Infrared Views of the Solar System from Space 693
FIGURE 21 Pluto-Charon were detected moving across the
infrared sky by IRAS. These images were constructed from 60
micron scans for (a) July 13, (b) July 23–24, and (c) August 16 in
- The predicted positions of Pluto-Charon at each of these
times are indicated by circles. The August 16 position is the
lower left circle in (a) and (b).
6. Pluto and Beyond
Thermal radiation from Pluto and its moon Charon was first
detected byIRAS(Fig. 21). The thermal flux of the system
was consistent with that of a rapidly rotating graybody hav-
ing an equatorial temperature of∼60 K. This information
in combination with ground-based spectroscopic measure-
ments and albedo maps derived from the mutual eclipses
between Pluto and its moon between 1984 and 1990 has
provided important insights into the nature and dynamics
of the surface of Pluto. [SeePluto.]
When methane was first detected in visible wavelengths
on the surface of these objects, it was thought that Pluto
must be completely covered by the frozen ice, and would
be isothermal because of the transport of heat as highly
insolated locations would be cooled by sublimation and less
insolated locations would be warmed by the condensation
of atmospheric methane. Charonwas thought to be a less
likely location of such a coating of methane frost because of
its lower gravity, from which methane would be expected
to escape over time.
The detection of an extended atmosphere from a stellar
occultation in 1988 and the subsequent detection in the
near-infrared of nitrogen ice on Pluto’s surface required
that the volatile surface ices be dominated by nitrogen with a
small fraction of the more spectroscopically active methane,
and that these surface ices must be very cold,∼35 K.
The spectroscopic andIRAS-derived temperatures ap-
pear to contradict each other. Nitrogen ice at the warmer
radiometric value would produce an enormous atmosphere
that would have been evident in the occultation observa-
tions. The surface albedo maps, however, show that Pluto’s
surface is segregated into bright and dark regions with
bright ices generally at higher latitudes. A high-albedo sur-
face is bright at visible wavelengths (reflected light) and
faint at thermal wavelengths, while a dark surface is faint at
visible wavelengths and bright in the thermal. On Pluto, vis-
ible wavelength spectroscopy samples primarily the bright
icy polar regions of the planet while space-based thermal
observations are dominated by the dark equatorial region.
IRAStells us that the volatile nitrogen ice—from which
the atmosphere derives—is segregated on the surface of
Pluto, away from the warmer regions, giving rise to the
thermal emission detected by the satellite. These warmer
regions are probably a mixture of water ice and carbona-
ceous residue resulting from the radiation processing of
methane ice over the age of the solar system. The dark re-
gions are not contributing significantly to the atmospheric
gases, which is consistent with a water/organic composition
that would have negligible vapor pressure at the tempera-
ture inferred.
Spitzerspectroscopic studies of Pluto are expected to
provide rotationally resolved information on the complex
organics comprising Pluto’s dark regions, as well as con-
stituents of its bright ice regions. These observations will