Pluto 553
FIGURE 7 Pluto and Charon spectra. Top panel shows
spectra of (a) Pluto+Charon made prior to eclipse; (b)
Pluto-only after second contact with Charon hidden; (c)
Charon-only smoothed to 80A resolution resulting from the ̊
subtraction of (a)−(b); and (d) the raw Charon-only
spectrum resulting from the subtraction of (a)−(b). Notice
that the strong methane absorption bands present in Pluto’s
spectrum are not detected in the Charon-only spectrum.
(Adapted from Fink and DiSanti, 1988.) Bottom panel shows
Marcialis et al.’s (1987) detection of water ice in Charon’s
reflectance spectrum (data points) against a laboratory
spectrum of water ice at 55 K. (Adapted from Marcialis et al.,
1987,Science 237 , 1349.)is quite neutral, unlike Pluto’s clearly reddish tint. Another
major set of advances that resulted from the eclipse events
was the first set of constraints on Charon’s basic surface com-
position. These came from the subtraction of spectra made
just prior to eclipse events from those made when Charon
was completely hidden behind Pluto. The resulting “net”
spectrum thus contains the Charon-only signal. As shown
in Fig. 7, this technique has been applied both in the visi-
ble (0.55–1.0μm) and infrared (1–2.5μm) bandpasses. The
visible light data show that Charon’s surface does not dis-
play the prominent CH 4 absorption bands that Pluto does,
indicating that Charon’s surface has little or (more likely)
no substantial methane on it.
Additionally, there is no evidence for strong absorptions
due to a number of other possible surface frosts, including
CO, CO 2 ,H 2 S, N 2 ,orNH 4 HS, on Charon. The IR spectra
of Charon do show that Charon does, however, display clear
evidence of water ice absorptions, which Pluto does not. It
is tempting to speculate (as some authors have) that Charon
may have lost its volatiles through the escape of a primordial
atmosphere or by heating resulting from its formation in a
giant impact.
Since the launch ofHubble Space Telescope, it has been
possible to routinely separate Charon’s light from Pluto’s,
and to learn Charon’s phase coefficient, UV albedo, and
rotational lightcurve. Most notably among these, Marc Buie
and Dave Tholen have determined that Charon displays
a small but significant lightcurve variation near 8% as it
rotates on its axis.
Because the major identified surface constituent of
Charon is water ice, which is not volatile at the expected
50–60 K surface radiative equilibrium temperature at peri-
helion, one does not expect Charon to have an atmosphere.
The fact that CH 4 is not present on the surface supports
this expectation. However, absence of evidence is not the
same as evidence of absence. One published interpreta-
tion of the 1980 Charon stellar occultation claims there is
some evidence for a weak atmospheric refraction signal. To
definitively resolve the issue of Charon’s atmosphere, either
a better-observed stellar occultation event or a spacecraft
flyby is required.
In 2001, groups led by Mike Brown of Cal Tech and
Will Grundy of Lowell Observatory used infrared ground-
based telescopes to find spectroscopic evidence for both
crystalline water ice and ammonia (NH 3 ) or ammonium
hydrates on Charon’s surface. If these identifications are
correct, they imply the possibility of recent geologic activity
on Charon.