by Stephen Smith
April 23, 2010
Mimas is sharply
outlined against the giant planet Saturn.
NASA/JPL/Space Science Institute
Saturn's moons are enigmatic, with
features that are difficult to explain using conventional theories.
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 (below image) 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
However, there is little debris
within the crater and not many boulders or other fragments surround
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
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
(below image) 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
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