All About Pluto & Charon:

By: Dr. S. Alan Stern
Principal Investigator NASA's New Horizons PKB Mission and Director of Southwest Research Institute's Depart of Space Studies in Boulder, CO.

Pluto-Charon. The Pluto-Charon system is the only planet-satellite system in our solar system that has not been explored by spacecraft. Therefore, the state of knowledge about this system is necessarily more primitive than at any other planet. Despite this limitation, however, many basic facts are established. These include the radius, mass, and density of Pluto (each known to better than 10%) and the radius of Charon (known to 5%), and the mass and density of Charon (known to about 25%). Importantly, Charon is almost precisely half the size of Pluto. Because the system barycenter is known to be outside Pluto (between the two bodies), the pair constitute a true double planet- something unique in our solar system.

Pluto-Charon orbit the Sun in an elliptical, inclined, 248-year orbit that is in the 3:2 mean motion resonance of Neptune. Perihelion was reached in 1989; the system is now receding from the Sun. The planet and satellite share a polar obliquity of 120 deg. Pluto-Charon have reached complete spin-spin-orbit synchronicity; the pair are the only fully tidally evolved planet-satellite pair in the solar system. Pluto's density, very near, 2 gm cm-3, indicates its bulk composition is dominated by hydrated rock, but contains up to 30% water ice. Light organics and other materials are predicted to be abundant minor constituents.

Pluto's surface is highly reflective, with a globally averaged normal albedo of 55%. The surface color is red, much like Triton. Reflectance spectroscopy has identified N2, CO, CH4, and H2O frosts on the surface, with N2 being the dominant constituent. Other light organics resulting from ice radiolysis and other processes are widely expected to be present. Photometric measurements have revealed a complex lightcurve with an amplitude higher than any other planet in the solar system. The surface has been mapped crudely (500 km resolution) by HST; the maps reveal polar caps and other high-contrast surface units. Thermal measurements indicate steep surface temperature gradients, with bright (presumably sublimation-cooled) areas being near 40 K, and dark (purely radiative equilibrium?) units being near 60 K.

Pluto's atmosphere was discovered by stellar occultation techniques. Its base surface pressure is at least 3 and perhaps as great as 50 microbars; the upper atmosphere has a temperature of 106 K owing to a near-surface inversion, but the details of this thermal structure are indeterminate. Hazes and/or discrete clouds may be present in the atmosphere. Model calculations predict an N2 dominated atmosphere, with traces of CH4, CO, and a complex suite of photolysis products. Owing to Pluto's high orbital eccentricity and its high axial tilt, strong thermal forcing results. Owing to coupled ice/atmosphere sublimation thermal balance, strong seasonal pressure cycles have been predicted, including possible seasonal atmospheric collapse. Escape rate calculations indicate that Pluto's atmosphere is likely to be in hydrodynamic escape, unlike any other planet (but like the archaen Earth).

Charon's surface albedo is much darker than Pluto (35%), its surface color is gray (neutrally reflecting), and it has only a low amplitude (8%) lightcurve. Its surface composition appears to be dominated by water ice, but new absorption features in the mid-infrared have been detected in recent years, indicating the presence of other, as yet unidentified, surface constituents. There has been no definitive detection of an atmosphere.

The origin of the Pluto-Charon binary is thought to have been caused by a giant impact, much like the Earth-Moon system. The evidence for this hypothesis is based on the system's high specific angular momentum, its high axial obliquity, and the large mass ratio of the binary. Pluto itself is thought to have been grown in heliocentric orbit during the epoch of planetary growth in the Kuiper Belt, some 4 Gyr ago. As such, and owing to its size, it is expected to represent a key sample of the bulk composition of planetesimals in the trans-Neptunian region.

The Kuiper Belt. The existence of the Kuiper Belt was first predicted by mid-20th century astronomers such as Kenneth Edgeworth and Gerard Kuiper. These and other astronomers of the 1930s, 1940s, and 1950s postulated that a debris belt of material left over from planetary formation might orbit the Sun beyond Neptune. However, the telescope and photographic technology of the mid-20th century was too primitive to give astronomers much hope of finding bodies our there- they were simply too faint to be found. By the late 1980s cometary astronomers, however, found strong evidence in the inclination distribution of the Jupiter family comets that they are coming from a disk-like reservoir just beyond Neptune's orbit. As a result, a number of searches were begun in the late 1980s for the belt of material that Kuiper predicted. The first Kuiper Belt Object (KBO) was subsequently discovered in 1992. This object, designated 1992QB1, is more than 1000 times fainter than Pluto, and probably about 10--15 times smaller in radius.

Over 500 KBOs has been discovered by late 2001, with estimated diameters ranging from 50 to 1200 km. It is expected that the KBO size distribution includes both smaller objects (comets) and larger ones (perhaps even up to Pluto's size).

Based on the amount of sky left to be searched and the number of faint, distant objects being found in faint CCD images, it is estimated that over 100,000 KBOs with diameters >50 km may orbit in a disk- or belt-like structure that stretches from 30 to at least 55 Astronomical Units (AU) from the Sun. This large population means that the Kuiper Belt is an even greater collection of objects than the asteroid belt between Mars and Jupiter.

The orbits of KBOs fall into three major categories: field objects (called the classical KBOs), objects in mean motion resonances (like Pluto, thus called the Plutinos), and the so-called Scattered Disk Objects orbit which have orbits stretching far beyond 50 AU (indeed some have orbits that stretch out beyond 1000 AU). The orbits of the Plutinos appear to provide evidence for the migration of Neptune, perhaps by many AU, early in the history of the solar system. The wide distribution of classical KBO orbital inclinations and the presence of the SDOs indicate that some violent dynamical event, perhaps involving a passing star, large migrating planetary embryos, or sweeping secular resonances likely also took place during the time that the KBOs were forming.

Based on analogy to cometary nuclei and recently-obtained millimeter wave detections, the surfaces of Kuiper Belt Objects are expected to be very dark, typically reflecting only 3% to 10% of the light that falls on them. It has been found the KBOs have a wide range of surface colors, varying from almost gray to very red, but it is not clear whether this is due to genetic differences among KBOs or evolutionary affects (e.g., space weathering, collisional resurfacing). There is some evidence for water ice and more exotic ices on KBOs. It is also not known if KBOs fall into compositional groups as the asteroid do, though some observing groups have claimed evidence to this effect. It is believed KBOs consist primarily of mixtures of water ice and rock, with some amount of organic and other complex compounds as well. Most KBOs rotate on their axes in a few hours, but some take days to rotate. In 2001 the first KBO satellites were discovered.

Collisional processes are known to play a key role in the Kuiper Belt. One significant result of collisional modeling is that KBOs smaller than ~50 km in diameter cannot have survived the collisional bombardment over time and therefore must be younger than the age of the solar system. As a result it is now widely accepted that the Jupiter Family comets, which have their source region in the Kuiper Belt, are chips off KBOs created in (comparatively) recent times by collisions in the Kuiper Belt.

How did the Kuiper Belt and KBOs form? Computer simulations indicate that the KBOs formed along with Pluto early in the history of the solar system. The total mass of the present-day Kuiper Belt is low, in the range of 0.5 to 1 Earth mass. This is known to be too low to have been able to form the KBOs in the age of the solar system. It is therefore surmised that the primordial Kuiper Belt was many (e.g., 50) times) its present day mass. This mass estimate indicates that the primordial solar nebula extended uninterrupted beyond Neptune's distance (30 AU), at least to the present-day edge of the main Kuiper Belt (55 AU). It is not clear if the relative dearth of large KBOs seen beyond 55 AU is due to a real edge in the Kuiper Belt near this distance, or instead simply a trough which may stretch only a few tens of AU with a larger, even more massive belt lying beyond.

Based on the sizes and orbits of KBOs, it appears that the Kuiper Belt was well on its way to growing one or more large planets, perhaps even something the size of the Earth, or even Neptune, when the growth process was interrupted. It is believed that the formation of Neptune is what disturbed the region gravitationally and interrupted this growth. One consequence of this disturbance is that Neptune's gravitational influence caused collisions between objects in the young Kuiper Belt to become very violent. As a result, much of the mass in the Kuiper Belt was eroded into dust and subsequently blown away into interstellar space. Similar processes have been observed to be taking place in what appear to be Kuiper Belts around many stars in the galaxy, such as Vega and Beta Pictoris. This strong connection between the Kuiper Belt and other solar systems adds impetus to the desire to explore the Kuiper belt and KBOs further.