546 Encyclopedia of the Solar System
density of 2.03±0.06 g cm−^3 , where the error bar is domi-
nated by the uncertainty in the radius of Pluto. Any density
of 1.8 g cm−^3 or higher implies that the system is compo-
sitionally dominated by rocky material, probably hydrated
chondrites, as opposed to ices. This result and its implica-
tions will be discussed in more detail in Section 5.
4. Pluto’s Surface Properties and Appearance
Pluto’s surface properties have been studied since the
1950s. Photometric, spectroscopic, and polarimetric tech-
niques have been applied, and the explorable wavelength
regime has expanded from the ground-based window to
the reflected IR and thespace ultraviolet. Thermal-IR
and millimeter-wave measurements have also been made.
4.1 Albedo and Color
Two of the most basic photometric parameters one desires
to know for any solid body are its albedo and color. Accurate
knowledge of Pluto’s albedo was obtained only after the
onset of the mutual events because until then Pluto’s radius
was unknown, and there was no definitive way of removing
Charon’s contribution.
The very first report of eclipse detections revealed a fac-
tor of 2 difference in depth between partial eclipses of
Charon and Pluto, indicating Pluto’s geometric albedo is
substantially higher than Charon’s. Once the eclipse season
was complete, a more complete data set became available
for analysis. Comprehensive models for the analysis of mu-
tual event lightcurve data simultaneously solve for the in-
dividual radii of Pluto and Charon, the individual albedos,
and Charon’s orbital elements. The modeling of these pa-
rameters is complicated by solar phase angle effects, the
presence of shadows during eclipse events, and instrumen-
tal and timing uncertainties. To derive the albedo lightcurve
for Pluto alone, Pluto’s albedo at the longitude of the total
superior eclipses (in which Charon was completely hidden)
must first be determined; albedos at other rotational epochs
are then derived from this anchor point, assuming Charon’s
rotational lightcurvecontributes only a small constant
to the combined Pluto+Charon lightcurve. The assump-
tion that a constant Charon contribution can be removed
is not unreasonable, because (1) its geometric cross section
is small (one fourth) of Pluto’s, and (2) its eclipsed hemi-
sphere has a geometric albedo only about 50–60% of Pluto’s.
However,HSTobservations have shown that Charon does
vary somewhat in brightness (≈8%) as it rotates on its axis.
Analysis of a large set of mutual event data in the way just
described has found that Pluto’s maximum, disk-integrated,
B-bandpass (∼ 4360 A) geometric albedo is 0.61. Rotational ̊
variations cause this albedo to range from values as low as
0.44 to values as high as 0.61 as Pluto rotates.
Information on Pluto’s color comes from both photome-
try and the mutual events. As described in Section 1, Pluto’s
visible-bandpass color slope has been known to be red since
the 1950s. Analysis of premutual event photometry yields
B-V and U-B color differences of 0.84 and 0.31, respectively,
for Pluto+Charon. There is only weak evidence that this
value has changed since the 1950s when photoelectric mea-
surements were first made. Eclipse data have revealed that
the B-V color of Pluto itself is very close to 0.85 astronomi-
cal magnitudes. By comparison, this color is much less red
than therefractorysurfaces of Mars (B-V=1.36) and Io
(B-V=1.17), and slightly redder than its closest analog in
the solar system, Triton (B-V=0.72).4.2 Solar Phase Curve
The photometric behavior of a planet or satellite as it
changes in brightness on approach to opposition can be
used to derive surface scattering properties, and therefore
its microphysical properties. Knowledge of the complete
solar phase curve is also required to transform geometric
albedos into bolometric Bond albedos.HSTobservations
in the 1990s gave linear phase coefficients for Pluto and
Charon of 0.029±0.001 magnitudes/deg and 0.866±0.008
magnitudes/deg, respectively.
Pluto’s maximum solar phase angle (φmax) as seen from
Earth is just≈1.9◦. Therefore, no measurements of the
large-angle scattering behavior have been possible. With-
out measurements at large phase angles, no definitive de-
termination of Pluto’s phase integralqor Bond albedoA
can be made. However, some improvement in estimates of
qandAcould become possible if theCassinispacecraft is
able to obtain Pluto phase curve observations from Saturn
orbit, whereφmax≈ 18 ◦. However, what is really needed
are flyby spacecraft measurements of Pluto at high phase
angles. For the present, the best available phase integral to
use for Pluto is probably Triton’s (Pluto and Triton also have
similar linear phase coefficients). Triton’sqhas been mea-
sured byVoyager, givingq= 1 .2 (at green wavelengths)
to 1.5 (at violet wavelengths). If Pluto is similar, then its
surface may have Bond albedos ranging from 0.3 to 0.7.4.3 Surface Composition
Progress in understanding Pluto’s surface composition re-
quired the development of sensitive detectors capable of
making moderate spectral resolution measurements in the
infrared, where most surface ices show diagnostic spectral
absorptions. Although this technology began to be widely
exploited as early as the 1950s in planetary science, Pluto’s
faintness (e.g., 700 times fainter than the jovian Galilean
satellites) delayed compositional discoveries about it until
the mid-1970s.