The Saturnian system, with its majestic rings, icy moons, and mysteriously veiled Titan, is the gemstone of the solar system. Ironically the system is full of clues that it was born, or rather reborn, out of a violent demolition derby less than 4 billion years ago.
A clue is the giant moon Titan, larger that the planet Mercury. Every month NASA’s prolific Cassini orbiter unveils more details about this orange hued moon that make it look intriguingly earthlike. Just last week scientist reported seeing a cap of ice clouds forming over the moon’s south pole as a harbinger of the approaching southern hemisphere winter.
With its massive nitrogen atmosphere, hydrocarbon seas, ice continents, tropical methane rain, and cryovolcanoes, Titan has been dubbed “Earth II.” What are the odds that potentially habitable Titan-sized worlds exist around the estimated billions of gas giant planets in our galaxy? And, why doesn’t Jupiter have such a massive moon?
Adding to the mystery is that Saturn’s family icy moons, once dismissed as dead snowballs, look geologically young and active. The moon Enceladus is spewing water geysers from a subsurface ocean that could be an abode for life.
The moons are small enough that they should have been blasted apart in a “baptism by fire” period called the Late Heavy Bombardment (LHB) 4 billion years ago. This tore up Earth’s moon with massive impacts, and blasted out giant impact basins on Mercury and Mars, and presumably Venus and Earth.
The LHB was triggered by final gravitational shuffle of the giant planets. They first moved inward and then outward under a gravitational tug-of-war. The giants shifting orbits forced asteroids and comets onto destructive high-speed eccentric orbits.
Erik Asphaug of Arizona State University and co-authors have tied Saturn observations together to propose that the planet shuffle lead to a major makeover of Saturn’s satellite system. Their research was published in the journal Icarus.
Computer modeling by Asphaug suggest that Saturn started out with several primordial moons that were the size of Jupiter’s four major satellites (first discovered in 1609 by Galileo Galilei). The theory is that during the LHB their orbits were disrupted and they collided to merge and form Titan in place of Jupiter’s Galilean-style system. Perhaps not coincidentally, Titan is similar in mass to all of the Galilean satellites put together. “The original physics a satellite formation may have been the same around Jupiter and Saturn,” Asphaug writes.
This smashup also left behind Saturn’s family of oddball major icy satellites that that would have been formed after the LHB. The moons’ chemical diversity and geology tell what parts of the larger parent moons they came from, say the researchers. The idea is that denser moons like Enceladus, with its silicate core under a water ocean, came from deep inside the parent body. Low density, purely ice moons like Tethys could have come from a water-ammonia mantle of the lost Galilean sized satellites.
The model predicts that the icy moons should have had their own rings and subsatellites that decayed and crashed onto the moons. This could explain the puzzling equatorial bulge on walnut-shaped Iapetus (pictured right) where a shattered moon might have literally rained out of the sky.
Previous research suggests that the LHB also tore up the satellite system of Uranus and Neptune. The original moons around Neptune may have been destroyed when it gravitationally captured a cousin of Pluto, as Kuiper belt object were swept toward the solar system’s outer rim. Uranus’ satellite system partially survived as the cyan planet was tipped on it side by a massive collision with a rogue, gravitationally scattered planet.
This kind of mayhem in the early solar system seems the norm. Even Earth’s moon and Pluto-Charon’s satellite system were born from collisions. This kind of planetary pinball may be common among billions of systems throughout our galaxy.
When it comes to building planets and moons, it seems that chaos rules.
Image credit: NASA