Geoscience Reference
In-Depth Information
help elucidate the processes that control the composition and evolution of
terrestrial-type atmospheres. An important first step is to understand the
present processes that maintain the long-term stability of CO
2
on Venus.
The primary constituent of the Venus atmosphere is CO
2
,
96
.
5%.
1
CO
2
photodissociates at wavelengths
<
205 nm to form CO and O. In an
initially pure CO
2
atmosphere, O preferentially combines to form O
2
and
within a few thousand years, an initially pure CO
2
atmosphere evolves to
have
∼
7% CO and 3
.
5% O
2
.
2
,
3
The observed abundances for CO and O
2
on
Venus and Mars, however, are orders of magnitude smaller, which suggests
ecient catalytic cycle(s) aid in re-forming CO
2
. For Mars, these catalytic
cycles are believed to involve HO
x
radicals
2
(HO
x
=OH+HO
2
+H),
and models have been able to reproduce the observed CO and O
2
using
observational constraints for a number of years.
2
Comparable agreement
has yet to be achieved for Venus, but recent modeling
4
has shown reasonable
agreement with the existing upper limit on O
2
.
5
The leading candidates for gas-phase catalysts that can stabilize CO
2
in the Venus atmosphere are chlorine compounds,
6
,
7
and recent laboratory
work
4
has validated key assumptions made in models since the 1980s. Sig-
nificant uncertainties remain, however, in the rates for critical reactions in
the chlorine catalytic pathways, in the photolysis rates for loss of CO
2
,and
in the potential ecacy of alternative catalytic pathways.
8
,
9
In addition,
new observations of oxygen airglow suggest current models of Venus' oxy-
gen chemisty may be too simplistic. This manuscript compares the results
from gas-phase chlorine catalytic chemistry with two more speculative cat-
alytic mechanisms:
∼
CO+O
2
(c
1
Σ)
CO
2
+O(
1
D)
,
→
(1)
CO+O+aerosol
→
CO
2
+ aerosol
(2)
and then discusses what laboratory and observational work is needed to
improve our understanding of Venus' oxygen chemistry.
The detailed discussion of Reaction (1) from a forthcoming publication
9
is summarized here. The primary net production of O
2
in Venus atmo-
spheric models is via Reaction (3),
O
2
+
M,
2O +
M
→
(3)
where
M
is any third molecule or atom that can collisionally stabilize the
intermediate complex and O
2
is one of the many excited states of O
2
. Labo-
ratory and theoretical studies
10
,
11
suggest a large majority of the O
2
formed
in Reaction (3) is initially in a highly excited state
A
3
Σ
+
u
,A
3
∆
u
,c
1
Σ
u
,or
