Pluto 551
atmosphere. The best measurements of the occultation
were obtained by Robert Millis and James Elliot, and their
various MIT, Lowell Observatory, and Australian collab-
orators. These teams used both NASA’s mobile,Kuiper
Airborne Observatory(which contained a 36-inch diam-
eter telescope) and ground-based telescopes to observe the
occultation event. They discovered that light from the star
was diminished far more gradually than it would be from an
airless body. The apparent extinction of starlight observed
during the occultation was caused by atmospheric refrac-
tion (i.e., the degree of bending of the starlight by the at-
mosphere), which varies with height. The rate at which the
refractivity of the atmosphere varies with altitude depends
on the ratio of atmospheric temperature (T) to atmospheric
mean molecular weight (m). The 1989 Pluto occultation
data impliedT/m=3.7±0.7 K/g at and above an altitude
of 1215 km. If the atmosphere were composed entirely of
methane (m=16 g/mole), the implied atmospheric tem-
perature would be 60 K, whereas an N 2 or CO atmosphere
(m=28 g/mole) would be at a temperature near 106 K.
From the stellar occultation data alone it was impossi-
ble to separately determine the mean atmospheric molec-
ular weight and temperature of Pluto’s atmosphere. How-
ever, theoretical calculations of the atmospheric tempera-
ture made by Roger Yelle and Jonathan Lunine of the Uni-
versity of Arizona indicated a value of 106 K in the upper
atmosphere, under a variety of assumed compositions. This
is relatively high compared with the surface temperature
(∼40–55 K) because the efficiency at which the atmosphere
radiates and cools is very small. An upper atmospheric tem-
perature near 106 K implies that the atmospheric mean
molecular weight is close to 28 g/mole. This is consistent
with an atmosphere dominated by N 2 or CO gas, with trace
amounts of other species.
The detection of N 2 ice absorption features on Pluto’s
surface (see Section 4), coupled with the discovery by
Voyager 2that Triton’s atmosphere also consists predomi-
nantly of N 2 and only a trace of CO, suggests that Pluto’s
atmosphere is likely to be N 2 -dominated. Nevertheless, if
the high-temperature (106 K) atmospheric model is correct,
then at least a few percent methane is thought to be required
because methane (which is efficient at atmospheric heating)
is thought to be responsible for the elevated atmospheric
temperatures.
A nitrogen-dominated atmosphere with only a minor
amount of methane was significantly strengthened in 1994
when Leslie Young and colleagues at MIT detected CH 4
gas in Pluto’s atmosphere for the first time. This discovery,
which was made possible by sensitive, high-resolution IR
spectroscopy of the 2.3μmCH 4 band system, indicated a
total methane mixing ratio of<1%, and perhaps as little as
0.1% in the atmosphere. Subsequent high-resolution ob-
servations by Young and colleagues revealed that the CO
abundance in Pluto’s atmosphere must also be very very
low.
6.2 Atmospheric Structure
The 1988 occultation data exhibited interesting behavior at
altitudes below 1215 km, as is shown in Fig. 6. The starlight,
which was decreasing gradually at higher altitudes, dropped
suddenly to a value close to zero below this level; this is
called lightcurve steepening. The drop is still not as sudden
as would be expected from the setting of a star behind the
limb of an airless planet, however. Two possible explanations
have been proposed for this change.
In one model, the steepening was caused by the presence
of aerosol hazes in the lower atmosphere. (Condensation
clouds can be ruled out as an explanation for the aerosol
layer because of the temperature structure of the atmo-
sphere.) Because reproducible albedo features have been
seen on Pluto’s surface, any such aerosol layer must be trans-
parent when viewed from above, but relatively opaque when
viewed horizontally. The aerosols must also extend around
most of the planet since the steepening of the occultationFIGURE 6 Stellar occultation data showing the refractive
signature of Pluto’s atmosphere and the steepening of the
lightcurve around the half-light level that is discussed in the
article. The upper panel is a ground-based data product; the
lower panel was obtained from theKuiper Airborne
Observatory. (Adapted from Elliot et al., 1989,Icarus, 77 , 148.)