E. Implications for the Atmosphere of Jupiter
The deuterium to hydrogen (D/H) ratio provides an important constraint to the chemical
and physical processes in the atmospheres of planets (Owen 2003). For example, the D/H
ratio of water (HDO and H2O) in the upper atmosphere of Mars can be satisfactorily
explained by photolytic (Cheng et al. 1999; Miller and Yung 2000) and
condensation/evaporation processes (Bertaux and Montmessin 2001). The D/H ratios for
HD/H2 (Encrenaz et al. 1996; Mahaffy et al. 1998; Lellouch et al. 2001) and CH3D/CH4
(Encrenaz et al. 1999; Lellouch et al. 2001) in the atmosphere of Jupiter are ~2.310-5.
The isotopic composition other than these two species in Jupiter has not been obtained. A
theoretical study conducted by Lee et al. (2001) shows that C2H6, which is the most
abundant hydrocarbon compound produced photochemically via the photolysis of CH4 in
the upper atmosphere of Jupiter, is enriched in D. The D/H ratio in C2H5D/C2H6 is about
15 times that of H2 or CH4. In the lower atmosphere, the photolytically initiated isotopic
fractionation processes for hydrocarbon chemistry that operate in the upper atmosphere
are severely curtailed by the CH4 and C2H6 shielding at wavelengths <160 nm. The
photolytic processes of NH3 in the lower stratosphere/upper troposphere, driven by
photons with wavelengths <230 nm, are the most significant disequilibrium processes. To
evaluate the process on the D/H ratios for NH3 photolytic products, we perform a
one-dimensional kinetics simulation incorporating the newly measured photoabsorption
cross sections of NH3 and NH2D for this region of the atmosphere of Jupiter.
The one-dimensional Caltech/JPL KINETICS model is used in our study. A detailed
description of the model has been given previously (e.g., Gladstone et al. 1996). Figure
xx1 shows the vertical profile of NH3 adopted in this study (solid line). It is based on data
from Edgington et al. (1999), as shown by crosses in Figure xx1. For comparison, we also
include results for CH4 and C2H6 from Gladstone et al. (1996) and Lee et al. (2001).
Figure xx2 shows the vertical profiles of the photoabsorption rates of CH4, C2H6, and
NH3. As stated earlier, the photoabsorption of CH4 is significant in the region above ~0.1
mbar pressure level, and NH3 molecules become the dominant UV absorbers below ~200
mbar. The photolytic products of NH3 such as NH and NH2 could react with C2H6 in the
lower stratosphere/upper troposphere, and thereby modify the D/H ratio in ethane that is
initially produced in the upper atmosphere of Jupiter. The fractionation of
photoabsorption coefficients (J-value) for ammonia is presented in Figure xx3. It is
shown that the photolysis of ammonia will cause the photolytic products of ammonia to
be isotopically depleted compared with their parent molecules, ammonia. Therefore, the
predicted enhanced D/H ratio in enhance from CH4-mediated processes alone (Lee et al.
2001) should be revisited.
In this work, we find that the maximum isotopic fractionation of ammonia due to
photolysis is ~-250 per mil (-250 parts in thousand) at ~300 mbar. However, it is
generally believed that ammonia could condense to form clouds in the troposphere (e.g.,
West et al. 1986). The condensation and evaporation of ammonia would be analogous to
that of water which produces water to be isotopically enriched (Bertaux and Montmessin
2001). Our work provides the magnitude of the isotopic effect of ammonia due to
photolysis. Future observations and modeling of the D/H ratios in ammonia are needed to
advance our understanding of the chemistry and the microphysics (and dynamics)
involving ammonia in the troposphere of Jupiter.
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Figure XX1: Vertical profile of NH3 (solid line) adopted in the model. Crosses are
measurements by Edgington et al. (1999).
Figure XX2: Vertical profiles of photoabsorption rates for NH3 (upper panel), CH4 (lower
panel; solid line), and C2H6 (lower panel; dashed line).
Figure XX3: Fractionation in J-value for ammonia (upper panel), methane (lower panel;
solid line), and ethane (lower panel; dashed line). J-value is defined by the product of the
species’ cross section and solar
1000[J(deuterated)/J(normal) – 1].
spectrum,
and
J
(per
mil)
=