Non-methane Hydrocarbons On Pluto’s Surface:
From near- and mid- infrared spectra of Pluto’s surface we have learned much about this
cold, distant world. Volatile ices including CH4, N2, and CO have been detected on Pluto’s
surface (Cruikshank et al. 1976; Owen et al. Xxxx). We know that there is methane diluted in
nitrogen on Pluto’s surface from the shift in band centers (xx) and that very little of Pluto’s
surface (<6%), at least on one side, has pure N2 ice (Olkin et al. 2007). But there are still
outstanding questions, such as “Where is the ethane?”. Ethane is expected on Pluto’s surface due
to photolysis of methane. Recently, Cruikshank et al (2006) has suggested the faint spectral
feature at 2.404 microns is due to ethane not 13CO as previously thought (xxref). However, this
detection of ethane is still an open question partially because the stronger 2.46 micron feature of
ethane has not been detected. Additionally, other wavelengths have not produced a definitive
detection. Olkine et al. (2007) presented spectra of one face of Pluto in the 3-4 micron region
with no detection of ethane, see Figure 1. There was a possible detection of ethane (the
wavelength of the features do not correspond well with the model) on a different face of Pluto
(Sasaki et al 2000), see Figure 2.
One of our scientific goals for this project is to make a comprehensive spectral model
from the visible to 4 microns covering all rotational phases of Pluto to further investigate the
possibility of tholin and ethane on Pluto’s surface. This proposal covers the wavelength range
from xx to xx microns. (For the 1 to 2.5 micron wavelength observations I am applying to the
IRTF with SpeX and in the visible xx.) In previous work (Olkin et al. 2007), we found that
combining 3-4 micron spectra with 1 to 2.5 micron spectra led to greater insight into the surface
features on Pluto. In particular, we were able to constrain the amount of pure N2 on Pluto’s
surface to less than 6% and identify a spectral signature consistent with Titan tholin (Khare et al.
1984xx). We are now proposing to observe Pluto’s spectrum over this range wavelengths for a
complete rotation and during the same season with concurrent photometric observations to
calibrate the spectra well against slit loss.
A Comprehensive Look at Pluto:
Our second scientific goal requires a long-term observational program. Pluto’s
atmosphere, insolation and surface properties are all intertwined and need to be considered
together to improve our understanding processes in the outer solar system. Pluto’s surface frosts
support the atmosphere by vapor pressure equilibrium, Pluto’s atmosphere transports the volatiles
through sublimation and condensation, and the insolation and surface properties determine the
temperatures on Pluto’s surface which controls the surface pressure of Pluto’s atmosphere. This is
a unique time for observing Pluto. We are able to combine the strength of an aggressive stellar
occultation campaign (to probe Pluto’s changing atmosphere), observations from New Horizons
that will be able to get high phase observations, and spectral observations that will span the
visible wavelengths out to 4 microns. All these observations will occur at a time when Pluto is
receding from the Sun reducing the amount of insolation that Pluto receives (xxquantify) and the
atmosphere is undergoing significant changes (Sicardy et al. Xx).
The first two pieces of this program are already in place. One of us (Leslie Young) leads
an active campaign to observe stellar occultations by Pluto. Now that Pluto has moved into the
galactic plane the number of opportunities has increased significantly. The first stellar occultation
by Pluto was observed in 1988 and it was 14 years later until another was observed. However
with the more dense star field in the background, we have observed an occultation by Pluto in
both 2006 and 2007 and these more favorable conditions will continue for the next xx years.
The second piece is the ability to get unprecedented high-phase observations. With the
launch of the New Horizons spacecraft in January 2006, there will be annual observations of
Pluto using the panchromatic visible wavelength imager to achieve photometric observations at
phase angles not achievable from Earth. In order to put these high-phase observations in context,
we need complementary observations.
The near-ir observations proposed here are the best-suited wavelength range to
understand the distribution and properties (such as grain size) of Pluto’s surfaces ices since that is
where these molecules interact the most with the light. These observations will (hopefully) be
supplemented with visible wavelength spectra and mid-IR spectra in the 3-4 micron range.
We propose a new long-term project to explore Pluto’s surface properties. Each year we
will use SpeX (big dog) to observe spectra from xx to xx microns covering a complete rotation of
Pluto near minimum phase and then another spectrum of a single Pluto face near maximum phase
angle (1.xx degrees). We will also take photometric observations using guide dog to characterize
Pluto’s rotational light curve and to calibrate our spectral observations. These observations will
complement two other observing programs that are already in place: (1) a campaign of stellar
occultation observations and (2) annual panchromatic visible-wavelength photometry using the
LORRI instrument on New Horizons at higher phase angles than achievable from Earth.