A photochemical model for the atmosphere of Jupiter, including 1-D vertical eddy diffusive transport, was developed. It extends from the upper troposphere through the homopause. The hydrocarbon chemistry involves species containing up to four carbon atoms (and polyynes through C8H2). The calculations show that a large fraction of photochemical carbon may be contained in molecules with more than two carbon atoms. At the tropopause, C2H6 is the major photochemical species and C2H2, C3H8, and C4H10 are of comparable abundance and down from C2H6 by a factor of ten. These species may be detectable with the mass spectrometer of the Galileo Probe. The vertical distributions of the photochemical species are sensitive to the magnitude of eddy diffusive mixing in the troposphere and stratosphere and the details of the interface region

Allen, M.

Gladstone, G. R.

Yung, Y. L.

English

NASA Technical Reports Server

N87-17636
AN UPDATED HYDROCARBON PHOTOCHEMICAL MODEL FOR THE JOVIAN ATMOSPHERE
FROM THE TROPOSPHERE THROUGH THE HOMOPAUSE: A PRELUDE TO GALILEO
M. Allen
Jet Propulsion Laboratory, California Institute of Technology
G. R. Gladstone and Y. L. Yung
California Institute of Technology
A photochemical model for the atmosphere of Jupiter, including 1-D
vertical eddy diffusive transport, has been developed. It extends
from the upper troposphere (pressure <5 bar) through the homopause.
The hydrocarbon chemistry involves species containing up to four
carbon atoms (and polyynes through C8H2). The calculations show
that a large fraction of photochemical carbon may be contained in
molecules with more than two carbon atoms. At the tropopause (_ 164
mb), C2H 6 is the major photochemical species and C2H2, C3H8, and
C4H10 are of comparable abundance and down from C2H 6 by a factor of
ten. These species may be detectable with the mass spectrometer of
the GaliZeo Probe. The vertical distributions of the photochemical
species are sensitive to the magnitude of eddy diffusive mixing in
the troposphere and stratosphere and the details of the interface
region.
I presume that our paper was placed in this session on future spaceflight
opportunities because of the last four words of our title. It is not
necessarily inappropriate because I wish at the end to throw out a challenge
to the Galileo Probe to detect something predicted by our models to exist but
which has not yet been detected. You may have noticed that in his review
Darrell Strobel did not emphasize the hydrocarbon chemistry modeling,
essentially because nothing much has occurred in the past few years. Most of
the action has been centered on phosphorus. There has been a fair amount of
new observational information coming from Voyager, and with an eye towards
Galileo we've decided to reopen the analysis of hydrocarbon modeling of
Jupiter. Some of you may have heard me talk about some of the results from
our work at the DPS Meeting in Hawaii. l'd like to talk about other new
results this morning.
As for the basic details of the model, we now run the model from the 5 bar
level in the troposphere up through the homopause. The Voyager data are used
to define the temperature-pressure profile. We have expanded the model to
include more complex hydrocarbon species than you find in other published
hydrocarbon models for Jupiter. Much of this is a result of our work for
Titan where, because of the complex molecules that were seen, it was
necessary for us to make a major effort to expand the reaction set. Now we
are going back to see if the same basic reactions will help us understand
Jupiter. One hopes that the chemistry is the same, independent of where you
are in the solar system.
https://ntrs.nasa.gov/search.jsp?R=19870008203 2020-03-20T12:35:56+00:00Z
Since this is still a classic one-dimensional model, the transport is vertical
mixing through eddy diffusion. We employ a stratospheric profile for the eddy
dlffuslvlty that Randy Gladstone used in his thesis; it is similar to what
other people have used in the past. Then there is the question of how fast is
the mixing in the troposphere? Darrell Strobel mentioned values of the eddy
diffuslvity for the upper troposphere on the order of 104 cm2s -I based on work
with Marry Tomasko, but dynamical arguments suggest that at some level in the
troposphere the mixing may imply a coefficient as large as 108 cm2s -I. The
modeling that I will describe assumes a constant value of 104 cm2s -I through-
out the troposphere, but I'ii surmise about what would be the effects if you
had more rapid mixing. So the basic eddy diffusion profile we use has fast
mixing (107 cm2s -I) up high as inferred by the Voyager UVS observations,
decreases to a more stagnant layer (<103 cm2s -I) in the stratosphere, and then
jumps up to faster mixing in the troposphere. This kind of profile is similar
to what is used in Earth modeling, with a sharp discontinuity occurring at the
tropopause.
We use diurnally averaged radiation field calculations, which should be valid
for most species on Jupiter given their lifetimes. In addition to the basic
reactions involving C 1 and C2, we have a reaction set for a host of C3 and C4
compounds. Rather than describe the many pathways, I want to emphasize the
fact that you get these more complex species through 3-body recombination
between CI and C2 radicals. So combination of C2H 3 and C2H 5 produces C4H8,
and the C4HI0 comes from C2H 5. With recombination between CI and C2H2, one
gets allene and methyl-acetylene. There are tentative detections reported
for some of these species, but many are undetected. I should note that our
reaction set at the C4 level is not thoroughly complete, so that the results
we obtain for butane may be an upper limit. Another point that I should make
is that some of these reactions were previously discussed by Jack McConnell
at a DPS meeting some years ago.
Among the basic results of our model is the fact that we have allene, methyl-
acetylene, and ethene, which are species that have been reportedly detected
very recently. However, I particularly want to point out that in the lower
stratosphere, we find abundances of propane and butane that are almost equal
to the abundance of acetylene.
In fact, when you count up the number of carbon atoms, you find that there
may be as much "photochemical" carbon contained in these species as is con-
tained in acetylene. It is very interesting that none of these species have
been reported so far. I would like to suggest a reason for that and relate
it to the question of potential Galileo measurements.
The molecules with multiple bonds such as acetylene, methyl-acetylene, and
ethene have spectroscopic characteristics that are significantly different
from molecules that are fully saturated and hence have no multiple bonds
(e.g., ethane, propane, and butane). The saturated molecules tend to have
very smooth absorption that cuts off very sharply between 1600 and 1700 A,
whereas those with multiple bonds have a tendency to absorb at much longer
wavelengths. This situation leads to problems in trying to detect the more
saturated species because of the rapid decrease in solar illumination at
shorter wavelengths in the ultraviolet. It makes things very difficult for
225
the IUE. The lack of structure also makes the identification of the signa-
ture for a particular species problematic. At the sametime, the molecules
with multiple bonds have fairly intense bands in the infrared, while those of
the saturated molecules are relatively weak (down by a factor of I0 to I00).
In fact, the only reason that one might see ethane is that it is so abundant
relative to acetylene. Whenyou start looking for saturated species which
have abundanceson the order of that for acetylene, then you will have
trouble becauseyou are fighting the spectroscopy.
NowI would like to pose the challenge for the mass spectrometer on the
Galileo Probe, namely, the detection of one or more of these trace species.
Because of the predicted decrease in mixing ratio towards the troposphere, a
crucial issue in addition to its sensitivity will be the altitude at which
the massspectrometer begins to obtain data. It is my understanding that the
mass spectrometer turns on at the I00 mb level and has a sensitivity of one
part in 109. Becausewe have added a rapidly mixed troposphere instead of
cutting off our model at the tropopause as most previously published models,
the vertical mixing propagates up into the stratosphere. Even though the
species in the lower stratosphere tend to be long-lived, there is sort of a
"vacuum cleaner" effect on the troposphere, leading to a decrease in the
mixing ratio profile starting somewhatabove the tropopause and extending
down into the troposphere. This is in contrast to the kinds of modeling that
are done for remote sensing analyses in which constant vertical mixing ratios
are assumedthroughout the stratosphere. If the mass spectrometer has
sufficient sensitivity starting at I00 mb, one might even see the decrease in
the profile above the tropopause predicted by our more realistic modeling of
the transport in the one-dimensional limit. If you increase the mixing in
the troposphere by taking an eddy diffuslvlty of 108 cm2s-I instead of 104
cm2s-I, then the tropospheric profiles will drop even further, and there will
be even sharper cut-offs in the stratosphere. In that case, we maymiss
completely any potential window for detection of the trace species by the
massspectrometer. Whether or not we see these trace species maytherefore
give us someinformation about the transport in the atmosphere.
DR. LUNINE: Mark, a lot of the photochemistry that you've talked about is
also applicable to the other outer planets and also to Titan. There are some
uncertainties I know, that have plagued these models for a while. Which of
these uncertainties will be at least constrained, or possibly eliminated by
the ability to measure vertical profiles through one of these atmospheres?
Or will all the uncertainties remain buried in vertical transport?
DR. ALLEN: I think that, given our ability to do remote sensing, and the
observations that are just coming out now based on someVoyager and IUE data,
there is a possibility that we can attempt to confirm our reaction sequence to
someextent in the upper stratosphere from remote sensing. Then hopefully, if
we gain moreconfidence from looking at species in the stratosphere, tying
that in with our understanding of Titan and the visibility of propane on
Titan, maybein the end we can have somehope of then saying that the ability
to detect species depends on the sensitivity threshold. So you see, the mass
spectrometer mayactually succeed and we can then back out somestatement
about transport and mixing on Jupiter.
226
DR. NIEMANN: I supposeyou would like to know what the probe massspectro-
meter can do to verify your predictions. This is quite a challenge to the
mass spectrometer. The present mission design does not cover the altitude or
pressure range where you predict the relatively high and measurable hydro-
carbon concentrations. However, we have a chemical enrichment cell in the
instrument, and our laboratory tests have shownthat we may get a factor of
200 or more enrichment depending on the species. This will be one sample
averaged over an altitude of several kilometers. Massspectrometers have a
problem, particularly when they are operated in hydrogen, of generating their
own hydrocarbons up to the C3 level, which effectively lowers the detection
threshold for these species. So in the direct measurementmode, without the
enrichment cells, we will be below the detection threshold of the instrument,
but with the enrichment cells we have a chance to see someof the species.
However, we will not have any altitude resolution.
DR. ORTON:The Stony Brook group and I are reporting a detection of propane
over the infrared bright region in the northern hemisphere in the Voyager
IRIS data.
DR. ALLEN: I was in fact talking with Ken and Richard Wagenerabout those
results and interestingly enough, the fact that you see these species in the
North Polar region maynot be a function of unique chemistry there, but may
be an observability factor. Whenyou are looking from Ken's point of view,
the contribution level of the atmosphere that you're seeing near the pole is
muchhigher than it is whenyou're looking towards the equatorial regions.
If you remember, our profiles show a maximumin the stratosphere rather than
a constant mixing ratio. That meansthat if you're looking up at <I0 mb,
you have sensitivity for these various species that I'm talking about. When
you're looking more at the 20 mblevel corresponding to observations at lower
latitudes, you have less sensitivity. So it may turn out that in fact there
is nothing funny in the chemistry at the poles that makespropane more abun-
dant; it may be which level in the atmosphereyou're observing.
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An updated hydrocarbon photochemical model for the Jovian atmosphere from the troposphere through the homopause: A prelude to Galileo

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