Encyclopedia of the Solar System 2nd ed

554 Encyclopedia of the Solar System

TABLE 3 Pluto and Charon Comparison

Parameter Pluto Charon

Rotation period 6.387223 days 6.387223 days

Radius 1150–1220 km 602–606 km

Density ≈ 2 .1gmcm–^3 ≈ 1 .3gmcm–^3

Perihelion,V 0 13.6 magnitude 15.5 magnitude

Mean B geometric

albedo

0.55 0.38

Rotational lightcurve 38% 8%

B-V color 0.85 mag 0.70 mag

V-I color 0.84 mag 0.70 mag

Known surface ices CH 4 ,N 2 ,CO H 2 O, NH 3?

Atmosphere Confirmed None detected

Table 3 compares some basic facts about Pluto and
Charon.

8. The Origin of Pluto’s Satellite System

In the past few years, much progress has been made in un-
derstanding Pluto’s likely origin and its context in the outer
solar system. This work began with theoretical considera-
tion in the late 1980s and early 1990s, and was advanced
considerably by the discovery of numerous 100 to 1600 km
diameter objects in the Kuiper Belt, where Pluto also re-
sides.
Any scenario for the origin of Pluto must of course pro-
vide a self-consistent explanation for the major attributes of
the Pluto–Charon system. These include (1) the existence
of the exceptionally low,∼8.5:1 planet:satellite mass ratio
of Pluto:Charon; (2) the synchronicity of Pluto’s rotation
period with Charon’s orbit period; (3) Pluto’s inclined, el-
liptical, Neptune-resonant orbit; (4) the high axial obliquity
of Pluto’s spin axis and Charon’s apparent alignment to it;
(5) Pluto’s small mass (∼ 10 −^4 of Uranus’s and Neptune’s);
(6) Pluto’s high rock content—the highest among all the
outer planets and their major satellites; and (7) the di-
chotomous surface compositions of Pluto and Charon. This
formidable list of constraints on origin scenarios is very
clearly dominated by Charon’s presence, the unique dynam-
ical state of the binary, and the low mass of Pluto/Charon
compared to other planets.

8.1 The Origin of Pluto’s Satellite System

Several scenarios have been examined for the origin of
the Pluto–Charon system. These include coaccretion in
the solar nebula, mutual capture via an impact between
proto-Pluto and proto-Charon, and rotational fission. Grav-
itational capture of Pluto by Charon without physical con-
tact is not dynamically viable. The formation of Pluto and

Charon together in a subnebular collapse is not consid-

ered realistic because of their small size; standard planetary

formation theory suggests bodies in the Pluto and Charon

size class formed via solid-body accretion of planetesimals.

Similarly, the rotational fission hypothesis is unlikely to be

correct because the Pluto–Charon system has too much an-

gular momentum per unit mass to have once been a single

body.

The more likely explanation for the origin of the Pluto

system is an inelastic collision between two bodies, Pluto

and proto-Charon, which were on intersecting heliocentric

orbits. A similar scenario has been proposed for the origin of

the Earth–Moon binary, based in part on its relatively high

mass ratio (81:1) and high specific angular momentum. In

the collision theory, Pluto and the Charon-impactor formed

independently by the accumulation of small planetesimals

and then suffered a chance collision that dissipated enough

energy to permit binary formation.

An important qualitative difference between the Pluto–

Charon and Earth–Moon giant impacts is that the relative

collision velocities, and hence the impact energies of the

Pluto–Charon event, were much smaller. This enormously

reduced the thermal consequences of the collision. Thus,

whereas the Earth may have been left molten by the Mars-

sized impactor necessary to have created the Moon, the

proto-Charon impactor would probably have raised Pluto’s

global mean temperature by no more than 50 to 75 K. This

would have been insufficient to melt either body, but may

have been sufficient to induce the internal differentiation

of either. It would have also produced a substantial short-

lived, hot, volatile atmosphere with intrinsically high escape

rates. Such an escaping atmosphere could have interacted

with the Charon-forming orbital debris, and also perhaps

affected Pluto’s present-day volatile content.

Until recently, only scaling calculations showing the plau-

sibility of the giant impact hypothesis has been performed,

and it was accepted largely because it is the only scenario

that remains at all viable given the various constraints—

most particularly the high specific angular momentum of

the binary. In 2005, however, Robin Canup of the South-

west Research Institute published the first detailed giant

impact simulations demonstrating the viability of Pluto–

Charon formation owing to the collision of Pluto with an-

other large body. Canup’s work further demonstrated that

the most promising candidate impacts involved an oblique

collision by an impactor with 30–100% of Pluto’s mass, ap-

proaching at a relative speed up to 1 km/s.

The discovery of Nix and Hydra in 2005 yielded addi-

tional support for the giant impact hypothesis. This support

comes in two forms: the fact that all three satellites orbit in

a single orbital plane and the near or perfect orbital period

resonance of the three. The orbital coplanarity would be un-

likely for other satellite formation mechanisms like capture,

but naturally result from the giant impact scenario. The or-

bital resonance line of evidence naturally suggests the three