Most of the familiar moons in the Solar System orbit their planets deferentially, with one side always facing their host worlds. As with our own Moon, the relentless tug of the central planet causes this “synchronous rotation.” This situation is akin to horses on a carousel, with one flank perpetually facing the central axis.
But these tranquil conditions don’t apply to Pluto’s small moons.
In the months leading up to the New Horizons flyby of Pluto on 14 July, astronomers – who were diligently searching for any new moons or faint rings – also had the opportunity to carefully measure the spin rates of Pluto’s known satellites. While no new moons or rings were discovered, their investigations revealed some startling behaviour for the four tiny outer satellites – Styx, Nix, Kerberos and Hydra. They were spinning wildly.
“These are four of the strangest moons in the Solar System,” says Mark Showalter, Senior Research Scientist at the SETI Institute and a co-investigator on the New Horizons mission. One moon, Nix, is tilted on its axis and spinning backwards. The outermost moon, Hydra, is spinning like a top, rotating 89 times every time it circles the dwarf planet. “If Hydra were spinning much faster, material would fly off its surface due to the centrifugal force.”
Showalter suspects that Charon, Pluto’s large inner moon, is responsible for this odd behaviour. Recently, he and collaborator Douglas Hamilton of the University of Maryland predicted that Charon’s strong gravitational force would disrupt synchronous rotation, causing the small moons to tumble chaotically. In the fields of physics and mathematics, “chaos” is a technical term indicating unpredictable behaviour. However, chaos alone – while describing the motion of these moons – is not an explanation.
“There’s clearly something fundamental about the dynamics of the system that we do not understand,” Showalter notes. “We expected chaos, but this is pandemonium.”