June 03, 2015

Pluto’s Moons Tumbling in Absolute Chaos

If you lived on one of Pluto’s moons, you might have a hard time determining when, or from which direction, the sun will rise each day. Comprehensive analysis of data from NASA’s Hubble Space Telescope shows that two of Pluto’s moons, Nix and Hydra, wobble unpredictably.

“Hubble has provided a new view of Pluto and its moons revealing a cosmic dance with a chaotic rhythm,” said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate in Washington. “When the New Horizons spacecraft flies through the Pluto system in July we’ll get a chance to see what these moons look like up close and personal.”

The moons wobble because they’re embedded in a gravitational field that shifts constantly. This shift is created by the double planet system of Pluto and Charon as they whirl about each other. Pluto and Charon are called a double planet because they share a common center of gravity located in the space between the bodies. Their variable gravitational field sends the smaller moons tumbling erratically. The effect is strengthened by the football-like, rather than spherical, shape of the moons. Scientists believe it’s likely Pluto’s other two moons, Kerberos and Styx, are in a similar situation.

The astonishing results, found by Mark Showalter of the SETI Institute in Mountain View, California and Doug Hamilton of the University of Maryland at College Park, will appear in the June 4 issue of the journal Nature.

This set of computer modeling illustrations of Pluto’s moon Nix shows how the orientation of the moon changes unpredictably as it orbits the “double planet” Pluto-Charon. Credits: NASA/ESA/M. Showalter (SETI)/G. Bacon (STScI)

This illustration shows the scale and comparative brightness of Pluto’s small satellites. The surface craters are for illustration only and do not represent real imaging data. Credits: NASA/ESA/A. Feild (STScI)

Nature - Resonant interactions and chaotic rotation of Pluto’s small moons



"Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system,” Showalter said. “Our research provides important new constraints on the sequence of events that led to the formation of the system.”

Showalter also found three of Pluto’s moons are presently locked together in resonance, meaning there is a precise ratio for their orbital periods.

“If you were sitting on Nix, you would see that Styx orbits Pluto twice for every three orbits made by Hydra,” noted Hamilton.

Hubble data also reveal the moon Kerberos is as dark as a charcoal briquette, while the other frozen moons are as bright as sand. It was predicted that dust blasted off the moons by meteorite impacts should coat all the moons, giving their surfaces a homogenous look, which makes Kerberos’ coloring very surprising.

NASA’s New Horizons spacecraft, which will fly by the Pluto system in July, may help settle the question of the asphalt-black moon, as well as the other oddities uncovered by Hubble. These new discoveries are being used to plan science observations for the New Horizons flyby.

The turmoil within the Pluto-Charon system offers insights into how planetary bodies orbiting a double star might behave. For example, NASA’s Kepler space observatory has found several planetary systems orbiting double stars.

“We are learning chaos may be a common trait of binary systems,” Hamilton said. “It might even have consequences for life on planets if found in such systems.”

Clues to the Pluto commotion first came when astronomers measured variations in the light reflected off Nix and Hydra. Analyzing Hubble images of Pluto taken from 2005 to 2012, scientists compared the unpredictable changes in the moons’ brightness to models of spinning bodies in complex gravitational fields.

“Pluto will continue to surprise us when New Horizons flies past it in July,” Showalter said. “Our work with the Hubble telescope just gives us a foretaste of what’s in store.”

NBC News Alan Boyle has coverage.

For a long time, Pluto was considered an oddball planet — and although that perception has changed, astronomers now report that its tiniest moons rank among the oddest in the solar system.

"The Pluto system is more interesting than we thought," said Mark Showalter, a planetary scientist at the SETI Institute who's one of the authors of a paper about Pluto's moons in this week's issue of the journal Nature.

A detailed analysis of observations by the Hubble Space Telescope suggests that the orbital dance between Pluto and its largest moon, Charon, throws off the orbits of four smaller moons ever so slightly — and sends them tumbling like cosmic footballs.

One moon, Nix, apparently flips around. "If you were living on the north pole, you would suddenly find yourself on the south pole," Showalter told NBC News. Later, he told reporters that "you would literally not know if the sun is coming up tomorrow."

Another moon, Kerberos, is markedly darker than Pluto's other moons — and that suggests its origins may be different from those of the other moons. Showalter said Kerberos' composition might well provide evidence for a cosmic collision that astronomers say took place billions of years ago, and gave rise to Pluto and Charon as we know them today.

"We could be seeing a piece of the 'bullet' that broke apart Pluto," he said. "But that's just speculation at this point."

Pluto was once considered the oddest of the solar system's nine planets, but over the past decade, more objects like Pluto have been discovered on the solar system's icy edge, more than 3 billion miles (5 billion kilometers) from the sun. Now Pluto is considered one of the largest objects in a category known as dwarf planets.

New Horizons could shed more light on the questions surrounding the tiny moons' quirky orbits, or on the nature of dark Kerberos, the astronomers said. Mission team member John Spencer said New Horizons' findings "will prove Mark and Doug right, or possibly wrong."

In addition to showing whether or not Pluto has additional moons or rings, New Horizons is expected to reveal never-before-seen details of the icy world. Pluto may or may not have ice volcanoes that well up from beneath the surface, or clouds in the atmosphere above. But in any case, the Pluto system is already showing that dwarf planets and their even more dwarfish moons can offer giant mysteries.

"Independent of the new discoveries in store, we have already learned that Pluto hosts a rich and complex dynamical environment, seemingly out of proportion to its diminutive size," Showalter and Hamilton wrote in Nature.

SOURCES - Nasa, Nature, NBC News

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