We know that Pluto is unlike anything we ever imagined, and its family of moons is no exception. Before arriving at the Pluto system, scientists made some predictions about Pluto’s small moons based on data collected by the Hubble Space Telescope and other observatories. Some were pretty spot on, and others were way off. Let’s take a look at what we’ve learned so far about this incredibly interesting and dynamic system.
The Pluto system is composed of the binary pair of Pluto and its largest moon Charon, with four smaller moons – Nix, Styx, Hydra, and Kerberos – orbiting the duo. Since Pluto and Charon orbit a common barycenter, it’s better to think of Pluto and Charon as a pair rather than a planet and a large moon. Prior to its closest approach, data from the Hubble Space Telescope predicted that Pluto’s small moons orbit in a chaotic fashion.
The vast majority of moons in the Solar System are very well-behaved: orbiting in synchronous rotation and tidally locked with their planet, keeping the same face toward the planet all the time. As the science team analyzes the New Horizons data, we are seeing time and time again that Pluto's small moons do not play by these rules; they write their own.
Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
During a briefing at the 47th annual meeting for the Division of Planetary Science, Mark Showalter of the SETI Institute said, “We’ve gone from dots to details in less than 10 years. It’s amazing what we have learned so far. It’s not chaos in this system, it’s pandemonium.”
New data from the New Horizons spacecraft reveals the small satellites are spinning much faster than first predicted. For example Hydra, the most distant moon, spins a whopping 89 times in one single orbit. “Hydra is acting like a typical Kuiper Belt Object,” Showalter said. “It spins 89 times in one time around Pluto; flaunting everything we know about objects in the Solar System,” Hal Weaver, New Horizons project scientist added.
Our first response to this might be to ask why Hydra is spinning so fast, but the better question is “Why hasn’t it slowed down?” The science team doesn’t quite understand all the dynamics of the Pluto system yet, but one hypothesis is that Charon exerts a strong torque on the smaller satellites, preventing them from slowing down and settling into a synchronous rotation.
Nix, Pluto’s innermost, potato-shaped moon could arguably be the most fascinating of the small moons. Spanning 48 kilometers by 32 kilometers (29 miles by 19 miles) – roughly the surface area of Los Angeles – the surface is void of any boulders or fractures, but we do see a few recognizable craters and a few potential craters that we can’t quite resolve. The wobbly potato moon also rotates backwards, and even increased its spin rate by as much as 10 percent between 2012 and 2014.
The small moons of Pluto behave like spinning tops. Credit: Nasa/Youtube
Styx and Kerberos are so small and low mass that it’s no wonder our predictions for Kerberos were completely off. Originally scientists believed the moon to be darker than the others, but recently returned data from New Horizons shows the tiny moon’s surface to be just as reflective as the rest.
Images of the small moons indicate that at least two of them – Kerberos and Hydra – appear to be dual-lobed and could have formed as a result of two different bodies merging together. This suggests that at one time Pluto may have had more moons. Showalter and Alan Stern, New Horizons principal investigator, suspect that the small moons formed during the same impact that created Charon.
Scientists combined data collected from New Horizons with computer models and concluded that an object smaller than Pluto but larger than Charon had to have struck the protoplanet Pluto in order to create the two bodies we see today. The data also indicates this impact had to have taken place a very long time ago, not long after the formation of the original body.