Artist's Illustration of the Chaotic Spin of Pluto's Moon Nix
This set of artist's illustrations of Pluto's moon Nix shows how the
orientation of the moon changes unpredictably as it orbits the "double
planet" Pluto-Charon. This illustration is based on dynamical models of
spinning bodies in complex gravitational fields — like the field
produced by Pluto and Charon's motion about each other. Astronomers
used this simulation to try to understand the unpredictable changes in
reflected light from Nix as it orbits Pluto-Charon. They also found
that Pluto's moon Hydra also undergoes chaotic spin. The football shape
of both moons contributes to their wild motion. The consequences are
that if you lived on either moon, you could not predict the time or
direction the sun would rise the next day. (The moon is too small for
Hubble to resolve surface features, and so the surface textures used
here are purely for illustration purposes.) Credit: NASA, ESA, M. Showalter (SETI Institute), and G. Bacon (STScI)
Nix, Hydra, Kerberos, Styx and Charon
This artist's illustration shows the scale and comparative brightness
of Pluto's small satellites, as discovered by the Hubble Space
Telescope over the past several years. Pluto's binary companion, Charon
(discovered in 1978), is placed at the bottom for scale. Two of the
moons are highly oblate. The reflectivity among the moons varies from
dark charcoal to the brightness of white sand. Hubble cannot resolve
surface features on the moons and so the cratered textures seen here
are purely for illustration purposes. Credit: NASA, ESA, M. Showalter (SETI Institute), and A. Feild (STScI)
If you lived on one of Pluto's moons Nix or Hydra, you'd have a hard time setting your alarm clock. That's because you could not know for sure when, or even in which direction, the sun would rise.
A comprehensive analysis of all available Hubble Space Telescope data
shows that two of Pluto's moons, Nix and Hydra, are wobbling
unpredictably. Scientists believe the other two small moons, Kerberos
and Styx, are likely in a similar situation, pending further study.
"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, D.C.
"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."
Why the chaos? Because the moons are embedded inside a dynamically
shifting gravitational field caused by the system's two central bodies,
Pluto and Charon, whirling about each other. The variable
gravitational field induces torques that send the smaller moons
tumbling in unpredictable ways. This torque is strengthened by the fact
the moons are football shaped rather than spherical.
The surprising results of the Hubble research, conducted by Mark
Showalter of the SETI Institute in Mountain View, California, and Doug
Hamilton of the University of Maryland at College Park, are appearing
in the June 4 issue of the British science journal Nature.
"Prior to the Hubble observations nobody appreciated the intricate
dynamics of the Pluto system," Showalter said. "Our report provides
important new constraints on the sequence of events that led to the
formation of the system."
Hubble's monitoring of Pluto's four outer moons has also revealed
that three of them, Nix, Styx, and Hydra, are presently locked together
in resonance where there is a precise ratio among their orbital
periods. "This ties together their motion in a way similar to that of
three of Jupiter's large moons," noted Hamilton. "If you were sitting on
Nix you would see that Styx orbits Pluto twice for every three orbits
made by Hydra."
Hubble provides observational evidence that the satellites are also
orbiting chaotically. "However, that does not necessarily mean that the
system is on the brink of flying apart," Showalter added. "We need to
know a lot more about the system before we can determine its long-term
fate."
To the surprise of astronomers, Hubble also found that the moon
Kerberos is as dark as a charcoal briquette, while the other satellites
are as bright as white sand. It was predicted that pollution by dust
blasted off the satellites by meteorite impacts should overcoat all the
moons, giving their surfaces a homogeneous look. "This is a very
provocative result," Showalter said.
NASA's New Horizons probe, which will fly by the Pluto-Charon system
in July 2015, 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 in the science planning for New Horizons's observations.
The chaos in the Pluto-Charon system offers insights into how planets
orbiting a double-star might behave. "We are learning that chaos may
be a common trait of binary systems," Hamilton said. "It might even
have consequences for life on planets in such systems." NASA's Kepler
space observatory has found several planetary systems orbiting double
stars.
Clues to the Pluto chaos first came when astronomers measured
variations in the light reflected off of the two moons Nix and Hydra.
Their brightness changed unpredictably. "The data were confusing; they
made no sense at all. We had an inkling something was fishy," Showalter
said. His team analyzed Hubble images of Pluto taken during 2005-2012.
They compared the unpredictable changes in the moons' reflectivity to
dynamical models of spinning bodies in complex gravitational fields.
Virtually all large moons, as well as small moons in close-in orbits,
keep one hemisphere facing their parent planet. This means that the
satellite's rotation is perfectly matched to the orbital period. This
is not coincidental, but the consequence of gravitational tides between
moon and planet. (Hyperion, which orbits Saturn, is the only other
solar-system example of chaotic rotation; it is due to the combined
gravitational tugs of the planet and it largest moon, Titan).
Pluto's moons are hypothesized to have formed by a collision between
the dwarf planet and another similar-sized body early in the history of
the solar system. The smashup flung material that coalesced into the
family of satellites observed around Pluto today. Its large binary
companion, Charon, was discovered in 1978. The object is almost half
the size of Pluto. Hubble discovered Nix and Hydra in 2005, Kerberos in
2011, and Styx in 2012. These little moons, measuring just tens of
miles across, were found as part of a Hubble search for potential
hazards to the New Horizons spacecraft flyby.
Pluto and Charon are called a double planet because they orbit about a
common center of gravity that is located in the space between the
bodies. Some regard the Earth-moon system as a double planet, too,
although the center of gravity falls beneath Earth's surface. (Our moon
has 1/80th of Earth's mass, whereas Charon has 1/8th of Pluto's mass.)
Researchers say that a combination of monitoring data from Hubble,
New Horizons's brief close-up look, and eventually, observations with
the James Webb Space Telescope will help settle many mysteries of the
Pluto-Charon system. No ground-based telescopes have yet been able to
detect the smallest moons.
"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."
Contacts:
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Mark Showalter
SETI Institute, Mountain View, California
605-810-0234
mshowalter@seti.org
Doug Hamilton
University of Maryland, College Park, Maryland
301-405-1548
dhamil@astro.umd.edu
Source: HubbleSite
