Pluto 549
as the southern cap. Later results, including some color in-
formation, were subsequently obtained by Eliot Young and
colleagues.
In 1990, theHubble Space Telescopeimaged Pluto, but
owing to its then-severe optical aberrations, these images,
obtained by R. Albrecht and a team of collaborators cleanly
separated Pluto and Charon, but it did not reveal significant
details about the surface of Pluto. AfterHSTwas repaired by
an astronaut crew in late-1993, its optics were good enough
to resolve crude details on Pluto’s surface. And in mid-1994,
it obtained the first actual images of Pluto that revealed sig-
nificant details about Pluto’s surface. These images were
made by Alan Stern, Marc Buie, and Laurence Trafton us-
ing the Faint Object Camera (FOC) of theHubble Space
Telescope. The 20-imageHSTdata set is longitudinally com-
plete and rotationally resolved and obtained at both blue
and ultraviolet wavelengths. The various images thatHST
obtained were combined to make blue and UV maps of the
planet, such as the one shown in Fig. 4. TheHSTimages and
derived maps reveal that Pluto has (1) a highly variegated
surface, (2) extensive, bright, asymmetric polar regions, (3)
large midlatitude and equatorial spots, and (4) possible lin-
ear features hundreds of kilometers in extent. The dynamic
range of albedo features across the planet detected at the
FOC’s resolution in both the 410 and 278 nm bandpasses
exceeds 5:1. NewHSTimages were obtained in 2002 by a
team led by Marc Buie usingHST’s Advanced Camera for
Surveys (ACS), but the results from these observations had
not been published as of late 2006.
9045− 454500 90 180 270 360− 45− 900LatitudeLatitudeEast LongitudeFIGURE 4 A map derived from direct imaging of Pluto using
HSTimages made in 1994. (Adapted from Stern et al., 1997,
Astronom. J. 113 , 827.)
5. Pluto’s Interior and Bulk Composition
5.1 Density
To determine the separate densities of Pluto and Charon,
one must either obtain precise astrometric measurements
that detect the barycentric wobble between Pluto and
Charon or use orbit solutions for Pluto’s small satellites.
Since 1992, bothHSTand ground-based measurements
have been gathered to address the mass ratio, and therefore
the relative masses and densities of Pluto and Charon. These
are very difficult measurements. The best available density
determination for Pluto is due to Buie and co-workers, who
analyzed the orbits of Nix and Hydra in a 2006 publication
that gave 2.03±0.06 g cm−^3 for a reference radius of 1153
km.5.2 Bulk Composition and Internal Structure
The 1980s discovery that the Pluto–Charon system’s aver-
age density is near2gcm−^3 was a major surprise resulting
from the mutual events. Many scientific papers had previ-
ously predicted values closer to the density of water ice (∼ 1
gcm−^3 ), or even lower. Thus, contrary to earlier thinking,
the Pluto–Charon pair is known to be mass-dominated
by rocky material. Based on this information, a three-
component model for Pluto’s bulk composition and internal
structure can be derived. In such a model, Pluto’s bulk
density is assumed to consist of three of the most common
condensates in the outer solar system: water ice (ρ= 1. 00
gcm−^3 ), “rock” (2.8<ρ < 3 .5gcm−^3 , depending upon its
degree of hydration), and methane ice (ρ= 0 .53gcm−^3 ).
From three-component models, it is believed that Pluto’s
rock fraction is in the range of 60–80%, with preferred
values close to 70%. By comparison, the large (e.g.,R>
500 km) icy satellites of Jupiter, Saturn, and Uranus have
typical rock fractions in the range 50–60% by mass. Only Io,
Europa, and Triton rival Pluto in terms of their computed
rock content. Pluto’s high rock (i.e., nonvolatile) mass frac-
tion is in contrast to the≈50:50% rock:ice ratio predicted
for objects formed from solar nebula material according to
many nebular chemistry models and our present-day un-
derstanding of the nebular C/O ratio. This high rock frac-
tion indicates that the nebular material from which Pluto
formed was CO-rich rather than CH 4 -rich. As such, roughly
half of the available nebular oxygen should have gone into
CO, rather than H 2 O formation, which in turn would lead
to a high rock:ice ratio.
There are two possible ways out of the apparent nebular
chemistry dilemma imposed by Pluto’s high rock fraction.
One is that Pluto’s minimum estimated radius of 1150 km
may be too small; a value near 1200 km, as suggested
by some stellar occultation models, would solve the prob-
lem. Alternatively, William McKinnon and the late Damon
Simonelli independently suggested that a giant impact may