Geoscience Reference
In-Depth Information
2. Observations and Data Reduction
A near-infrared spectroscopic observation of Pluto was performed by the
8-m Subaru telescope with IRCS in conjunction with its AO system on May
28, 2002 (UT) near the opposition (RA = 17 h 05 m 33 s, dec = 12
◦
39
25
).
The sub-Earth longitude was 40-50
◦
and the separation from Charon was
0.9
. Spectral resolving power was approximately 400 at the
L
band in our
observation, meanwhile that was about 60 in Ref. 1. The typical psf width
was 0.3-0.4
during the observation and the total on-source integration time
was 2600 s. For the cancellation of telluric absorption features, a reference
star (G3V star SAO141540) was observed just after Pluto observation. The
air mass differences between Pluto and the reference star were less than
0.035 throughout the observation.
We used NOAO IRAF astronomical software package to reduce near-
infrared spectra obtained by IRCS. Argon lamp frames were used for the
wavelength calibration. Since Pluto was clearly separated from Charon with
the FWHM as sharp as 0.4
, the resulting spectrum includes no contami-
nation from Charon. For details refer to Ref. 5.
3. Results
The spectrum of Pluto is shown in Fig. 1 along with the previous low
resolution data
1
and synthetic spectra of simple ternary intimate mixture
CO. As Nakamura
et al.
,
2
we employed Hapke's bidirectional
model
6
to calculate the spectrum of a uniform half-infinie layer covering
the whole surface of Pluto. It should be noted that the detailed model
calculations are beyond the scope of this paper because we cannot account
for the realistic solid solutions, vertically layered structure and spatially
segregated patches unlike some previous approaches. Apparently, the shape
of the synthetic specta is dominated by CH
4
because N
2
and CO have no
significant absorptions in the wavelength range.
Grundy
et al.
1
found a decrease of the reflectance lower than 3.2
µ
mand
beyond 3.95
µ
m. They interpreted the result as the contribution from non-
volatile ices, such as H
2
O for the former, SO
2
and CO
2
for the latter. While
we cannot confirm the putative band of SO
2
and CO
2
due to the limited
wavelength coverage, our spectra shortward of 3.2
µ
m are consistent with
their results.
Our observations show lower reflectance around 3.45
µ
m and additional
absorptions around 3.1, 3.2, and 3.35
µ
m. In order to reproduce the features,
of N
2
−
CH
4
−
