Charon (Moon)

Charon (Moon) (officially designated (134340) Pluto I Charon) is the largest of the five known natural satellites of the dwarf planet Pluto and the largest satellite relative to its primary body in the Solar System. Discovered on 22 June 1978 by the American astronomer James W. Christy at the United States Naval Observatory Flagstaff Station while examining photographic plates of Pluto that showed a periodic astrometric elongation every 6.39 days, Charon has a mean radius of R = 606.0 ± 0.5 km (51.0% of Pluto's radius of 1,188.3 km), a total mass of M = (1.586 ± 0.015) × 10²¹ kg (12.17% of Pluto's mass), and a mean bulk density of ρ = 1.702 ± 0.017 g/cm³.

Because the Charon-to-Pluto mass ratio (q = 0.1217) is nearly ten times larger than the Moon-to-Earth mass ratio (q = 0.0123), the gravitational center of mass (barycenter) of the Pluto–Charon system does not lie inside Pluto, as is the case for every major planet–satellite pair in the Solar System, but rather in free space 2,126 km above Pluto's surface along the line connecting the two bodies. Furthermore, Pluto and Charon have evolved into complete mutual tidal locking: both bodies rotate synchronously with their 6.38723-day orbital period across a circular separation of a = 19,591.4 km (e < 0.0002), keeping the same hemispheres permanently facing one another across the sky while the four smaller circumbinary moons (Styx, Nix, Kerberos, and Hydra) orbit the central Pluto–Charon binary. Imaged at sub-kilometer resolution during the flyby of NASA's New Horizons spacecraft on 14 July 2015, Charon revealed a dichotomic world lacking a detectable atmosphere, split by a 1,000-km equatorial belt of extensional tectonic chasms (Serenity Chasma and Mandjet Chasma), resurfaced across its southern hemisphere by the expansive cryolava plains of Vulcan Planitia, and capped at its north pole by the dark reddish organic macula Mordor Macula.

Discovery and Mutual Eclipse Campaign (1985–1990)

When James Christy examined magnified photographic plates of Pluto taken with the 1.55-meter Kaj Strand Astrometric Reflector in April and May 1978, he noticed that Pluto's image appeared slightly elongated on certain nights while remaining circular on others, with the position angle of the bulge cycling with Pluto's 6.387-day photometric rotation period. Ruling out plate defects because background stars remained point-like, Christy deduced the presence of a close, synchronously locked companion and named it Charon—referencing both the mythological ferryman who carried souls across the river Styx into the underworld of Pluto and the nickname ("Char") of his wife, Charlene Christy. Archival plate searches subsequently confirmed pre-discovery elongations dating back to 1965, and between 1985 and 1990 Earth crossed the orbital plane of the Pluto–Charon system, producing a five-year series of mutual transits and occultations that yielded the first accurate determinations of both bodies' radii, masses, bulk densities, and surface albedos.

Grazing Giant Impact and Tidal Orbital Expansion

Dynamicists attribute the origin of the Pluto–Charon binary and its four outer small moons to a low-velocity, grazing giant impact hypothesis event in the early Kuiper Belt roughly 4.0 to 4.5 billion years ago (Canup, 2005, 2011). In smoothed-particle hydrodynamics simulations, a differentiated or partially differentiated proto-Charon (radius ~700–800 km) collided obliquely with proto-Pluto at an impact velocity near the mutual escape velocity (v_imp ≈ 0.8 to 1.1 km/s, far gentler than the ~10 km/s giant impact that formed Earth's Moon). Rather than vaporizing the colliding bodies, this grazing collision launched an intact icy-rocky fragment (Charon) into a close, eccentric orbit at ~3 to 5 Pluto radii alongside a compact debris disk that accreted into Styx, Nix, Kerberos, and Hydra.

Over the subsequent 1 to 10 Myr, strong tidal forces and viscoelastic tidal heating inside both bodies transferred rotational angular momentum from Pluto's rapid initial spin into Charon's orbit, driving Charon outward from ~4 R_P to its modern synchronous dual-locked state at a = 19,591 km (16.5 R_P) while damping the orbital eccentricity to zero.

Oz Terra and the Equatorial Tectonic Canyon Belt

When New Horizons imaged Charon's Pluto-facing hemisphere on 14 July 2015 from a minimum distance of 28,858 km, it revealed a dramatic north–south geological dichotomy separated by a continuous, >1,000-km-long equatorial belt of northeast–southwest-trending normal fault scarps, graben, and chasms reaching 5 to 10 km in depth—four to six times deeper than Earth's Grand Canyon (Moore et al., 2016; Beyer et al., 2017). The northern hemisphere (Oz Terra) consists of rugged, heavily fractured, and moderately cratered crustal blocks cut by polygonal troughs, whereas the equatorial transition zone is dominated by two colossal tectonic troughs: Serenity Chasma (a flat-floored graben 50–60 km wide and ~200 km long bounded by 5-km-high fault walls) and Mandjet Chasma (reaching 7 to 9 km deep), extending toward the northeastern limb where Argo Chasma drops nearly 9 km below its rim.

Noticeably absent anywhere on Charon are compressional tectonic features such as thrust faults or wrinkle ridges. Instead, normal fault dips (~60°) across the equatorial chasm belt indicate a 1% to 2% global areal expansion of Charon's surface (an increase in radius of ~3 to 6 km). Because radiogenic heating from Charon's ~55–60 wt% silicate core combined with early post-impact tidal heating melted an internal water–ammonia layer into a subsurface ocean 50 to 150 km thick, the subsequent secular cooling and freezing of this subsurface ocean into low-density Ice I_h expanded the interior volume by ~5% to 8%, placing the overlying brittle ice shell under global isotropic tensile stress and splitting the crust open along the equatorial chasm belt (Beyer et al., 2017).

Vulcan Planitia and Kubrick Mons Moat Mountain

South of Serenity and Mandjet Chasmata lies Vulcan Planitia, a vast, rolling, comparatively smooth plain exceeding 400,000 km² whose crater size-frequency distribution yields an ancient emplacement age of ~4.0 billion years—only slightly younger than the northern highlands—indicating that Charon's geologic resurgence occurred during the first ~500 Myr of its history as its subsurface ocean froze. Vulcan Planitia exhibits flow-front lobes, rilles, and uniquely "moated mountains" such as Kubrick Mons (3.5 to 4 km tall and 40 × 30 km wide) and ** Clarke Mons**, which rise from deep peripheral depressions 1 to 2 km below the surrounding plain. Geophysical modeling indicates that as the freezing subsurface ocean pressurized an ammonia-rich residual liquid lens, dense crustal blocks foundered while viscous water–ammonia cryolava erupted through deep fractures across Vulcan Planitia via effusive cryovolcanism, flooding the basin to a depth of several kilometers and viscously terminating as a sunken moat around pre-existing upstanding mountain blocks before solidifying in place.

Near-infrared hyperspectral mapping by the New Horizons Linear Etalon Imaging Spectral Array (LEISA) confirmed that Charon's neutral gray surface (geometric albedo 0.38, contrasting sharply with Pluto's golden-red nitrogen and methane glacio-plains) is dominated by crystalline water ice (H₂O) (1.65 μm absorption band) mixed with ammonia hydrates (NH₃·nH₂O) and ammonium salts (2.21 μm absorption feature), which are concentrated inside fresh young impact craters such as Dorothy and Organa where impacts excavated pristine volatile-rich crustal ice (Grundy et al., 2016). Because Charon's surface gravity (g = 0.288 m/s²) is less than half of Pluto's (g = 0.620 m/s²), volatile ices of N₂, CO, and CH₄ escaped thermally to space early in Charon's history; New Horizons Alice ultraviolet occultations placed a strict upper limit on any neutral Charonian atmosphere of less than 10⁻¹¹ bar (forty million times thinner than Pluto's atmosphere).

Despite lacking an intrinsic atmosphere, Charon's north polar region (and a matching south polar cap observed in Pluto-shine) is stained by Mordor Macula, a dark reddish-brown cap ~475 km across with an albedo half that of the equatorial plains. Atmospheric escape and surface-thermal models (Grundy et al., 2016) proved that Mordor Macula is fed directly by atmospheric transfer from Pluto: gaseous methane (CH₄) escaping hydrodynamically from Pluto's upper atmosphere is gravitationally intercepted by Charon. During Charon's 124-year polar winter night, temperatures in the shadowed polar basin plummet from ~55 K down to ~15–25 K, cold-trapping intercepted CH₄ molecules as a thin polar frost before they can sublime. Over the 124-year winter, solar Lyman-α ultraviolet photons backscattered by interplanetary hydrogen atoms photolyze this trapped methane ice into heavier, involatile hydrocarbons and macromolecular reddish tholins, which remain behind as the permanent dark stain of Mordor Macula when spring sunlight warms the pole back to 53 K and sublimates the residual unreacted methane.

Why are Pluto and Charon often called a binary dwarf planet?

Charon contains 12.17% of Pluto's mass—nearly ten times the Moon-to-Earth mass ratio (1.23%)—which places the gravitational center of mass (barycenter) of the Pluto–Charon system in open space 2,126 km outside Pluto's surface. In addition, both worlds have achieved complete mutual tidal locking, orbiting their shared external barycenter every 6.387 days with the same hemispheres permanently facing each other.

What formed the giant canyons and smooth plains on Charon?

Early in its history, radiogenic and tidal heat melted Charon's interior into a subsurface water–ammonia ocean beneath an icy crust. As this internal ocean cooled and froze into crystalline water ice, its volume expanded by 5% to 8%, stretching and fracturing Charon's outer ice shell into the 5-to-10-km-deep equatorial canyons of Serenity Chasma and Mandjet Chasma while forcing pressurized water–ammonia cryolava onto the surface to flood Vulcan Planitia.

How did Charon get its dark red north polar cap, Mordor Macula?

Methane gas escaping from Pluto's atmosphere streams across space and hits Charon. During Charon's century-long polar winter, the shadowed pole cools below 25 K and cold-traps the methane gas as solid frost; ultraviolet light then converts the trapped methane into non-volatile reddish organic macromolecules (tholins) that remain behind when the pole warms in summer.

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