548 Encyclopedia of the Solar System
4.5 Surface Appearance and Markings
Because Pluto is less than 0.1 arcsec across as seen from
Earth, its disk could not be resolved until the advent of
theHubble Space Telescope. However, evidence for sur-
face markings has been available since the mid-1950s, when
lightcurve modulation was first detected. Because Pluto is
large enough to be essentially spherical (and indeed, mu-
tual event and stellar occultation data show it actually is),
the distinct variation in this lightcurve must be related to
large-scale albedo features.
From the lightcurve in Fig. 2, it can be seen that
Pluto’s surface must contain at least three major longitu-
dinal provinces. Information on the latitudinal distribution
of albedo can be gained by observing the evolution of this
lightcurve as Pluto moves around its orbit while the pole
position remains inertially fixed, assuming, of course, that
the surface albedo distribution is time invariant.
The most complete mapping products obtained from
photometric data inversions (variously using rotational
lightcurves and mutual event lightcurves) have been
obtained by two teams. The first team, led by Marc Buie of
Lowell Observatory has used both mutual event lightcurves
and rotational lightcurve data compiled from 1954 to 1986
to compute a complete map of Pluto. The second group,
consisting of Eliot Young and Richard Binzel, of MIT and
SwRI, numerically fit a spherical harmonic series to each
element of a finite element grid using the Charon transit
mutual event lightcurve data as the model input. Because
Young and Binzel used only mutual event data, their map
is limited to the hemisphere of Pluto that Charon eclipses.
Because the two groups used different data sets and differ-
ent numerical techniques, their results are complementary
and serve to check one another on the Charon-facing hemi-
sphere they share in common.
These two maps are shown in Fig. 3. There are differ-
ences between the two maps, but it must be remembered
that each map has intrinsic noise. The common features of
these maps are (1) a very bright south polar cap, (2) a dark
band over mid-southern latitudes, (3) a bright band over
mid-northern latitudes, (4) a dark band at high northern
latitudes, and (5) a northern polar region that is as brightFIGURE 3 Two maps of Pluto’s
Charon-facing hemisphere. The map on the
left was derived by M. Buie, K. Horne, and
D. Tholen using both mutual event and
lightcurve data. The map on the right was
derived by E. Young and R. Binzel from their
mutual event data. Although the fine details
of these maps differ, their gross similarities
are striking. See R. P. Binzel et al., 1997, in
“Pluto & Charon (S.A. Stern and D.J.
Tholen, eds.), Univ. Arizona Press, Tucson,
for additional details.