Geoscience Reference
In-Depth Information
Observation
Grundy et al. 2002
ternary, diluted CH4
pure CH4
1
0.8
0.6
0.4
0.2
3
3.2
3.4
3.6
3.8
Wavelength (micron)
Fig. 1. Reflectance spectrum of Pluto with the previous low-resolution data and
synthetic spectra of intimate ternary mixture of N
2
−
CO with the mass ratio
1:0.01:0.002. The circles represents our observation smoothed by taking a running aver-
age of 31 pixels (0.08
µ
m) with their error bars of standard deviation. The dotted and
dashed curves indicate synthetic spectra for pure CH
4
and for CH
4
diluted in the solid
molecular nitrogen, respectively. We normalized all the data at 3.58
µ
m and the synthetic
spectra were convolved to the same wavelength resolution as the observation.
CH
4
−
we incorporated some non-methane hydrocarbons, namely C
2
H
2
,C
2
H
4
,
C
2
H
6
,andC
3
H
8
into the model calculations. Assuming their mass ratio to
CH
4
as 10%, we computed the model spectra in Fig. 2. The optical constants
were measured by Quirico
et al.
7
Although HCN, CH
3
OH, and H
2
CO are
also found in both interstellar and cometary ices, the optical constants for
them are not available. In Fig. 2, CH
4
is assumed to be diluted in the
solid molecular nitrogen. It is found that C
2
H
2
,C
2
H
4
,andC
2
H
6
would
produce the 3.1
µ
m absorption if their relative concentration to CH
4
is
approximately 10 wt%. This value should be regarded as the upper limit
for C
3
H
8
because the absorption at 3.0
µ
m is not seen in the observed
spectrum. Figure 2 indicates that C
2
H
4
and C
2
H
6
could be associated with
the absorptions around 3.2 and 3.35
µ
m.
Adding C
2
H
4
as the fourth component, we obtained a better agreement
with the observations than the simple ternary mixture model, but there
still remains discrepancy between 3.4 and 3.55
µ
m. On the other hand,
C
2
H
6
improves the fit in this range, although we see no clear absorption
around 3.65
µ
m in the observed spectrum. So we plotted two synthetic spec-
tra (ternary + C
2
H
6
and ternary + C
2
H
6
+C
2
H
2
) in Fig. 3 for the detailed
