The Moon is an impact crater museum. Craters range from microscopic to hemispheric in scale. Many lunar impact craters have common features that formed with the crater, such as bowl-shaped profiles, central peaks, terraced walls, bright rays, and deposits of impact melt. Others were reshaped by geologic processes after they formed. Among the latter are the Floor-Fractured Craters, or FFCs.
Geologists have so far discovered nearly 200 FFCs. All include signs that internal heat has extensively reshaped them. This makes FFCs important for understanding the internal thermal history of the Moon. No one is certain how FFCs form, though geologists have proposed two major models. It's likely that both play a role in FFC evolution.
The igneous intrusion model would have magma rise beneath a crater along a subsurface fissure. As the magma reached the zone beneath the crater where the crust was broken into small pieces by the force of the impact, the molten rock would spread out horizontally, filling gaps and cracks. The magma would continue to push upward like a piston, raising, flexing and fracturing the crater floor. In some FFCs, magma reached the surface, forming lava lakes on the crater floor. The lava lakes then cooled to become small basalt plains.
The viscous relaxation model involves more widespread heating of the lunar landscape over long periods of time. If the crust were kept warm over billions of years, then it would tend to sag and flex very slowly under the pull of gravity. This might explain why some FFCs have rims shorter than they should be if only impacts were responsible for eroding them. FFC floors, more deeply embedded in the warm crust, might change shape even more than FFC rims. Viscous relaxation would not exclude the surface eruptions of the igneous intrusion model.
Today's Featured Image highlights an FFC that could tell us much about the lunar crust. An asteroid or comet impact is thought to have excavated 146.6-kilometer-wide Compton crater about 3.85 billion years ago. Igneous intrusion or viscous relaxation — or perhaps both processes — subsequently produced branching fractures and small basalt plains within Compton crater. The latter are darker than their surroundings.
Compton crater is close to the Compton-Belkovich Volcanic Complex (CBVC) (61.2°N lat, 99.5°E lon), a farside volcanic landform about 35 kilometers wide and 25 kilometers long. The CBVC is a major site of silicic volcanism, which is otherwise rare on the Moon. On Earth, silicic volcanism produces silicon-rich minerals — predominantly rhyolite and dacite. Terrestrial silicic volcanism requires plate tectonics and water. The Moon lacks tectonic plates and oceans, which raises the question of how silicic volcanism could occur there. We might need to obtain samples from the CBVC before we can solve this mystery.
Geologists count craters to determine the age of lunar surface features. The more craters a feature has, the older it is. It turns out that the small basalt plains inside Compton crater and the CBVC are both about 3.65 billion years old. This might mean that related magmatic intrusions reshaped Compton crater and caused the CBVC silicic eruptions.
Compton crater is one of the few craters named for more than one person. It honors physicist brothers Arthur Compton (1892-1962), who won a Nobel Prize in Physics in 1927, and Karl Taylor Compton (1887-1954), who was President of the Massachusetts Institute of Technology from 1930 to 1948.
You can explore more of the central part of floor-fractured Compton crater in the zoomable image below. North is to the left.
Sources
Jozwiak, L. M., Head, J. W., Neumann, G. A., and Wilson, L. (2017), Observational constraints on the identification of shallow lunar magmatism: insights from floor-fractured craters, Icarus 283, 224-231
Shirley, K. A., Zanetti, M., Jolliff, B., van der Bogert, C. H., and Hiesinger, H. (2016), Crater size-frequency distribution measurements and age of the Compton-Belkovich Volcanic Complex, Icarus 273, 214-223
Schultz, P. H. (1976), Floor-Fractured Lunar Craters, The Moon 15, 241-273
Related Featured Images
The Fractured Floor of Compton
NAC Anaglyph: Floor Fractures
Age of the Compton-Belkovich Volcanic Complex
Farside Highlands Volcanism
New View of Rare Volcanism on the Moon
Atlas
Gassendi's Fractures
Alphonsus Crater Mantled Floor Fracture
Published by David Portree on 10 April 2019