June 3, 2015 | David F. Coppedge

What to Look for at Pluto

As New Horizons closes in on Pluto for its July 14 encounter, what questions should be asked?

The nine-plus-year flight of the New Horizons spacecraft, managed for NASA by Johns Hopkins University’s Applied Physics Laboratory (JHUAPL), is coming to its climax: a close flyby of Pluto on July 14. Back in 2007, New Horizons sent back stunning shots of Jupiter’s moon Io in eruption. Its instruments are now bringing in data from Pluto and its 5 moons (Hydra, Kerberos, Nix, Styx and the large moon Charon that makes Pluto almost a double planet).  Today’s fuzzy images will be improving dramatically as the encounter date approaches.

This week, Nature published three articles about Pluto in advance of the encounter. A Nature Editorial (“To Pluto”) wastes words on Greek mythology, trying to clarify the history of names given to Pluto’s entourage, as if knowing that will inform today’s deep mysteries of the planet’s underworld. The editors also dwell on Pluto’s evolving status as a planet, dwarf planet, plutoid, or trans-Neptunion object. They then discuss some of the scientific questions that are poised for new light from New Horizons.

Little is known about Pluto’s creation, but astronomers had assumed that it formed from the remains of a collision between proto-Pluto and a proto-Charon. The smaller moons may have then come together from bits among the swirling impact debris. The 2012 discovery of Styx was already something of a surprise, because studies had suggested the other three smaller moons were packed so closely together that there was no room for another.

On page 45 of this issue, planetary scientists Mark Showalter and Douglas Hamilton describe how they analysed Hubble Space Telescope images to build up a picture of the orbital configurations and brightnesses of Pluto’s small moons. They find that Styx, Nix and Hydra are locked together in what astronomers call three-body resonance, a phenomenon that links the timing of their orbits and usually makes their movements stable.

They also suggest that Kerberos is a little out of place. Although Nix and Hydra have bright surfaces similar to that of Charon, Kerberos appears as dark as coal, and this raises questions about how this mixed satellite system might have formed. (Pluto is the brightest of the lot, with a reflectivity roughly that of sea ice.) Their findings are discussed in a News & Views article on page 40.

The News & Views article by Scott Kenyon elaborates on these problems, and discusses in more detail the paper by Showalter and Hamilton.  Of particular interest is how the data from Pluto will affect formation theories. “The shapes and compositions of Pluto–Charon’s four moons provide crucial tests of models of planet and satellite formation,” Kenyon says. Also, the tight packing of the orbits of the smaller moons will generate thinking about how long the arrangement could remain stable, given that some of the bodies (Nix and Hydra) exhibit chaotic rotation.  Showalter and Hamilton say,

According to integrations spanning a few centuries, a moon that begins in synchronous rotation will stay there, albeit with large librations. It is therefore possible for synchronous rotation about Pluto and Charon to be stable. However, the large and regular torques of Pluto and Charon probably swamp the small effects of tidal dissipation within the moons, so they never have a pathway to synchronous lock.

Planetary scientists will also be keen at looking for evidence of an active surface (8/25/14) or subsurface ocean (3/13/15). Hints of activity have been inferred from brightness variations on Pluto, and from the activity of a similar outer solar system body: Neptune’s moon Triton.

Update 6/7/15: Live Science posted a Space.com video showing excited mission scientists expressing what they want to find out about Pluto from the New Horizons flyby. Animations of Pluto and its moons by artist Mark Garlick play in the background, providing a means to compare expectations to reality in a little over a month. One thing scientists are eager to learn is how the atmospheric variations observed by the Hubble telescope connect to geology on the surface.

At the end of the clip, while another animation plays, Alan Stern simply asserts that that the impact theory popular for the Earth-moon system also applies to Pluto and its moons. The article, though, quotes Showalter puzzling over the fact that “Nix and Hydra are basically ‘dirty snowballs‘ in terms of reflectivity,” whereas “‘Kerberos is a charcoal briquet‘” by comparison—a result that “took us completely by surprise, because everybody has been assuming all along that the moons would be pretty similar.” From what Showalter says next, this doesn’t jive with the impact theory. “They all probably formed at the same time; they all are made of the same stuff,” he says. A helpful infographic in the article shows what is known about the moons’ sizes, albedos and orbital characteristics, compared to Earth’s moon and dwarf planet Eris (which is even larger than Pluto). The tight packing of the outer moonlets in a 1:3:4:5:6 ratio is stunning. The arrangement is barely stable, allowing almost no space for any other objects.

Prediction 1: Evidence of surface activity will be found that will be hard to explain continuing for billions of years. Prediction 2: The orbital dance of the moons will defy stability for billions of years. Prediction 3: Compositional features of the bodies will challenge theories of accretion from the same region of a dust disk. Prediction 4: Secular planetary scientists will invoke collisions to explain all the anomalies.

Regardless of whose predictions will match reality, the Pluto encounter is a historic event of great interest to the world. It will be the end of an era, completing the reconnaissance of the nine “classical” planets of our solar system. There will continue to be major discoveries. Most future missions, however, will supplement what has been found by these historic “firsts” in space exploration. If history is a guide, scientists will be astonished—even bewildered—by what they see. Stay tuned!


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