Iron sulfate, sulfuric acid, and water are major components in Venus’ aerosols

Introduction

In this work, we describe new and transformative insights into the composition of Venus’ aerosols using uncharacterized data acquired by the Pioneer Venus Large Probe (PVLP), which descended through Venus’ atmosphere in 1978 [1]. During the descent, the PVLP inadvertently collected cloud aerosols and measured the decomposition of their contents [2]. In the initial post-flight PVLP investigations, aerosol collection and analysis were inferred for the Large Probe Neutral Mass Spectrometer (LNMS) [2] and considered for the Large Probe Gas Chromatograph (LGC) [3].

Here, we demonstrate that the LNMS data contain several uncharacterized mass signals, which directly relate to the thermal decomposition of aerosols possessing a heterogeneous chemical composition. Accordingly, we re-analyzed and re-interpreted the altitude trends from the LNMS data and LGC results as the evolved gas analyses that followed the temperature gradient of the PVLP descent (-30 to 462 ˚C, ~ 65 to 0 km [4]).

Methods

Thermal decomposition profiles from the LNMS data were constructed by plotting the number densities for SO₂, H₂O, SO₃, and O₂ against the LNMS inlet temperatures, which were heated during operation. The LNMS inlet temperatures were obtained using data from the LNMS project reports [5] and PVLP descent timeline [6]. The mass spectra obtained at the electron ionization energies of 70, 30, and 22 eV were independently treated using the procedures described in [7-9]. The peaks for SO₂, H₂O, SO₃, and O₂ in the thermal profiles were fit to Gauss functions and the regressions minimized by least squares analysis. Using the stoichiometry in Reactions 1-3, the peak areas for SO₂ and H₂O were converted to weight percent (w%) and mass loading (mg m^-3) for H₂SO₄, Fe₂(SO₄)₃, and H₂O (after correction for the H₂O from Reaction 1). Review of the LNMS CO₂ density profile [9] revealed that aerosols were collected between ~ 51.0 to 48.4 km. The collection column was treated as a cylinder with a height of 2.6 km and diameter of 0.98 m, which equaled the outer PVLP diameter [10].

H₂SO₄  →  SO₃  +  H₂O                   (1)

SO₃  →  SO₂  +  0.5 O₂                    (2)

Fe₂(SO₄)₃  →  Fe₂O₃ +  3 SO₃       (3)

Results and Discussion

The LNMS mixing ratios (x) for SO₂ and H₂O (Fig. 1) are inconsistent with other Venus measurements obtained spectroscopically. Yet, after aerosol capture (< 51.0 km, Fig. 1), the LNMS and LGC xSO₂ and xH₂O are within error (or comparable). Moreover, the cloud xH₂O from the LNMS are lower than other Venus measurements obtained by direct analyses (Fig. 1). These combined results suggest that the xSO₂ and xH₂O from the LNMS and LGC do not represent atmospheric gases. Instead, the LNMS and LGC xSO₂ and xH₂O the represent the gases released during the thermal decomposition of the captured aerosols. The differing maxima observed in the LNMS xSO₂ and xH₂O (at ~ 35 and 10 km) suggest the decomposition of ≥ 2 sulfate-bearing compounds from the captured aerosols.

Re-expression of the LNMS data as evolved gas profiles (Fig. 2) revealed the temperature-dependent release of SO₂, H₂O, SO₃, and O₂ from the captured aerosols. Parsing of the LNMS data additionally revealed signals consistent with the release of metal-bearing species (e.g., FeO⁺ and MgSO₄⁺). These results are consistent with an aerosol composition including H₂SO₄, hydrated ferric sulfates, possibly hydrated magnesium sulfate, and other hydrates. The thermal decomposition reactions for H₂SO₄ and ferric sulfates are provided in Reaction 1-3. The inferred aerosol relative abundances amounted to 22 ± 4 wt% H₂SO₄, 16 ± 3 wt% Fe₂(SO₄)₃ (projected), and 62 ± 8 wt% H₂O. The aerosols H₂O predominantly released from hydrates between ~ 270–460 ˚C (59 ± 8 wt%), consistent with the loss of H₂O from hydrated iron and magnesium sulfates and other hydrates (e.g., [11, 12]).

Thus, this work reveals that Venus’ aerosols contain a previously underestimated reservoir of water and possible altered materials derived from iron and magnesium of cosmic origin. These results provide new considerations for cloud chemistry models and cloud habitability discussions.

 

Figure 1. The LNMS apparent mixing ratios for (A) SO₂ and (B) H₂O compared to the LGC and other Venus measurements. Brackets to the right side of the plots indicate where the differing aerosol components decomposed. The LNMS values (squares) were obtained from the spectra obtained at 70 eV (red squares) and 30 and 22 eV (green squares). The LGC (yellow circles) error bars represent the reported 3 s confidence intervals. The comparative Venus values (green triangles) for (A) SO₂ and (B) H₂O are respectively labeled in the top legends. Pertinent abbreviations include NIR (near infrared spectroscopy), GC (gas chromatograph), UVS (UV spectroscopy), V12 (Venera 12), V13/14 (Venera 13 and 14), and V11/13/14 (Venera 11, 13, and 14).

 

Figure 2 The LNMS evolved gas profiles for (A) SO₂ and (B) H₂O expressed against the LNMS inlet temperatures (lower x-axis) and associated altitudes (upper x-axis). Number densities are from the 70 (circles), 30 (squares), and 22 eV (diamonds) spectra. Fits to the LNMS data (solid red lines) represent the sum of the numbered Gauss peaks (dashed lines). Temperatures at maximum decomposition (T_D) for each peak (n) are listed. Dotted lines demark the LNMS clog (~ 50-25 km).

 

References

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[5] Final Report, Large Probe Neutral Mass Spectrometer, August 31, 1978, NASA Ames Research Center History Archives, Collection Number AFS8100.15A.

[6] Seiff A. et al. (1980) JGR Space Sci., 85, 7903-7933.

[7] Mogul R. et al. (2021) GRL, 48, e2020GL091327.

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[10] Dutta S. et al. In AIAA SCITECH 2023 Forum (2023), 1165.

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[12] Spratt H. et al. (2014) J. Therm. Analysis Calorim., 115, 101-109.