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Adding C
2
H
2
as the fifth component in the model calculation makes the
spectrum match the observed spectrum quite well, as shown in Fig. 3.
4. Discussion
In the the protoplanetay disk and the interstellar cloud that proceeded
it, the gas-phase ion-molecule reactions could yield C
2
H
2
. The reaction
of H with C
2
H
2
on cold dust grains could produce C
2
H
6
more effectively
than C
2
H
4
.
9
In fact, C
2
H
2
and C
2
H
6
have been found in the comas of
Oort-cloud comets,
10
,
11
but the detection of C
2
H
4
has not been reported
so far. The C
2
H
6
/CH
4
ratio estimated from Fig. 3 is consistent not only
with the Oort-cloud comets but also with a value of a short-period comet
21P/Giacobini-Zinner
12
and the upper limit for interstellar materials.
13
Moreover, the C
2
H
2
/CH
4
ratio approximately agrees with the values for
Oort-cloud comets.
10
,
14
Non-methane hydrocarbons could be the secondary products generated
from methane. Krasnopolsky and Cruikshank
15
simulated photochemical
reactions in Pluto's tenuous atmosphere and the subsequent precipitation
to the surface. C
2
H
2
and C
4
H
2
have the highest precipitation rates among
numerous photochemical products, but C
2
H
2
cannot produce the observed
3.2 and 3.35
µ
m features and we do not have the optical constants for solid
C
4
H
2
. The precipitation rates of C
2
H
4
and C
2
H
6
are smaller than those
of C
2
H
2
and C
4
H
2
by an order of magnitude. If the 3.2 and 3.35
µ
mfea-
tures are indication of C
2
H
4
and/or C
2
H
6
produced by the photochemical
reactions, we must increase their relative concentrations to C
2
H
2
and C
4
H
2
somehow. Moore and Hudson
16
conducted the systematic infrared study of
proton-irradiated N
2
-rich ices containing CH
4
and CO. They found C
2
H
2
,
C
2
H
4
,C
2
H
6
,andC
3
H
8
along with HCN and HNC in the irradiated analog
materials of Pluto's surface. It should be emphasized that they identified
C
2
H
6
with the absorption at 3.35
µ
m but the expected absorption around
3.65
µ
m was not detected as our observation. In addition, HCN is expected
to decrease the gap between the models and observation around 3.1
µ
m.
Therefore, we do not rule out possibilities that observed features were asso-
ciated with non-methane hydrocarbons produced by the
in situ
surface
reactions induced by cosmic-ray irradiation to the original ternary mixture
of N
2
,CH
4
, and CO. In either case, thin deposits are vulnerable to microme-
teorite impact. The relative mass ratio to the parent methane, derived from
our observations, could be a key to understanding the gardening process on
Pluto, such as poorly known dust flux
17
and vertical mixing timescale.
15