On the estimation of Sulfuric Acid Vapor concentrations below the Venus cloud deck using the Akatsuki Radio Science Experiment

TL;DR

This paper addresses the challenge of quantifying sulfuric acid vapor below Venus’ clouds using Akatsuki’s X-band radio occultation. It introduces a PSD-based signal-processing pipeline that extracts signal power and Doppler shifts, applies FFZ averaging, and uses Abel inversion to derive refractivity, temperature, and pressure while separating defocusing, mispointing, and absorption losses. By subtracting known absorbers (CO2, N2, SO2) from total X-band absorptivity, the authors retrieve H2SO4 vapor profiles, constrained with in-situ VEGA SO2 data and an indirect Oschlísniok-based approach, yielding H2SO4 abundances up to ~15 ppm below ~43 km and near-zero above ~50 km. The results align with Venus cloud structure models and demonstrate that RO, coupled with optimized spectral analysis, can provide quantitative constraints on trace absorbers in optically thick atmospheres, with implications for atmospheric dynamics and potential future dual-frequency missions.

Abstract

We report new constraints on the vertical distribution of sulfuric acid vapor in the Venusian atmosphere, derived from a refined analysis of radio occultation (RO) data. The method estimates the power spectral density (PSD) of the received signal to recover both the signal intensity and the Doppler shift. The received signal power is estimated at 1-sec cadence which enhances the sensitivity and detection of the signal at lower altitudes of Venus, even in regions of high atmospheric opacity. After correcting total attenuation for refractive losses, absorption by known microwave absorbers is removed, leaving a residual signal attributable to sulfuric acid vapor. Two different methods of estimating the absorption due to Sulfur Dioxide have been presented, including one which incorporates in-situ data, which should better constrain the sulfuric acid vapor abundance below the clouds. Retrieved profiles for altitudes of 40 - 50 km reveal an increasing vapor abundance to more than 10 ppm below the clouds, and a sharp decline above 50 km in line with the expected saturation profile. These measurements agree with current models of the Venusian cloud structure and composition, and demonstrate that RO data, when coupled with optimized spectral analysis, can yield quantitative constraints on trace absorbers in optically thick atmospheres.

Figures (10)

• Figure 1: Schematics of the occultation geometry. Radio signal initially emitted along R$_2$ direction gets bent by an angle $\delta$ by the Venusian atmosphere and reaches the ground station on Earth. The perpendiculars to the ray-path asymptotes designated as $\textbf{a}$ the impact parameters are also shown
• Figure 2: Signal Attenuation time series data for 22 July 2020. The signal (red curve) is steady and strong in the initial stages of the experiment and starts falling drastically when the probed altitude goes below 100 km, at 18720s. The ingress cut-off is marked by the blue dashed line, after which only the noise floor is seen in the power spectrum until 20442s, when the signal again emerges from the noise at the egress phase of the occultation experiment.
• Figure 3: Frequency Residuals for 22 July 2020 ingress. The first figure (top left) shows the combined residuals of the ingress and egress phases of the occultation experiment. The second plot (top right) shows the linear trend in the residuals (green line) often observed in planetary occultation experiments primarily due to prevailing background conditions, and how a linear baseline correction is done to remove its effect. Next, (bottom left) it is seen that even though large fluctuations in the residuals exist, the predominant linear (or non-linear) trend in the residuals in the deep atmosphere is still maintained even through the fluctuations. A linear trendline (red line), as shown in the bottom right plot, in the non fluctuating region of the lower atmosphere (between $18750s$ and $18840s$) is extrapolated forward in time till the ingress cutoff (blue dashed line) is reached, and a threshold of around 40 Hz is used to pick out the "correct residuals" from the data.
• Figure 4: Frequency Residuals for 22 July 2020 Egress. The bottom panel shows the non linear trend in the residuals, that has been captured by using a second order polynomial fit in the non-fluctuating regions of the egress experiment when the ray is still probing the neutral Venusian atmosphere.
• Figure 5: Signal Attenuation Altitude Profile Unaveraged (top panel) and 1 km Averaged (bottom panel) for 22 July 2020 ingress. The red curve is the total signal attenuation ($\phi$), the blue is the defocusing loss (L) and the total absorption loss $\tau$ = $\phi$ - L, is represented by the green curve. $\tau$ is clearly seen to increase exponentially below 50 km altitudes, primarily due to the Sulfuric acid vapors present below the clouds.