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November 10, 2015

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“Pandemonium” seen in Pluto’s moon system

Nov. 10, 2015
Courtesy of the SETI Institute
and World Science staff

Most of the fa­mil­iar moons in the so­lar sys­tem or­bit their plan­ets calm­ly. Nor­mal­ly, one side of the moon al­ways faces the host world, the same way the same side of a horse on a car­ou­sel al­ways faces the cen­ter. This “syn­chronous rota­t­ion,” in the case of moons, is due to the gravita­t­ional tug of the cen­tral plan­et.

NASA's New Horizons captured this enhanced-color view of Charon just before its closest approach on July 14. (Credits: NASA/JHUAPL/SwRI)


But Plu­to’s small moons seem to break those rules and more, a study has found.

In the months lead­ing up to the July 14 fly­by of Plu­to by NASA’s New Hori­zons space­craft, as­tronomers—who were search­ing for any new satel­lites around Plu­to—also had a chance to care­fully meas­ure the spin rates of the known ones. 

The in­ves­ti­ga­t­ions, they said, re­vealed some startling be­hav­ior for the four ti­ny out­er moons: Styx, Nix, Ker­beros and Hy­dra. They were spin­ning wild­ly.

“These are four of the strang­est moons in the So­lar Sys­tem,” said Mark Showal­ter, Sen­ior Re­search Sci­ent­ist at the SETI In­sti­tute in Moun­tain View, Ca­lif. and a co-in­ves­ti­ga­tor on the New Hori­zons mis­sion. 

One moon, Nix, is tilted on its ax­is and spin­ning back­wards, he said. The out­ermost moon, Hy­dra, is spin­ning ex­tra­or­di­narily fast, turn­ing 89 times eve­ry time it cir­cles the dwarf plan­et. “If Hy­dra were spin­ning much faster, ma­te­ri­al would fly off its sur­face,” the way dust would fly off a spin­ning top, Showal­ter said.

He sus­pects that Char­on, Plu­to’s large in­ner moon, is re­spon­si­ble for the odd be­hav­ior. 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 be­hav­ior. But 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 said. “We ex­pected cha­os, but this is pan­de­mo­ni­um.”


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Most of the familiar moons in the solar system orbit their planets calmly. Normally, one side of the moon always faces the host world, the same way the same side of a horse on a carousel always faces the center. This “synchronous rotation,” in the case of moons, is due to the gravitational tug of the central planet. But Pluto’s small moons seem to break all the rules, a study has found. In the months leading up to the July 14 flyby of Pluto by NASA’s New Horizons spacecraft, astronomers—who were searching for any new satellites around Pluto—also had a chance to carefully measure the spin rates of the known ones. The investigations, they said, revealed some startling behavior for the four tiny outer moons: Styx, Nix, Kerberos and Hydra. They were spinning wildly. “These are four of the strangest moons in the Solar System,” said Mark Showalter, Senior Research Scientist at the SETI Institute in Mountain View, Calif. and a co-investigator on the New Horizons mission. One moon, Nix, is tilted on its axis and spinning backwards, he said. The outermost moon, Hydra, is spinning extraordinarily fast, turning 89 times every time it circles the dwarf planet. “If Hydra were spinning much faster, material would fly off its surface,” the way a grain of sand would fly off a spinning top, Showalter said. He suspects that Charon, Pluto’s large inner moon, is responsible for the odd behavior. 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 behavior. But 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 said. “We expected chaos, but this is pandemonium.”