Venus’ Aerosol Composition Extracted from Pioneer Venus Data

In this work, we show that Venus’ aerosols possess a complex chemical composition and contain previously underestimated reservoirs of water and iron sulfate. These assessments are based on re-analyses of data acquired in Venus’ atmosphere in 1978 by the Pioneer Venus Large Probe (PVLP). Data from the Large Probe Neutral Mass Spectrometer (LNMS) and Gas Chromatograph (LGC) are consistent with evolved gas analysis. During descent through the clouds, aerosols likely collected into the intake inlet assemblies of the LNMS and LGC. The collected aerosols then differentially decomposed through the increasingly hot atmosphere and released gases into the LNMS and LGC. Our treatment of LNMS data indicates that aerosols from ~ 51-48 km contain sulfuric acid (H₂SO₄) and iron sulfate(s) (e.g., Fe₂(SO₄)₃) in similar masses (~ 1 mg m^-3) and 3-fold higher abundances of H₂O (~ 3 mg m^-3). The substantial aerosol-phase H₂O likely arises from hydrates such as hydrated sulfates of iron and magnesium. Our inferred total aerosol mass loading, H₂SO₄ mass loading, and relative abundances of H₂SO₄ and H₂O in the volatile fraction of the aerosol (sulfuric acid solution) are consistent with all preceding measurements. We suggest that all direct measurements conducted in Venus’ clouds – to date – sampled and analyzed the cloud aerosols. Aerosol-phase H₂O was likely measured by the LNMS, LGC (Oyama et al., JGR, 85, 1980), Venera 13 and 14 gas chromatographs (Gel'man et al., Cosm. Res., 17, 1980, Mukhin et al., Sov. Astron. Let., 8, 1982), Venera 13 and 14 hygrometers (Surkov et al., Sov. Astron. Lett.l, 8, 1982), and Vega 1 and 2 moisture meters (Surkov et al., JGR Solid Earth, 91, 1986) – which independently measured high abundances of water in the clouds. Aerosol-phase iron was likely measured by the LNMS and suggested by X-ray radiometric data from Venera 12 (Petryanov et al., Soviet Physics Doklady, 260, 1981) and Vega 1 and 2 (Andreichikov et al., Cosm. Res., 25, 1987). Hence, these combined assessments highlight reservoirs of bulk water, iron sulfate, and possible cosmic materials (e.g., Fe and Mg) in Venus’ aerosols. This aerosol composition presents new considerations for Venus’ cloud chemistry, spectroscopy (e.g., refractive index and UV absorption), and habitability assessments. Further, these results apply to the upcoming DAVINCI mission and Venus Orbiter Mission, which plan to sample within and above the clouds, respectively.