|
Items in cart [1] |
Questions? Call 888-624-8373 |
| Copyright © 2009. National Academy of Sciences. All rights reserved. Terms of Use and Privacy Statement |
Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 218
Lithospheric and Atmospheric
Interaction on the Planet Venus
VLADISLAV P. VOLKOV
V.I. Vernadskiy Institute of Geochemistry and Analytical Chemistry
ABSTRACT
A host of interesting problems related to the probability of a global
process of chemical interaction of the Venusian atmosphere with that
planet's surface material has emerged in the wake of flights by the Soviet
space probes, "Venera4, -5, -6, and -7" (1967-70~. It was disclosed during
these flights that the temperature of Venus' surface attains 750 K, pressure
is approximately 90 aim., and CO2 constitutes 97% of the atmosphere. We
shall explore several of these issues which were discussed in the pioneering
works of Mueller (1963, 1969) and Lewis (1968, 1970~:
· Is Venus' troposphere in a state of chemical equilibrium?
· Can we assume that the chemical composition of the troposphere
is buffered by the minerals of surface rock?
.
What are the scales and mechanisms involved as exogenic processes
take place?
· 1b what degree is the composition of cloud particles tied to the
process of lithospheric-atmospheric interaction?
We have succeeded in resolving a number of these problems over
the past 20 years. At the same time, critical issues such as the chemical
constituents of the near-surface layer of Venus' atmosphere, cloud particle
chemistry, and the mineralogy of iron and sulfur in surface rock obviously
cannot be definitely resolved until further landing craft will have been sent
to the surface of Venus.
Several research projects have been conducted in the USSR and the
United States, which used physical-chemical and thermodynamic methods
218
OCR for page 219
AMERICAN AND SOVIET RESEARCH
219
for computing multi-component systems. These projects have helped us to
understand the particularity of the natural process occurring on the surface
of Venus (Lewis and Kreimendahl 1980, Barsukov et al. 1980; Volkov et al.
1986; and Zolotov 1985~.
Factual material from the studies of the atmosphere and surface of
Venus, gathered with the "Venera" series spacecraft and during the "Pioneer
Venus" mission, can be used to compare our view of the distribution,
chemical composition, and physical properties of products of lithospheric-
atmospheric interaction on Venus.
MANIFESTATION OF EXOGENIC PROCESSES USING
PHOTOGEOLOGICAL DATA
The following conclusions on the nature and scope of exogenic pro-
cesses were made after the probes "Venera-15" and "Venera-16" finished
mapping Venus:
· The present surface relief of Venus was formed as a result of the
combined processes of crater formation, volcanism, and tectonic activity;
· The rate of renewal of Venus' relief is estimated to take a million
years for the first several centimeters (in the last three billion years), as
compared to hundreds of meters on Earth and the first several meters on
Mars (Nikolaeva et al. 1986~;
There is no evidence of exogenic processes on a global scale, such
as lunar regolith;
· There are no traces of fluvial or aeolian processes having occurred
on a scale that matches the resolution of the radar images (one kilometer).
At the same time, microscale exogenic processes have been quite clearly
manifested. TV-panoramas from "Venera-9" and "Venera-10" recorded
three types of processes: the formation of cracks; degradation with the
emergence of desert aeolian weathering ridges; and corrosion, akin to
porous aeolian or chemical weathering. The "Venera-13" and "Venera-14"
images show laminated formations which have been interpreted (F-loren-
skiy et al. 1982) as aeolian-sedimentation rock. Their formation can be
described as a cycle: weathering transport—deposition lithiphication
weathering. . .
Experiments to estimate such physical properties of surface rock on
Venus as porosity, and carrying capacity such as "Venera-13" and "Venera-
14" (Kemurdgian et al. 1983) confirmed the existence of loose, porous
bedrock. Loose, porous bedrock with an estimated thickness of 10 cen-
timeters exists at the landing site of the Soviet "Venera-13" and "Venera-14"
probes. The question of their geological nature is still unanswered: Are
they products of chemical weathering or aeolian activity?
OCR for page 220
220
PLANETARY SCIENCES
There is no direct evidence of the existence of aeolian forms as yet. No
global aeolian, martian-Wpe structures were revealed in the area mapped
by the radars of the "Venera-15" and"Venera-16" probes. Nor do any of
the four TV-panoramas show aeolian forms.
Experimental simulation (Greely et al. 1984) demonstrated that in
an atmosphere of CO2, with a pressure of approximately 100 atm. and
wind speed of up to 3 m/see-i, signs of rippling occur when saltation of
particles of up to 75 Am in diameter takes place. Theoretical estimates
of the threshold rates of the separation of particles of varying dimensions
produced similar results. Dust fraction, transported as suspension, will have
a diameter of < 30 ~m.
Scrupulous investigation of the TV-panoramas, incorporating data
from measurements of the optical properties of the near-surface atmo-
sphere, have demonstrated that the formation of dust clouds from the
aerodynamic landing of Soviet probes is a reality. It is considered that the
nature of particle behavior during wind activity on Venus is similar to the
sorting of material at the bottom of the ocean at a depth of about 1000
meters.
Let us sum up the information on exogenic processes that was gen-
erated by research on the morphology and properties of the surface of
Venus:
· The rate of exogenic processing of the Venusian surface relief Is
extremely low; the morphology of the ancient (0.5 to one billion years)
relief has been excellently preserved;
· physical weathering (the equivalent of terrestrial, geological pro-
cesses) has not been found: there is no aqueous water, living matter, or
climatic contrasts;
· Regolith-like forms of relief are not developed;
· Aeolian activity on present-day Venus does not lead to the forma-
tion of global forms which can be differentiated on radar maps;
· The television images show traces of chemical weathering in the
form of rock corrosion and degradation.
The findings from X-ray-fluorescent analyses on the Soviet "Venera-
13" and "Venera-14" and "Vega-2" probes and K, U. and Th determinations
on the Soviet"Venera-S, -9, and -10," and "Vega-l, and -2" probes (Surkov
1985) have given us information regarding the chemical nature of the
surface rock. It is merely important for this paper to note that all of this
rock belongs to the basalt group and contains almost 10 times more sulfur
than their terrestrial equivalents.
OCR for page 221
AMERICAN AND SOV7ET RESEARCH
THE CHEMICAL COMPOSITION AND A CHEMICAL MODEL OF
THE TROPOSPHERE OF VENUS
221
It became clear, following the flight by the Soviet "Venera4" probe
in 1967, that CO2 accounts for 97% of the Venusian troposphere, N2 is
approximately 3%, and the remaining constituents account for appron-
mately 0.1% (by volume). Unfortunately, we lack instrumental data on
chemical composition at elevations below 20 kilometers. This creates con-
siderable difficulty as we attempt to understand the chemical processes at
the boundary between the atmosphere and the surface.
The troposphere of Venus can be seen as a homogenous, well mixed,
gaseous envelope for the major constituents (CO2 and N2) and the inert
gases. It is clear that complex relationships exist between the physical
(turbulent mixing, and horizontal and vertical planetary circulation of gas
masses) and chemical (condensation and vaporization of cloud particles,
gas-phase reactions, and gas-mineral types of interaction) processes in the
atmosphere which lead to the existence of vertical and horizontal gradients
of microconstituent concentrations (H2O, SO2 and CO; see Figure 1~.
Venus' high surface temperature can be regarded as a factor which
enhances the chemical interaction of the atmosphere with surface rock and,
as a consequence, yields a dependency of the atmosphere's composition on
heterogenic chemical reactions at the atmosphere-surface boundary.
Mueller (1964) proposed 25 years ago that three zones may exist in
the vertical profile of Venus' atmosphere, depending on the predominance
of varying types of chemical processes:
.
The zone of thermochemical reactions in which the composition of
the atmosphere is buffered by surface rock minerals;
· The zone of "frozen" chemical equilibrium, where the composition
of gases corresponds to their equilibrium ratios in the near-surface layer of
the troposphere;
The zone of photochemical reactions in the upper atmosphere.
According to this model, chemical reactions at the planet's surface take
place amid a constant influx of reactive matter from the crust reservoir, as
geological and tectonic activity also occur. Using the principle of global
chemical quasiequilibrium in the atmosphere-crust system, we can apply
thermodynamic computations to estimate the equilibrium concentrations
of atmospheric gases that are not accessible to direct measurements.
Lewis (1970) obtained more complete data on calculations of the
chemical composition of the near-surface atmosphere; he took into account
the results of the atmospheric analyses performed by the Soviet "Venera-4,
-5 and Hi" probes. Unlike Mueller (1964), he only considered chemical
equilibrium at the atmosphere-surface boundary Table 1~. Both of the
OCR for page 222
222
By
90
LLJ
~ 70
An
LL
~ 50
US
>
o
m
LLJ 30
PLANETARY SCIENCES
N2 ~ CO2 |
night
HE SO2 HCI ``
night / I
/ CO
day I
// day
°2,'1
s8
10
i,,,
}~1//:
' ~
i
2
_ _ _ 3
I / COS
1/ 1
1/ .
1/ 1
1 1 1 1 1 1,
i
10-5 10-3 10-1 10 103 105
RELATIVE CONTENT BY VOLUME, PARTS PER MILLION
FIGURE 1 A schematic vertical cross-section of the troposphere of Venus. It shows
the distribution of macro- and microconstituents based on data from measurements per-
formed by the "Venera" series and the "Pioneer Venus" probes. 1: microconstituents; 2:
macroconstituents; 3: data requiring refinement.
above models used the existence of chemical equilibrium throughout the
troposphere, to its upper cloud boundary.
The literature has been discussing Urey's (1951) hypothesis for quite
some time. He proposed "Wollastonite" equilibrium as a mechanism for
buffering PCO2 in the global, equilibrium atmosphere-crust system (Mueller
1963; Vinogradov and Volkov 1971; Lewis and Kreimendahl 1980~:
CaC03 + SiO2 ~ CaSiO3 + CO2
calcite quartz Wollastonite
The thermodynamic calculations performed in these studies demonstrated
that the mineral association of calcite-quartz-Vollastonite on the surface of
Venus can buffer Pcog (~ ~ bar) at a temperature of 742 K This is Estuary
OCR for page 223
AMERICAN AND SOVIET RESEARCH
TABLE 1 Chemical Models of Venus' Troposphere
223
Quasi ne Zone of "Frozen"
Authors of Chemical Chemical Initial
Equilibrium Equilibrium Data
R. Mueller Troposphere + Troposphere to an Spectroscopic measurements
1963, 1969 lithosphere altitude of 80 km* of CO, HCI, HE, and H2O
J. Lewis 1970 Troposphere + " " Spectroscopic measurements
surface rock of CO, HCI, HF; H2O
(t'Venera-5 & -6" data)
Florenskiy " " Troposphere to lower Chemical analysis of the
et al. 1976 cloud boundary atmosphere on Soviet "Venera
4 & -10" probes
Khodakovskiy " " Near-surface Chemical analysis of the
et al. troposphere atmosphere on Soviet "Venera
1979 -11 & -12" probes & "Pioneer
Venus"
Krasnopol'skiy
and Parshev
1979
.. ..
Troposphere to an
altitude of 60 km$*
.. ..
Supper boundary of the cloud layer
**According to Krasnopol'skiy and Parshev (1979): to the "zone of
photochemical reactions"
commensurate with the surface conditions. However, interpretation of the
multisystem computations has shown that carbonates are unstable. Yet, the
high concentration of SO2 in the troposphere is one of the determining
factors of this process (Zolotov 1985; Volkov et al. 1986~. Consequently,
"Wollastonite" equilibrium can scarcely be seen as the basis for a chemical
model of Venus' atmosphere.
F"lorenskii e! al. (1976) developed the idea in 1976 that there may
be chemical equilibrium in the subcloud portion of the troposphere. The
lower atmosphere was divided into three zones:
The stratosphere with an upper layer of clouds, which is the zone
of photochemical processes;
· The main cloud layer zone, where photochemical (above) and
thermochemical (below) processes compete;
· The portion of the troposphere below the cloud base, which is the
zone where thermochemical equilibria are predominant.
This model brought us to a closer understanding of the Venusian
troposphere as a complex, predominantly nonequilibrious system, even
though numerical estimates of microconstituent concentrations (primarily
SO2) departed greatly from the actual values Cable 2~.
OCR for page 224
224
PLANETARY SCIENCES
TABLE 2 Chemical Composition of the Venusian Near-Surface Troposphere from
Computational Data (Relative Levels of Microconsiituents by Volume).
Gas 1 2 3 4 5 6
CO 2.104 5.l0-S l.7.l0-s l.5.l0-S 7.210~ l.7.l0-s
H2O 5.104 3.2.104 2.l0-s 2~104 2.l0-s 2.10-5
so2 3.10-7 8.lo-6 1.3.104 1.3.104 1.3.104 1.3.104
H2S 5.lo-6 1.2~10-6 52.lo-8 3.10-7 8~10-9 8.l0-s
COS s.lo-s 3.2-10-S 2.3-10-S 2.l0-s 3.lo-6 4elo-s
s2 2~10-8 4 10-8 1.8~10-? 10-7 1 3-10-8 2~10-8-8~10-7
H2 7.10-7 10-7 2.410-9 2.lo-8 10-9 2.5.l0-S
O2 8 10-26 10-24 10-23 10-23 1.8.10-S
Notes: The underlined figures are initial data of measurements on space probes or ground-
based facilities; 1: Mueller (1969); 2: Lewis (1970); 3: Khodakovskiy et al. (1979); 4:
Krasnopol'skiy and Parshev (1979); 5: Zolotov (1985).
Column 6 tabulates data of measurements made an He "Venera" and "Pioneer Venus" probe
senes; no measurements were performed below an altitude of 20 kilometers (Figure 1).
Following measurements of the chemical composition of the tropo-
sphere by the Soviet "Venera-ll and -12" probes and the "Pioneer Venus"
probe, the computational and experimental values of microconstituent con-
centrations were compared. Khodakovskii et al. (1979) and Krasnopol'skii
and Parshev (1979) concurrently and independently proposed models (Ta-
ble 2). These models were the first to compare gas-phase reaction rates
with troposphere mixing rates. These consequences were generated:
· The troposphere is generally in nonequilibrium, with the exception
of the near-surface layer with a thickness of the first kilometer, where
the highest temperatures are dominant. However, the processes of het-
erogeneous catalysis at the atmosphere-surface boundary may favor the
establishment of chemical equilibrium in relation to certain constituents;
· The chemical composition of the microconstituents in the vertical
cross-section below the cloud base region of the troposphere does not
vary: it corresponds to the "frozen" equilibrium at the atmosphere-surface
boundary (T = 735 K; P = 90 atm).
The principal of "frozen" equilibrium was applied in order to theoret-
ically estimate the chemical composition of cloud particles; this enables us
to better understand the sulfur and chlorine cycles in the atmosphere-crust
system (Volkov 1983; VoLkov e! al. 1986~.
The lack of instrumental determinations of microconstituents in the
OCR for page 225
AMERICAN AND SOVIET RESEARCH
225
troposphere at altitudes below 20 kilometers prevents us from solving the
critical problem of the ratio of gaseous sulfur: H2S ~ COS ~ SO2 (Lewis
1970) or H2S + COS < SO2 (Khodakovskii et al. 1~79; see Table 2~.
Furthermore, gas chromatographic determination of oxygen by the Soviet
"Venera-13, and -14" probes cannot be reconciled with the concurrent pres-
ence in these same samples of 80 ppm H2S and 40 ppm COS (see VoLkov
and Khodakovskii 1984 for greater detail). The only original attempt to
experimentally estimate the oxidation-reduction regime on Venus' surface,
using a "Kontrast" detector on the Soviet "Venera-13, and -14" probes
(Florenskii et al. 1983) pointed to the presence in the near-surface layer of
the troposphere of a reducing agent (CO). However, it does not give us a
clear-cut solution to the oxygen problem.
The results from estimations of the chemical composition of the tro-
posphere and the nature of the processes occurring in its near-surface layer
can be summarized in three conclusions:
(1) Chemical equilibrium in the troposphere of Venus has generally
not been reached.
(2) The vertical gradients of SO2, H2O and CO concentrations are
a function of the competition between physical and chemical processes in
the troposphere.
(3) The near-surface troposphere can be seen as a layer in a state of
"frozen" chemical equilibrium.
Unfortunately, we have yet to resolve the question of the oxidation-
reduction regime on Venus' surface, as well as the problem of the existence
of free oxygen in the troposphere.
THE MINERAL COMPOSITION OF SURFACE ROCK ON VENUS
Many investigations have attempted to estimate the possible mineral
associations on the surface of Venus using chemical thermodynamic meth-
ods.
Mueller published the first such study as a component of the afore-
mentioned chemical model of the atmosphere (Mueller 1963) and obtained
the following results:
· Temperature and pressure on Venus' surface are consistent with
silicate-carbonate equilibrium, and carbon is bound in the rock in COCOS
form;
Oxygen partial pressure is buffered by Fe-containing minerals;
· Graphite and the native metals are not stable;
· Nitrogen is not bound in the condensed phases;
OCR for page 226
226
PLANETARY SCIENCES
· A number of chlorine- and fluorine-containing minerals are stable
at the surface.
Lewis (1970) calculated 64 mineral equilibria in order to estimate P
and T on the surface before the probes performed these measurements.
One out of three proposed options for the P and T values was in satisfactory
agreement with the actual values obtained a year later. Lewis yielded the
following, additional forecast estimates:
· Surface rock contains H2O molecules bound in the form of tremo-
lite;
.
sulfur is bound in the cloud layer in the form of mercury sulfides.
Carbonyle-sulfide is the dominant form in which sulfur is found in the
troposphere. This prediction proved only partially true: sulfur is actually
the main component of cloud particles, but the latter consist primarily of
H2SO4.
A series of studies to calculate mineral composition was conducted in
1979-83 at the VI. Vernadskiy Institute using the computation of the phase
ratios in multicomponent, gaseous systems, modeling the atmosphere/sur-
face-rock system. The computations were based on troposphere chemical
analysis data from the Soviet "Venera" series of probes, "Pioneer Venus,"
thermodynamic constants of about 150 phases, the chemical components
of terrestrial magmatic rock, and the results of x-ray-fluorescent analysis of
rock at three probe landing sites ("Venera-13," "Venera-14," and "Vega-
2"~. Compilation of this material can be found in VoLkov et al. (1986~. It
should be stressed that three important predictive conclusions were made
before the first data on the chemical composition of Venus' bedrock were
obtained:
· Sulfur may be bound as sulfates (CaS04) and/or sulfides (FeS2),
and its concentration greatly exceeds known sulfur levels in terrestrial
equivalents;
.
Water-containing minerals are unstable;
· Carbonates are unstable;
· Magnetite Fe3O4 must be a widespread constituent both as primary
and as altered bedrock
These conclusions were generally confirmed, albeit with some refine-
ment, by comparing them with X-ray-fluorescent analyses at "Venera-13,"
"Venera-14" and "Vega-2" landing sites, and by further, more detailed
theoretical investigations (Zolotov 1985, 1989~.
Sulfur at the surface probe landing sites, if we judge from the data
of additional, postflight calibration investigations (Surkov et al. 1985), is
in an anhydrite form (Cased. Sulfur content may serve as a measure of
convergence to the state of chemical equilibrium relative to SO2 in the
OCR for page 227
AMERICAN AND SOWET RESEARCH
227
atmosphere-crust system (Lewis and Prinn 1984; VoLkov et al. 1986~. It
may be possible that rock with a maximum level of sulfur (1.9 mast %,
"Vega-2") were in contact with the atmosphere longer than the bedrock at
the landing sites of the Soviet "Venera-13" and "Venera-14" probes.
In his 1985 study, Zolotov conducted thermodynamic assessments of
carbonate stability depending on the concentration of SO2, since a reaction
such as:
CaC03 + 1.5SO2 ~ CaSO4 + CO2 + 0.25S2
takes place in Venus surface conditions free of kinetic constraints. As it
turned out, the presence of SO2 in quantities exceeding 1 ppm excludes
the existence of calcite and dolomite. However, magnesite (MgCO3), as a
product of the alteration of pure forsterite, MgSiO4 (Folk), may be stable
at altitudes of 1.5 to eight kilometers.
Zolotov demonstrated in this same study (1985) that hematite (Fe2O3)
may even be stable at an altitude of more than 1.5 kilometers (Figure 2), in
addition to magnetite (Fe3O4) (the product of water vapor-driven oxidation
of Fe-containing silicates, CO2 and SON. Hematite stability is apparently
confirmed by the results obtained from interpreting the surface color on
the TV images from "Venera-13 and -14" (Shkuratov e! al. 19~7~.
Nevertheless, in their 1980 study Lewis and Kreimendahl retain the
conclusion regarding calcite (CaC03) and wustite (Fe O) stability, while
allowing for the prevalence of H2S and COS over SO2 in conditions of
total chemical equilibrium at the surface-atmosphere boundary. They come
to the same logical conclusion that in this case, the surface rock of Venus'
crust is characterized by an extremely low degree of oxidation (Fe+3/Fe+2
at one to two orders lower than the terrestrial value). Strictly speaking, the
ultimate solution to the problem of the oxidation of Venus' crust has not
been found due to the lack of instrumental data.
In 1975, WaLker (1975) drew attention to the possible dependence of
the mineral constituents of Venus' surface on the hypsometric level. The
pressure (= 65 atm.) and temperature (= 100 K) gradients are actually
so great that they may alter the composition of the phases of rock during
their exogenous cycle, that is, under the influence of aeolian transport. If
we take into account the fact that our knowledge of Venus' mineralogy
does not go beyond the framework of theoretical forecasting, the factor of
"hypsometric control" must still be considered hypothetical
Let us summarize the theoretical investigations of the chemical inter-
action of Venus' rock with its atmosphere.
· Alteration of the composition of Venus' basalts during interaction
with the aunosphere is highly probable;
OCR for page 228
228
T. K
690
705
720
735
750
765
PLANETARY SCIENCES
:
-25 -24 -23 -22 -21 -20 IgPO2
H,KM
+6
+4
+2
o
-2
-4
FIGURE 2 Estimates of the oxidation-reduction regime in the troposphere and on the
surface of Venus from data produced lay measurements (1,2,5) and computations (3,4~.
(Zolotov 1985~. 1. CO2 = CO + 2O2 (CcO2 = 96.53to; Coo = 20 ppm). 2. SO2 =
2S2 + O2 (CSO2 = 130 . 185 ppm; CS2 = 20 ppb). 3. 3Fe2O3 = 2Fe3O4 + 2O2
(buffer HM). 4. 2Fe304+ 3SiO2 = 3Fe2SiO4 + O2. S. "Kontrast" detector (Florenskiy
et al. 1983~.
· Apparently, the primary outcome stemming from this interaction
will be the sink of sulfur in the crust as anhydrite (CaSO4) and/or iron
sulfides (FeS and FeS2~;
The existence of carbonates (besides MgCO3), free carbon and
nitrogen compounds on the surface of Venus is thermodynamically prohib-
ited;
· The lack of complete factual data prevents our making a clear-cut
conclusion as to the stability of water-bearing minerals and the degree of
oxidation of the Venusian crust.
THE CYCLES OF VOLATILE COMPONENTS
Interpretation of data on Venus' atmospheric chemistry, and in par-
ticular, consideration of the photochemical processes in the stratosphere
(Krasnopolskii 1982; Yung and De More 1982) demonstrated that nitrogen
and carbon cycles are completed in the atmosphere. The H2O cycle poses
more problems, since we are not yet clear on the vertical profile of H2O
concentrations in the near-surface atmosphere.
Clearly, sulfur is the only volatile element on Venus which, in the
OCR for page 229
AMERICAN AND SOVIET RESEARCH
h, KM
80 -
50 -
30
229
catalysis
, _ _ _ _ _ _ ,
,
LCOz ~ MARCO, +O:1 ~ hv ~~'x
At/
~ 1 S(lS812.
hv ~ ~ ~
~ C 1 h`, ~ I
! tH2+~+CO I `~ +CO2
· I |SO2, H2S, COS|
~ Weathering crust ~ ~
W~
~_FeSiO3 ~, FeS2 ~CaAl2si2O-8- ~:CaSO8
///
1 -- - 2 —3 4 —- -5
~ i ~ ~
~ ///
FIGURE 3 Diagram of the cycles of C02, sulfur and chlorine in the Venusian atmosphere
1: chlorine cycle; 2: CO2 cycle; 3: rapid sulfur cycle; 4: slow sulfur cycle; 5: sulfur flux
into the crust.
contemporary geological epoch, participates in the Cyclical mass exchange
between the atmosphere and the crust. Sulfur's behavior as a constituent
of the cloud layer essentially determines its structure and dynamics. Three
cycles (Figure 3) have been discerned, depending on the rates at which
these processes unfold (Lewis and Prinn 1984~.
The rapid cycle takes place in the stratosphere and the clouds and
sets the stage for the photochemical emergence and thermal destruction of
sulfuric acid aerosols. The residence time for the SO2 molecule is estimated
OCR for page 230
230
PLANETARY SCIENCES
TABLE 3 Mineral Composition of Venusian Surface Rock Based on Theoretical Assessments
(Secondary Minerals)
1 2 3
Carbon in CaCO3 Carbonates Ca and Mg
Cow) iS
unstable
Carbonates are unstable
MgCO, (?)
H2O in amphiboles H2O in amphiboles Amphiboles (?)
and micas
Fe2O,, Fe3O4 FeO, Fe3O4 where
Xcos + XH25 > XSO2
Fe304. Fe2O3 (?) where
XCOS + XH2S < XSO2
Sulfur in sulfides of Fe and anhydatc (CaS04)
Predominantly CaS04,
sulfides of Fe are stable
Nitrogen-beanug minerals are unstable
Chlonne- and fluonde-beanng minerals are stable (fluonte/apatite?)
1. Mueller 1963, 1969
2. Lewis 197~, Lewis and Kreimendshl 1980
3. Khodalcovskiy c' al. 1978; Volicov 1983; Zolotav 1989.
to be from several hours to several years. Two alternative scenarios are
proposed in Table 4. It is difficult to select between the two because of the
lack of experimental data on the rates of certain photochemical reactions.
The slow atmospheric cycle is most likely a function of photochemical
and thermodynamic reactions in the lower atmosphere which lead to the
existence of reduced forms: H2S and COS and elementary sulfur. Ap-
parently, the stratosphere is the region of H2S and COS flux: they either
photodissociate there or are oxidized by molecular oxygen to SO3. The
time span of sulfur molecules in the cycle is estimated to be several dozen
years (Lewis and Prinn 1984~.
The mass exchange between Venus' crust and its atmosphere is carried
out in a "geological" sulfur cycle. The source of sulfur is crust matter which
produces sulfur-bearing gases through both volcanism and the interaction
of minerals with atmospheric gases, such as FeS2 with CO2, H2O, and CO.
These gases repeatedly participate in photo- and thermodynamic pro-
cesses in the atmosphere. The rapid atmospheric Cycle brings about the
long-term existence of a cloud cover made up of condensed H2SO4 parti-
cles. The competition of photo- and thermochemical reactions in the slow
cycle apparently support the existence of SO2 as the dominant form of
sulfur in the atmosphere. An excess of SO2 compared with its equilibrium
concentration in the atmosphere-crust system create an SO2 flux in the
form of sulfates in surface rock.
Two factors determine the scales and rates of flux:
OCR for page 231
AMERICAN AND SOVIET RESEARCH
TABLE 4 Sulfur Cycles on Venus
231
Cycle
Timeframe I. Fast cycle (stratosphere and cloud layer)
(Winick and Stewart 1980) (Krasnopol'skiy 1982)
CO2 + he ~ CO + O
SO2+ hv ~ SO + O
< 10 yrs. SO2 + O + M ~ SO3 + M SO + O + M ~ SO2 + M
(OH, HO2 are catalysers) SO2 + O + M ~ SO3 + M
SO3 + H2O ~ H2SO4 (Sol)
SO3 + H2O ~ H2SO4 (Sol)
II. Slow cycle (lower atmosphere and cloud layer)
SO3 + 4CO ~ COS + 3SO2 SO3 + H2 + 3CO ~ H2S + 3CO2
· e
> 10 yrs. COS + hv ~ CO + S H2S + hv ~ HS + H
COS + 1-5O2 ~ SO3 + CO
H2S + 1.502 ~ SO3 + H2
m. Geological Cycle
CaSiO3, CaAl2Si2O8 + SO2 ~ CaSO4
~ 106 yrs. FeSiO3, Fe3O4 + COS (H2S) ~ FeS(FeS2)
The time frame for a cycle to run its course depends on:
1) Mineral gas reaction rates on the planet's surface
2) Length of time during which mineral particles are in contact with the atmosphere
such as surface relief renewal rates
.
surface;
The rate of heterogeneous mineral = gas reactions on the planet's
The residence span in which mineral particle are in contact with
the atmosphere, for example, the surface relief renewal rate.
The completing of the "geological" cycle probably occurs as the altered
surface rock (rich CaS04) is resmelted in the deep regions of the crust.
Attenuated volcanic and tectonic activity on Venus ultimately reduces the
thickness of the cloud layer because sulfur is fixed in the crust and depleted
in the atmospheric reservoir.
GENERAL CONCLUSIONS
We can cite at least four firmly established facts that determine the
existence of the chemical interaction of Venus' atmosphere with its surface
rock. These are:
OCR for page 232
232
PL4NETARY SCIENCES
· Loosely porous rock on the planet's surface is developed; massive
rock display traces of corrosion and degradation;
· Inhere is no global regolith; aeolian transport on a limited scale is
supported by weak winds in the near-surface atmosphere;
· The troposphere contains reactive gases (microconstituents): SO2,
H2O, CO, and others;- Venus' basalts contain one to 1.5 more orders of
sulfur than their terrestrial equivalents.
We can make the following conclusions based on our interpretation of
the entire set of observational data:
(1) The processes of lithospheric-atmospheric interaction substantially
alter primary basalts and subject them to chemical weathering. The scale
of this process cannot be estimated;
(2) The troposphere is generally not in a state of chemical equilibrium
with the surface rock and the chemical composition of the near-surface
layer may correspond to a "frozen" equilibrium which is buffered by the
minerals.
(3) Sulfur is in a state of cyclical mass exchange between the atmo-
sphere and the crust.
(4) Nitrogen and oxygen in the crust's rock do not form stable phases.
Their cycles become completed in the atmosphere.
REFERENCES
Barsukov, V.L, V.P. Volkov, and I.L. Khodakovsky. 1982. The crest of Venus: theoretical
models of chemical and mineral composition. J. Geophys. Res. 87(suppl. Part 1~:A3-
A9.
Florenskiy, K.P., A.T. Bazilevskiy, and AS. Selivanov. 1982. Panoramas of the landing
sites of "Venera-13" and "Venera-14" (a preliminary analysis). Astron. Vestnik.
16(3):131-138.
Florenskiy, K.P., V.P. Volkov., and O.V. Nikolaeva. 1976. Towards a geochemical model of
the troposphere of Venus. Geokhimiya 8: 1135-1150.
Florenskiy, IMP., O.V. Nikolaeva, V.P. Volkov, et al. 1983. On the oxidation-reduction
conditions on the surface of Venus, based on data from the "Kontrast" geochemical
detector deployed on the "Venera-13" and "Venera-14" probes. Kosmich. issled.
21~3~:351-354.
Greely, R., J. Iverson, R. Leach, et al. Windblown sand on Venus: preliminary results of
laboratory simulations. Icarus 57~1~:112-124.
Kemurdjian, A.L., P.N. Brodskiy, V.V. Gromov, et al. 1983. Preliminary results in determining
the physical and mechanical properties of bedrock by the Soviet automated "Venera-13"
and "Venera-14" probes. Kosmich. issled. 21~3~:323-330.
Khodakovskiy, I.L., V.P. Volkov, Yu.I. Sidorov, et al. 1978. The mineralogical composition
of bedrock, and the processes of hydration and oxidation of the outer envelope of
the planet Venus (a preliminary forecast). Geokhimiya 12:1821-1835.
Khodakovskiy, I.L., V.P. Volkov, Yu.I. Sidorov, et al. 1979. A geochemical model of
the troposphere and crust of the planet Venus, based on new data. Geokhimiya
12:1748-1758.
Krasnopol'skiy, V.^ 1982. Photochemistry of the atmospheres of Mars and Venus. Nauka,
Moscow.
OCR for page 233
AMERICAN AND SOVIET RESEARCH
Z33
Krasnopol'skiy, V.A., and V.A. Parshev. 1979. On the chemical composition of the Venusian
troposphere and cloud layer, based on measurements made by "Venera-11 and -12"
and "Pioneer Venus." Kosmich. issled. 17~5~:763-771.
Lewis, J.S. 1968. An estimate of the surface conditions of Venus. Icarus 8~3~:434~57.
Lewis, J.S. 1970. Venus: atmospheric and lithospheric composition. Earth Planet. Sci. Lett.
10(1~:73 80.
Lewis, J.S., and F.A. Kreimendahl. 1980. (oxidation state of the atmosphere and crust of
Venus from Pioneer-Venus results. Icarus 42~3~:330-337.
Lewis, J.S., and R.G. Prinn. 1984. Planets and their atmospheres. Origin and evolution.
Academic Press, Orlandi.
Mueller, R.F. 1963. Chemistry and petrology of Venus: preliminary deductions. Science
141~3585~:1046 1047.
Mueller, R.F. Planetary probe: origin of the atmosphere of Venus. Science 163~3873~:1322-
1324.
Nikolaeva, O.V., LB. Ronka, and A T. Bazilevskiy. 1986. Circular formations on the plains
of Venus as evidence of its geological history. Geokhimiya 5:579-589.
Surkov, Yu.A. 1985. Cosmic investigations of the planets and their satellites. Nauka,
Moscow.
Surkov, Yu.A., L-P. Moskaleva, O.P. Shcheglov, et al. 1985. The method, equipment and
results from determinations of the elementary composition of the Venusian bedrock
made by the "Vega-2" space probe. Astron. Vestnik. 19~4~:275-288.
Shkuratov, Yu.G., M.A. Kreslavskiy, and O.V. Nikolaeva. 1987. A diagram of the albedo-
color of a parcel on the surface of Venus and its interpretation. Astron. vestnik.
21~2~:152-154.
Urey, H.C. 1951. The origin and development of the Earth and other terrestrial planets.
Geochim. et C:osmochim. Acta. 1~2~:209-277.
Vinogradov, AP., and V.P. Volkov. 1971. On Vollastonite equilibrium as a mechanism
determining the composition of Venus' atmosphere. Geochimiya 7:755-759.
Volkov, V.P. 1983. The Chemistry of the atmosphere and surface of Venus. Nauka, Moscow.
Volkov, V.P., and I.L. Khodakovskiy. 1984. Physical-Chemical modeling of the mineral com-
position of the surface rock of Venus. Pages 32-37. In: Geokhimiya i kosmochimiya.
Papers at the 27th session of the Int'l. Geol. Congr. Section C. 11. Nauka, Moscow.
Volkov, V.P., I.L Khodakovsly, and M.Yu. Zolotov. 1986. Lithospheric-atmospheric inter-
action on Venus. Chemistry and physics of terrestrial planets. Adv. Phys. Geochim.
6:136-190.
Walker, J.C 1975. Evolution of the atmosphere of Venus. J. Atm. Sci. 32~6~:1248-1256.
Schick, J.R., and A.I.F. Stewart 1980. Photochemistry of S02 in Venus' upper cloud
layers. J. Geophys. Res. 85(A 13~:7849-7860.
Yung, Y.L, and W.B. de More. 1982. Photochemistry of the stratosphere of Venus:
implications for atmospheric evolution. Icarus 51~2~:199- 247.
Zolotov, M.Yu. 1989. Chemical weathering on Venus and Mats: Similarities and Differ-
ences. Pages 71-79. In: Cosmochemistry and comparitive planetology. Proceedings of
Soviet scientists for the 28th session of the Int'l. Geol. Congr. Nauka, Moscow.
Zolotov, M.Yu. 1985. Sulfur-containing gases in the Venus' atmosphere and stability of
carbonates. Pages 942-944. In: Abstr. XVI Lunar and Planet. Sci. Conf. Houston,
part 2.
Representative terms from entire chapter:
surface rock
