Pluto 547
The first identification of a surface constituent on Pluto
was the discovery by Dale Cruikshank, Carl Pilcher, and
David Morrison in 1976 of CH 4 ice absorptions between
1 and 2μm (a wavelength of 1μm=10,000A). Cruik- ̊
shank et al. made this discovery using infrared photome-
ters equipped with customized, compositionally diagnos-
tic filters. In their report, Cruikshank et al. also presented
evidence against the presence of strong H 2 O and NH 3 ab-
sorptions in Pluto’s spectrum. Confirmation of the methane
detection came in 1978 and 1979 when both additional CH 4
absorption bands and true IR spectra of Pluto became avail-
able.
In mid-1992, another breakthrough occurred when Toby
Owen, Dale Cruikshank, and other colleagues made ob-
servations using a new, state-of-the-art IR spectrometer at
the UK Infrared Telescope (UKIRT) on Mauna Kea. These
data revealed the presence of both N 2 and CO ices on Pluto.
These molecules are much harder to detect than methane
because they produce much weaker spectral features. Their
presence on Pluto indicates the surface is chemically more
heterogeneous, and more interesting than had previously
been thought. Because N 2 and CO are orders of magnitude
morevolatile(i.e., have higher vapor pressures) than CH 4 ,
their presence also implies they play a highly important role
in Pluto’s annual atmospheric cycle. Abundance inversions
of Pluto reflectance spectra make clear that N 2 dominates
the composition of much of Pluto’s surface, with CO and
CH 2 being trace constituents.
In 2006, ethane (C 2 H 6 ) was detected on Pluto’s surface.
This and other hydrocarbons and nitriles had long been
predicted to reside on Pluto as a result of photochemical
and radiological processing of Pluto’s surface ices and at-
mosphere. Future surface reflectance studies are expected
to yield additional surface constituent detections.
Rotationally resolved spectra of Pluto’s CH 4 absorption
bands have been reported by a number of groups. Their
studies showed that Pluto’s methane is present at all rota-
tional epochs, but the band depths are correlated with the
lightcurve so that the minimum absorption occurs at min-
imum light. Mutual event spectroscopy has now demon-
strated that Charon is not the cause of this variation, since
Charon’s surface is devoid of detectable CH 4 absorptions
(see Section 6). This important discovery suggests that
Pluto’s dark regions could contain reaction products result-
ing from the photochemical or radiological conversion of
methane and N 2 to complex nitriles and higher hydrocar-
bons.
We thus have the following basic picture of Pluto’s sur-
face composition: CH 4 appears rotationally ubiquitous, but
with its surface coverage more widespread in regions of high
albedo. In many areas, the methane is dissolved in a matrix
of other ices, but in some locations the CH 4 is seen as pure
ice. CO and N 2 have also been detected. In the bright ar-
eas of the planet where these ices are thought to mainly be
located, N 2 dominates the surface abundance, and the CO is
more abundant than the previously known (but more spec-
troscopically detectable) CH 4. Ethane, a byproduct of CH 4
chemistry, was detected in 2006. Pluto’s strong lightcurve
and red color demonstrate that other widespread, probably
involatile surface constituents exist. This may either be due
to rocky material, or hydrocarbons resulting from radiation
processing of the CH 4 due to long-term exposure to ultra-
violet sunlight, or both. Whether the volatile frost we are
seeing is a surface veneer or the major component of Pluto’s
crust has not yet been established.4.4 Surface Temperature
Results from theInfrared Astronomical Satellite(IRAS) in-
dicated that Pluto’s perihelion-epoch surface temperature
was in the range of 55 to 60 K, close to that expected in radia-
tive equilibrium with solar insolation. However, it has sub-
sequently become appreciated that the situation on Pluto’s
surface is more complicated.
One line of evidence for this conclusion comes from
millimeter-wave measurements of Pluto’s Rayleigh–Jeans
blackbody spectrum. Such measurements, reported first by
Wilhelm Altenhoff and collaborators, and then later by Alan
Stern, Michel Festou, and David Weintraub, and indepen-
dently confirmed by David Jewitt, indicate that a significant
fraction of Pluto’s surface is significantly colder than 60 K,
most likely in the range 35–42 K. A second line of evi-
dence came in 1994 from high-resolution spectroscopy of
the temperature-sensitive 2.15μmN 2 ice absorption band,
which Kimberley Tryka and her co-workers found indicates
a surface temperature of about 40 K for the widespread ni-
trogen ices on Pluto. As described in Stern et al. reported
in 1993, although the surface pressure of N 2 is not well
known, it must be less than≈ 60 μbar. This is consistent with
an N 2 ice temperature of≈40 K, assuming vapor pressure
equilibrium between the N 2 ice and the atmosphere. This,
combined with theIRASmeasurements, led to the conclu-
sion that Pluto’s surface must exhibit both warm and cold
regions. This was subsequently confirmed by rotationally re-
solved studies of Pluto’s thermal emission spectrum by the
Infrared Space Observatory(ISO) and theSpitzer Infrared
Space Telescope Facility(SIRTF). These space telescopes
also revealed that Pluto’s coldest regions are correlated with
bright surface units, and that the warmer regions are cor-
related with darker surface units with lower abundances of
sublimating ices.
It is now well established that Pluto’s surface tempera-
ture varies from place to place on the surface, with≈ 40
K regions where N 2 ice is sublimating and≈55–60 K re-
gions where N 2 ice is not present in great quantities. The
strong temperature contrasts across Pluto’s surface imply
strong wind speeds and significant lateral transport of ma-
terial across the surface.