[meteorite-list] SwRI Study Finds That Pluto Satellites' Orbit Ballet May Hint of Long-Ago Collisions

[Ron Baalke]

http://www.swri.org/9what/releases/2013/pluto-moon.htm 

SwRI study finds that Pluto satellites' orbital ballet may hint of long-ago collisions
Southwest Research Institute
Embargoed for release at 1 p.m. CDT on Wednesday, Oct. 9, 2013

Boulder, Colo. - Oct. 9, 2013 - A large impact 4 billion years ago may 
account for the puzzling orbital configuration among Pluto's five known 
satellites, according to a new model developed by planetary scientists 
from Southwest Research Institute (SwRI).

Starting with Charon, Pluto's nearest and largest moon, each of the successively 
more distant - and much smaller - moons orbits Pluto according to a steadily 
increasing factor of Charon's own orbital period. The small satellites, 
Styx, Nix, Kereberos and Hydra, have orbital periods that are almost exactly 
3, 4, 5 and 6 times longer than Charon's.

"Their distance from Pluto and the orbital arrangement of the satellites 
has been a challenge for theories of the small satellites' formation," 
said lead investigator Dr. Harold "Hal" Levison, an Institute scientist 
in SwRI's Planetary Science Directorate at Boulder, Colo.

Models for the formation of Charon leave plenty of small satellites, but 
all of them are much closer to Pluto than the current system that we see 
today,' said Levison. A major problem has been understanding how to move 
these satellites outward, but not lose them from the Pluto-Charon system 
or have them crash into Charon. He said, 'This configuration suggests 
that we have been missing some important mechanism to transport material 
around in this system."

The SwRI study, funded by a grant from NASA's Outer Planetary Research 
program and Lunar Science Institute, considered the earliest and most 
dynamic epoch of the Pluto/Charon system. It is thought that Charon was 
formed by a large impact during a period in solar system history when 
such collisions were dramatically more frequent. Any initially surviving 
satellites would likely be destroyed in collisions, but these shattered 
moons wouldn't be lost; rather, their remains would stay in the Pluto/Charon 
system and become the starting point for building new satellites. Thus 
there would have been many generations of satellite systems over the history 
of Pluto and Charon.

In modeling the destruction of the satellites, the SwRI study found that 
there may be a method for moving them, or their building blocks, outward, 
due to the competing effects of Charon's gravitational kicks and collisions 
among the debris of the disrupted satellites. Charon is the largest satellite 
of any planet or dwarf-planet, weighing in at 1/10 the mass of Pluto (the 
Moon is just 1/81 the mass of Earth), and so it could rapidly slingshot 
the small satellites outward if they were to approach too closely. Meanwhile, 
collisions among small satellites can change orbits to keep things away 
from Charon. When combined, this leads to a series of satellites colliding, 
breaking to pieces, moving outward and then rebuilding.

"The implications for this result are that the current small satellites 
are the last generation of many previous generations of satellites," said 
Dr. Kevin Walsh, another investigator and a research scientist in SwRI's 
Planetary Science Directorate at Boulder, Colo. "They were probably first 
formed around 4 billion years ago, and after an eventful million years 
of breaking and rebuilding, have survived in their current configuration 
ever since."

This work used the Extreme Science and Engineering Discovery Environment 
(XSEDE) digital collection, which is supported by National Science Foundation 
grant number OCI-1053575.