The Strangeness of the Pluto System
Gravitational Resonances and Unexpected Albedos
Pluto and its largest moon, Charon, as discussed in a recent post, have a dynamic that's somewhat unique in the solar system. Like all planet-moon systems, they actually rotate around their shared center of mass, a point called the "barycenter," as they travel around the Sun. Unlike most planet-moon systems, Pluto and Charon are close enough in mass that this barycenter point lies outside of the larger body. The point where Pluto and Charon's mass balances is an empty point space, unlike, for example, the Earth-Moon system, where that point is well beneath the surface of the much more massive Earth. Pluto and Charon's rotation around the barycenter, consequently, was readily visible to New Horizons, which is on approach and taking clearer images by the day.
Animation of Pluto and Charon from images acquired by the New Horizons spacecraft. Note the "wobble" as both Pluto and Charon revolve around the point that is their shared center of mass. Credit: NASA
Besides the obvious wobble, one result of this is that Pluto's other, much smaller satellites -- Styx, Nix, Kerberos, and Hydra, in order of their distance from Pluto-Charon -- are effectively orbiting the Pluto-Charon system. At a teleconference held by NASA today, scientists observing Pluto using the Hubble Space Telescope detailed some very interesting findings that result from the phenomenon.
Digging through Hubble observations dating back to 2005, which included previously unknown detections of later-discovered moons like Kerberos and Styx, Mark Showalter, senior research scientist at the SETI Institute, and Douglas Hamilton, professor of astronomy at the University of Maryland, found some attributes that differed significantly from what they would expect of a system like Pluto's.
Most of the moons in our solar system are "tidally locked" with the planet they orbit. This is a natural result of mutual tidal forces the moon and planet impart on each other over very long periods of time, that cause the moon's rate of rotation around the planet to match its own rotation around its axis. Since it spins at the same speed that it rotates around the planet, it only ever shows one face to the planet. This is the case with the Earth and our Moon.
The left animation demonstrates the tidal locking of the Earth and Moon. The Moon rotates once around its axis in the same amount of time it takes to revolve around the Earth once, therefore showing the same face to Earth constantly. If the Moon didn't rotate, or rotated at a different speed, as demonstrated on the right, the face it showed to Earth would vary as it orbited. Credit: Stigmatella Aurantiaca
Very small objects like Nix and Hydra are expected to be of an irregular shape, rather than neatly spherical, because they're not massive enough for their own gravity to trump all other forces during their formation. For objects like these, tidal locking still occurs, and an observer would expect that the long axis would always point to the parent body. This is how Hamilton and Showalter expected the irregularly shaped Nix and Hydra to orbit the Pluto-Charon system:
Credit: NASA
Note the light bulbs of varying brightness. These represent how the team expected to observe the moons as a result of their irregular shape and tidally-locked rotation. At the 3 and 9 o'clock positions, when the greatest part of the moon was facing Hubble, they expected it to appear the brightest. At noon and six, where only the small pointed end of the moon was exposed to sunlight, they expected it to appear the dimmest. Instead, their observations did not reveal this brightness pattern. In fact, it revealed no pattern at all in the brightness of Nix and Hydra.
The resulting conclusion is that these moons are actually not tidally locked, but have a "chaotic rotation," following no particular axis. They tumble through their orbit randomly. If you were standing on the surface of Nix or Hydra, Showalter said, you would "literally not know if the Sun is coming up tomorrow."
Numerical simulation of the chaotic, random rotation of Nix and Hydra. Credit: NASA
Showalter also noted that Kerberos, the second most distant of the moons, has a starkly different albedo (amount of light reflected) than its brothers. Most of the bodies in the system reflect about 40% of the light they receive, akin to dirty snow, or sand. But based on their observations, Kerberos reflects no more than 4% of the light it receives, which they likened to a charcoal briquette. Showalter called this a "very very strange result" that is "extremely hard to understand." Kerberos must either be coated with some darker substance than its snowball companions, or else is composed of something entirely different. This is one of many questions that the New Horizons mission hopes to answer when it flies by the system about a month and a half from now.
Showalter's co-author, Douglas Hamilton, discussed the impact that Charon had on the orbits of the four outer moons. Charon's orbital period, the amount of time it takes to make one orbit of Pluto, is about one week. Styx, Nix, Kerberos, and Hydra all take 3 weeks or more to make one orbit of Pluto. So for every orbit the outer moons make of their parent body, Charon, with its comparatively large gravitational pull, is passing by 3 times, cyclically perturbing their orbits from what would otherwise be mostly circular into irregular ellipses. Typically this would not be a stable system, for if all of the bodies were to line up on one axis on the same side of Pluto, their gravitational pull on one another would be at its strongest, possibly disrupting the arrangement. But this never happens, and Hamilton and Showalter were able to discover why in their observations: 3 of Pluto's outer moons are in an orbital resonance.
Gravitational interaction has locked Nix, Styx, and Hydra into a special orbital relationship. For every one rotation Nix makes around Pluto, Styx makes two, and Hydra makes three. This cycle never changes, and it is what keeps the moons from ever lining up on the same axis on the same side of Pluto. It's called a "three-body resonance."
The orbital resonance of Jupiter's moons Io, Europa, and Ganymede, which is analogous to the resonance of Nix, Styx, and Hydra. Note how it ensures the three moons never align on one axis on the same side of Jupiter. Credit:WolfmanSF
The researchers compared this resonance to that experienced by Jupiter's moons Io, Europa, and Ganymede. Interestingly, Pluto's outer moons are roughly the same sizes relative to one another and Pluto as Jupiter's Galilean moons and Jupiter (emphasis on "relative to one another;" obviously the Jovian system is much, much larger). This presents an easy analogy to understand the perturbations that Charon causes in the orbits of the outer moons: imagine how disrupted the Jovian satellites would be if an object twice the size of Neptune orbited very close to Jupiter.
In less than two months, the New Horizons spacecraft will be right on Pluto's doorstep. At that time, images of the tiny outer moons of Pluto will be anywhere from 50 to 100 pixels in size, and mission scientists will collect a lot more data on the orbits, masses, and compositions of the outer moons. They may even discover one or more previously unknown outer moons in the process. This will be a watershed moment of discovery for one of the most unique and complicated planetary systems that we know of. The reason for Kerberos's surprising darkness, and the dynamics of the intricately balanced orbits of the outer moons, will be just two data points in an avalanche of new knowledge about a relic from the beginning of our solar system.