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Composition of the Venus atmosphere below 100 km altitude
Adv. Space Res. Vol.5, No.11, pp.173—195, 1985 Printed in Great Britain, All rights reserved.
0273-1177/85 $0.00 + .50 Copyright ©COSPAR
CHAPTER V COMPOSITION OF THE VENUS ATMOSPHERE BELOW 100 km ALTITUDE U. von Zahn’ and V. I. Moroz2 Physikalisches Institut, Universität Bonn, Nussallee 12, 5300 Bonn 1, F.R.G. 2 Space Research Institute, U.S.S.R. Academy of Sciences, Moscow 117810, U.S.S.R. ABSTRACT
Our current knowledge on the composition of the Venus atmosphere in the altitude range from the surface to 100 km is compiled. Gases
that have been measured, and whose mixing ratios are
assumed to be constant with altitude, are CO 2, N2, He, Ne, Ar, and Kr.
Gases that have been identified
in the lower and/or
middle atmosphere, but whose mixing ratios may depend on altitude,
latitude and/or local time, are CO, H2O, HC1, HF, and
SO2.
Conflicting data or only upper
limits exist on some
important trace gases, such as 021 H2, and Cl2.
The latter two
are key constituents in the photochemistry of the middle atmosphere of Venus.
The chapter concludes with a listing of the
isotopic abundances of elements measured in the Venus atmosphere.
1.
Scope of Chapter
This chapter summarizes the available measurements of the composition of the Venus
lower and middle atmosphere.
latter are defined as the regions of the Venus
The
atmosphere lying
between the surface and 100 km altitude (corresponding to ambient pressures
between appr. 95 bar and 0.03 mbar).
The
chapter deals with only the gaseous constituents of the Venus atmosphere.
Hence, it will not dwell on the particulate matter
(haze, cloud, or aerosol particles)
contained
in the Venusian
atmosphere, as this is the subject of a separate chapter of
173
174
U. von Zahn and V. I. Moroz
this book.
Where appropriate, the chapter will briefly
reference current theoretical models of the chemistry of the Venus atmosphere.
2.
Presentation
of Data
In past decades a large body of information on the chemical and isotopic composition of the Venus atmosphere
has been acquired.
It is now known that more than 99.9% of the Venus atmosphere made up of only CO
2 and N2.
Thus,
the sum of all trace gases
accounts for less than 0.1% of the total atmosphere. less,
Neverthe-
it is just this group of gases which attracts most of our
attention, 021
is
H2O,
as it includes many interesting species such as H2,
CO, SO2,
The number of
H2S, COS, HC1, HF, and all the noble gases.
individual measurements pertaining
atmospheric composition has become quite numerous measurements of the water atmosphere have been published. is so extensive now that
large.
to the Venus For example,
vapor content of this
The literature on this subject
it would be worth
a review in itself.
For a few trace gases, the abundance data, which were collected by quite different measurement satisfying manner.
techniques, agree in a very
This is the case,
for example,
for CO,
the
abundance of which has been determined by remote sensing techniques
in the near—IR and microwave region, and in—situ
measurements using gas chromatographs. however,
the situation
For other trace gases,
is much less appealing.
Different
measurements of the same species may be in conflict with each other
(e.g., for Kr or COS).
In other cases the measured
abundances of chemically active gases cannot be reconciled with chemical or thermodynamical equilibrium conditions O2—CO-S02 or NH3—H2O-HC1
(e.g.,
for
in the lower atmosphere).
t atmosphere Comprehensive of the We composition ofparticular, Venus have been givenreviews elsewhere. mention, in articles by Moroz
(l981b) and by von Zahn et al.
(1983) which were
written after the deluge of new data obtained in 1978/79 had been analyzed and published. treated by Marov
In the l970s the subject was
(1972) and Young
(1974).
With those and
additional reviews of the Venus atmosphere being readily available,
it cannot be the intention of this chapter to
attempt giving again something like a complete treatment of the subject matter. Instead, we aim at providing a concise
Composition below 100km Altitude
175
tabulation of those results which we still consider significant. With our intention of conciseness and easy usage of the table entries, we give for some of the gases only a single abundance value which
is to be understood as a weighted mean of the
original measurements.
For further details the reader is
referred to the literature cited in the tables.
We presume that CO
2, N2, and the noble gases each have a
constant mixing ratio throughout the altitude region under consideration.
On the other hand, we expect that the mixing
ratios of virtually all other gases depend on altitude and possibly local time and latitude.
For only a few gases,
like
H2O, CO, and SO2, such variations have been clearly established by measurements.
To link and evaluate data taken at different
locations and times, one needs geochemical and photochemical models of the entire Venus
atmosphere or parts thereof.
ly the following model studies have been published: Kreimendahl
(1980), Barsukov et al.
Lewis and
(1980), Khodakovsky
Yung and DeMore (1982), and Krasnopolsky and Parshev
Recent-
(1982),
(1983).
One more type of natural variation in mixing ratios needs special attention:
possible mid—term changes in the atmospheric
trace gas budget caused by episodic volcanic activity. (1984) has argued that a significant
Esposito
decrease of the SO2 mixing
ratio at the cloud tops had occurred during the time period 1978 through 1983.
He suggests this effect to be caused by a compar-
ative lull in the activity of volcanos at the surface of Venus. On the other hand,
the possibility of a change of the observed
SO2 band intensity without an actual change of the SO2 mixing ratio
(through cloud structure variability for example) still
cannot be ruled out.
The aforementioned observations
indicate that at times measured
data on mixing ratios might turn out to lie above previously established upper
limits and yet both can be correct at the
epoch of their measurement.
Hence, even if such a seemingly
conflicting situation exists, we usually list both values in Table
5—1, which
follows.
In Table 5—i we have collected both measured mixing ratios and column abundances, as well as upper limits for the abundances of the gases
in the Venus
atmosphere below 100 km altitude.
The species are grouped loosely according
to chemical families,
176
0. von Zahn and V. I. Moroz
although we readily admit these data.
that there
Footnotes assembled at the end of the table add
some brief details.
In those cases where
comment is deemed appropriate chapter. Table
is no ideal way to arrange
a more extensive
it can be found in the next
Information on isotopic abundances
is assembled
in
5—2.
3.
Comments on Table 5—1
3.1
Carbon Dioxide and Nitrogen
As mentioned before, CO
2 and N2 make up more than 99.9% of the
entire Venus atmosphere.
Von Zahn et al.
(1983) review details
of the many nitrogen measurements performed in the course of the Venera and Pioneer Venus missions.
From the mixing ratios given
in Table 5—1, one calculates the mean molecular mass of the Venus atmosphere
below 95 km altitude to be:
rn 3.2
=
43.44
±
0.15 kg/kmole.
Helium
Helium has been measured extensively
in the altitude range 130
km to about 600 km (Kumar and Broadfoot, 1980;
Niemann et al., 1980;
et al., 1984).
1975;
von Zahn et al.,
Burgin and Sholokhov,
In the upper atmosphere
1981;
its mixing ratio
Bertaux is,
however, strongly enhanced with respect to the lower atmosphere due to diffusive separation.
Hence, an extrapolation of the
upper atmosphere data into the well—mixed lower atmosphere requires one to make assumptions on the altitude profiles of eddy mixing and temperature.
Such extrapolations have been
done independently by von Zahn et al. (1983).
(1980) and Hedin et al.
unfortunately, their results turn out quite
differently
(see Table
5—1).
The mass spectrometer aboard the Pioneer Venus obtained data on helium in the bulk atmosphere
Large Probe of Venus. The
data suggest an extraordinarily high mixing ratio Pollack,
1983), which may be biased, however,
(Donahue and
by contamination
from helium used for engineering reasons within the Large Probe. 3.3
Neon and Argon
Both of these noble gases have been measured by the mass
Composition below 100km Altitude spectrometers,
gas chromatographs,
spectrometers Pioneer
aboard
Venus Large
the Venera Probe,
Measured
argon
mixing
(Gelman
et al.,
1979)
combining
25) ppm.
We have used this
to normalize
spectrometers
and by X-ray
11,
12,
the results
the
13,
by a total
fall
into
median
sensitivities
et
fluorescence
14
landers
of
12
in the range
to 200 ppm (Surkov
recommend
ratio
hence
ratios
177
instruments
from 40 ppm
al.,
1982).
a mean value
value of
for
arid the
of
We
(70
±
the Ar mixing
the various
mass
and gas chromatographs used for argon and neon
measurements.
After this normalization, measured neon mixing
ratios fall in the range from 4.3 ppm (Oyama et al., 1980) 10 ppm (Hoffman, Hodges, et al., weighted mean of
3.4
(7
±
1980).
to
We recommend a
3) ppm.
Krypton
One measurement of the mixing ratio of Kr yielded a value the upper
in
l0~ range, two more gave values in the middle
io8 range.
We expect, however,
constant in space and time,
the Kr mixing ratio to be
at least in the lower atmosphere.
Hence,
the two data sets are incompatible. We note here, that t measurement by Istomin et al. (1980) has an additional ‘high’ been withdrawn (V.G. Istomin, private communication).
3.5
Molecular Oxygen
Ground state 02 has been undetectable so far near and above the cloud tops.
The upper
limit for the mixing ratio of 02
in this altitude region has been pushed down to 3 x
by
Trauger and Lunine (1983).
In the Venus airglow, however, two different emissions excited oxygen molecules have been observed which nate from the middle atmosphere.
from
in part origi-
The visible nightglow spectrum
was first measured by the Venera 9 and 10 orbiters (Krasnopolsky et al., 1976) and its main component identified as the 02 Herzberg II band system (Lawrence et al., 1977). (1979) reported earth—based observations of at 1.27 pm from
o
(~ 2
Connes et al.
intense emissions
).
g
Although these emissions
indicate clearly that 02 is produced
above approximately 75 km by photochemical reactions, nevertheless impossible
it is
to ascertain the steady state abundance
178
U. von Zahn and V. I. Moroz
of 02 from these sink
strength
be done by still
measurements
for 02
invoking
somewhat
in this
without altitude
specific
Large
Venera 14
regime.
photochemical
also
the
This can only
models
which are
speculative.
Two gas chromatographic measurements, Venus
specifying
Probe
(Oyama et
(and 13?)
Lander
one aboard the Pioneer
al., 1980) (Mukhin et
and the other aboard the al.,
1983), have yielded
positive identifications of molecular oxygen within and below the main cloud deck of Venus
(see Table
5—1).
amounts of 02 which have been claimed to exist
The substantial in the lower
atmosphere of Venus, on the other hand, are very difficult to reconcile with both the upper limit of 3 x l0’~ for the mixing ratio near the cloud top
(Trauger and Lunine, 1983), and
with the simultaneous observation of significant amounts of CO, SO2~ and H
2.
On the latter question, Oyama et al.,
(1980) comment as follows:
ttThe
coexistence of 02 and CO in
the lower atmosphere would seem to involve a distinctly non— equilibrium process.
Only by postulating an elemental
S/0 ratio
of about 3 instead of 2 were we able to generate nontrivial amounts of 02
in our thermodynamic
calculations.
Several tens
of parts per million of 02 near the surface were obtained only with models that also had SO3 as the dominant sulfur—bearing species in this portion of the atmosphere and had a mixing ratio of about
1000 ppm.’
The idea of saving the situation by allowing for the presence of high abundances of SO3 in the lowest part of the Venus atmosphere has been studied further by Craig et al.
(1983).
This study, however, presumes CO to be inert, which in our opinion is questionable in the light of the high reactivity of SO3 towards CO.
We conclude that currently available measurements of CO, 02, H2,
SO2,
and H2O are not compatible with conditions of thermo-
dynamic equilibrium among these constituents in the lower atmosphere.
3.6
Molecular Hydrogen
The one existing positive identification of H2 (Mukhin et al., 1983) yields a comparatively high mixing ratio, which implies a fairly strong reducing state of the Venus atmosphere, within the clouds.
at least
Photochemical models of the middle atmo—
Composition below 100km Altitude
179
sphere, such as the one of Yung and DeMore (1982), can reconcile most available observational data on the middle atmosphere with the presence of 20 ppm of H
2, except for the observed diurnal
variation of CO near 80 to 90 km altitude (Schloerb et al., 1980; Wilson and Klein,
1981).
Much more controversial
is the situation for the lower atmo-
sphere, where strong non-equilibrium conditions would exist
if
the measured mixing ratios for H2, 021 CO, and H2O are all correct.
For example,
for 20 ppm of CO and 20 ppm of H20 the
water—gas reaction
CO
+
H2O
CO2
++
112
+
yields an equilibrium value for 112 of 0.002 ppm (Kumar et al., 1981) near the surface of Venus.
For the large amounts of
H2 which are claimed to exist within the clouds, no viable source has been offered so far to balance the strong known sinks
3.7
in the middle and upper parts of the atmosphere.
Water
A large body of observations and upper water abundance
limits exists for the
in the Venus atmosphere.
In fact,
large to give a full account here or in Table 5—1.
it is too For a
recent review of the subject we refer the reader to von Zahn et al.
(1983).
For the regions above the 90 mbar level (about 65 km altitude) we wish to point out in particular the local time and latitude variations of the 1120 abundance Schofield et al.
(1982).
(Figure 5—1)
as determined by
They analyzed the radiances measured
by the Pioneer Venus Orbiter Infrared Radiometer in the 45
pm
channel, which is centered in the water vapor rotation band, and the 11.5 pm channel,
which
opacity.
clearly localizes a wet region at
The experiment
is chiefly affected by cloud
equatorial latitudes in the mid-afternoon sector.
But mixing
ratios throughout most of the nightside are below the detection limit of 6 ppm for the experiment. A latitude—dependence of the 1120 mixing ratio below the clouds has been inferred by Revercomb et al.
(1985)
from
thermal IR net flux measurements aboard the four entry probes of the NASA Pioneer Venus mission.
180
U. von Zahn and V. I. Moroz
There
are
a number
of contradictions
the results
of
different
experiments
concerning
profiles
the 1120 mixing ratio were obtained from the anal-
of
H
in
2O mixing
ratios
below 50 km.
Several
ysis of spectra of solar scattered radiation obtained on board Veneras 11,
12, 13,
and 14
(Moroz, Parfentev, et al., 1979;
Moroz, Moshkin, et al., 1983).
These data indicate a maximum
of the water mixing ratio near the cloud bottom (~200 ppm) and a fall—off towards the surface where the mixing ratio approaches 20 ppm (Figure 5—2).
But at least two recent experiments
et al., 1980; Surkov et al., 1983)
gave mixing ratios above
1000 ppm at altitudes between 22 and 50 km. dances are, however,
(Oyama
Such large abun-
incompatible with measured temperature
profiles and surface temperatures of Venus
(Pollack et al.,
1980) and are fully inconsistent with the optical data.
3.8
Sulfur Dioxide
Because SO2
is the only sulfur—bearing gas discovered so far
in the middle atmosphere of Venus, its absolute abundance
and
scale height place very stringent constraints for photochemical and dynamical models of this region of the Venus atmosphere. Further, since
its discovery
in 1978,
(Barker,
1979) UV bands
of SO2 have been nearly continuously monitored by the Pioneer Venus Ultraviolet Spectrometer experiment. time period Esposito
Results for the
from late 1978 until early 1983 were presented by
(1984)
(Figure 5—3), and interpreted as indicating a
large decrease of the SO2 mixing ratio at the 40 mbar level (about 69 km altitude) as an explanation
over this time period.
for the observed
Esposito offers
temporal variability of SO2
the episodic injection of SO2 into the Venus atmosphere by volcanic activity.
3.9
Ammonia
An experiment for the determination of the ammonia mixing
ratio
in the 44 to 32 km altitude region was carried out on board the Venera 8 lander. Surkov et al.
From the color change of “bromophenol blue”
(1974) estimated the ammonia mixiny ratio to be
between 10~ and 1O~. with the upper mm
This value
is, however,
incompatible
limit of 16 ppm established by Smirnova and Kuz-
(1974) trom the absence of an absorption feature at 1.25 cm
in the radio emission spectrum of Venus.
Composition below 100km Altitude Goettel and Lewis NH
3
in the Venus
181
(1974) examined the equilibrium chemistry of atmosphere and concluded that the high NH3
mixing ratios reported by Surkov et al.
(1974)
are inconsistent
with the observation of rather low abundances of other gases Venus’ al.
atmosphere,
in particular 1120 and HC1.
in
Florensky et
(1978), on the other hand, argued that the upper part of
the Venus
troposphere can hardly be assumed to be in chemical
equilibrium and that the tropospheric distribution of minor constituents can be strongly nonuniform, thus leaving open the possibility of a high NH3 abundance in the lower cloud layer.
4.
Comments on Table 5-2
4.1
Deuterium
McElroy et al.
(1982)
suggested that the mass 2 ion observed by
the Pioneer Venus Orbiter Ion Mass Spectrometer et al.,
1980) is in fact D~ rather than
pretation implies a D/H ratio of
Subsequently Donahue et al. (1.6
±
0.2)
4.
(H. A. Taylor
This inter-
~io_2 in the bulk atmosphere.
(1982) determined a D/H ratio of
x io2 from the water—related peaks in the
spectra of the Pioneer Venus Large Probe Mass Spectrometer. Full details of the latter analysis have, however, not yet been published.
A detailed analysis of the height variation and local time dependence of the light ions observed by the Pioneer Venus Orbiter Ion Spectrometer led Hartle and Taylor (1983) to the confirmation of the earlier suggestion that the mass 2 ion observed the Venus
ionosphere
is predominantly D+.
From their
iono-
spheric measurements they interred a D/H ratio, extrapolated into the lower atmosphere, of
(2.2
±
0.6) x l0_2.
in
U. von Zahn and V. I. Moroz
182
Table 5—1.
Gas
CO
2
Measurea Volume Mixing Ratios and upper Limits of Gases*
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
<100
0.965 ±0.008
see text von Zahn et al.
(1983)
N2
<100
0.035 ±0.008
see text von Zahn et al.
(1983)
He
<100 <100
5 l.2xl0 6x107 4.5xl04
(a),+ (a),+ +
von Zahn et al. (1980) Hedin et al. (1983) Donahue and Pollack
(1983) Ne
<100
7xl06
see text von Zahn et al.
(1983)
Ar
<100
7x105
see text von Zahn et al.
(1983)
Kr
l.a.** 49 to 37 26 to 0
5xl08 7xl07 2xl08
(b),+
Donahue et al. (1981) Mukhin et al. (1983) Istomin et al. (1983)
(b)
Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980) Hoffman et al. (l980b)
+ +
upper limits: 52 42 22 <24 Xe
<4xl05
upper limits only: <26 <20 <20
*footnotes
<7xl09
at end of table)
(c)
Istomin et al. (1983) Donahue et al. (1981) Donahue and Pollack (1983)
Composition below 100km Altitude
Table
Gas
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
CO
100
Mixing Ratio
c.t.* c.t. 52 42 to 36 12 42 22 upper
(d)
Schloerb et al. (1980) Moroz (1963; 1964) Connes et al. (1968) Oyama et al. (1980) Gelman et al. (1979) Gelman et al. (1979) Oyama et al. (1980) Oyama et al. (1980)
4.5xl05 3xl05 3xl05 2xl05 3x105 2xlO—5
Wilson and Klein
4.4xl05 l.6xl05 l.8xl05
(1981) (1981)
see text see text see text
(1981)
Oyama et al. (1980) Oyama et al. (1980) Mukhin et al. (1983)
limits:
c.t.
<57
c.t. c.t. c.t.
<8xl05 <2xl05 <1xl06
c.t.
<3xl07
60 to 0 52
<5xl05 <3xl05
46 42 to 0 <24
< l0~ <2xl05 <3x105
*footnotes
Wilson and Klein Wilson and Klein
limit:
52 42 58 to 35 upper
Reference
(d)
1.5
75
02
Column Remark/ Abundance Footnote (cm atm)
3.5 to4 l4x10 2x104 <0.4 to llxlO5
90 75
183
at end of table
<1.4 <6
(e) (f)
Spinrad and Richardson (1965) Belton and Hunten (1968) Belton and Hunten (1968) Traub and Carleton (1974) Trauger and Lunine
(1983) Moroz (1981) Hoffman, Oyama, et al. 1980) Vinogradov et al. (1971) Gelman et al. (1979) Hoffman, Hodges, et al. (1980)
184
U. von Zahn and V. I. Moroz
Table
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Gas
Altitude Range (km)
Mixing Ratio
03
upper limits
only:
>95 95 to 70 94 85 77 c.t. c.t. c.t.
Column Remark/ Abundance Footnote (cm atm)
6 <2xl0 <6x106 < 10—6
Reference
Wilson et al. (1981) Wilson et al. (1981) Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky (1980) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972)
C 302
112
upper limits c.t. c.t. c.t. c.t. c.t. c.t. c.t. 58 to 49
only: 7
<2
<5xl0 <5 <5xl07 <1x106 <0.5
Cruikshank Moroz (1963)and Sill (1967) Owen (1968) Kuiper (1969) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972) see text Mukhin et al.
(1963)
upper limits: 52 42 22
HC1
c.t.
<2xl04 <7x105
4xlO7
Oyama et al. Oyama et al. Oyama et al.
(1980) (1980) (1980)
Connes et al.
(1967)
upper limit: c.t.
*footnotes
<10—6
at end of table
Owen and Sagan
(1972)
Composition below 100km Altitude
Table
Gas
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
c.t.
5xl0
HF HCN
5—1.
Column Remark/ Abundance Footnote (cm atm)
9
Reference
Connes et al. (1967)
upper limits only: <95 95 to 70 c.t.
<2xl08 <4xl08 < 10—6
Wilson et al. Wilson et al. Comes et al.
H
6 2O
185
60 c.t. to 58 58 58 to 49 50 50 to 46 45 42 22 0
(d)
<1 > to lx106 40x10 3x105 7xl04 3xl04 2xl03 2xl04 5.2xl03 l.4xl03 2xl05
+
see text + + +
(1981) (1981) (1967)
see text Oertel et al. (1984) Moroz et al. (1985) Mukhin et al. (1983) Ustinov and Moroz (1978) Surkov et al. (1983) Moroz (l983a,b) Oyama et al. (1980) Oyama et al. (1980) Moroz (1983a,b)
upper limits: > 95 95 to 70 c.t. c.t. c.t. c.t. 52 42 to 0 42 to 0 < 24
<4xl06 <4xl05 < l0~
< 70 <125 < 2 < 16
pm pm pm pm
< l0~ <5xl05
Wilson et al. (1981) Wilson et al. (1981) Spinrad (1962) Belton and Hunten (1966) Connes et al. (1967) Owen (1967) Hoffman et al. (l980a) Gelman et al. (1979) Gelman et al. (1980) Hoffman, Hodges, et al.
(1980) H
6 2S
*footnotes
appr.55
lxlO
37 to 29 < 20
8xl05 3x106
at end of table
.JASR 5:11-N
Hoffman, Hodges, et al. +
(1980) Mukhin et al. (1983) Hoffman, Hodges et al. (1980)
186
U. von Zahn and V. I. Moroz
Table
Gas
H
2S
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
(cont.) upper limits: 4 <3xl07 <2xl0
c.t. c.t. 52 42 22
S
<50
+
1 3
SO
23 upper
2
69 66 c.t. 65 58 22 22
Sanko
(1980)
4x10’ limits only:
>95 95 to 70
SO
Cruikshank (1967) Anderson et al. (1969) Owen and Sagan (1972) Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980)
<2x108 <5xl08
7 0.3lxlO to 9x108 2xl0’6 <0.2 to 5xl07 <1x106 4x106 2xl04 l.3x104
*footnotes at end of table
Wilson et al. Wilson et al.
see text
(1981) (1981)
Esposito et al. (1979) Esposito (1984) Moroz et al. (1985) Barker (1979) Oertel et al. (1984) Oyama et al. (1980) Gelman et al. (1979)
Composition below 100km Altitude
Table
Gas
5-1.
187
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
SO
2 (cont.) upper
limits:
95 >95to 70 94 85 77 c.t.
7 <6xl07 <2x10 <3xl06 <3x107
c.t. c.t. c.t. c.t.
<3x107 <1x108 <1xl08 <5x109
55
<1xl05
52 50 <50 <24
<6x104 <2x10~ <2xl04 <3xl04
112504 upper
<9x105
SF
7
CS
(g)
limit only: 48
6
<5 l0~
Wilson et al. (1981) Wilson et al. (1981) Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky (1980) Cruikshank and Kuiper (1967) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972) Shaya and Caldwell (1976) Hoffman, Hodges, et al. (1980) Oyama et al. (1980) Good and Schloerb (1983) Janssen and Klein (1981) Hoffman, Hodges, et al. (1980)
58 to 35 upper
Steffes and Eshleman (1982)
Mukhin et al.
(1983)
Wilson et al. Wilson et al.
(1981) (1981)
2x10
limits only:
>95 95 to 70
<9xl09 <5xl08
*tootnotes at end of table
188
U. von Zahn and V. I. Moroz
Table
Gas
CS
2
5—1.
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
upper limits only: 94 85 77 c.t.
COS
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
< l0~ 5 <5xl0 <4x105 <5xl08
37 to 29
Krasnopolsky (1980) Krasnopolsky (1980) Barker (1979)
4x105
+
Mukhin et al.
(1983)
upper limits: >95 95 to 70 c.t. c.t. c.t. c.t. 52 42 >24
<5x107 <8x108 < 10~ < l0~ <2x107 < l0~ <4x105
22 0 to 20
<2xl06 <5xl04
Wilson and Klein (1981 Wilson and Klein (1981 Cruikshank (1967) Kuiper (1969) Anderson et al. (1969) Owen and Sagan (1972) Oyama et al. (1980) Oyama et al. (1980) Hoffman, Hodges, et al (1980) Oyama et al. (1980) Hoffman, Hodges, et al (1980)
<0.3
+ + +
NH 3
44 to 32
l0~ to l0~
+
Surkov et al.
(1974)
(see text) upper c.t. c.t. c.t. c.t. l.a.
limits: 8 <3x10
*footnotes at end of table
<3 <9xl03 +
Moroz Kuiper (1963) (1969) Jenkins et al. (1969) Owen and Sagan (1972) Smirnova and Kuzmin (1974)
Composition below 100km Altitude
Table
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Gas
Altitude Range (km)
NO
upper limits only: c.t. c.t.
NO
2
189
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
<0.1
Jenkins et al. Owen and Sagan
limits only: 6 85 <3x107 94 <4xl0 77 <2x107 c.t. <8xl07 c.t. c.t. < 10 c.t. <6x109
Reference
(1969) (1972)
upper
l.a. l.a.
Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky (1980) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972) Krasnopolsky and Parshev (1979) Moroz et al. (1979a,b) Moroz et al. (l981b, 1983a)
<6xl02
<5xl0~° <4xlO’10
N 20
upper
limits only:
95 >95to 70 c.t. c.t. 52 42 22~
6 <1xl05 <5x10 <4x105 <2xl04 <7xl05 <1x105
<2xl02 <4 (h)
Wilson et al. (1981) Wilson et al. (1981) Kuiper (1949) Moroz (1963) Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980)
N 204
upper
limit only: 8
c.t. *footnotes
at end of
<4xl0 table
Owen and Sagan
(1972)
190
U. von Zahn and V. I. Moroz
Table
Gas
Cl
2
5-1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
upper limits only: 7 < l0~ <7xl0 <4xl07 <3xlO’7
94 85 77 c.t. <24
<
l0~
l.a.
<
10—8
l.a.
<
l0~
l.a.
<4x108
(i)
Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky and Parshev (1979) Hoffman, Hodges, et al. (1980) Moroz, Parfentev, et al. (1979) Moroz (198lb); Moroz, Ekonomov, et al. (1981) Moroz, Golovin, et al.
(1981, 1983) Br 2
Hg
upper limits only: 94 85 77 l.a.
< l0~ 6
l.a.
< 2xl01°
l.a.
< 2xlOU
upper limit <24
Krasnopolsky (1980) (1980) Krasnopolsky (1980) Moroz, Moshkin, et al. (1979) Moroz, Golovin, et al. (1981) Moroz, Ekonomov, et al. (1983)
only: <5xl06
Hoffman, Hodges, et al. (1980)
CH 3 and higher aldehydes: c.t.
upper limit
<10—6
*footnotes at end of table
only Owen and Sagan (1972)
Composition below 100km Altitude
Table
5—1.
Measured Volume Mixing
Ratios
191
and Upper Limits
of
Gases (continued) Gas
CH
4
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
upper limits only: c.t. c.t. c.t. 52 42 22
<20 < 3 5 < 10—6
Kuiper (1949) Moroz (1963) Oyama Connes et et al. al. (1980) (1967) Oyama et a1. (1980) Oyama et al. (1980)
C 2H2
upper limit only: c.t.
C2H4
Connes et al.
upper limits only: c.t. c.t. 52 42 32
5 <2xl0 <7xl06 <1x106
<24
<2x10
C
<3 <2
6 2H6
(1967)
Kuiper (1949) dyama et Moroz (1963) al. (1980) Oyama et al. (1980) Oyama et al. (1980) Hoffman, Hodges, et al.
(1980) upper limits: c.t. 52 42 22
<1 <2xl05 <7x106
Kuiper
(1949)
Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980)
C 3H8
upper limits only: 5 52 <2x10 42 <3xl05 22 <5x106
*footnotes at end of table
Oyama et al. Oyama et al. Oyama et al.
(1980) (1980) (1980)
192
U. von Zahn and V. I. Moroz
Table 5—1.
Gas
CH
3F
Altitude Range (km)
upper
Column Remark/ Abundance Footnote (cm atm)
Reference
<10—6
Comes
et al.
(1967)
Comes
et al.
(1967)
upper limit only: c.t.
CH2O
Mixing Ratio
limit only:
c.t.
CH3C1
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
upper limits only: 7 <7xl0 95 >95 to 70 <2xl06 c.t. c.t. <10—6
<0.3
Wilson et al. (1981) Wilson et al. (1981) Wildt (1940) Owen and Sagan (1972)
CH 3COCH3 and higher ketones:
6 c.t. pm **
+
(a) (b) (c) (d) (e) (f) (g) (h) (i)
upper
limit only Owen and Sagan
(1972)
column abundance given in pm of precipitable water vapor c.t. = near cloud top (about 65 km altitude) l.a. = lower atmosphere conflicting data exist which cannot be resolved yet value extrapolated from data taken above 130 km altitude mixing ratio for only 84Kr mixing ratio for only 1311-Xe + 132Xe the quoted range of values encompasses the observed diurnal variation for scattering model for reflecting—layer model assuming constant mixing ratio below 50 km altitude optimum case may be considerably higher depending on other gas concentrations (Oyama et al., 1980) Cl
Composition below 100km Altitude
Table 5—2.
193
Isotopic Composition of the Elements (in percent)
Mass Terrestrial Element Number Abundance*)
Venus Abundance
Reference
H (D)
1 2
99.985 0.015
He
3 4
0.000138 99.999862
C
12 13
98.90 1.10
1.12
14 15
99.634 0.366
0.366
16 17 18
99.762 0.038 0.200
0.20
±
0.01
20 21 22
90.51 0.27 9.22
91.5 <0.7 7.8
±
0.5
±
0.5
Cl
35 37
75.77 24.23
no significant Connes et al. difference from Young (1972) terrestrial value
Ar
36
0.337
43.6
±
1
38 40
0.063 99.600
8.4 48.0
±
0.3 1
Hoffman, Hodges, et al. (1980) Istomin et al. (1979; 1980; 1983)
78 80 82 83 84 86
0.35 2.25 11.6 11.5 57.0 17.3
7 23 14 48 8
± ± ±
7 12 14 10 8
Donahue Donahue Donahue Donahue Donahue
N
0
Ne
Kr
1.6 2.2
± ±
0.2 0.6
0.03
Donahue et al. (1982) Hartle and Taylor (1983) (see text) Hoffman, Hodges, et al. (1980)
±
0.02
±
0.075
Istomin et al.
Hoffman, et al.
± ±
(1979)
Hoffman, Hodges, et al. (1980)
Istomin et al.
±
(1980)
et et et et et
al. al. al. al. al.
(1983)
(1967)
(1981) (1981) (1981) (1981) (1981)
*Values from Holden et al., Isotopic compositions of the elements, 1983, Pure Appl. Chem., 56, 675—694, 1984.
194
U. von Zahn and V. I. Moroz 50
ABUNDANCES ~2O”
—
MICRONS.
\~
~~2g
—‘-10 180
210
MIDNIGHT.
Figure 5—1.
240
270 300 330 0 30 60 90 120 LONGITUDE — DEGREES (SOLAR—FIXED COORDINATES). SUNSET. NOON. SUNRISE.
150
180 MIDNIGHT.
The Horizontal Variation of Water Vapor Column Abundance Above the 90-mbar Level as Derived from Observations of the Pioneer Venus Orbiter Infrared Radiometer (Schofield et al., 1982). Column abundances are given in pm of precipitable water vapor (1 pm corresponds to a mixing ratio of approximately 2 ppm).
•
A2~~2
fli -
I-I
-
.11
20-
.3
//
/4:1 ii
10 Figure 5—2.
HZ
II
Hf ______
100 75j2Q
1000 ,
ppm
Recent Measurements (1 — 4) of Water Vapor Mixing Ratios Inside and Below the Clouds (from Moroz, Moshkin, et al., 1983). 1 — Venera 13 and 14 LiCl humidity sensor (Surkov et al., 1983); 2 — Venera 13/14 gas chromatograph (Mukhin et al., 1983); 3 — Pioneer Venus gas chromatograph (Oyama et al., 1980); 4 — Venera spectrophotometer (Moroz, Mosh— kin, et al., 1983). Lines Al—A3 and 111—112 are suggested altitude profiles which are to fit all or part of the available data on the water vapor mixing ratio.
Composition below 100km Altitude
~
100
-
80
-
60
-
195
S
E 0 ‘~
S
C
~
+
201
~
+
1+’
0 0
400
+
I+.~ I
+4
+~
+
I
800
I 1200
+
I
I 1600
Days since orbit insertion Figure 5—3.
Mean SO
2 Mixing Ratio (in ppb) at 40 mb as Derived by Esposito (1984) from SO2 Band Intensities Observed by the Pioneer Venus Ultraviolet Spectrometer. Orbit insertion took place Dec. 4, 1978; day no. 1600 is April 20, 1983.
0273-1177/85 $0.00 + .50 Copyright ©COSPAR
CHAPTER V COMPOSITION OF THE VENUS ATMOSPHERE BELOW 100 km ALTITUDE U. von Zahn’ and V. I. Moroz2 Physikalisches Institut, Universität Bonn, Nussallee 12, 5300 Bonn 1, F.R.G. 2 Space Research Institute, U.S.S.R. Academy of Sciences, Moscow 117810, U.S.S.R. ABSTRACT
Our current knowledge on the composition of the Venus atmosphere in the altitude range from the surface to 100 km is compiled. Gases
that have been measured, and whose mixing ratios are
assumed to be constant with altitude, are CO 2, N2, He, Ne, Ar, and Kr.
Gases that have been identified
in the lower and/or
middle atmosphere, but whose mixing ratios may depend on altitude,
latitude and/or local time, are CO, H2O, HC1, HF, and
SO2.
Conflicting data or only upper
limits exist on some
important trace gases, such as 021 H2, and Cl2.
The latter two
are key constituents in the photochemistry of the middle atmosphere of Venus.
The chapter concludes with a listing of the
isotopic abundances of elements measured in the Venus atmosphere.
1.
Scope of Chapter
This chapter summarizes the available measurements of the composition of the Venus
lower and middle atmosphere.
latter are defined as the regions of the Venus
The
atmosphere lying
between the surface and 100 km altitude (corresponding to ambient pressures
between appr. 95 bar and 0.03 mbar).
The
chapter deals with only the gaseous constituents of the Venus atmosphere.
Hence, it will not dwell on the particulate matter
(haze, cloud, or aerosol particles)
contained
in the Venusian
atmosphere, as this is the subject of a separate chapter of
173
174
U. von Zahn and V. I. Moroz
this book.
Where appropriate, the chapter will briefly
reference current theoretical models of the chemistry of the Venus atmosphere.
2.
Presentation
of Data
In past decades a large body of information on the chemical and isotopic composition of the Venus atmosphere
has been acquired.
It is now known that more than 99.9% of the Venus atmosphere made up of only CO
2 and N2.
Thus,
the sum of all trace gases
accounts for less than 0.1% of the total atmosphere. less,
Neverthe-
it is just this group of gases which attracts most of our
attention, 021
is
H2O,
as it includes many interesting species such as H2,
CO, SO2,
The number of
H2S, COS, HC1, HF, and all the noble gases.
individual measurements pertaining
atmospheric composition has become quite numerous measurements of the water atmosphere have been published. is so extensive now that
large.
to the Venus For example,
vapor content of this
The literature on this subject
it would be worth
a review in itself.
For a few trace gases, the abundance data, which were collected by quite different measurement satisfying manner.
techniques, agree in a very
This is the case,
for example,
for CO,
the
abundance of which has been determined by remote sensing techniques
in the near—IR and microwave region, and in—situ
measurements using gas chromatographs. however,
the situation
For other trace gases,
is much less appealing.
Different
measurements of the same species may be in conflict with each other
(e.g., for Kr or COS).
In other cases the measured
abundances of chemically active gases cannot be reconciled with chemical or thermodynamical equilibrium conditions O2—CO-S02 or NH3—H2O-HC1
(e.g.,
for
in the lower atmosphere).
t atmosphere Comprehensive of the We composition ofparticular, Venus have been givenreviews elsewhere. mention, in articles by Moroz
(l981b) and by von Zahn et al.
(1983) which were
written after the deluge of new data obtained in 1978/79 had been analyzed and published. treated by Marov
In the l970s the subject was
(1972) and Young
(1974).
With those and
additional reviews of the Venus atmosphere being readily available,
it cannot be the intention of this chapter to
attempt giving again something like a complete treatment of the subject matter. Instead, we aim at providing a concise
Composition below 100km Altitude
175
tabulation of those results which we still consider significant. With our intention of conciseness and easy usage of the table entries, we give for some of the gases only a single abundance value which
is to be understood as a weighted mean of the
original measurements.
For further details the reader is
referred to the literature cited in the tables.
We presume that CO
2, N2, and the noble gases each have a
constant mixing ratio throughout the altitude region under consideration.
On the other hand, we expect that the mixing
ratios of virtually all other gases depend on altitude and possibly local time and latitude.
For only a few gases,
like
H2O, CO, and SO2, such variations have been clearly established by measurements.
To link and evaluate data taken at different
locations and times, one needs geochemical and photochemical models of the entire Venus
atmosphere or parts thereof.
ly the following model studies have been published: Kreimendahl
(1980), Barsukov et al.
Lewis and
(1980), Khodakovsky
Yung and DeMore (1982), and Krasnopolsky and Parshev
Recent-
(1982),
(1983).
One more type of natural variation in mixing ratios needs special attention:
possible mid—term changes in the atmospheric
trace gas budget caused by episodic volcanic activity. (1984) has argued that a significant
Esposito
decrease of the SO2 mixing
ratio at the cloud tops had occurred during the time period 1978 through 1983.
He suggests this effect to be caused by a compar-
ative lull in the activity of volcanos at the surface of Venus. On the other hand,
the possibility of a change of the observed
SO2 band intensity without an actual change of the SO2 mixing ratio
(through cloud structure variability for example) still
cannot be ruled out.
The aforementioned observations
indicate that at times measured
data on mixing ratios might turn out to lie above previously established upper
limits and yet both can be correct at the
epoch of their measurement.
Hence, even if such a seemingly
conflicting situation exists, we usually list both values in Table
5—1, which
follows.
In Table 5—i we have collected both measured mixing ratios and column abundances, as well as upper limits for the abundances of the gases
in the Venus
atmosphere below 100 km altitude.
The species are grouped loosely according
to chemical families,
176
0. von Zahn and V. I. Moroz
although we readily admit these data.
that there
Footnotes assembled at the end of the table add
some brief details.
In those cases where
comment is deemed appropriate chapter. Table
is no ideal way to arrange
a more extensive
it can be found in the next
Information on isotopic abundances
is assembled
in
5—2.
3.
Comments on Table 5—1
3.1
Carbon Dioxide and Nitrogen
As mentioned before, CO
2 and N2 make up more than 99.9% of the
entire Venus atmosphere.
Von Zahn et al.
(1983) review details
of the many nitrogen measurements performed in the course of the Venera and Pioneer Venus missions.
From the mixing ratios given
in Table 5—1, one calculates the mean molecular mass of the Venus atmosphere
below 95 km altitude to be:
rn 3.2
=
43.44
±
0.15 kg/kmole.
Helium
Helium has been measured extensively
in the altitude range 130
km to about 600 km (Kumar and Broadfoot, 1980;
Niemann et al., 1980;
et al., 1984).
1975;
von Zahn et al.,
Burgin and Sholokhov,
In the upper atmosphere
1981;
its mixing ratio
Bertaux is,
however, strongly enhanced with respect to the lower atmosphere due to diffusive separation.
Hence, an extrapolation of the
upper atmosphere data into the well—mixed lower atmosphere requires one to make assumptions on the altitude profiles of eddy mixing and temperature.
Such extrapolations have been
done independently by von Zahn et al. (1983).
(1980) and Hedin et al.
unfortunately, their results turn out quite
differently
(see Table
5—1).
The mass spectrometer aboard the Pioneer Venus obtained data on helium in the bulk atmosphere
Large Probe of Venus. The
data suggest an extraordinarily high mixing ratio Pollack,
1983), which may be biased, however,
(Donahue and
by contamination
from helium used for engineering reasons within the Large Probe. 3.3
Neon and Argon
Both of these noble gases have been measured by the mass
Composition below 100km Altitude spectrometers,
gas chromatographs,
spectrometers Pioneer
aboard
Venus Large
the Venera Probe,
Measured
argon
mixing
(Gelman
et al.,
1979)
combining
25) ppm.
We have used this
to normalize
spectrometers
and by X-ray
11,
12,
the results
the
13,
by a total
fall
into
median
sensitivities
et
fluorescence
14
landers
of
12
in the range
to 200 ppm (Surkov
recommend
ratio
hence
ratios
177
instruments
from 40 ppm
al.,
1982).
a mean value
value of
for
arid the
of
We
(70
±
the Ar mixing
the various
mass
and gas chromatographs used for argon and neon
measurements.
After this normalization, measured neon mixing
ratios fall in the range from 4.3 ppm (Oyama et al., 1980) 10 ppm (Hoffman, Hodges, et al., weighted mean of
3.4
(7
±
1980).
to
We recommend a
3) ppm.
Krypton
One measurement of the mixing ratio of Kr yielded a value the upper
in
l0~ range, two more gave values in the middle
io8 range.
We expect, however,
constant in space and time,
the Kr mixing ratio to be
at least in the lower atmosphere.
Hence,
the two data sets are incompatible. We note here, that t measurement by Istomin et al. (1980) has an additional ‘high’ been withdrawn (V.G. Istomin, private communication).
3.5
Molecular Oxygen
Ground state 02 has been undetectable so far near and above the cloud tops.
The upper
limit for the mixing ratio of 02
in this altitude region has been pushed down to 3 x
by
Trauger and Lunine (1983).
In the Venus airglow, however, two different emissions excited oxygen molecules have been observed which nate from the middle atmosphere.
from
in part origi-
The visible nightglow spectrum
was first measured by the Venera 9 and 10 orbiters (Krasnopolsky et al., 1976) and its main component identified as the 02 Herzberg II band system (Lawrence et al., 1977). (1979) reported earth—based observations of at 1.27 pm from
o
(~ 2
Connes et al.
intense emissions
).
g
Although these emissions
indicate clearly that 02 is produced
above approximately 75 km by photochemical reactions, nevertheless impossible
it is
to ascertain the steady state abundance
178
U. von Zahn and V. I. Moroz
of 02 from these sink
strength
be done by still
measurements
for 02
invoking
somewhat
in this
without altitude
specific
Large
Venera 14
regime.
photochemical
also
the
This can only
models
which are
speculative.
Two gas chromatographic measurements, Venus
specifying
Probe
(Oyama et
(and 13?)
Lander
one aboard the Pioneer
al., 1980) (Mukhin et
and the other aboard the al.,
1983), have yielded
positive identifications of molecular oxygen within and below the main cloud deck of Venus
(see Table
5—1).
amounts of 02 which have been claimed to exist
The substantial in the lower
atmosphere of Venus, on the other hand, are very difficult to reconcile with both the upper limit of 3 x l0’~ for the mixing ratio near the cloud top
(Trauger and Lunine, 1983), and
with the simultaneous observation of significant amounts of CO, SO2~ and H
2.
On the latter question, Oyama et al.,
(1980) comment as follows:
ttThe
coexistence of 02 and CO in
the lower atmosphere would seem to involve a distinctly non— equilibrium process.
Only by postulating an elemental
S/0 ratio
of about 3 instead of 2 were we able to generate nontrivial amounts of 02
in our thermodynamic
calculations.
Several tens
of parts per million of 02 near the surface were obtained only with models that also had SO3 as the dominant sulfur—bearing species in this portion of the atmosphere and had a mixing ratio of about
1000 ppm.’
The idea of saving the situation by allowing for the presence of high abundances of SO3 in the lowest part of the Venus atmosphere has been studied further by Craig et al.
(1983).
This study, however, presumes CO to be inert, which in our opinion is questionable in the light of the high reactivity of SO3 towards CO.
We conclude that currently available measurements of CO, 02, H2,
SO2,
and H2O are not compatible with conditions of thermo-
dynamic equilibrium among these constituents in the lower atmosphere.
3.6
Molecular Hydrogen
The one existing positive identification of H2 (Mukhin et al., 1983) yields a comparatively high mixing ratio, which implies a fairly strong reducing state of the Venus atmosphere, within the clouds.
at least
Photochemical models of the middle atmo—
Composition below 100km Altitude
179
sphere, such as the one of Yung and DeMore (1982), can reconcile most available observational data on the middle atmosphere with the presence of 20 ppm of H
2, except for the observed diurnal
variation of CO near 80 to 90 km altitude (Schloerb et al., 1980; Wilson and Klein,
1981).
Much more controversial
is the situation for the lower atmo-
sphere, where strong non-equilibrium conditions would exist
if
the measured mixing ratios for H2, 021 CO, and H2O are all correct.
For example,
for 20 ppm of CO and 20 ppm of H20 the
water—gas reaction
CO
+
H2O
CO2
++
112
+
yields an equilibrium value for 112 of 0.002 ppm (Kumar et al., 1981) near the surface of Venus.
For the large amounts of
H2 which are claimed to exist within the clouds, no viable source has been offered so far to balance the strong known sinks
3.7
in the middle and upper parts of the atmosphere.
Water
A large body of observations and upper water abundance
limits exists for the
in the Venus atmosphere.
In fact,
large to give a full account here or in Table 5—1.
it is too For a
recent review of the subject we refer the reader to von Zahn et al.
(1983).
For the regions above the 90 mbar level (about 65 km altitude) we wish to point out in particular the local time and latitude variations of the 1120 abundance Schofield et al.
(1982).
(Figure 5—1)
as determined by
They analyzed the radiances measured
by the Pioneer Venus Orbiter Infrared Radiometer in the 45
pm
channel, which is centered in the water vapor rotation band, and the 11.5 pm channel,
which
opacity.
clearly localizes a wet region at
The experiment
is chiefly affected by cloud
equatorial latitudes in the mid-afternoon sector.
But mixing
ratios throughout most of the nightside are below the detection limit of 6 ppm for the experiment. A latitude—dependence of the 1120 mixing ratio below the clouds has been inferred by Revercomb et al.
(1985)
from
thermal IR net flux measurements aboard the four entry probes of the NASA Pioneer Venus mission.
180
U. von Zahn and V. I. Moroz
There
are
a number
of contradictions
the results
of
different
experiments
concerning
profiles
the 1120 mixing ratio were obtained from the anal-
of
H
in
2O mixing
ratios
below 50 km.
Several
ysis of spectra of solar scattered radiation obtained on board Veneras 11,
12, 13,
and 14
(Moroz, Parfentev, et al., 1979;
Moroz, Moshkin, et al., 1983).
These data indicate a maximum
of the water mixing ratio near the cloud bottom (~200 ppm) and a fall—off towards the surface where the mixing ratio approaches 20 ppm (Figure 5—2).
But at least two recent experiments
et al., 1980; Surkov et al., 1983)
gave mixing ratios above
1000 ppm at altitudes between 22 and 50 km. dances are, however,
(Oyama
Such large abun-
incompatible with measured temperature
profiles and surface temperatures of Venus
(Pollack et al.,
1980) and are fully inconsistent with the optical data.
3.8
Sulfur Dioxide
Because SO2
is the only sulfur—bearing gas discovered so far
in the middle atmosphere of Venus, its absolute abundance
and
scale height place very stringent constraints for photochemical and dynamical models of this region of the Venus atmosphere. Further, since
its discovery
in 1978,
(Barker,
1979) UV bands
of SO2 have been nearly continuously monitored by the Pioneer Venus Ultraviolet Spectrometer experiment. time period Esposito
Results for the
from late 1978 until early 1983 were presented by
(1984)
(Figure 5—3), and interpreted as indicating a
large decrease of the SO2 mixing ratio at the 40 mbar level (about 69 km altitude) as an explanation
over this time period.
for the observed
Esposito offers
temporal variability of SO2
the episodic injection of SO2 into the Venus atmosphere by volcanic activity.
3.9
Ammonia
An experiment for the determination of the ammonia mixing
ratio
in the 44 to 32 km altitude region was carried out on board the Venera 8 lander. Surkov et al.
From the color change of “bromophenol blue”
(1974) estimated the ammonia mixiny ratio to be
between 10~ and 1O~. with the upper mm
This value
is, however,
incompatible
limit of 16 ppm established by Smirnova and Kuz-
(1974) trom the absence of an absorption feature at 1.25 cm
in the radio emission spectrum of Venus.
Composition below 100km Altitude Goettel and Lewis NH
3
in the Venus
181
(1974) examined the equilibrium chemistry of atmosphere and concluded that the high NH3
mixing ratios reported by Surkov et al.
(1974)
are inconsistent
with the observation of rather low abundances of other gases Venus’ al.
atmosphere,
in particular 1120 and HC1.
in
Florensky et
(1978), on the other hand, argued that the upper part of
the Venus
troposphere can hardly be assumed to be in chemical
equilibrium and that the tropospheric distribution of minor constituents can be strongly nonuniform, thus leaving open the possibility of a high NH3 abundance in the lower cloud layer.
4.
Comments on Table 5-2
4.1
Deuterium
McElroy et al.
(1982)
suggested that the mass 2 ion observed by
the Pioneer Venus Orbiter Ion Mass Spectrometer et al.,
1980) is in fact D~ rather than
pretation implies a D/H ratio of
Subsequently Donahue et al. (1.6
±
0.2)
4.
(H. A. Taylor
This inter-
~io_2 in the bulk atmosphere.
(1982) determined a D/H ratio of
x io2 from the water—related peaks in the
spectra of the Pioneer Venus Large Probe Mass Spectrometer. Full details of the latter analysis have, however, not yet been published.
A detailed analysis of the height variation and local time dependence of the light ions observed by the Pioneer Venus Orbiter Ion Spectrometer led Hartle and Taylor (1983) to the confirmation of the earlier suggestion that the mass 2 ion observed the Venus
ionosphere
is predominantly D+.
From their
iono-
spheric measurements they interred a D/H ratio, extrapolated into the lower atmosphere, of
(2.2
±
0.6) x l0_2.
in
U. von Zahn and V. I. Moroz
182
Table 5—1.
Gas
CO
2
Measurea Volume Mixing Ratios and upper Limits of Gases*
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
<100
0.965 ±0.008
see text von Zahn et al.
(1983)
N2
<100
0.035 ±0.008
see text von Zahn et al.
(1983)
He
<100 <100
5 l.2xl0 6x107 4.5xl04
(a),+ (a),+ +
von Zahn et al. (1980) Hedin et al. (1983) Donahue and Pollack
(1983) Ne
<100
7xl06
see text von Zahn et al.
(1983)
Ar
<100
7x105
see text von Zahn et al.
(1983)
Kr
l.a.** 49 to 37 26 to 0
5xl08 7xl07 2xl08
(b),+
Donahue et al. (1981) Mukhin et al. (1983) Istomin et al. (1983)
(b)
Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980) Hoffman et al. (l980b)
+ +
upper limits: 52 42 22 <24 Xe
<4xl05
upper limits only: <26 <20 <20
*footnotes
<7xl09
at end of table)
(c)
Istomin et al. (1983) Donahue et al. (1981) Donahue and Pollack (1983)
Composition below 100km Altitude
Table
Gas
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
CO
100
Mixing Ratio
c.t.* c.t. 52 42 to 36 12 42 22 upper
(d)
Schloerb et al. (1980) Moroz (1963; 1964) Connes et al. (1968) Oyama et al. (1980) Gelman et al. (1979) Gelman et al. (1979) Oyama et al. (1980) Oyama et al. (1980)
4.5xl05 3xl05 3xl05 2xl05 3x105 2xlO—5
Wilson and Klein
4.4xl05 l.6xl05 l.8xl05
(1981) (1981)
see text see text see text
(1981)
Oyama et al. (1980) Oyama et al. (1980) Mukhin et al. (1983)
limits:
c.t.
<57
c.t. c.t. c.t.
<8xl05 <2xl05 <1xl06
c.t.
<3xl07
60 to 0 52
<5xl05 <3xl05
46 42 to 0 <24
< l0~ <2xl05 <3x105
*footnotes
Wilson and Klein Wilson and Klein
limit:
52 42 58 to 35 upper
Reference
(d)
1.5
75
02
Column Remark/ Abundance Footnote (cm atm)
3.5 to4 l4x10 2x104 <0.4 to llxlO5
90 75
183
at end of table
<1.4 <6
(e) (f)
Spinrad and Richardson (1965) Belton and Hunten (1968) Belton and Hunten (1968) Traub and Carleton (1974) Trauger and Lunine
(1983) Moroz (1981) Hoffman, Oyama, et al. 1980) Vinogradov et al. (1971) Gelman et al. (1979) Hoffman, Hodges, et al. (1980)
184
U. von Zahn and V. I. Moroz
Table
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Gas
Altitude Range (km)
Mixing Ratio
03
upper limits
only:
>95 95 to 70 94 85 77 c.t. c.t. c.t.
Column Remark/ Abundance Footnote (cm atm)
6 <2xl0 <6x106 < 10—6
Reference
Wilson et al. (1981) Wilson et al. (1981) Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky (1980) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972)
C 302
112
upper limits c.t. c.t. c.t. c.t. c.t. c.t. c.t. 58 to 49
only: 7
<2
<5xl0 <5 <5xl07 <1x106 <0.5
Cruikshank Moroz (1963)and Sill (1967) Owen (1968) Kuiper (1969) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972) see text Mukhin et al.
(1963)
upper limits: 52 42 22
HC1
c.t.
<2xl04 <7x105
4xlO7
Oyama et al. Oyama et al. Oyama et al.
(1980) (1980) (1980)
Connes et al.
(1967)
upper limit: c.t.
*footnotes
<10—6
at end of table
Owen and Sagan
(1972)
Composition below 100km Altitude
Table
Gas
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
c.t.
5xl0
HF HCN
5—1.
Column Remark/ Abundance Footnote (cm atm)
9
Reference
Connes et al. (1967)
upper limits only: <95 95 to 70 c.t.
<2xl08 <4xl08 < 10—6
Wilson et al. Wilson et al. Comes et al.
H
6 2O
185
60 c.t. to 58 58 58 to 49 50 50 to 46 45 42 22 0
(d)
<1 > to lx106 40x10 3x105 7xl04 3xl04 2xl03 2xl04 5.2xl03 l.4xl03 2xl05
+
see text + + +
(1981) (1981) (1967)
see text Oertel et al. (1984) Moroz et al. (1985) Mukhin et al. (1983) Ustinov and Moroz (1978) Surkov et al. (1983) Moroz (l983a,b) Oyama et al. (1980) Oyama et al. (1980) Moroz (1983a,b)
upper limits: > 95 95 to 70 c.t. c.t. c.t. c.t. 52 42 to 0 42 to 0 < 24
<4xl06 <4xl05 < l0~
< 70 <125 < 2 < 16
pm pm pm pm
< l0~ <5xl05
Wilson et al. (1981) Wilson et al. (1981) Spinrad (1962) Belton and Hunten (1966) Connes et al. (1967) Owen (1967) Hoffman et al. (l980a) Gelman et al. (1979) Gelman et al. (1980) Hoffman, Hodges, et al.
(1980) H
6 2S
*footnotes
appr.55
lxlO
37 to 29 < 20
8xl05 3x106
at end of table
.JASR 5:11-N
Hoffman, Hodges, et al. +
(1980) Mukhin et al. (1983) Hoffman, Hodges et al. (1980)
186
U. von Zahn and V. I. Moroz
Table
Gas
H
2S
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
(cont.) upper limits: 4 <3xl07 <2xl0
c.t. c.t. 52 42 22
S
<50
+
1 3
SO
23 upper
2
69 66 c.t. 65 58 22 22
Sanko
(1980)
4x10’ limits only:
>95 95 to 70
SO
Cruikshank (1967) Anderson et al. (1969) Owen and Sagan (1972) Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980)
<2x108 <5xl08
7 0.3lxlO to 9x108 2xl0’6 <0.2 to 5xl07 <1x106 4x106 2xl04 l.3x104
*footnotes at end of table
Wilson et al. Wilson et al.
see text
(1981) (1981)
Esposito et al. (1979) Esposito (1984) Moroz et al. (1985) Barker (1979) Oertel et al. (1984) Oyama et al. (1980) Gelman et al. (1979)
Composition below 100km Altitude
Table
Gas
5-1.
187
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
SO
2 (cont.) upper
limits:
95 >95to 70 94 85 77 c.t.
7 <6xl07 <2x10 <3xl06 <3x107
c.t. c.t. c.t. c.t.
<3x107 <1x108 <1xl08 <5x109
55
<1xl05
52 50 <50 <24
<6x104 <2x10~ <2xl04 <3xl04
112504 upper
<9x105
SF
7
CS
(g)
limit only: 48
6
<5 l0~
Wilson et al. (1981) Wilson et al. (1981) Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky (1980) Cruikshank and Kuiper (1967) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972) Shaya and Caldwell (1976) Hoffman, Hodges, et al. (1980) Oyama et al. (1980) Good and Schloerb (1983) Janssen and Klein (1981) Hoffman, Hodges, et al. (1980)
58 to 35 upper
Steffes and Eshleman (1982)
Mukhin et al.
(1983)
Wilson et al. Wilson et al.
(1981) (1981)
2x10
limits only:
>95 95 to 70
<9xl09 <5xl08
*tootnotes at end of table
188
U. von Zahn and V. I. Moroz
Table
Gas
CS
2
5—1.
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
upper limits only: 94 85 77 c.t.
COS
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
< l0~ 5 <5xl0 <4x105 <5xl08
37 to 29
Krasnopolsky (1980) Krasnopolsky (1980) Barker (1979)
4x105
+
Mukhin et al.
(1983)
upper limits: >95 95 to 70 c.t. c.t. c.t. c.t. 52 42 >24
<5x107 <8x108 < 10~ < l0~ <2x107 < l0~ <4x105
22 0 to 20
<2xl06 <5xl04
Wilson and Klein (1981 Wilson and Klein (1981 Cruikshank (1967) Kuiper (1969) Anderson et al. (1969) Owen and Sagan (1972) Oyama et al. (1980) Oyama et al. (1980) Hoffman, Hodges, et al (1980) Oyama et al. (1980) Hoffman, Hodges, et al (1980)
<0.3
+ + +
NH 3
44 to 32
l0~ to l0~
+
Surkov et al.
(1974)
(see text) upper c.t. c.t. c.t. c.t. l.a.
limits: 8 <3x10
*footnotes at end of table
<3 <9xl03 +
Moroz Kuiper (1963) (1969) Jenkins et al. (1969) Owen and Sagan (1972) Smirnova and Kuzmin (1974)
Composition below 100km Altitude
Table
5—1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Gas
Altitude Range (km)
NO
upper limits only: c.t. c.t.
NO
2
189
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
<0.1
Jenkins et al. Owen and Sagan
limits only: 6 85 <3x107 94 <4xl0 77 <2x107 c.t. <8xl07 c.t. c.t. < 10 c.t. <6x109
Reference
(1969) (1972)
upper
l.a. l.a.
Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky (1980) Anderson et al. (1969) Jenkins et al. (1969) Owen and Sagan (1972) Krasnopolsky and Parshev (1979) Moroz et al. (1979a,b) Moroz et al. (l981b, 1983a)
<6xl02
<5xl0~° <4xlO’10
N 20
upper
limits only:
95 >95to 70 c.t. c.t. 52 42 22~
6 <1xl05 <5x10 <4x105 <2xl04 <7xl05 <1x105
<2xl02 <4 (h)
Wilson et al. (1981) Wilson et al. (1981) Kuiper (1949) Moroz (1963) Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980)
N 204
upper
limit only: 8
c.t. *footnotes
at end of
<4xl0 table
Owen and Sagan
(1972)
190
U. von Zahn and V. I. Moroz
Table
Gas
Cl
2
5-1.
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
upper limits only: 7 < l0~ <7xl0 <4xl07 <3xlO’7
94 85 77 c.t. <24
<
l0~
l.a.
<
10—8
l.a.
<
l0~
l.a.
<4x108
(i)
Krasnopolsky (1980) Krasnopolsky (1980) Krasnopolsky and Parshev (1979) Hoffman, Hodges, et al. (1980) Moroz, Parfentev, et al. (1979) Moroz (198lb); Moroz, Ekonomov, et al. (1981) Moroz, Golovin, et al.
(1981, 1983) Br 2
Hg
upper limits only: 94 85 77 l.a.
< l0~ 6
l.a.
< 2xl01°
l.a.
< 2xlOU
upper limit <24
Krasnopolsky (1980) (1980) Krasnopolsky (1980) Moroz, Moshkin, et al. (1979) Moroz, Golovin, et al. (1981) Moroz, Ekonomov, et al. (1983)
only: <5xl06
Hoffman, Hodges, et al. (1980)
CH 3 and higher aldehydes: c.t.
upper limit
<10—6
*footnotes at end of table
only Owen and Sagan (1972)
Composition below 100km Altitude
Table
5—1.
Measured Volume Mixing
Ratios
191
and Upper Limits
of
Gases (continued) Gas
CH
4
Altitude Range (km)
Mixing Ratio
Column Remark/ Abundance Footnote (cm atm)
Reference
upper limits only: c.t. c.t. c.t. 52 42 22
<20 < 3 5 < 10—6
Kuiper (1949) Moroz (1963) Oyama Connes et et al. al. (1980) (1967) Oyama et a1. (1980) Oyama et al. (1980)
C 2H2
upper limit only: c.t.
C2H4
Connes et al.
upper limits only: c.t. c.t. 52 42 32
5 <2xl0 <7xl06 <1x106
<24
<2x10
C
<3 <2
6 2H6
(1967)
Kuiper (1949) dyama et Moroz (1963) al. (1980) Oyama et al. (1980) Oyama et al. (1980) Hoffman, Hodges, et al.
(1980) upper limits: c.t. 52 42 22
<1 <2xl05 <7x106
Kuiper
(1949)
Oyama et al. (1980) Oyama et al. (1980) Oyama et al. (1980)
C 3H8
upper limits only: 5 52 <2x10 42 <3xl05 22 <5x106
*footnotes at end of table
Oyama et al. Oyama et al. Oyama et al.
(1980) (1980) (1980)
192
U. von Zahn and V. I. Moroz
Table 5—1.
Gas
CH
3F
Altitude Range (km)
upper
Column Remark/ Abundance Footnote (cm atm)
Reference
<10—6
Comes
et al.
(1967)
Comes
et al.
(1967)
upper limit only: c.t.
CH2O
Mixing Ratio
limit only:
c.t.
CH3C1
Measured Volume Mixing Ratios and Upper Limits of Gases (continued)
upper limits only: 7 <7xl0 95 >95 to 70 <2xl06 c.t. c.t. <10—6
<0.3
Wilson et al. (1981) Wilson et al. (1981) Wildt (1940) Owen and Sagan (1972)
CH 3COCH3 and higher ketones:
6 c.t. pm **
+
(a) (b) (c) (d) (e) (f) (g) (h) (i)
upper
limit only Owen and Sagan
(1972)
column abundance given in pm of precipitable water vapor c.t. = near cloud top (about 65 km altitude) l.a. = lower atmosphere conflicting data exist which cannot be resolved yet value extrapolated from data taken above 130 km altitude mixing ratio for only 84Kr mixing ratio for only 1311-Xe + 132Xe the quoted range of values encompasses the observed diurnal variation for scattering model for reflecting—layer model assuming constant mixing ratio below 50 km altitude optimum case may be considerably higher depending on other gas concentrations (Oyama et al., 1980) Cl
Composition below 100km Altitude
Table 5—2.
193
Isotopic Composition of the Elements (in percent)
Mass Terrestrial Element Number Abundance*)
Venus Abundance
Reference
H (D)
1 2
99.985 0.015
He
3 4
0.000138 99.999862
C
12 13
98.90 1.10
1.12
14 15
99.634 0.366
0.366
16 17 18
99.762 0.038 0.200
0.20
±
0.01
20 21 22
90.51 0.27 9.22
91.5 <0.7 7.8
±
0.5
±
0.5
Cl
35 37
75.77 24.23
no significant Connes et al. difference from Young (1972) terrestrial value
Ar
36
0.337
43.6
±
1
38 40
0.063 99.600
8.4 48.0
±
0.3 1
Hoffman, Hodges, et al. (1980) Istomin et al. (1979; 1980; 1983)
78 80 82 83 84 86
0.35 2.25 11.6 11.5 57.0 17.3
7 23 14 48 8
± ± ±
7 12 14 10 8
Donahue Donahue Donahue Donahue Donahue
N
0
Ne
Kr
1.6 2.2
± ±
0.2 0.6
0.03
Donahue et al. (1982) Hartle and Taylor (1983) (see text) Hoffman, Hodges, et al. (1980)
±
0.02
±
0.075
Istomin et al.
Hoffman, et al.
± ±
(1979)
Hoffman, Hodges, et al. (1980)
Istomin et al.
±
(1980)
et et et et et
al. al. al. al. al.
(1983)
(1967)
(1981) (1981) (1981) (1981) (1981)
*Values from Holden et al., Isotopic compositions of the elements, 1983, Pure Appl. Chem., 56, 675—694, 1984.
194
U. von Zahn and V. I. Moroz 50
ABUNDANCES ~2O”
—
MICRONS.
\~
~~2g
—‘-10 180
210
MIDNIGHT.
Figure 5—1.
240
270 300 330 0 30 60 90 120 LONGITUDE — DEGREES (SOLAR—FIXED COORDINATES). SUNSET. NOON. SUNRISE.
150
180 MIDNIGHT.
The Horizontal Variation of Water Vapor Column Abundance Above the 90-mbar Level as Derived from Observations of the Pioneer Venus Orbiter Infrared Radiometer (Schofield et al., 1982). Column abundances are given in pm of precipitable water vapor (1 pm corresponds to a mixing ratio of approximately 2 ppm).
•
A2~~2
fli -
I-I
-
.11
20-
.3
//
/4:1 ii
10 Figure 5—2.
HZ
II
Hf ______
100 75j2Q
1000 ,
ppm
Recent Measurements (1 — 4) of Water Vapor Mixing Ratios Inside and Below the Clouds (from Moroz, Moshkin, et al., 1983). 1 — Venera 13 and 14 LiCl humidity sensor (Surkov et al., 1983); 2 — Venera 13/14 gas chromatograph (Mukhin et al., 1983); 3 — Pioneer Venus gas chromatograph (Oyama et al., 1980); 4 — Venera spectrophotometer (Moroz, Mosh— kin, et al., 1983). Lines Al—A3 and 111—112 are suggested altitude profiles which are to fit all or part of the available data on the water vapor mixing ratio.
Composition below 100km Altitude
~
100
-
80
-
60
-
195
S
E 0 ‘~
S
C
~
+
201
~
+
1+’
0 0
400
+
I+.~ I
+4
+~
+
I
800
I 1200
+
I
I 1600
Days since orbit insertion Figure 5—3.
Mean SO
2 Mixing Ratio (in ppb) at 40 mb as Derived by Esposito (1984) from SO2 Band Intensities Observed by the Pioneer Venus Ultraviolet Spectrometer. Orbit insertion took place Dec. 4, 1978; day no. 1600 is April 20, 1983.