550 Encyclopedia of the Solar System
FIGURE 5 Typical interior structural
models for Pluto and Charon. Adapted from
W. B. McKinnon et al., 1997, in “Pluto &
Charon” (S. A. Stern and D. J. Tholen, eds.),
Univ. Arizona Press, Tucson.have induced volatile loss from an already differentiated
Pluto, which may have raised Pluto’s rock fraction some-
what (perhaps 20%) to reach its present value. As we discuss
in Section 8, such an impact is thought to be responsible
for the formation of Pluto’s satellite system.
The gross internal thermal structure of Pluto depends on
several factors, virtually all of which are uncertain. These in-
clude material viscosities in the interior, the internal convec-
tion state, the actual rock fraction and radioisotope content,
and the internal density distribution (i.e., most fundamen-
tally, the differentiation state). It would appear likely that
Pluto’s deep interior reaches temperatures of at least 100–
200 K, but not much higher. Whether or not Pluto is warm
enough to exhibit convection in its ice mantle depends on
both the internal thermal structure and the radial location
of water ice in its interior.
Based on the results just given and laboratory equations
of state, Pluto’s central pressure can be estimated to lie be-
tween 0.6 and 0.9 GPa (gigapascals) if the planet is undiffer-
entiated, or 1.1–1.4 GPa if differentiation has occurred. As
such, the high-pressure water ice phase Ice VI is expected in
the deep interior if the planet has not differentiated. If dif-
ferentiation has occurred, as is likely, then a higher pressure
form of water ice called Ice II may be present, but only near
the base of the convection layer. If Pluto did differentiate,
then its gross internal structure may be represented by a
model like that shown in Fig. 5.6. Pluto’s Atmosphere
6.1 Atmospheric Composition
The existence of an atmosphere on Pluto was strongly sus-
pected after the discovery of methane on its surface in
1976, largely because at the predicted surface tempera-
tures (∼40–60 K), sufficient methane vapor pressure should
obtain to constitute a significant atmosphere. This circum-
stantial argument was supported by the high reflectivity of
Pluto’s surface, which suggested some kind of resurfacing—
most plausibly due to volatile laundering through an or-
bitally cyclic atmosphere. Still, however, there was no defini-
tive evidence for an atmosphere until the late 1980s.
The formal proof of Pluto’s atmosphere came from the
occultation of a 12th magnitude star by Pluto in 1988,
by providing the first direct observational evidence for an