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.
“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.