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Charon & company: Pluto's five moons proved to be more complicated than the New Horizons team ever imagined.

THIRD IN A SERIES

Sky & Telescope's October issue (p. 14) details New Horizons' results about Pluto's surface, and November's issue (p. 18) explores Pluto's atmosphere and its solar-wind interactions. This final article examines Charon and its four smaller siblings.

As NASA's New Horizons spacecraft closed in on its prime objective early last year, mission scientists were expecting to be surprised by Pluto. Still, no one was prepared for the sculpted-ice wonderland and off-the-charts geologic complexity revealed by New Horizons during its historic flyby on July 14, 2015 (S&T: Nov. 2015, p. 18).

But Pluto's moons were a different story. Telescopic study had shown that Charon, by far the largest, is dark-hued and spectroscopically bland (topped with water and ammonia ices only). It seemed likely to prove little different than dozens of other bleak, cratered, outer-planet moons seen at close range over the past few decades. Nonetheless, the New Horizons team planned a full range of observations--even timing the craft's arrival so it could duck briefly into Charon's shadow for a pair of occultations to detect any hint of atmosphere.

This moon's four small siblings, all found with the Hubble Space Telescope, were unresolved blips before the encounter. The two larger of these, Nix and Hydra, had been discovered in mid-2005 by a "Pluto Companion Search Team" led by Harold Weaver (Johns Hopkins Applied Physics Laboratory) and Alan Stern (Southwest Research Institute, or SWRI). The timing of those finds, about a half year before New Horizons left Earth, gave the mission team a chance to tweak the encounter plan and work in some detailed observations of them.

However, the discoveries of tiny Kerberos and Styx came in 2011 and 2012, respectively. With the spacecraft already more than halfway to Pluto, all the team could do was dedicate a small reserve of "Unknown TBD" imaging slots to the two latecomers--not optimal, but it was at least something.

What the mission's scientists hoped to find, if nothing else, were clues as to why all the orbits and spin axes of everything in the Pluto system are swung way over to one side--much like the situation with Uranus and its family of moons. And how did Pluto end up with Charon, a moon big and massive enough to skew the system's center of gravity into the empty space between them?

A Manufactured Family

Almost as soon as U.S. Naval Observatory astronomer James Christy discovered Charon in 1978 (delightfully recounted by Govert Schilling in the June 2008 issue), theorists hypothesized that something big had slammed into Pluto eons ago, spurting out enough debris to form Charon and dramatically altering the system's angular momentum and overall geometry.

In 2005, Robin Canup (SWRI) used the computer-simulation techniques she'd developed to explore a giant-impact origin for the Moon to model a similarly cataclysmic Charon-forming impact. The details were different, involving smaller, ice-rich Kuiper Belt bodies that collided at only about 1 km per second (2,000 mph) and generated hardly any melting. But the model's outcome satisfyingly left Pluto with one relatively big, close-in moon in a dramatically reoriented equatorial plane.

Six years later, after the discovery of Nix and Hydra, Canup went back to the digital drawing board and reworked the collision's specs. The key tweak was requiring both Pluto and the impactor to be compositionally layered (differentiated), with rocky centers and icy exteriors. This time the computer simulations spit out Pluto and Charon, as before, and enough icy debris to yield a handful of smaller satellites as well.

New Horizons' hi-and-bye flyby didn't provide lots of opportunities to view Pluto's four small moons closely, but images taken at long range provided enough coverage to establish their sizes, refine their orbits, and clock their rotation rates.

Mark Showalter (SETI Institute), who led the team that discovered Styx and Kerberos, thinks the moons' elongated shapes (see the table below) imply that they're clumps of smaller objects that merged at low speeds. In fact, he imagines that Pluto once had hundreds of small moons that eventually got swept up into Charon and its four smaller siblings.

Thanks to strong tidal interactions with their parent planet, small, close-in moons typically end up forced to orbit with only one face pointing at the planet. That's the norm everywhere else in the solar system. But not so with Pluto's small moons. "Tidal locking isn't what's going on--we knew that," Showalter explains. "But we didn't anticipate what is going on."

All four appear to be spinning chaotically, at rates much faster than their orbital periods--in fact, Hydra spins around 89 times during each 38-day-long orbit. Moreover, an analysis led by Simon Porter (SWRI) shows that all four are rolling on their sides, with rotation axes nearly in their orbital planes--though the pole directions have been especially difficult to pin down. Nix has a spin axis that appears to be precessing (wobbling) randomly, and the pole for Kerberos might actually point in the direction opposite that of the present estimate. "It's not just chaos but pandemonium," Showalter admits. "We've not seen anything like this before."

Compositionally, the evidence for a big-splat origin seems clear. "All four moons--especially Hydra--have albedos that are very, very high, between 60% to 80%, near that of pure water ice," explains Hal Weaver (Johns Hopkins University Applied Physics Laboratory), who headed the Hubble effort that found Nix and Hydra. That's a nice fit to Canup's giant-impact model, in which the small moons assembled from clumps of icy mantle that rocketed away from the impact zone.

One puzzle, Weaver notes, is how these little moons have managed to stay uncontaminated by the darkening effects of "space weathering" and infalling carbon-rich dust over billions of years. One plausible idea is they exert so little gravitational force that small-scale impacts simply chip away the topmost layers over time, exposing the pristine ice that lies underneath.

It's worth noting that, in 2012, a trio of dynamicists led by Andrew Youdin (Harvard-Smithsonian Center for Astrophysics) predicted, on purely dynamical grounds, that these four moons would be small and icy rather than big and dark. Their orbits are spaced so closely, the team reasoned, that if they were large, massive bodies, then they would perturb one another's orbital motion noticeably--an effect that isn't happening.

Of the four moonlets, New Horizons only recorded Nix particularly well. It's pocked by more than a dozen impact craters--enough of them, SWRI investigator Kelsi Singer concludes, to imply that its surface is roughly 4 billion years old. This, together with the moons' other shared properties (such as circular orbits in the same plane and similarly bright surfaces), makes a strong case that not only did a giant, system-forming impact create the whole family but also that this cataclysmic event occurred very early in solar-system history.

Charon: Witness to History

Having assumed all along that Pluto's big splat was an ancient event, planetary geologists expected that the icy (but rock-hard) surface of Charon would preserve eons of cratering history--and it does. As was the case with Pluto, the spacecraft saw only one hemisphere of Charon well during its brief visit, and that half displays two broad areas with lots of impact scars. The southern one, informally named Vulcan Planum by the team, appears smoother than the northern one, dubbed Oz Terra. But based on the abundant craters they bear, both appear to be at least 4 billion years old.

Vulcan Planum is relatively smooth with distinct sets of parallel furrows and, here and there, relatively few craters. New Horizons geologists suspect that parts of Charon were "refreshed" soon after its formation. Perhaps the surface was pushed and pulled around by tectonic forces, or maybe torrents of sluggish, nearly frozen brine erupted from its interior in several episodes of cryovolcanism. Especially curious are a few isolated, high-standing mountains 3 or 4 km high that are surrounded by depressed "moats" up to 2 km deep.

What really got the team's immediate attention, however, are huge gashes that separate the northern and southern plains. This enormous fracture system girds the moon's midsection for at least 1,600 km (1,000 mi)--four times the Grand Canyon's length and twice as deep in spots--and likely continues onto the "farside" of Charon not seen well during the flyby. The part nicknamed Serenity Chasma can be more than 50 km wide and 5 km deep; Mandjet Chasma, to its west, reaches depths of 7 km.

"It looks like the entire crust of Charon has been split open," observes John Spencer (SWRI)--the kind of crustal rending seen on Earth (the East African Rift, for example) and Mars (Valles Marineris). "Every tectonic feature that we see is being opened up and pulled apart," adds Ross Beyer (SETI Institute and NASA Ames).

The most logical explanation is that Charon once had an interior ocean of water that expanded as it slowly froze. The overlying crust would have cracked wide open to accommodate the increased volume. Beyer explains: "If you blow up a balloon, then cover it with papier-mache, and then pump it up some more, it'll crack."

Charon's added girth amounts to a 1% enlargement in surface area (roughly the size of Newfoundland), corresponding to an increase in radius of 3 km. Achieving that, Beyer says, would have required an outer shell of water about 35 km thick to freeze from the top down.

Other mission scientists, meanwhile, have fixated on an enigmatic, red-stained depression at Charon's north pole. Mordor Macula, as it's informally named, has a dark inner zone about 275 km across and a less-dark outer zone, 450 km across, that gradually fades at its margin. Although there are hints of ridges or faults along the outer margin, the reddish coating seems too thin and spread out to have come from inside Charon.

Instead, the team's leading explanation implicates Pluto as the cause. The idea, says Will Grundy (Lowell Observatory), is that some of the methane gas (CH4) escaping to space from Pluto's upper atmosphere ends up as a thin deposit of frost at Charon's colder-than-cold poles. Over time, ultraviolet sunlight and cosmic radiation convert these simple molecules into much more complicated long-chain hydrocarbons, called tholins, that have a characteristically red color.

But that's just at the poles. Overall Charon has the water-ice-dominated composition that researchers expected. There aren't any deposits of nitrogen, methane, or carbon monoxide (like those on Pluto). Charon is too warm on average--a balmy 44 Kelvin (-380[degrees]F)--to keep these volatile compounds from sublimating into gas, and its gravity too weak to hang onto them once airborne. In fact, Charon appears to have no atmosphere at all: the upper limit as measured by the spacecraft is no greater than a few picobars of surface pressure (a few trillionths of what we enjoy here on Earth).

In contrast, ammonia (NH3) is quite stably solid at Charon's temperature and isn't going anywhere, and ground-based observations first spotted ammonia on the moon's surface in 2007. The spacecraft's LEISA spectrometer detected widespread hints of ammonium hydrate (NH3-H20) and a particularly strong concentration of ammonia near a particularly fresh-looking crater nicknamed Organa. It's not clear yet whether a reservoir of ammonia-rich material lies hidden just below Charon's surface, is, gurgling up from great depth, or has been arriving from afar aboard impacting bodies.

Clues to the Kuiper Belt

Although New Horizons has long since left Pluto's vicinity, the rich scientific results from Charon and its small siblings will pay bonus dividends in future studies of the Kuiper Belt. In particular, the distribution of crater sizes found on Vulcan Planum hints that the billions of bodies floating out there are bigger than thought. Scientists had suspected the Kuiper Belt's basic building blocks were predominantly only about 1 km across, but based on Charon's craters the norm should instead be tens of kilometers in size--more akin to what's found in the asteroid belt. That's also the likely diameter of New Horizons' next target, 2014 [MU.sub.69], which it'll fly past at close range on January 1, 2019.

ILLUSTRATION BY CASEY REED

Official Feature Names? Not Yet

The feature names used iri this article resulted from suggestions and votes made by the public via ourpluto.org. Those for Charon, for example, commemorate fictional explorers, their vessels, and their destinations, as well as the authors and artists who envisioned those journeys. But none of them has been approved yet by the International Astronomical Union's Working Croup for Planetary System Nomenclature. On August 9th members of the WGPSN finally sat down with key New Horizons scientists--the first in what promises to be an extended series of meetings to hammer out what we'll ultimately call the peaks, valleys, and uniquely quirky features in the Pluto system.

Senior Editor Kelly Beatty wasn't around when Clyde Tombaugh spotted Pluto in 1930, but he was all over the discoveries of Charon, Nix, Hydra, Kerberos, and Styx.

Pluto's Moons at a Glance

            Year of    Semimajor      Orbital
Moon       discovery   axis (km)   period (days)

Charon       1978       19,596         6.387
Styx         2012       42,413        20.162
Nix          2005       48,690        24.855
Kerberos     2011       57,750        32.168
Hydra        2005       64,721        38.202

             Rotation      Rotations       Diameter
Moon       period (days)   per orbit         (km)

Charon         6.387          1.0      1,212 [+ or -] 2
Styx           3.24           6.2        14 x 10 x 8
Nix            1.829         13.6        50 x 31 x 26
Kerberos       5.31           6.0        18 x 11 x 9
Hydra          0.430         88.9        56 x 41 x 37

                       Obliquity
Moon                  (axial tilt)

Charon                 0[degrees]
Styx       121[degrees] [+ or -] 30[degrees]
Nix        108[degrees] [+ or -] 20[degrees]
Kerberos    95[degrees] [+ or -] 10[degrees]
Hydra       94[degrees] [+ or -] 30[degrees]


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Please note: Some tables or figures were omitted from this article.
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Title Annotation:New Horizons: Part 3
Author:Beatty, J. Kelly
Publication:Sky & Telescope
Date:Dec 1, 2016
Words:2288
Previous Article:The comets of Edgar Allan Poe.
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