On Pluto, the moons can be a bit ec­cen­tric

Hub­ble Tele­scope data show Nix, Hy­dra, Ker­beros and Styx tum­ble un­pre­dictably, thanks to Charon.

Los Angeles Times - - CALIFORNIA - DEB­O­RAH NETBURN deb­o­rah.netburn @la­times.com

A closer look at the far­away Pluto sys­tem has re­vealed that it’s a lot more pe­cu­liar than sci­en­tists ex­pected.

Ob­ser­va­tions made with the Hub­ble Space Tele­scope sug­gest the dwarf planet’s four small­est moons — Nix, Hy­dra, Ker­beros and Styx — tum­ble un­pre­dictably as they fol­low their wob­bly or­bits around Pluto and its largest moon, Charon.

“If you stood on Nix, the sun might rise in the west and set in the east one day, and rise in the east and set in the north on an­other,” said Mark Showalter, a se­nior re­search sci­en­tist at the SETI In­sti­tute in Moun­tain View. “And if you bought real es­tate in the north pole, you might dis­cover one day that you are now in the south pole.”

Saturn’s moon Hype­r­ion is the only other moon in the so­lar sys­tem known to ex­hibit a sim­i­larly chaotic ro­ta­tion.

In a study pub­lished this week in the jour­nal Na­ture, Showalter and Dou­glas Hamil­ton of the Uni­ver­sity of Mary­land, Col­lege Park, wrote that this un­pre­dictable lu­nar be­hav­ior is the re­sult of the grav­i­ta­tional pull ex­erted by Charon, the in­ner­most of Pluto’s moons.

“Charon formed in the mid­dle of this sys­tem some 4 bil­lion years ago,” Showalter said. “Ev­ery­thing else has been chaotic ever since.”

Pluto and Charon are close enough in size that in­stead of one or­bit­ing the other, they jointly or­bit a com­mon cen­ter of grav­ity.

Pic­ture two un­equal weights at the ends of a dumb­bell that is ro­tat­ing. This is the type of sys­tem that as­tronomers call a “bi­nary planet.”

Adding to the ab­sur­dity, Pluto’s four small moons ap­pear to be shaped more like foot­balls than spheres. That’s prob­a­bly be­cause they don’t have enough mass for their in­ter­nal grav­ity to pull them into a round shape.

Each time th­ese ob­long moons pass Charon, that keeps them from set­tling down, lead­ing to the chaos the sci­en­tists ob­served.

Com­puter mod­els show that if Charon weren’t there, the other moons would fol­low smooth, cir­cu­lar or­bits. But Charon is there.

“Each time Charon passes by one of th­ese moons, it pulls on it a lit­tle bit more strongly,” Hamil­ton said. “The or­bits are nearly cir­cu­lar, but with Charon go­ing by ev­ery so of­ten, they move in less pre­dictable ways.”

Pluto’s strange, small moons were dis­cov­ered quite re­cently. Nix and Hy­dra were found in 2005, fol­lowed by Ker­beros in 2011 and Styx in 2012.

The en­tire Pluto sys­tem lies 3 bil­lion miles from Earth, and so even a tele­scope as pow­er­ful as Hub­ble can see them only as tiny points of light.

Showalter and Hamil­ton first re­al­ized that Nix and Hy­dra were mov­ing in un­pre­dictable ways by watch- ing those points move and grow ever so slightly brighter and dim­mer in nearly a decade’s worth of Hub­ble data. They ex­pected to find pat­terns in the way the light changed, but they couldn’t.

“It is a fun­da­men­tally un­pre­dictable sit­u­a­tion,” Showalter said.

Styx and Ker­beros are too dim to mea­sure the same way, but Showalter said there is no rea­son to think that they would be­have dif­fer­ently from their lu­nar sib­lings.

The Hub­ble ob­ser­va­tions re­vealed an­other in­trigu­ing find­ing as well. The as­tronomers re­ported that Nix, Hy­dra and Charon are the color of dirty snow or desert sand, while Ker­beros is as dark as char­coal.

This dis­cov­ery is par­tic­u­larly baf­fling be­cause the re­searchers be­lieve that all of Pluto’s moons formed as the re­sult of an im­pact be­tween the dwarf planet and an­other body early in the so­lar sys­tem’s his­tory. The im­pact would have caused a cloud of dust to or­bit the planet, even­tu­ally co­a­lesc­ing into the moons we see to­day.

But if all the moons were formed of the same stuff, why would one be black and the oth­ers light?

One pos­si­ble ex­pla­na­tion is that the ob­ject that col­lided with Pluto bil­lions of years ago was dark as coal, and per­haps Ker­beros is made up of ma­te­rial just from that ob­ject.

As­tronomers may soon gain new in­sight into th­ese mys­ter­ies with the ar­rival of the first space­craft to visit the re­mote dwarf planet.

Af­ter a 91⁄ -year jour­ney through the so­lar sys­tem, NASA’s New Hori­zons mission is less than six weeks from its clos­est ap­proach to Pluto. The space­craft will take ex­tremely high-res­o­lu­tion images of Pluto and Charon as well as col­lect in­for­ma­tion about the smaller moons that will help as­tronomers un­der­stand how big they are, what they are made of and how much light they re­flect.

John Spencer, a sci­en­tist at the South­west Re­search In­sti­tute in Boul­der, Colo., who works on the New Hori­zons mission, said it would add valu­able in­for­ma­tion to the study of the Pluto sys­tem.

“It will rev­o­lu­tion­ize our knowl­edge of th­ese moons,” Spencer said. “It will prove Mark and Doug right — or pos­si­bly wrong.”

NASA

A SET of artist’s il­lus­tra­tions shows changes in Nix as it or­bits. “If you stood on Nix, the sun might rise in the west and set in the east one day, and rise in the east and set in the north on an­other,” a sci­en­tist says.

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