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date: 05 December 2019

The Pluto−Charon System

Summary and Keywords

Pluto orbits the Sun at a mean distance of 39.5 AU (astronomical units; 1 AU is the mean distance between the Earth and the Sun), with an orbital period of 248 Earth years. Its orbit is just eccentric enough to cross that of Neptune. They never collide thanks to a 2:3 mean-motion resonance: Pluto completes two orbits of the Sun for every three by Neptune. The Pluto system consists of Pluto and its large satellite Charon, plus four small satellites: Styx, Nix, Kerberos, and Hydra. Pluto and Charon are spherical bodies, with diameters of 2,377 and 1,212 km, respectively. They are tidally locked to one another such that each spins about its axis with the same 6.39-day period as their mutual orbit about their common barycenter. Pluto’s surface is dominated by frozen volatiles nitrogen, methane, and carbon monoxide. Their vapor pressure supports an atmosphere with multiple layers of photochemical hazes. Pluto’s equator is marked by a belt of dark red maculae, where the photochemical haze has accumulated over time. Some regions are ancient and cratered, while others are geologically active via processes including sublimation and condensation, glaciation, and eruption of material from the subsurface. The surfaces of the satellites are dominated by water ice. Charon has dark red polar stains produced from chemistry fed by Pluto’s escaping atmosphere.

The existence of a planet beyond Neptune had been postulated by Percival Lowell and William Pickering in the early 20th century to account for supposed clustering in comet aphelia and perturbations of the orbit of Uranus. Both lines of evidence turned out to be spurious, but they motivated a series of searches that culminated in Clyde Tombaugh’s discovery of Pluto in 1930 at the observatory Lowell had founded in Arizona. Over subsequent decades, basic facts about Pluto were hard-won through application of technological advances in astronomical instrumentation. During the progression from photographic plates through photoelectric photometers to digital array detectors, space-based telescopes, and ultimately, direct exploration by robotic spacecraft, each revealed more about Pluto. A key breakthrough came in 1978 with the discovery of Charon by Christy and Harrington. Charon’s orbit revealed the mass of the system. Observations of stellar occultations constrained the sizes of Pluto and Charon and enabled the detection of Pluto’s atmosphere in 1988. Spectroscopic instruments revealed Pluto’s volatile ices. In a series of mutual events from 1985 through 1990, Pluto and Charon alternated in passing in front of the other as seen from Earth. Observations of these events provided additional constraints on their sizes and albedo patterns and revealed their distinct compositions. The Hubble Space Telescope’s vantage above Earth’s atmosphere enabled further mapping of Pluto’s albedo patterns and the discovery of the small satellites. NASA’s New Horizons spacecraft flew through the system in 2015. Its instruments mapped the diversity and compositions of geological features on Pluto and Charon and provided detailed information on Pluto’s atmosphere and its interaction with the solar wind.

Keywords: planetary science, solar system, trans-Neptunian objects, Kuiper belt, dwarf planets, volatile ices, cryovolcanism, glaciers, sublimation, space exploration

The possibility of undiscovered outer planets was an obvious consequence of the theoretical prediction and subsequent discovery of Neptune in the mid-19th century. Predictions of additional planets were made by Bostonian rivals Percival Lowell and William Pickering in the early 20th century. Lowell’s predictions led to a series of searches that, 14 years after his death, led to the discovery of Pluto in 1930 by Clyde Tombaugh at the Arizona observatory Lowell had founded. For the next half century, Pluto was seen as a solitary oddball in the outer solar system: small, faint, and with an unusually inclined and eccentric orbit. Facts about it were laboriously accumulated through application of successive generations of technological advances in astronomical instrumentation. Eventually it became clear that Pluto was a much lower mass object than Lowell and Pickering had been imagining. It could not possibly have accounted for the perturbations they had attributed to it. But it also opened the door to a new, third zone of the solar system, beyond the inner zone of rocky terrestrial planets and the middle zone of gaseous giant planets. By the 1990s, enough had been learned about Pluto to fill a 728-page technical book from the University of Arizona’s Space Science Series, and many more objects began to be discovered beyond the orbit of Neptune. Pluto remains the largest of them, although it is not the most massive.

Despite all that had been learned during the 20th century, Pluto remained a barely resolved point of light until NASA’s New Horizons spacecraft explored the system in 2015, sending back a wealth of details about the planet and its retinue of satellites. That encounter finally made it possible to read the geological history recorded in the surfaces of Pluto and its moons and to begin to make sense of the processes that sculpted them in the ancient past, and those that continue to do so today. From the initial discovery of Pluto as a tiny speck on 1930 photographic plates through its detailed exploration by a robotic spacecraft in 2015, a spectacular explosion in knowledge had taken place over the time of just a single human lifespan.

A key factor enabling Pluto’s continuing planetary activity is the presence on its surface of volatile ices of N2, CO, and CH4. These molecules are heavy enough not to have escaped to space over the age of the solar system, despite Pluto’s weak gravity, but are volatile enough to be easily mobilized by the weak energy sources available on a small planet far from the Sun. Charon has lost most or all of its original inventory of these volatiles, but its surface still bears the telltale signs of activity during an earlier era.

History and Issues

Discovery

The 18th and 19th centuries saw a spectacular expansion of the Sun’s retinue. William Herschel discovered Uranus in 1781 and Guiseppe Piazzi discovered Ceres in 1801. The asteroids Pallas, Juno, and Vesta were discovered within a few more years, followed a few decades later by many more asteroids. Urbain Le Verrier and John Couch Adams independently predicted the existence and location of an even more distant major planet based on discrepancies in the orbital motion of Uranus. The resulting discovery of Neptune by Johann Gottfried Galle in 1846 underscored the idea that even more planets may be awaiting discovery and that key clues could be gleaned from the orbits of the known ones.

In the early years of the 20th century, Percival Lowell pointed to two lines of evidence for another major planet beyond Neptune: apparent residual discrepancies in the position of Uranus and clustering in the aphelion distances of comets (Lowell, 1915). Neither of these effects ultimately turned out to be real, but at the time, they looked convincing and Lowell used them to compute possible orbits and positions of a trans-Neptunian Planet X. His computations informed a series of searches at the observatory he founded in Flagstaff, Arizona. William Pickering at Harvard University made competing predictions and instigated searches of his own (Pickering, 1909, 1919). The race to discover the next planet was on!

Lowell died in 1916 without having spotted his Planet X, although, unbeknownst to him, Pluto actually had been captured on a pair of photographic plates from his Planet X search in 1915. The search eventually resumed in the late 1920s under the leadership of observatory director Vesto Slipher. Slipher oversaw the construction of a new special-purpose telescope with a 13-inch lens that projected a wide swathe of sky onto 14 × 17-inch glass photographic plates. In 1929, he hired Kansas farm boy and amateur telescope maker Clyde Tombaugh to help with the search. By night, Tombaugh exposed photographic plates using the new telescope. By day he used a blink comparator microscope to compare pairs of plates taken a few days apart to search for a lone point of light moving among the myriad fixed stars. On February 18, 1930, Tombaugh discovered a faint object moving much too slowly to be an asteroid. It was in a region of the sky where Lowell and Pickering had predicted Planet X may be found, and subsequent follow-up observations (as well as the pre-discovery plates from 1915) made it clear that its orbit was beyond that of Neptune. Following the timely suggestion of 11-year-old English school girl Venetia Burney, the new planet was named for the Roman underworld god Pluto.

Characterization

Beyond its faintness and orbit around the Sun, very little was initially known about Pluto. The basic facts of its size, mass, rotation rate, and composition were beyond the capabilities of astronomical instrumentation of the 1930s. Looking through the eyepiece of Lowell Observatory’s 24-inch Clark telescope, Vesto Slipher’s younger brother Earl estimated that Pluto’s diameter could be no larger than 11,000 km. In subsequent visual observations at the Palomar 200-inch telescope, Gerard Kuiper and Milton Humason estimated its size as 5,900 km (Kuiper, 1950). That erroneous result is more than double Pluto’s true diameter of 2,377 km, but it was already awkwardly small for a supposedly Earth-mass object capable of perturbing the orbits of Uranus and Neptune. Ideas were floated involving light reflecting from a small spot near the center of a larger disk, as a shiny ball bearing might appear, or that Pluto could be made of some sort of exotic, ultra-dense material to reconcile a small size with a large mass. The scientific community was not yet ready to embrace the more prosaic possibility that Pluto was simply small, icy, and not massive enough to perturb Uranus.

Additional evidence for Pluto’s small size came from attempts to observe it occulting background stars. Ian Halliday, Robert Hardie, and John Priser observed a potential event from three sites in 1965. None of them saw the star wink out behind Pluto, setting a firm upper limit on Pluto’s diameter of 6,800 km and effectively eliminating the possibility of the large, shiny, ball bearing Pluto with a small central reflection (Halliday, Hardie, Franz, & Priser, 1966).

As astronomical technology advanced, scientists applied new innovations to the study of Pluto, painstakingly winning more and more facts about the system. For instance, in the early 1950s, Merle Walker and Robert Hardie used then state-of-the-art photomultiplier tubes to measure Pluto’s brightness much more accurately than could be done with photographic plates. They found a repeating pattern of brightening and darkening, called a light curve, from which they determined Pluto’s 6.39-day rotation period (Walker & Hardie, 1955). That observation demonstrated the existence of surface features with contrasting brightnesses. Over subsequent decades, Pluto’s light curve continued to be observed, leading to refinements in the rotation period and also indicating that the surface features responsible for the light curve were persistent on decadal time scales (Hardie, 1965). A gradual increase in the light curve amplitude, the difference between minimum and maximum brightness as Pluto rotates, indicated that the view of Pluto from Earth was becoming more equatorial over time. Pluto was evidently tilted over on its side, with its pole lying close to the plane of its orbit around the Sun (Andersson & Fix, 1973). A declining overall brightness indicated that Pluto’s equatorial latitudes were darker than its poles.

A key technique for learning the compositions of distant objects is spectrometry, the measure of how light reflected or emitted from an object varies with the wavelength or color of the light. Pluto was too faint for the photographic spectrometers of the early 20th century, but as the technology progressed to exploit more sensitive electronic detectors, the compositions of Pluto and Charon were gradually revealed. The first chemical ingredient to be definitively detected was frozen methane (CH4; Cruikshank, Pilcher, & Morrison, 1976), consistent with a snowy, highly reflective surface. The high volatility of CH4 also implied that Pluto should possess a thin atmosphere. The variation of Pluto’s CH4 absorptions as it rotated about its axis indicated that the CH4 ice was not uniformly distributed (Buie & Fink, 1987). Ices of nitrogen (N2) and carbon monoxide (CO) were subsequently discovered using more advanced infrared spectrometers (Owen et al., 1993). Their even greater volatility underscored the importance of seasonal surface−atmosphere interactions on Pluto.

Gerard Kuiper and Milton Humason had closely examined Pluto through the eyepiece of the Palomar 200-inch telescope under excellent seeing conditions in 1950 and had not seen any satellites. They also searched unsuccessfully for satellites using photographic techniques. Charon evaded detection until the summer of 1978, when Jim Christy at the United States Naval Observatory (USNO) examined photographic plates taken at USNO’s 61-inch Kaj Strand telescope, located outside Flagstaff, Arizona. Some of the photographs of Pluto appeared to be defective, with an asymmetric lump off to one side or the other. Puzzlingly, no such lump was seen in star images on the same plates. The lump seemed to alternate between the north side of Pluto and the south side, and Christy soon recognized that it must be a satellite orbiting Pluto with the same 6.39-day period as Pluto’s rotational period (Christy & Harrington, 1978). Charon’s orbit about Pluto enabled the mass of the pair to be directly measured as 0.2% of an Earth mass, far too small to account for the supposed perturbations of Uranus that had motivated Percival Lowell’s search and ultimately Clyde Tombaugh’s 1930 discovery.

The plane of the mutual orbit of Pluto and Charon crossed the inner solar system during Pluto’s equinox in the 1980s, leading to a season of mutual events where Pluto and Charon alternated in passing in front of one another as seen from Earth. This alignment occurs only twice during Pluto’s 248-year orbit around the Sun. By luck, Charon was discovered just in time for the events to be observed. They enabled determination of the sizes of Pluto and Charon and allowed the surfaces of their mutually facing hemispheres to be mapped at unprecedented spatial resolution (e.g., Binzel & Hubbard, 1997; Young, Binzel, & Crane, 2001). Comparing spectra of the system when Charon was hidden behind Pluto with those when both bodies were visible revealed Charon’s surface to be dominated by frozen water (H2O), unlike the volatile ice-rich surface of Pluto (Buie, Cruikshank, Lebofsky, & Tedesco, 1987).

Also in the 1980s, Pluto’s atmosphere was directly detected through observations of a stellar occultation (Hubbard, Hunten, Dieters, Hill, & Watson, 1988; Elliot et al., 1989). The star’s light faded out gradually, instead of abruptly, as a result of being refracted through the atmosphere instead of being abruptly cut off by the hard surface. Subsequent stellar occultations showed Pluto’s atmospheric pressure to be increasing over time, due to seasonal effects (e.g., Sicardy et al., 2003).

Telescope technology continued to improve, and with the advent of the Hubble Space Telescope, it became possible to produce coarse maps of the distribution of light and dark regions across Pluto’s surface (Stern, Buie, & Trafton, 1997; Buie, Grundy, Young, Young, & Stern, 2010). Combining these maps with spectrometry enabled a bright spot on Pluto’s anti-Charon hemisphere to be identified as being especially rich in N2 and CO ices (Grundy & Buie, 2001). That anomalous spot would eventually be revealed as the colossal glacial deposit of Sputnik Planitia. Separate spectroscopy of Pluto and Charon revealed that all of the N2, CO, and CH4 absorption was associated with Pluto, while Charon’s surface had some admixture of ammonia in addition to the H2O ice (NH3; Brown & Calvin, 2000; Buie & Grundy, 2000; Cook, Desch, Roush, Trujillo, & Geballe, 2007).

Trans-Neptunian Population

For decades after its discovery, Pluto had been perceived as a lone oddball on a weirdly eccentric and inclined orbit at the edge of the solar system. Researchers had speculated that many smaller trans-Neptunian objects (TNOs) could be out there (e.g., Edgeworth, 1943), but being so small and faint, individual objects eluded discovery for a long time. The first of them, Albion, was finally spotted in 1992 (Jewitt & Luu, 1993). Thousands have since been found, indicative of a much larger population still to be discovered.

The orbits of TNOs fall into several distinct dynamical categories (e.g., Elliot et al., 2005; Gladman, Marsden, & VanLaerhoven, 2008; Bannister et al., 2018). Objects referred to as Cold Classical TNOs occupy low inclination, low eccentricity orbits at mean heliocentric distances between 42 and 48 AU (1 AU is the mean distance between Earth and the Sun). Others are in more dynamically excited orbits with high inclinations and eccentricities. Many are in n:m mean motion resonances with Neptune such that for every n orbits the object completes around the Sun, Neptune completes m (Pluto is in the 2:3 mean motion resonance with Neptune). The existence of numerous objects in mean motion resonances implies that Neptune migrated outward, sweeping objects up into the resonant orbits (e.g., Malhotra, 1995).

The trans-Neptunian region constitutes an enormous third zone of the solar system, after the inner terrestrial planet zone and the middle giant planet zone. The vast majority of known TNOs are in the tens to hundreds of kilometers size range, but a number of larger planet-sized objects have also been found, notably Eris, Makemake, Haumea, Sedna, Orcus, Quaoar, Varda, Gǃkúnǁ’hòmdímà, Ixion, and Varuna, along with many others, not yet named. Even more in this size range likely remain to be discovered, and the possibility of undiscovered outer solar system objects that are larger still continues to motivate observational searches (e.g., Trujillo & Sheppard 2014; Batygin & Brown, 2016).

Planetary Status

Spurred by the growing wealth of newly discovered bodies in the outer solar system, a controversial vote was held on the last day of the 2006 General Assembly of the International Astronomical Union (IAU) to place these objects into a separate category of “dwarf planets” and to redefine the word “planet” so as to exclude the dwarf planets by adding a requirement for a planet to have cleared other objects from its orbital zone. This redefinition was rejected by many planetary scientists, both for its lack of scientific utility and for the extraordinary nature of its imposition on the planetary science community by vote of a small group of astronomers. The IAU was perceived to be attempting to suppress diverse popular and scientific cosmologies that had previously shared the broader use of the term (Messeri, 2010). Within days of the announcement of the new IAU definition, over 300 planetary scientists (more than the number of astronomers who had voted for it) had signed an open letter saying they found the new definition unhelpful and would not use it. The scientific literature on Pluto mostly continues to use the word “planet” in the more inclusive, geophysical sense of a large, round, substellar object, regardless of its dynamical environment, from orbiting any star (not just the Sun), another planet, or even floating free through the galaxy (e.g., Runyon et al., 2017). This article follows the convention of the planetary science literature.

Spacecraft Exploration

As early as the 1960s, the American space agency NASA had begun considering ideas to send robotic spacecraft to explore Pluto. These concepts culminated in the development of the Pioneer and Voyager missions to explore the outer solar system. Pluto had been a potential target for Voyager I, subsequent to its flyby of Saturn, but the decision was made to instead send the spacecraft on a close flyby of Saturn’s largest moon, Titan. The maneuver to do that deflected the probe’s trajectory out of the plane of the solar system, making Pluto unreachable. Voyager II continued in the plane of the solar system (albeit not in the right direction to visit Pluto), flying past Uranus in early 1986 and Neptune in 1989, before it too was deflected out of the plane of the solar system. Neptune’s largest moon, Triton, is similar in size and surface composition to Pluto. Voyager’s images of Triton’s exotic surface features and actively erupting plumes helped spur interest in exploring Pluto, as did the discovery of additional trans-Neptunian objects starting a few years later. A variety of mission concepts were considered through the 1990s, culminating with the 2001 selection of New Horizons for flight (Neufeld, 2014). The New Horizons spacecraft was launched in January 2006, obtained a gravity boost from a flyby of Jupiter in February 2007, and flew past its primary target, the Pluto system, in July 2015. That encounter revealed much of what is known about the system as well as raising many new questions (e.g., Stern et al., 2015).

Current State of Knowledge

Pluto orbits the Sun with an orbital period of 248 Earth years and a semimajor axis of 39.5 AU (astronomical units, the mean separation between the Earth and the Sun). The orbit is inclined to the plane of the solar system (mean inclination 16°) and is eccentric (mean eccentricity 0.24). At its closest approach to the Sun at perihelion, Pluto comes within the orbital distance of Neptune, but the mean motion resonance prevents the two from having potentially disruptive close encounters.

The Pluto system consists of six bodies: Pluto and Charon, plus four much smaller, irregularly shaped outer satellites: Styx, Nix, Kerberos, and Hydra. All six objects orbit the barycenter, or center of mass of the system. Pluto accounts for about 89% of the mass of the system, but the barycenter lies outside of Pluto’s volume, so in a sense it too is a satellite orbiting around the barycenter, and Pluto and Charon can be described as a double planet. Pluto and Charon spin on their axes in exactly the same amount of time as they take to orbit one another, and the orientations of their spins are aligned. The pair are fully tidally locked. One face of Charon, known as the sub-Pluto hemisphere, is permanently oriented toward Pluto, while the sub-Charon hemisphere of Pluto is permanently oriented toward Charon. Basic physical parameters of Pluto, Charon, and the four small satellites are presented in Table 1.

The small satellites (see Figure 1) are thought to be remnants of debris from the aftermath of a giant glancing collision of two large progenitor bodies that produced the Pluto−Charon binary (Canup, 2005, 2011; McKinnon et al., 2017). They have highly reflective surfaces (Weaver et al., 2016) with compositions dominated by H2O ice, plus an admixture of ammonia (NH3) ice or some ammoniated compound, or both, based on an absorption feature at 2.21 µm wavelength (Cook et al., 2018). This composition suggests that one or more of the progenitors had been at least partially differentiated, meaning that denser, rocky material has settled to their cores, leaving ice-rich exteriors (McKinnon et al., 2017). All of the small satellites have irregular shapes and they spin more rapidly than their orbital periods, with spin axes unaligned with their orbital axes (Porter et al., 2016). Nix and Hydra, the two small satellites imaged with the best spatial resolution, exhibit highly cratered and thus presumably ancient surfaces. The other two were relatively poorly resolved, having not been discovered until after the New Horizons encounter sequence had already been designed (Showalter et al., 2011, 2012). An enigmatic reddish spot near the equator of Nix has not been explained, but may be associated with an impact crater (Weaver et al., 2016).

Table 1. Properties of the Six Bodies in the Pluto System

Object

Size (km)

Orbital Period (Days)

Rotation Period (Days)

Mass (1021 kg)

Mean Density (kg m−3)

Pluto

2,376.6 ± 1.6

6.387227

6.387227

13.03 ± 0.03

1,854 ± 6

Charon

1,212.0 ± 1.0

6.387227

6.387227

1.586 ± 0.015

1,702 ± 17

Styx

16 x 9 x 8

20.1616

3.24 ± 0.07

?

?

Nix

48 x 33 x 30

24.85463

1.829 ± 0.009

?

?

Kerberos

19 x 10 x 9

32.1676

5.31 ± 0.10

?

?

Hydra

50 x 36 x 32

38.20177

0.4295 ± 0.0008

?

?

Note. Adapted from Stern et al. (2018).

The Pluto−Charon SystemClick to view larger

Figure 1. Composite image showing all five of Pluto’s satellites at a common scale.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA20033.

Pluto

Unlike the frozen heavily cratered ice ball many had expected Pluto to be, it is an active world. A complex blend of surface and atmospheric processes work together to produce a spectacular diversity of landscapes despite the extremely limited energy budget to drive activity. Sunlight provides 0.87 W m−2 at Pluto’s mean heliocentric distance of 39.5 AU. Owing to the eccentricity of Pluto’s orbit, this varies from 0.57 W m−2 at aphelion to 1.5 W m−2 at perihelion. Decay of radioactive elements in Pluto’s interior (see Figure 2) provides an even more feeble 0.003 W m−2, assuming Pluto’s interior contains 65.5% rock by mass along with H2O ice making up the remainder of its 1,854 ± 6 kg m−3 bulk density, and that the rock component has similar abundances of radioactive elements to chondritic meteorites (McKinnon et al., 2017; Stern et al., 2018). Although this energy budget is far below Earth’s 1,360 W m−2 from sunlight and 0.09 W m−2 from the interior, it is sufficient to drive activity thanks to the extreme volatility of N2, CO, and CH4.

The Pluto−Charon SystemClick to view larger

Figure 2. Interior structures of Pluto and Charon inferred from their bulk densities.

Adapted from McKinnon et al. (2017).

Most of Pluto’s N2, CO, and CH4 is frozen solid on its surface, but some also exists as gas, forming a thin atmosphere in vapor pressure equilibrium with the surface (see Figure 3). N2, being the most volatile of the three, dominates the composition of the lower atmosphere. At the time of the New Horizons encounter, the atmospheric pressure at Pluto’s surface was about 1 Pa (Gladstone et al., 2016; Young et al., 2018), about a factor of 100,000 less than Earth’s atmospheric pressure. The New Horizons radio occultation experiment showed the temperature of Pluto’s atmosphere to be coldest near the surface at about 38 K. Above the surface, the temperature rises quickly to a maximum of around 107 K at an altitude of 20 to 30 km before slowly falling off again to around 65 to 68 K at higher altitudes. The coldness of Pluto’s upper atmosphere limits its loss to space to around 1 to 2 kg per second, a paltry amount on planetary scales. Most of the escaping gas is CH4, which is abundant at the top of Pluto’s atmosphere due to its low molecular weight compared with N2 and CO.

The Pluto−Charon SystemClick to view larger

Figure 3. Atmospheric temperature as a function of altitude and pressure.

CH4 and N2 in Pluto’s upper atmosphere provide feedstock for photochemical production of heavier hydrocarbons and nitriles that in turn combine to form aerosol particles consisting of complex organic macromolecules. These particles are thought to be the main source of cooling of the upper atmosphere (Zhang, Strobel, & Imanaka, 2017) and they produce a spectacular display of layered hazes in Pluto’s atmosphere (see Figure 4; Cheng et al., 2017). Ultimately, the haze particles settle through the atmosphere to Pluto’s surface, their complex chemistry providing the main source of the planet’s reddish-brown coloration (Grundy et al., 2018).

The Pluto−Charon SystemClick to view larger

Figure 4. Image captured shortly after New Horizons flew past Pluto, looking back at the sunlit crescent showing rugged mountains and numerous layers of atmospheric haze.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA19948.

Pluto’s surface temperatures are partly governed by ice-vapor equilibrium. Heat is efficiently transported from N2 ice that is warmed by sunlight to any place on Pluto’s surface that becomes colder than the global mean N2 ice temperature, such as through radiation to space. This heat transport works through sublimation and condensation of N2, with the latent heat of sublimation−condensation being transferred. Vapor pressure equilibrium sets the minimum temperature for Pluto’s surface, about 38 K at the time of the New Horizons flyby.

Regions that are free of volatile ices are not cooled by sublimation and so can become warmer, especially if they are dark so that they absorb more of the sunlight striking them. Equatorial regions never undergo long polar nights, helping them resist condensation of bright, volatile ices (Earle et al., 2018). An irregular belt of dark regions was observed to encircle Pluto along its equator, the largest and best-imaged of which is informally named Cthulhu (names of Pluto system features mentioned in this article include a mix of official and informal names). Parts of Cthulhu are among the most heavily cratered of Pluto’s terrains, indicative of ancient surfaces lacking erosive processes able to erase the craters over time (Robbins et al., 2017, Singer et al., 2019). The main process modifying Cthulhu appears to be accumulation of haze particles settling out of the atmosphere, but this is a slow process. Extrapolating from present-day haze production rates, tens of meters of this dark material could accumulate over the age of the solar system, far too little to fill in large craters, but more than enough to rapidly paint over any fresh, bright ice excavated by occasional impacts (e.g., Grundy et al., 2018).

Volatile ices N2, CO, and CH4, identified by their characteristic infrared absorption bands (see Figure 5), dominate Pluto’s surface. Their distribution is controlled by the seasonal variation of sunlight, in concert with local factors such as altitude, slope, and thermal inertia, as well as differences in the volatilities of the three ices (Grundy et al., 2016a). Lowell Regio, Pluto’s northern polar region receiving continuous spring and summer sunshine at the time of the flyby, shows abundant CH4 ice. Little N2 or CO is detected there (Protopapa et al., 2017). N2 and CO are more volatile than CH4. They are more abundant at mid-northern latitudes, primarily in low-altitude regions such as valleys and crater floors where the pressure is high and temperature is low (Schmitt et al., 2017). Closer to the equator, N2 and CO become scarce again, but CH4 ice outcrops remain common, especially at high-altitude settings such as crater rims and the crests of mountains. The equator is dominated by dark expanses like Cthulhu that are mostly free of volatile ices. Latitudes south of the equator are experiencing fall and winter. CH4 ice appears to be ubiquitous there. High southern latitudes were in winter darkness at the time of the encounter, limiting what New Horizons could learn about them, but the radio science system observed thermal emission indicative of relatively warm temperatures below Pluto’s surface at high southern latitudes (Linscott et al., in press). That heat was presumably sequestered during the previous southern summer. As the southern hemisphere continues to cool, N2 and CO ices should eventually begin to condense there. Global circulation models indicate how the various volatile ices condense and sublimate over time (e.g., Forget et al., 2017; Bertrand et al., 2018).

The Pluto−Charon SystemClick to view larger

Figure 5. Near-infrared reflection spectrum of Pluto showing absorptions by CH4, N2, and CO ices. These features enabled the different ices to be mapped by the infrared spectral imaging instrument aboard New Horizons.

A much longer cycle of “mega-seasons” occurs on Pluto due to the slow evolution of the planet’s orbital parameters (Earle & Binzel, 2015). At present, Pluto’s perihelion coincides with the equinox, when the Sun is over the equator. But over three million years, the season coinciding with perihelion cycles from the equinox, to northern summer, to the equinox, to southern summer, and so on. Since sunlight is 2.6 times more intense at perihelion as it is at aphelion, these mega-seasonal cycles make a huge difference to the seasonal cycling of Pluto’s volatile ices. The observed latitudinal distribution of ices results from a combination of annual seasonal effects as well as longer-term mega-seasonal effects.

Exceptions to the latitudinal distribution of Pluto’s ices point to some of Pluto’s most interesting and curious geological features. On the east side of the encounter hemisphere is Tartarus Dorsae, a series of rugged, approximately north−south aligned ridges, rich in CH4 ice (see Figure 6). These features are hypothesized to have formed through sunlight-controlled sublimation of a once-thick CH4 ice deposit, analogous to the penitentes that occur in high-altitude snow and ice on Earth, only much larger (Moores, Smith, Toigo, & Guzewich, 2017; Moore et al., 2017, 2018).

The Pluto−Charon SystemClick to view larger

Figure 6. https://photojournal.jpl.nasa.gov/jpeg/PIA19957.jpg Bladed terrain in Tartarus Dorsae. North is to the left.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA19957.

Near the center of the encounter hemisphere, Sputnik Planitia is a vast basin filled mostly with N2 as well as CO and CH4 ices (Protopapa et al., 2017; Schenk et al, 2018a). Its surface exhibits a polygonal pattern of convection cells tens of kilometers across (see Figure 7; Moore et al., 2016). The convective motion is slow, centimeters per year, comparable to plate tectonic motion on Earth. The resulting overturning time scale is under a million years, rapid enough to erase all traces of impact craters from Sputnik’s surface (McKinnon et al., 2016). Sputnik underscores that even Pluto’s weak interior heat can drive active geology on a grand scale. This giant convecting glacier is unlike anything seen elsewhere in the solar system, though its existence raises the possibility that similar features could occur on other, as yet unexplored outer solar system bodies with surfaces dominated by volatile ices, such as Eris and Makemake (e.g., Stern et al., 2018). At scales of hundreds of meters, the surface of Sputnik can be seen to be sculpted by processes that act even more rapidly than the convection overturn time, including wind-blown dunes of CH4 ice (Telfer et al., 2018) and sublimation pits (White et al., 2017).

The Pluto−Charon SystemClick to view larger

Figure 7. Convection cells in Sputnik Planitia, appearing as low blister-like domes, often separated by double grooves. Much smaller sublimation pits are especially prominent at lower right.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA20726.

The coincidence of Sputnik’s location with Pluto’s anti-Charon point provides clues about the interior of Pluto. The odds of the largest impact basin coinciding with the tidal axis are slim. The feature could have migrated to the tidal axis, but only if it is a mass concentration rather than the mass deficit that would be created by an unfilled impact basin. Nitrogen ice now fills part of the basin and is a little more dense than water ice, but it would take a lot more N2 ice than is thought to occupy Sputnik to turn the topographic low into a positive mass anomaly. An internal liquid water ocean is the more likely explanation, having been uplifted below the impact basin to create a positive mass anomaly because liquid water is denser than ice (Nimmo et al., 2016).

More familiar-looking valley glaciers also occur on Pluto. Several of them flow westward into Sputnik Planitia from the rugged highlands of eastern Tombaugh Regio. Nitrogen ice, thought to be the primary material in these glaciers, is able to flow readily under Pluto surface conditions because it is not too far from its melting temperature (Umurhan et al., 2017). Landscapes thought to have been carved by glaciers can be seen in many places across Pluto’s surface, pointing to more such glacial activity in the past (Howard et al., 2017).

Pluto’s surface also has more familiar features seen on other icy satellites throughout the outer solar system, such as impact craters and tectonic scarps (Schenk et al., 2018a). Impact craters on Pluto are not unlike those seen elsewhere in that smaller craters have simple bowl-like shapes while some larger ones feature central uplifts. However, distinct modifications are seen for some craters. Several exhibit a radial, fluted pattern of erosion (Moore et al., 2016). Many show bright CH4 ice deposits on their rims, giving them a distinctive haloed appearance, especially in Vega Terra and Bird Planitia, north of Cthulhu (Moore et al., 2017). Many of Pluto’s craters lack obvious ejecta blankets, presumably due to obscuration by seasonal volatile ice deposits. In the walls of a few craters, dark horizontal strata can be seen, pointing to early geological history recorded in subsurface stratigraphic layers. Similar layers are exposed in cliffs associated with long scarps that extend across Pluto’s surface, such as Virgil, Inanna, and Dumuzi Fossae. To the east of the encounter hemisphere, Mwindo Fossae has an intriguing radial pattern. Some of these fracture patterns are conjectured to be associated with the Sputnik Planitia basin and the reorientation of Pluto (Hamilton, Stern, Moore, & Young, 2016; Keane, Matsuyama, Kamata, & Steckloff, 2016). But they look too well-preserved to have formed that early in Pluto’s history and may instead be a much more recent reflection of global expansion due to partial freezing of Pluto’s interior ocean (McKinnon et al., 2017; Schenk et al., 2018a).

Wright Mons, a candidate cryovolcano, towers over the southern edge of the encounter hemisphere (see Figure 8). It stands some 5 km tall and 150 km across, with an enormous 5 km deep, 45 km wide, central depression and an unusual lumpy or hummocky surface texture. Few, if any craters on its flanks suggest it could be a relatively recent feature (Singer et al., 2016). Picard Mons was only imaged in twilight, but appears to be a very similar feature just to the south of Wright Mons.

The Pluto−Charon SystemClick to view larger

Figure 8. Wright Mons.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA20361.

Charon

Charon is a little too small to retain the volatile N2, CO, and CH4 that enable such a complex interplay of processes between Pluto’s surface and atmosphere. Instead, Charon’s surface is dominated by water ice, a material that is inert at Charon’s low surface temperatures of 15 to 60 K. The most detailed New Horizons observations of Charon were of the northern part of its Pluto-facing hemisphere that was oriented toward the Sun and toward the approaching spacecraft at the time of the encounter (see Figure 9). Most of Charon’s features that have been described in the scientific literature are located on that encounter hemisphere.

The Pluto−Charon SystemClick to view larger

Figure 9. Charon’s Pluto-facing hemisphere.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA19968.

An especially striking feature of Charon’s encounter hemisphere is a belt of steep scarps and cliffs that cuts diagonally across the hemisphere for more than a thousand kilometers, with vertical relief as great as 11 km (7 miles) between the highest and lowest points (see Figure 10; Moore et al., 2016). This tectonic belt is interpreted as having formed through extension or stretching of Charon’s crust when an interior ocean froze, causing Charon to expand. An increase in Charon’s total surface area by around 1% is required to account for the feature (Beyer et al., 2017).

The Pluto−Charon SystemClick to view larger

Figure 10. Enlargement of a portion of Charon’s tectonic belt with an elevation map.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA20467.

The tectonic belt divides the rugged northern terrains called Oz Terra from the distinctly smoother area called Vulcan Planitia to the south. Although not obvious in individual images, an elevation map based on stereo imaging reveals Oz Terra to be dissected into high-standing blocks separated by wide troughs ranging from 3 to as much as 13 km deep (see Figure 11; Schenk et al., 2018b). This pattern could point to an earlier episode of planetary expansion (e.g., Desch & Neveu, 2017), since the chasms dissecting Oz Terra look much more eroded and indistinct than the relatively crisp appearance of the diagonal tectonic belt, implying earlier formation with subsequent degradation. Both Oz Terra and Vulcan Planitia are moderately heavily cratered and thus ancient, although craters are much easier to identify on the smooth plains to the south, thanks both to the smoothness and to the angle of the lighting (Robbins et al., 2017, Singer et al., 2019).

The Pluto−Charon SystemClick to view larger

Figure 11. A map of Charon showing the encounter hemisphere in the center and the non-encounter hemisphere at much lower resolution to the left and right. Colors indicate elevation, revealing large polygonal high regions in Oz Terra separated by depressions. Caleuche Chasma (C) is the deepest of these.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA21860.

Vulcan Planitia shows a number of distinctive features suggestive of a thick slurry or even solid-state flow having slowly flooded the region (see Figure 12). Along its northern margin, elevations drop off to a few kilometers lower than elsewhere across the region and a few isolated mountains are surrounded by low-elevation moats. These features suggest that the material that filled Vulcan Planitia had a high viscosity that impeded its flow toward and around obstacles. One possible formation scenario involves warm, plastically deforming mantle material flowing up and around foundering crustal blocks, with the moated mountains representing the last exposed corners of some of the sinking blocks (Beyer et al., 2019). Various rilles, wrinkles, and puckers across the surface of Vulcan Planitia hint at the flow of highly viscous material (Robbins et al., 2019).

The Pluto−Charon SystemClick to view larger

Figure 12. Higher resolution image of Vulcan Planitia, showing moated mountains, rilles, wrinkles, small pits, and hills.

New Horizons mapped Charon’s absorption feature at a 2.21 µm wavelength that had been attributed to ammonia. This absorption is seen at a low level across much of Charon’s surface, but it is strongly enhanced in the ejecta blankets of a handful of craters (see Figure 13), raising questions about whether those particular craters excavated especially ammonia-rich subsurface material or the ammonia was delivered by the impactors (Grundy et al., 2016a; Dalle Ore et al., 2018). Ammonia ice is thought to be relatively quickly destroyed by exposure to the space radiation environment (e.g., Cook et al., 2007), suggesting the craters could be recent. However, the fact that the ancient surfaces of the small satellites also show the feature (Cook et al., 2018) suggests ammonia may be able to survive irradiation longer than had been thought, or that the ammonia is actually in the form of some other, more durable compound such as an ammoniated salt.

The Pluto−Charon SystemClick to view larger

Figure 13. A region in Oz Terra, Charon’s rugged northern province, showing a pair of similarly sized craters. The black and white image from the long-range imager is overlaid in green with a map of the 2.2 µm absorption attributed to ammonia. One of the two craters shows a strong spectral signature at that wavelength.

Adapted from NASA/JHUAPL/SwRI. https://photojournal.jpl.nasa.gov/catalog/PIA20036.

Mordor Macula is the large reddish stain seen around Charon’s north pole. CH4 gas escaping from Pluto’s atmosphere is hypothesized to be responsible. Gas that reaches Charon is temporarily bound by Charon’s gravity, forming an extremely thin atmosphere (Hoey et al., 2017). Charon’s poles get so cold during the multi-decades-long polar winter nights that this gas temporarily freezes onto the ground around Charon’s winter pole. Exposure to space radiation there leads to the formation of larger, less volatile organic molecules that remain behind after the pole re-emerges into sunlight in the spring and the remaining CH4 ice sublimates away (Grundy et al., 2016b). Further radiation processing eventually leads to the production of dark red macromolecules analogous to those in Pluto’s haze. Extrapolating from present-day conditions, it is estimated that only a few tens of centimeters of this material would have accumulated around Charon’s poles over the age of the solar system, but being a strongly pigmented material, it does not take much to control the visual appearance of Charon’s high latitudes (Grundy et al., 2016b).

Additional hints of a late or perhaps even ongoing role for volatiles on Charon are clusters of small pits and hills in various places in Vulcan Planitia (e.g., Robbins et al., 2019; Beyer et al., 2019). These are most evident near the terminator, where the lighting was favorable for highlighting low-relief features and high spatial resolution imagery was obtained. These peculiar features could be related to escape of volatiles from the foundering crustal blocks soon after emplacement of Vulcan Planitia, or continuing volatile escape from Charon’s deeper interior.

Conclusion

Pluto and Charon are the first to be explored of a new type of small, icy planet in the distant third zone of the solar system. As the first, they offer a template for what characteristics may be expected on other icy dwarfs. These planets are very different from the rock- and metal-rich terrestrial planets close to the Sun and from the gas- and ice-rich giant planets in the middle zone of the solar system. Pluto amply demonstrates that despite their small sizes and great distances from the Sun, these small worlds are able to host the complex webs of physical and chemical processes that make planets such fascinating environments. Of the known icy dwarfs, a few, like Eris and Makemake, share Pluto’s inventory of volatile ices, and thus could feature volatile ice-enabled active geology like Pluto (e.g., Grundy & Umurhan, 2017). Others, like Haumea and Orcus, have surfaces dominated by water ice. They could be more like Charon in having surfaces that bear witness to activity early in their history that has by now mostly subsided.

Having just one instance of a small trans-Neptunian planet large enough to retain its volatiles as a template has the potential to mislead. Expectations for Pluto had been strongly shaped by Neptune’s largest moon, Triton, which is comparable in size and surface composition to Pluto and is thought to have formed in a similar region of the protoplanetary nebula as Pluto prior to its capture into orbit around Neptune. Yet many of Triton’s most striking features discovered by Voyager II, including active plumes, cantaloupe terrain, guttae surrounded by aureoles, and double ridges, were not seen at all on Pluto. Likewise, many of Pluto’s most striking features were not seen on Triton and had thus not been widely anticipated. The lesson is that bodies that share similar sizes, compositions, and locations of origin can still be astonishingly diverse.

A legacy of the geological activity on Pluto and Charon is the erasure of their primordial surface features. The progenitor bodies that collided to form the Pluto system are presumed to have been assembled from smaller planetesimals that accreted from icy dust particles in the protoplanetary nebula. The accretion process would have left characteristic imprints in the resulting planetesimals. Although these traces were overprinted by subsequent events in the Pluto system, they likely survive in the numerous, much smaller TNOs that did not grow large enough to reach elevated internal temperatures and differentiate.

Satellites have been discovered around almost all of the known planet-sized objects in the outer solar system, but only Haumea has more than one known satellite, making the Pluto system an outlier in terms of the richness of its satellite system, at least so far. But discoveries of rings around Chariklo and Haumea (Braga-Ribas et al., 2014; Ortiz et al., 2017) hint at further surprises to be found.

Future spacecraft exploration of more of these worlds offers spectacular scope for further discovery and deeper understanding of the processes that shape this distinct class of planet. The same is true for future, more in-depth follow-up exploration of the Pluto system. These small icy worlds are the new frontier for comparative planetology and for cutting-edge solar system exploration.

Further Reading

Barucci, A., Boehnhardt, H., Cruikshank, D. P., & Morbidelli, A. (Eds.). (1997). The solar system beyond Neptune. Tucson: University of Arizona Press.Find this resource:

Cruikshank, D. P., & Sheehan, W. (2018). Discovering Pluto: Exploration at the edge of the solar system. Tucson: University of Arizona Press.Find this resource:

Hoyt, W. G. (1980). Planets X and Pluto. Tucson: University of Arizona Press.Find this resource:

Levy, D. H. (1991). Clyde Tombaugh: Discoverer of planet Pluto. Tucson: University of Arizona Press.Find this resource:

Schilling, G. (2009). The hunt for planet X: New worlds and the fate of Pluto. New York, NY: Copernicus Books.Find this resource:

Schindler, K., & Grundy, W. (2018). Pluto and Lowell Observatory: A history of discovery at Flagstaff. Charleston, SC: History Press.Find this resource:

Stern, A., & Grinspoon, D. (2018). Chasing new horizons: Inside the epic first mission to Pluto. New York, NY: Picador.Find this resource:

Stern, A., & Mitton, J. (1998). Pluto and Charon: Ice worlds on the ragged edge of the solar system. New York, NY: Wiley.Find this resource:

Stern, S. A., & Tholen, D. J. (Eds.). (1997). Pluto and Charon. Tucson: University of Arizona Press.Find this resource:

Tombaugh, C. W., & Moore, P. (1980). Out of darkness: The planet Pluto. Harrisburg, PA: Lutterworth Press.Find this resource:

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