Highest resolution image of Rhea. Credit: NASA/JPL/Malin Space Science Systems.
Apr 8, 2015
Rhea bears examination in light of electrical theories.
Rhea is one of Saturn’s medium-sized moons, second only in size to Titan, with a mean diameter of 1528 kilometers. The entire visible surface of Rhea is covered in craters, making it a candidate for the most heavily cratered object in the Solar System: one crater is 360 kilometers across. If the crater resulted from an asteroid impact, it was the most cataclysmic event in the moon’s history, since it would have been blasted to rubble by anything larger.
A prominent bright splotch covers almost an entire hemisphere, and resembles rayed structures found on Earth’s Moon or on Mercury. Planetary scientists believe that an impact event hurled subsurface debris outward in long ejecta blankets. As previous Picture of the Day articles have noted, however, rayed formations are more likely to be the result of electric arcs and not impacts.
For instance, arround a large central crater, the rays are not deep but look more like a thin layer of dust. They were probably deposited by an “ionic wind” as plasma arcs reduced the surface rocks to fine powder and then blew them away as ionized particles.
Nearly all craters on Rhea occur in multiples, the majority of the larger ones are in pairs with many smaller craters distributed around them non-randomly; they are found on the rims of the large craters, corresponding to what happens in electric discharge machining (EDM) when a spark will “stick” to one spot for a split second, carving small craters as the main discharge rotates within the large one. The impact of a space rocks could never create such formations.
Another eye-catching feature on Rhea is an oval-shaped crater approximately 115 kilometers on its longest axis. The conventional explanation is that it was formed when a meteor struck the moon and scooped out the crater along its direction of travel. Where is the gouge that should be at the bottom? Why is the leading rim not built up from the movement of rock and ice as the meteor exploded through it at a shallow angle? And why is the crater – and all the craters surrounding it – flat on the bottom with sharp, perpendicular walls?
These are only a few out of many such huge structures that indicate Rhea did not undergo a slow, steady formation out of a nebular cloud, but probably tore through rapidly evolving conditions that hammered its surface. What has been left behind is the record of those conditions in the form of fractures, chaotic terrain and gigantic caldera as big as Colorado, but with no debris.
Despite what appears to be explosions large enough to rock the planet, there are no fragments. If the craters and rilles were formed in giant electric arcs, then rock and ice were probably vaporized or turned into tiny grains and thrown into space.
Stephen Smith
