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On the iron chloride aerosol in the clouds of Venus
Accepted Manuscript
On the Iron Chloride Aerosol in the Clouds of Venus Vladimir A. Krasnopolsky PII: DOI: Reference:
S0019-1035(16)30650-9 10.1016/j.icarus.2016.10.003 YICAR 12219
To appear in:
Icarus
Received date: Revised date: Accepted date:
27 March 2016 10 September 2016 2 October 2016
Please cite this article as: Vladimir A. Krasnopolsky , On the Iron Chloride Aerosol in the Clouds of Venus, Icarus (2016), doi: 10.1016/j.icarus.2016.10.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
On the Iron Chloride Aerosol in the Clouds of Venus Vladimir A. Krasnopolsky Department of Physics, Catholic University of America, Washington, DC 20064, USA
Highlights
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Moscow Institute of Physics and Technology (PhysTech), Dolgoprudnyy 141700, Russia
Mode 1 aerosol consists of FeCl3 in the middle and lower clouds Loss of FeCl3 by coagulation with sulfuric acid
Iron chloride fractions are 17 and 19 ppbv in the atmosphere and rocks
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Abstract
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Iron chloride in the Venus clouds is under discussion for three decades, and the saturated vapor pressure of this species is of crucial importance for its modeling. There is a great scatter in the published data, and the preferable results are by Rustad and Gregory (1983, J. Chem. Eng. Data 28, 151-155) and those based on thermodynamic parameters by Chase (1998, J. Phys. Chem. Ref. Data Monograph 9). Using these data, loss by coagulation with sulfuric acid, transport by eddy diffusion, and the Stokes precipitation, the model confirms conclusions of our early study (Krasnopolsky 1985, Planet. Space Sci. 33, 109-117) that FeCl3 in the Venus clouds (1) agrees with the near UV and blue reflectivity of Venus (Zasova et al. 1981, Adv. Space Res. 1, #9, 1316), (2) was observed by the direct X-ray fluorescent spectroscopy, (3) explains the altitude profiles of the mode 1 aerosol in the middle and lower cloud layers and (4) the decrease in the NUV absorption below 60 km. Here we add to these conclusions that (5) the delivery of FeCl3 into the upper cloud layer and the production of sulfuric acid are just in proportion of 1 : 100 by mass that is required to fit the observed NUV albedo. Furthermore, (6) the mode 1 and 2 particle sizes fit this proportion as well. Finally, (7) the required Fe2Cl6 mixing ratio is 17 ppbv in the atmosphere and the FeCl3 mole fraction is 19 ppbv in the Venus surface rocks.
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Keywords
Atmospheres, composition; Venus; Venus, atmosphere 1. Introduction
Zasova et al. (1981, see also Krasnopolsky (1986, 2006)) argued that the blue and near ultraviolet absorption in the upper clouds of Venus can be caused by iron chloride FeCl 3 diluted in the sulfuric acid droplets with concentration of ~1% (Figure 1). Absorption spectra of some other species were calculated as well. According to the figure, sulfur aerosol, chlorine, and nitric dioxide may contribute but cannot explain the NUV absorption. Furthermore, photochemical 1
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estimates of their abundances are far below the values used to fit the Venus albedo. This also refers to Sa + SCl2 that fits the NUV spectrum. For example, all photochemical models starting from Yung and DeMore (1982) give either no sulfur aerosol or its very low abundance. This abundance in model A of Zhang et al. (2012) is smaller than the mode 1 aerosol by a factor of 5×104 at 65 km. The most abundant sulfur aerosol is in the model by Krasnopolsky (2012); however, it is smaller than the mode 1 aerosol by more than a factor of 200 (Krasnopolsky 2016). Carlson et al. (2016) analyzed the VIRTIS-M spectra from Venus Express at 350-640 nm. They are similar to those previously observed with weaker NUV absorption. The absorption by sulfur is claimed without justification in their report. Chemical production of free sulfur is significant below the clouds with the S 8 fraction of 2.5 ppm near the cloud bottom (Krasnopolsky 2013). This sulfur condenses in the lower cloud layer and constitutes ~10% of its mass loading. The lower cloud layer is not seen from outside, and sulfur is not the NUV absorber (Krasnopolsky 2016). Although FeCl3 looks exotic, the surface of Venus is so hot that some rocks can generally deliver significant abundances of their vapors into the atmosphere. X-ray fluorescent spectroscopy of the cloud particulate matter collected at the Venera 12 probe revealed peaks of chlorine at 2.62 keV and iron at 6.9 keV with an intensity proportion that matched FeCl3 (Petryanov et al. 1981). Krasnopolsky (1985, hereafter Paper I) calculated mass loading of the FeCl3 aerosol assuming the dimer Fe2Cl6 mixing ratio of 15 ppbv in the lower atmosphere. Then the calculated FeCl3 aerosol perfectly fitted the mode 1 mass loading profile in the lower and middle cloud layers observed by the Pioneer Venus particle size spectrometer (Knollenberg and Hunten 1980). Here we return to this problem and consider it in more detail.
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Figure 1. Albedo of Venus at 300-500 nm is compared with those calculated for various absorbers. From Krasnopolsky (1986). 2. Saturated vapor of iron chloride
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The problem critically depends on the iron chloride saturated vapor composition and pressure at temperatures of the lower and middle cloud layers that cover a range of 280-370 K. Here we will use 350 K as a reference temperature for comparison of data from different sources. The iron chloride vapor has never been measured at the temperatures of our interest, and the measurements and compilations for the higher temperatures will be inevitably extrapolated. The ―CRC Handbook of Chemistry and Physics‖ (Haynes 2015) gives the FeCl3 saturated vapor pressure of 1, 10, and 100 Pa at T = 391, 426, and 463 K, respectively. The data are copied from a compilation by Stull (1947). The values at the lowest temperatures are approximated by (
)
(1)
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that gives p = 0.37 μbar at T = 350 K. (1 μbar = 1 dyne cm-2.) The ―Lange’s Handbook of Chemistry‖ (Speight 2005) recommends the FeCl3 vapor pressure at 433-577 K by a relationship (
)
;
(2)
then p = 0.0067 μbar at T = 350 K. The data are also based on the measurements before 1960. The Russian ―Handbook of Chemistry‖ (1971) stated that the dimer Fe2Cl6 dominates in the iron chloride vapor, and its pressure is 10-6 and 10-5 torr at T = 349.9 and 365.6 K, respectively. Then (
)
3
(3)
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and = 0.00135 μbar at T = 350 K. These data were used in Paper I to calculate the FeCl3 aerosol mass loading. The latest measurements of the iron chloride vapor were made by Rustad and Gregory (1983) in a temperature range of 422 to 575 K. They confirmed the dimer as a dominant species in the iron chloride vapor and recommended the following relationship for the saturated vapor pressure: )
(4)
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(
= 0.0104 μbar at T = 350 K. Saturated vapor is in thermodynamic equilibrium with the condensed phase. Therefore its pressure can be calculated using the thermodynamic parameters for the solid iron chloride and its FeCl3 and Fe2Cl6 vapors (Table 1). Table 1. Thermodynamic data Species ΔfH° (kJ/mol) S° (J/mol*K) FeCl3 solid -399.41 142.22 FeCl3 gas -253.13 344.20 Fe2Cl6 gas -654.38 536.93 From Chase (1998).
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The calculated relationships are (
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Then
)
.
(6)
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(
(5)
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Evidently the dimer is more abundant than the monomer by three orders of magnitude. Their pressures at 350 K are 0.0043 μbar and 5.2×10-6 μbar, respectively. Relationship (6) gives the pressure at 350 K that is between those from (3) and (4). Furthermore, thermodynamic data at the standard conditions (the room temperature) may be better applicable to the clouds of Venus. Finally, this is the latest reference, and we choose (6) for our modeling. However, uncertainties of (6) in the temperature range of our interest may be evaluated as a factor of 3. 3. Iron chloride aerosol
FeCl3 reacts with H2SO4 to form the colorless ferric sulfate. However, this process is slow in concentrated sulfuric acid, and Zasova et al. (1981) evaluated the lifetime of FeCl3 solution of about a week at the room temperature. The Venera 14 descent probe revealed the NUV absorber in a layer of 58-62 km (Ekonomov et al. 1983), that is, the observations started at 62 km, and the true absorption at 55-58 km was smaller than that at 58-62 km by a factor of 20. Residence time of the mode 2 particles of the colored sulfuric acid is equal to ~107 s ≈ 15 weeks near 60 km, taking into account both the Stokes precipitation and eddy diffusion with K ≈ 10 4 4
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cm2 s-1. However, temperature near 60 km is lower than the room temperature by ~50 K, and the colored droplets may survive during the precipitation. Nevertheless the FeCl3 solution does not exist in the middle and lower cloud layers that are warm enough. Flux of iron chloride in two phases within an aerosol layer is *
(
)
(
)(
)+
(7)
*
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Here ns is the number density of the saturated vapor and na of that in the aerosol, K is eddy diffusion, H is the scale height, and V is the Stokes-Davies aerosol precipitation velocity: (
)+
-3
(8)
-2
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where ρ = 2.9 g cm is the FeCl3 aerosol density, g = 872 cm s is the gravity at 50-60 km, r is the particle radius, η is the CO2 viscosity (1.75×10-4 g cm-1 s-1 at 350 K), and l is the mean free path (3.9×10-6 cm in CO2 at the standard conditions), The gas-phase reaction between FeCl3 and H2SO4 is slower than that in solution by a factor of ~1000 (a typical ratio of liquid-to-gas density) scaled by the H2SO4 fraction of a few ppm and is therefore negligible. However, collisions between the sulfuric acid droplets and the FeCl3 particles in the middle and lower cloud layers may result coagulation and loss of FeCl3. Loss term for coagulation (Gao et al. 2014) is .
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and
(9) (10)
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is the continuity equation. Here N2 = 50 cm-3 and r2 = 1.2 μm are the number density and radius of the mode 2 particles in the middle and lower cloud layers, and r1 = 0.2 μm is that of mode 1 (Esposito et al. 1983, their table 1). Relationship (7) means that the vertical transport of a species by eddy diffusion is partly compensated by the Stokes precipitation, and the difference is spent as condensation nuclei for sulfuric acid in the upper cloud layer. It was solved analytically in Paper I using some approximations for the saturated vapor density, temperature profile, and assuming a constant V/K within 47-57 km. Here we will solve equation (10) numerically with a vertical step of 0.2 km. Zasova et al. (2006) made some adjustments to the Venus International Reference Atmosphere (VIRA) based on the Venera 15 infrared sounding and the accurate temperature profiles measured by the Vega probes. They also accounted for the Vega balloon data and radio occultations by the Venera 15 and Magellan orbiters. We apply their temperature profile at 50 to 60 km and extend it down to 47 km using the temperature gradient from VIRA (Seiff et al. 1985). Eddy diffusion is that from the basic model for sulfuric acid in the clouds of Venus (Krasnopolsky 2015): K = 104 cm2 s-1 above 54 km decreasing to 4800 cm2 s-1 at 47 km.
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The mode 1 particle modal radius is 0.2 μm in the middle and lower cloud layers (Knollenberg and Hunten 1980). However, r >> l, the precipitating flow is proportional to ( )
, and we adopt the effective mean radius of 0.3 μm in the calculations.
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The Fe2Cl6 gas mixing ratio is the lower boundary condition. It is chosen at 17 ppbv to fit the observed bottom of the mode 1 aerosol (Figure 2). The upward flux Φ is another fitting parameter, and solutions for three values of Φ are shown in the figure. Evidently the value of Φ = 1.2×10-12 g cm-2 s-1 is preferable. 4. Discussion
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Critical analysis of the existing data on the iron chloride vapor pressure demonstrates that the vapor has never been measured in the temperature range of the Venus clouds. The extrapolated values show a large scatter, and the most reliable are those from Rustad and Gregory (1983) and based on the thermodynamic parameters from Chase (1998). The calculated profile of the FeCl3 aerosol agrees with that observed by the Pioneer Venus particle size spectrometer for the mode 1 particles in the middle and lower cloud layers. There are two free parameters in the model. The first one is Fe2Cl6 abundance below the clouds, and the value of 17 ppbv is adjusted to fit the bottom of the mode 1 aerosol that is at 47.6 km in our model. The calculated mass loading peak is similar to that observed, and this is in favor of the chosen species.
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Fig. 2. Mode 1 mass loading observed by the PV particle size spectrometer (red line, Knollenberg and Hunten (1980)) is scaled by a factor of 1.5 to account for the higher FeCl3 density and compared with the calculated vertical profiles of the iron chloride aerosol. Upward fluxes of Fe2Cl6 are shown for the calculated profiles.
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Upward flux of iron chloride is the other parameter, and the best value is 1.2×10-12 g cm-2 s-1 of Fe2Cl6. This flux may be compared with the production of H2SO4 that is equal to 5.7×1011 cm-2 s-1 in the photochemical models (Krasnopolsky 2012 and references therein). Concentration is 80% in the upper cloud layer (Krasnopolsky 2015), and the flow is 5.7×1011*98/0.8/6×1023 = 1.2×10-10 g cm-2 s-1. The iron chloride aerosol being diluted in the sulfuric acid flow results in a concentration of 1%, just that required to explain the blue and near UV absorption (Zasova et al. 1981). Furthermore, if the FeCl3 particles are condensation centers for sulfuric acid, then the concentration is equal to ( )
1.3%,
(9)
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again similar to that required. Here r1 = 0.2 μm and r2 = 1.0 μm are the mode 1 and 2 modal radii in the upper cloud layer, ρ1 = 2.9 and ρ2 = 1.8 g cm-3 are their densities, respectively. However, it is necessary to bear in mind that the iron chloride particles are a small population within the mode 1 aerosol in the upper cloud layer (see Figure 2). The required mixing ratio of Fe2Cl6 is 17 ppbv, that is, 127 ppbm in the atmosphere. Using the relationship (4) from Rustad and Gregory (1983), the saturated vapor of iron chloride has a pressure of 164 bar near the surface of Venus. According to Raoult’s law, the species partial vapor pressure is equal to the saturated vapor pressure times its mole fraction in the evaporating mixture. Then the FeCl3 mole fraction in the Venus rocks is 2×17×92/164 = 19 ppbv. Here 2 is the correction for the dimer and 92 bar is the atmospheric pressure at the surface of Venus. The value looks very modest compared with 9% of FeO in the Venus rocks observed by X-ray fluorescent spectroscopy from Veneras 13 and 14 (Surkov et al. 1983). Generally the ferrous species are more stable thermodynamically than the ferric species at the surface of Venus, and FeCl2 should be more abundant than FeCl3. However, the surface color observed at the Venera and Vega probes indicates the presence of ferric rocks (Pieters et al. 1986) as well. 5. Conclusions Here we confirm the conclusions of Paper I that the presence of FeCl3 in the Venus clouds (1) agrees with the near UV and blue reflectivity of Venus, (2) was observed by the direct X-ray fluorescent spectroscopy, (3) explains the altitude profiles of the mode 1 aerosol in the middle and lower cloud layers and (4) the decrease in the NUV absorption below 60 km. Now these conclusions are based on the best available data on the saturated vapor of iron chloride and inclusion of the FeCl3 aerosol loss by coagulation with sulfuric acid. 7
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We add to these conclusions that (5) the delivery of FeCl3 into the upper cloud layer and the production of sulfuric acid are just in proportion of 1 : 100 by mass that is required to fit the observed NUV albedo. Furthermore, (6) the mode 1 and 2 particle sizes fit this proportion as well. Finally, (7) the required Fe2Cl6 mixing ratio is 17 ppbv in the atmosphere and the FeCl3 mole fraction is 19 ppbv in the Venus rocks. All these facts make it possible to consider iron chloride as a component of the Venus atmosphere and clouds with some confidence.
References
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Acknowledgment. This work is supported by Grant 16-12-10559 of the Russian Science Foundation to Moscow Institute of Physics and Technology (PhysTech) and V.A. Krasnopolsky.
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Carlson, R.W., Piccioni, G., Filacchione, G., Yung, Y.L., Gao, P., 2016. Venus's ultraviolet absorber: cyclo-octal (S8) and polymeric sulfur (Sx) and their latitudinal behavior. International Venus Conference, Oxford, UK, p.27 (abstract). Chase Jr., M.W., 1998. NIST-JANAF Thermodynamic Tables, fourth ed. J. Phys. Chem. Ref. Data Monograph 9. Ekonomov, A.P., Moshkin, B.E., Moroz, V.I., Golovin, Yu.M., Gnedykh, V.I., Grigoriev, A.V., 1983. UV photometry at the Venera 13 and 14 descent probes. Cosmic Res. 21, 254-268. Esposito, L.W., et al., 1983. The clouds and hazes of Venus. In ―Venus‖ (Hunten, D.M., Colin, L., Donahue, T.M., Moroz, V.I., editors), Univ. Arizona Press, pp. 484-564. Gao, P., et al., 2014. Bimodal distribution of sulfuric acid aerosols in the upper haze of Venus. Icarus 231, 83-98. Handbook of Chemistry, 1971. Himia, Leningrad (in Russian). Haynes, W.M., Editor-in-Chief, 2015. CRC Handbook of Chemistry and Physics, 96th Edition. Taylor and Francis, LLC. Knollenberg, R.G., Hunten, D.M., 1980. Microphysics of the cloudsof Venus: results of the Pioneer Venus particle size spectrometer. J. Geophys. Res. 85, 8039–8058. Krasnopolsky, V.A., 1985. Chemical composition of Venus clouds. Planet. Space Sci. 33, 109117. Krasnopolsky, V.A., 1986. Photochemistry of the Atmospheres of Mars and Venus. Springer, Berlin, 304 pages. Krasnopolsky, V.A., 2006. Chemical composition of Venus atmosphere and clouds: Some unsolved problems. Planet. Space Sci. 54, 1352-1359. Krasnopolsky, V.A., 2012. A photochemical model for the Venus atmosphere at 47–112 km. Icarus 218, 230–246. Krasnopolsky, V.A., 2013. S3 and S4 abundances and improved chemical kinetic model for the lower atmosphere of Venus. Icarus 225, 570-580. Krasnopolsky, V.A., 2015. Vertical profiles of H2O, H2SO4, and sulfuric acid concentration at 45-75 km on Venus. Icarus 252, 327-333. Krasnopolsky, V.A., 2016. Sulfur aerosol in the clouds of Venus. Icarus 274, 33-36. 8
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Petryanov, I.V., Andreychikov, B.M., Korchuganov, B.N., Ovsyankin, E.I., Ogorodnikov, B.I., Skitovich, V.I., Khristianov, V.K., 1981. Iron in the Venus clouds. Dokl. AN SSSR 260, 834. Pieters, C.M., et al., 1986. The color of the surface of Venus. Science 234, 1379–1381. Rustad, D.S., Gregory, N.W., 1983. Vapor pressure of iron (III) chloride. J. Chem. Eng. Data 28, 151-155. Seiff, A. et al., 1985. Models of the structure of the atmosphere of Venus from the surface to 100 km altitude. Adv. Space Res. 5 (11), 3–58. Speight, J.G., 2005. Lange’s Handbook of Chemistry, 16th Edition. McGraw-Hill, USA. Stull, D.R., 1947. Vapor pressure of pure substances—Inorganic compounds. Ind. Eng. Chem. 39, 540–550. Surkov, Yu.A., et al., 1983. Elemental composition of Venus’ rocks. Cosmic Res. 21, 308-315. Yung, Y.L., DeMore, W.B., 1982. Photochemistry of the stratosphere of Venus: Implications for atmospheric evolution. Icarus 51, 199-247. Zasova, L.V., Krasnopolsky, V.A., Moroz, V.I., 1981. Vertical distribution of SO2 in the upper cloud layer of Venus and origin of UV absorption. Adv. Space Res. 1, #9, 13–16. Zasova, L.V., Moroz, V.I., Linkin, V.M., Khatuntsev, I.V., Maiorov, B.S., 2006. Structure of the Venusian atmosphere from surface up to 100 km. Cosmic Res. 44, 364-383. Zhang, X., Liang, M.C., Mills, F.P., Belyaev, D.A., Yung, Y.L., 2012. Sulfur chemistry in the middle atmosphere of Venus. Icarus 217, 714-739.
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On the Iron Chloride Aerosol in the Clouds of Venus Vladimir A. Krasnopolsky PII: DOI: Reference:
S0019-1035(16)30650-9 10.1016/j.icarus.2016.10.003 YICAR 12219
To appear in:
Icarus
Received date: Revised date: Accepted date:
27 March 2016 10 September 2016 2 October 2016
Please cite this article as: Vladimir A. Krasnopolsky , On the Iron Chloride Aerosol in the Clouds of Venus, Icarus (2016), doi: 10.1016/j.icarus.2016.10.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
On the Iron Chloride Aerosol in the Clouds of Venus Vladimir A. Krasnopolsky Department of Physics, Catholic University of America, Washington, DC 20064, USA
Highlights
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Moscow Institute of Physics and Technology (PhysTech), Dolgoprudnyy 141700, Russia
Mode 1 aerosol consists of FeCl3 in the middle and lower clouds Loss of FeCl3 by coagulation with sulfuric acid
Iron chloride fractions are 17 and 19 ppbv in the atmosphere and rocks
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Abstract
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Iron chloride in the Venus clouds is under discussion for three decades, and the saturated vapor pressure of this species is of crucial importance for its modeling. There is a great scatter in the published data, and the preferable results are by Rustad and Gregory (1983, J. Chem. Eng. Data 28, 151-155) and those based on thermodynamic parameters by Chase (1998, J. Phys. Chem. Ref. Data Monograph 9). Using these data, loss by coagulation with sulfuric acid, transport by eddy diffusion, and the Stokes precipitation, the model confirms conclusions of our early study (Krasnopolsky 1985, Planet. Space Sci. 33, 109-117) that FeCl3 in the Venus clouds (1) agrees with the near UV and blue reflectivity of Venus (Zasova et al. 1981, Adv. Space Res. 1, #9, 1316), (2) was observed by the direct X-ray fluorescent spectroscopy, (3) explains the altitude profiles of the mode 1 aerosol in the middle and lower cloud layers and (4) the decrease in the NUV absorption below 60 km. Here we add to these conclusions that (5) the delivery of FeCl3 into the upper cloud layer and the production of sulfuric acid are just in proportion of 1 : 100 by mass that is required to fit the observed NUV albedo. Furthermore, (6) the mode 1 and 2 particle sizes fit this proportion as well. Finally, (7) the required Fe2Cl6 mixing ratio is 17 ppbv in the atmosphere and the FeCl3 mole fraction is 19 ppbv in the Venus surface rocks.
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Keywords
Atmospheres, composition; Venus; Venus, atmosphere 1. Introduction
Zasova et al. (1981, see also Krasnopolsky (1986, 2006)) argued that the blue and near ultraviolet absorption in the upper clouds of Venus can be caused by iron chloride FeCl 3 diluted in the sulfuric acid droplets with concentration of ~1% (Figure 1). Absorption spectra of some other species were calculated as well. According to the figure, sulfur aerosol, chlorine, and nitric dioxide may contribute but cannot explain the NUV absorption. Furthermore, photochemical 1
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estimates of their abundances are far below the values used to fit the Venus albedo. This also refers to Sa + SCl2 that fits the NUV spectrum. For example, all photochemical models starting from Yung and DeMore (1982) give either no sulfur aerosol or its very low abundance. This abundance in model A of Zhang et al. (2012) is smaller than the mode 1 aerosol by a factor of 5×104 at 65 km. The most abundant sulfur aerosol is in the model by Krasnopolsky (2012); however, it is smaller than the mode 1 aerosol by more than a factor of 200 (Krasnopolsky 2016). Carlson et al. (2016) analyzed the VIRTIS-M spectra from Venus Express at 350-640 nm. They are similar to those previously observed with weaker NUV absorption. The absorption by sulfur is claimed without justification in their report. Chemical production of free sulfur is significant below the clouds with the S 8 fraction of 2.5 ppm near the cloud bottom (Krasnopolsky 2013). This sulfur condenses in the lower cloud layer and constitutes ~10% of its mass loading. The lower cloud layer is not seen from outside, and sulfur is not the NUV absorber (Krasnopolsky 2016). Although FeCl3 looks exotic, the surface of Venus is so hot that some rocks can generally deliver significant abundances of their vapors into the atmosphere. X-ray fluorescent spectroscopy of the cloud particulate matter collected at the Venera 12 probe revealed peaks of chlorine at 2.62 keV and iron at 6.9 keV with an intensity proportion that matched FeCl3 (Petryanov et al. 1981). Krasnopolsky (1985, hereafter Paper I) calculated mass loading of the FeCl3 aerosol assuming the dimer Fe2Cl6 mixing ratio of 15 ppbv in the lower atmosphere. Then the calculated FeCl3 aerosol perfectly fitted the mode 1 mass loading profile in the lower and middle cloud layers observed by the Pioneer Venus particle size spectrometer (Knollenberg and Hunten 1980). Here we return to this problem and consider it in more detail.
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Figure 1. Albedo of Venus at 300-500 nm is compared with those calculated for various absorbers. From Krasnopolsky (1986). 2. Saturated vapor of iron chloride
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The problem critically depends on the iron chloride saturated vapor composition and pressure at temperatures of the lower and middle cloud layers that cover a range of 280-370 K. Here we will use 350 K as a reference temperature for comparison of data from different sources. The iron chloride vapor has never been measured at the temperatures of our interest, and the measurements and compilations for the higher temperatures will be inevitably extrapolated. The ―CRC Handbook of Chemistry and Physics‖ (Haynes 2015) gives the FeCl3 saturated vapor pressure of 1, 10, and 100 Pa at T = 391, 426, and 463 K, respectively. The data are copied from a compilation by Stull (1947). The values at the lowest temperatures are approximated by (
)
(1)
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that gives p = 0.37 μbar at T = 350 K. (1 μbar = 1 dyne cm-2.) The ―Lange’s Handbook of Chemistry‖ (Speight 2005) recommends the FeCl3 vapor pressure at 433-577 K by a relationship (
)
;
(2)
then p = 0.0067 μbar at T = 350 K. The data are also based on the measurements before 1960. The Russian ―Handbook of Chemistry‖ (1971) stated that the dimer Fe2Cl6 dominates in the iron chloride vapor, and its pressure is 10-6 and 10-5 torr at T = 349.9 and 365.6 K, respectively. Then (
)
3
(3)
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and = 0.00135 μbar at T = 350 K. These data were used in Paper I to calculate the FeCl3 aerosol mass loading. The latest measurements of the iron chloride vapor were made by Rustad and Gregory (1983) in a temperature range of 422 to 575 K. They confirmed the dimer as a dominant species in the iron chloride vapor and recommended the following relationship for the saturated vapor pressure: )
(4)
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(
= 0.0104 μbar at T = 350 K. Saturated vapor is in thermodynamic equilibrium with the condensed phase. Therefore its pressure can be calculated using the thermodynamic parameters for the solid iron chloride and its FeCl3 and Fe2Cl6 vapors (Table 1). Table 1. Thermodynamic data Species ΔfH° (kJ/mol) S° (J/mol*K) FeCl3 solid -399.41 142.22 FeCl3 gas -253.13 344.20 Fe2Cl6 gas -654.38 536.93 From Chase (1998).
)
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The calculated relationships are (
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Then
)
.
(6)
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(
(5)
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Evidently the dimer is more abundant than the monomer by three orders of magnitude. Their pressures at 350 K are 0.0043 μbar and 5.2×10-6 μbar, respectively. Relationship (6) gives the pressure at 350 K that is between those from (3) and (4). Furthermore, thermodynamic data at the standard conditions (the room temperature) may be better applicable to the clouds of Venus. Finally, this is the latest reference, and we choose (6) for our modeling. However, uncertainties of (6) in the temperature range of our interest may be evaluated as a factor of 3. 3. Iron chloride aerosol
FeCl3 reacts with H2SO4 to form the colorless ferric sulfate. However, this process is slow in concentrated sulfuric acid, and Zasova et al. (1981) evaluated the lifetime of FeCl3 solution of about a week at the room temperature. The Venera 14 descent probe revealed the NUV absorber in a layer of 58-62 km (Ekonomov et al. 1983), that is, the observations started at 62 km, and the true absorption at 55-58 km was smaller than that at 58-62 km by a factor of 20. Residence time of the mode 2 particles of the colored sulfuric acid is equal to ~107 s ≈ 15 weeks near 60 km, taking into account both the Stokes precipitation and eddy diffusion with K ≈ 10 4 4
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cm2 s-1. However, temperature near 60 km is lower than the room temperature by ~50 K, and the colored droplets may survive during the precipitation. Nevertheless the FeCl3 solution does not exist in the middle and lower cloud layers that are warm enough. Flux of iron chloride in two phases within an aerosol layer is *
(
)
(
)(
)+
(7)
*
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Here ns is the number density of the saturated vapor and na of that in the aerosol, K is eddy diffusion, H is the scale height, and V is the Stokes-Davies aerosol precipitation velocity: (
)+
-3
(8)
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where ρ = 2.9 g cm is the FeCl3 aerosol density, g = 872 cm s is the gravity at 50-60 km, r is the particle radius, η is the CO2 viscosity (1.75×10-4 g cm-1 s-1 at 350 K), and l is the mean free path (3.9×10-6 cm in CO2 at the standard conditions), The gas-phase reaction between FeCl3 and H2SO4 is slower than that in solution by a factor of ~1000 (a typical ratio of liquid-to-gas density) scaled by the H2SO4 fraction of a few ppm and is therefore negligible. However, collisions between the sulfuric acid droplets and the FeCl3 particles in the middle and lower cloud layers may result coagulation and loss of FeCl3. Loss term for coagulation (Gao et al. 2014) is .
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(9) (10)
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is the continuity equation. Here N2 = 50 cm-3 and r2 = 1.2 μm are the number density and radius of the mode 2 particles in the middle and lower cloud layers, and r1 = 0.2 μm is that of mode 1 (Esposito et al. 1983, their table 1). Relationship (7) means that the vertical transport of a species by eddy diffusion is partly compensated by the Stokes precipitation, and the difference is spent as condensation nuclei for sulfuric acid in the upper cloud layer. It was solved analytically in Paper I using some approximations for the saturated vapor density, temperature profile, and assuming a constant V/K within 47-57 km. Here we will solve equation (10) numerically with a vertical step of 0.2 km. Zasova et al. (2006) made some adjustments to the Venus International Reference Atmosphere (VIRA) based on the Venera 15 infrared sounding and the accurate temperature profiles measured by the Vega probes. They also accounted for the Vega balloon data and radio occultations by the Venera 15 and Magellan orbiters. We apply their temperature profile at 50 to 60 km and extend it down to 47 km using the temperature gradient from VIRA (Seiff et al. 1985). Eddy diffusion is that from the basic model for sulfuric acid in the clouds of Venus (Krasnopolsky 2015): K = 104 cm2 s-1 above 54 km decreasing to 4800 cm2 s-1 at 47 km.
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The mode 1 particle modal radius is 0.2 μm in the middle and lower cloud layers (Knollenberg and Hunten 1980). However, r >> l, the precipitating flow is proportional to ( )
, and we adopt the effective mean radius of 0.3 μm in the calculations.
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The Fe2Cl6 gas mixing ratio is the lower boundary condition. It is chosen at 17 ppbv to fit the observed bottom of the mode 1 aerosol (Figure 2). The upward flux Φ is another fitting parameter, and solutions for three values of Φ are shown in the figure. Evidently the value of Φ = 1.2×10-12 g cm-2 s-1 is preferable. 4. Discussion
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Critical analysis of the existing data on the iron chloride vapor pressure demonstrates that the vapor has never been measured in the temperature range of the Venus clouds. The extrapolated values show a large scatter, and the most reliable are those from Rustad and Gregory (1983) and based on the thermodynamic parameters from Chase (1998). The calculated profile of the FeCl3 aerosol agrees with that observed by the Pioneer Venus particle size spectrometer for the mode 1 particles in the middle and lower cloud layers. There are two free parameters in the model. The first one is Fe2Cl6 abundance below the clouds, and the value of 17 ppbv is adjusted to fit the bottom of the mode 1 aerosol that is at 47.6 km in our model. The calculated mass loading peak is similar to that observed, and this is in favor of the chosen species.
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Fig. 2. Mode 1 mass loading observed by the PV particle size spectrometer (red line, Knollenberg and Hunten (1980)) is scaled by a factor of 1.5 to account for the higher FeCl3 density and compared with the calculated vertical profiles of the iron chloride aerosol. Upward fluxes of Fe2Cl6 are shown for the calculated profiles.
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Upward flux of iron chloride is the other parameter, and the best value is 1.2×10-12 g cm-2 s-1 of Fe2Cl6. This flux may be compared with the production of H2SO4 that is equal to 5.7×1011 cm-2 s-1 in the photochemical models (Krasnopolsky 2012 and references therein). Concentration is 80% in the upper cloud layer (Krasnopolsky 2015), and the flow is 5.7×1011*98/0.8/6×1023 = 1.2×10-10 g cm-2 s-1. The iron chloride aerosol being diluted in the sulfuric acid flow results in a concentration of 1%, just that required to explain the blue and near UV absorption (Zasova et al. 1981). Furthermore, if the FeCl3 particles are condensation centers for sulfuric acid, then the concentration is equal to ( )
1.3%,
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again similar to that required. Here r1 = 0.2 μm and r2 = 1.0 μm are the mode 1 and 2 modal radii in the upper cloud layer, ρ1 = 2.9 and ρ2 = 1.8 g cm-3 are their densities, respectively. However, it is necessary to bear in mind that the iron chloride particles are a small population within the mode 1 aerosol in the upper cloud layer (see Figure 2). The required mixing ratio of Fe2Cl6 is 17 ppbv, that is, 127 ppbm in the atmosphere. Using the relationship (4) from Rustad and Gregory (1983), the saturated vapor of iron chloride has a pressure of 164 bar near the surface of Venus. According to Raoult’s law, the species partial vapor pressure is equal to the saturated vapor pressure times its mole fraction in the evaporating mixture. Then the FeCl3 mole fraction in the Venus rocks is 2×17×92/164 = 19 ppbv. Here 2 is the correction for the dimer and 92 bar is the atmospheric pressure at the surface of Venus. The value looks very modest compared with 9% of FeO in the Venus rocks observed by X-ray fluorescent spectroscopy from Veneras 13 and 14 (Surkov et al. 1983). Generally the ferrous species are more stable thermodynamically than the ferric species at the surface of Venus, and FeCl2 should be more abundant than FeCl3. However, the surface color observed at the Venera and Vega probes indicates the presence of ferric rocks (Pieters et al. 1986) as well. 5. Conclusions Here we confirm the conclusions of Paper I that the presence of FeCl3 in the Venus clouds (1) agrees with the near UV and blue reflectivity of Venus, (2) was observed by the direct X-ray fluorescent spectroscopy, (3) explains the altitude profiles of the mode 1 aerosol in the middle and lower cloud layers and (4) the decrease in the NUV absorption below 60 km. Now these conclusions are based on the best available data on the saturated vapor of iron chloride and inclusion of the FeCl3 aerosol loss by coagulation with sulfuric acid. 7
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We add to these conclusions that (5) the delivery of FeCl3 into the upper cloud layer and the production of sulfuric acid are just in proportion of 1 : 100 by mass that is required to fit the observed NUV albedo. Furthermore, (6) the mode 1 and 2 particle sizes fit this proportion as well. Finally, (7) the required Fe2Cl6 mixing ratio is 17 ppbv in the atmosphere and the FeCl3 mole fraction is 19 ppbv in the Venus rocks. All these facts make it possible to consider iron chloride as a component of the Venus atmosphere and clouds with some confidence.
References
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Acknowledgment. This work is supported by Grant 16-12-10559 of the Russian Science Foundation to Moscow Institute of Physics and Technology (PhysTech) and V.A. Krasnopolsky.
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