or central peaks in TYC class craters (Tycho - 85km dia.). The sequence ends with multi-ring lunar basins such as the Orientale Basin (930km dia.) The left-hand animation above shows this progression of morphology along the main lunar crater sequence after Wood and Andersson (1978) and Hiesinger and Head (2006). The red vertical bar on the right-side of the left-hand animation shows the relative size of the crater where the 930 kilometer diameter of Mare Orientale is 100 percent.

An analogy to elastic rebound of the lunar surface during impact events is seen in daily life in the form of a raindrop striking a puddle surface. The right-hand videoclip by UltraSlo.com of a water drop striking a liquid surface illustrates the rebound effect.

A practical geologic application of this rebound effect is that subsurface lunar rock layers are uplifted into central peaks. Tycho class craters with prominent central peaks also can be viewed as natural probes into lunar soil layers 8 to 20 kilometers beneath the Moon's surface.

The main lunar crater sequence is:

• ALC- bowl-shaped craters with smooth rims, diameters up to 20 km; prototype Albategnius C.
• BIO - flat-floored craters with similar morphology and diameter range as ALC but with a flat floor having a clear break in slope at the contact with the crater wall; prototype Biot.
• SOS - relatively shallow craters with broad flat floors and terrace-free narrow walls, diameters 5-35 km; prototype Sosigenes.
• TRI - scalloped-walled craters typically 15-50 km in diameter, with sub-angular or scalloped outline; broad often roughly concentric slump masses in wall with flat floor partially or completely obscured by slumped material; prototype Triesnecker.
• TYC - craters with multiple tiers of teraces, crenulated rim crest, large flat floor, diameters 30 - 175 km; prototype Tycho.
• Central Peak Ring Basins - Basins with a single ring of central peaks, at the center of the basin, the central peak disappears or is nascent, diameters 175 - 450km; prototype Schrˆdinger Basin.
• Multi-ring basins - More than one developed peak ring, diameter > 400km; prototype Orientale Basin.

Kurt A. Fisher

Yesterday's LPOD: Radial Fractures?

Tomorrow's LPOD: Mapping Names

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