Casualty of War
Apr 23, 2010

Mimas Is a tiny world, only 397 kilometers in diameter. It compares in size to another of its sisters, Enceladus, and to one of Neptune's moons, Proteus. Mimas looks like the “Death Star” space station seen in a popular movie. The giant crater that dominates one of its hemispheres is about one-eighth the diameter of the entire moon. A crater of similar scale would cover almost half of Earth's Pacific basin. Why such an enormous shock did not disrupt the moon's material structure is a mystery that continues to baffle planetary scientists.

Herschel crater, named for Sir William Herschel who discovered Mimas in 1789, is 130 kilometers wide with a towering central peak. Such craters are theorized to form from asteroid impacts. However, there is little debris within the crater and not many boulders or other fragments surround it.

Researchers think that one reason for the lack of debris is that Mimas has little gravitational attraction, so the blast remnants did not remain nearby. It sounds like a plausible explanation, except that the craters on large planets like Earth and Mars—some many hundreds of kilometers wide—also demonstrate little eruptive fallback, their floors and sidewalls are swept clean, though some glassified breccia is often incorporated into them.

The greatest puzzle of all is the hexagonal shape of Herschel crater. How can an asteroid explosion cause a hexagonal crater? No high velocity gun experiments have demonstrated a polygonal crater after an explosive event. Impact events do not result in such formations. Instead, they are chaotic and leave behind circular depressions with conical bottoms.

Another recent discovery on Mimas is the peculiar temperature distribution. Mimas is an extremely cold place. Infrared measurements of its surface by the Cassini spacecraft reveal it to range from -146 Celsius to -160 Celsius. It is the strange pattern of cold that is confusing NASA mission team members.

It was expected that Mimas would be the warmest in the area where the Sun's energy shines straight down. However, the infrared map generated by Cassini indicates that the warmest temperature is along the western limb.

Other false-color images seem to suggest that the temperature differences correspond to surface composition, but no one is sure why. It could be that the ice grains vary in size, causing them to change the way they reflect light.

In previous Picture of the Day articles, we have attempted to provide evidence for plasma discharges on Saturn's moons. Lightning bolts, diffuse glow-mode clouds of energetic particles, and rotating Birkeland currents have been suggested as causative agents for the bizarre conditions found there.

It could be that Mimas has collected a coating of some compounds that were eroded from the other moons in the Saturnian system, especially Phoebe. It was argued in previous Picture of the Day articles that the splotches of dark red and sooty black coloring the faces of Rhea, Tethys, Iapetus, and Mimas are made of ultra fine dust electrically etched from Phoebe.

Mimas might have once been caught in the grip of an interplanetary particle beam that excavated Herschel crater and the other geological features incised on its face. Due to the plasma instabilities in the discharge, a hexagon was cut deeply into its crust. When the electrical energy was withdrawn, Herschel crater remained, a “fossilized” geometric shape permanently burned in.

The electric currents that cut the craters and rilles on Mimas most likely left evidence of their passages in other ways. The anomalous temperature measurement that cannot be attributed to the Sun's influence is probably one sign of those past catastrophes. It may be that the unusual V-shaped pattern in the false color images from Cassini is a warmer layer of dust and ice that was excavated from Herschel crater and ionically deposited "downwind" by a high-energy plasma discharge in the recent past.

Stephen Smith