of 9
Methanol
Formation
in Hyperthermal
Oxygen
Collisions
with
Methane
Clathrate
Ice
Robert
W. Grayson,
Konstantinos
P. Giapis,
and William
A. Goddard,
III
*
Cite This:
J. Phys.
Chem.
A
2024,
128,
10250−10258
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ABSTRACT:
The
presence
of small
organic
molecules
at airless
icy
bodies
may
be significant
for
prebiotic
chemistry,
yet
uncertainties
remain
about
their
origin.
Here,
we consider
the
role
of hyperthermal
reactive
ions
in modifying
the
organic
inventory
of ice. We employ
molecular
dynamics
using
the ReaxFF
formalism
to simulate
bombardment
of carbon-bearing
ice by
hyperthermal
water
group
molecules
(H
x
O,
x
= 0
2)
with
kinetic
energy
between
2 and
58 eV.
Methanol
is the dominant
closed-
shell
organic
product
for a CH
4
clathrate
irradiated
at low
dose
by
atomic
oxygen.
It is produced
at yields
as high
as 10%,
primarily
by
a novel
hot-atom
reaction
mechanism,
while
radiolysis
makes
a
secondary
contribution.
At high
irradiation
doses
(
1.4
×
10
15
cm
2
), the composition
is driven
toward
greater
carbon
oxidation
states
with
formaldehyde
being
favored
over
methanol
production.
Other
water
group
impactors
are less
efficient
at inducing
chemistry
in the ice, and
alternate
clathrate
guest
species
(CO,
CO
2
) are very
robust
against
hydrogenation.
INTRODUCTION
The
presence
and
origin
of organics
at airless
icy bodies
is a
topic
of critical
importance
in planetary
science.
Methanol
in
particular
is an important
precursor
to complex
organics
with
prebiotic
significance
(e.g.,
amino
acids),
1,2
found
commonly
in cometary
comae
(typical
abundance
of 2%
relative
to
water)
3
and
detected
on
Enceladus’
surface
and
in its
plume.
4
6
In the cometary
context,
methanol
is presumed
to
be formed
primarily
prior
to accretion,
through
successive
hydrogen
addition
reactions.
7
For example,
in dense
molecular
clouds
and,
to a lesser
extent,
in the
protoplanetary
disk,
7
methanol
can be synthesized
via exposure
of pure
CO
or mixed
CO
H
2
O ice grains
to cold
atomic
hydrogen.
8
12
Comets
are
widely
regarded
as relatively
pristine
environments,
believed
to
closely
resemble
their
initial
compositions,
and
are used
to
infer
conditions
in the solar
system’s
distant
past.
13
Similarly,
the
composition
of Enceladus’
plume
can
indicate
the
composition
of its interior
ocean
and
thus
its habitability.
14
In both
cases,
it is vital
to understand
how
the composition
is
modified
by the radiation
environment
before
the moment
of
observation.
Low-energy
reactive
ions
(<1
keV,
e.g.,
H
x
O
+
, x = 0
3)
are
abundant
in these
same
environments,
15,16
where
they
bombard
ice surfaces
to largely
unknown
effect.
At Comet
67P
for instance,
the
Rosetta
mission
discovered
that
water
group
ions
are accelerated
by interaction
with
the solar
wind
to
a mean
energy
of several
hundred
eV and
impinge
on the
nucleus’
surface
at fluxes
comparable
to or even
exceeding
the
solar
wind
flux.
15,17
Since
cometary
ices
contain
volatile
carbon-bearing
species,
including
CO
2
(4
30%,
relative
to
water),
CO
(0.4
30%),
and
CH
4
(0.4
1.6%),
13
nonthermal
chemistry
can
enable
further
methanol
production.
Indeed,
methanol
synthesis
can
continue
after
accretion
due
to
radiolytic
processing
of CO
H
2
O ice grains
by cosmic
rays
18
or through
photolysis
of CH
4
H
2
O ices
by UV
photons.
19
The
contribution
of low-energy
reactive
ions,
however,
has not
been
explored.
Research
on ion
irradiation
of ices
is generally
limited
to
higher
kinetic
energies
(>1
keV),
where
radiolysis
is the
primary
driver
of chemistry
(see
ref
20).
Ions
in the
hyperthermal
energy
regime
(10
300
eV)
are
relatively
radiolytically
inert
because
most
recoils
in the collision
cascade
fail to transmit
enough
energy
for bond
dissociation.
20
When
the impinging
ions
are themselves
reactive,
however,
the excess
kinetic
energy
can
drive
chemistry
via alternative
mechanisms
such
as Eley
Rideal
and/or
hot-atom
reactions.
21,22
Unfortu-
nately,
the
literature
is very
sparse
on how
reactive
hyper-
thermal
impactors
modify
the organic
inventory
of ices,
in part
due
to serious
experimental
obstacles.
Ennis
et al. (2011)
irradiated
methane
ice with
5 keV
O
+
and
detected
methanol
Received:
September
8, 2024
Revised:
October
29, 2024
Accepted:
October
31, 2024
Published:
November
18,
2024
Article
pubs.acs.org/JPCA
© 2024
The Authors.
Published
by
American
Chemical
Society
10250
https://doi.org/10.1021/acs.jpca.4c06078
J. Phys.
Chem.
A
2024,
128,
10250
10258
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