Powerful lightning on Venus constrained by atmospheric NO

TL;DR

This study tackles whether Venus hosts lightning by constraining the needed lightning power to sustain observed NO in the atmosphere. It employs a 1D photochemical-kinetic model (ARGO) with the STAND2022 network to propagate NO production and destruction through the atmosphere, linking NO flux to lightning energy via a yield parameter. The analysis indicates that sustaining the measured below-cloud NO requires a global lightning power of about $P \approx 2.4 \times 10^{11}$ J s$^{-1}$, i.e., at least three times Earth's total lightning power, which can be achieved by more frequent strikes, higher energy per strike, or a combination of both. The results also suggest Lightning may occur deeper in Venus’s atmosphere, potentially explaining the lack of unambiguous optical detections, and highlight the need for future missions to confirm NO abundances below the clouds or detect additional lightning signatures to establish definitive evidence of Venusian lightning and its implications for planetary atmospheres.

Abstract

Signs of lightning on Venus have long been sought, including by space missions and ground-based telescopes searching for optical flashes, plasma waves, or radio signatures. These efforts have yielded conflicting findings regarding the presence or absence of lightning in Venus's atmosphere. In this study we adopt an indirect approach to constrain the prevalence of lightning on Venus, using the chemical by-products it produces in Venus's atmosphere. Nitric oxide (NO) is a key tracer species of lightning, being exclusively generated by lightning in Venus's lower atmosphere. By calculating the present rate of atmospheric destruction of NO in Venus's atmosphere through photochemical-kinetic modelling, we constrain the lightning power required to sustain the estimated NO abundances on modern Venus. The reported NO constraints require lightning to generate at-least three times the power released on Earth; consistent with either a higher rate of strikes, or greater energy per strike, or a combination of both. Limited detections of optical flashes within the clouds could point to lightning striking deeper in the atmosphere and nearer the surface -- with the result that its optical flashes are obscured by the clouds -- driven by triboelectric charging during volcanic eruptions or wind interactions with surface sediments. Our findings underscore the importance for future missions of confirming lightning on Venus, either by verifying the below-cloud NO abundance, or by detecting another unambiguous lightning signature, to provide the first definitive evidence of lightning on a rocky planet other than Earth.

Figures (9)

• Figure 1: Modelled NO mixing ratios in Venus's atmosphere compared with observational constraints. a, Comparison of atmospheric chemistry models. Our complete model (black solid line) integrates the rimmer2021hydroxide network with additional reactions from Krasnopolsky2012. Other models shown include those from Krasnopolsky2012 (turquoise dashed), bierson2020chemical (blue dashed), dai2024investigation (orange dashed), and the original rimmer2021hydroxide network without additions (pink dashed, primarily hidden under black dashed line). Grey shaded regions represent NO measurements from the TEXES spectrograph at NASA’s Infrared Telescope Facility KRASNOPOLSKY200680 and upper limits derived from non-detections by Venus Express (VEx) MAHIEUX2024115862. All models exceed the upper limits in the cloud top region ($\sim$60--70 km), underscoring a persistent discrepancy between model predictions and observations. b, Sensitivity of the complete network model to variations in the in-cloud eddy diffusion coefficient (100--1000 cm$^{2}$ s$^{-1}$). The nominal profile (black dashed) over-predicts NO abundances above the cloud layer, while reduced in-cloud mixing (coloured lines) lowers NO to within the VEx upper limits. eddy profiles used are plotted in Supplementary Figure \ref{['fig:eddy_profs']}.
• Figure 2: Sensitivity of modelled NO mixing ratios to individual reactions in the chemical network. Profiles show the NO mixing ratio as a function of altitude when specific reactions (and their reverse reactions) are removed from the network to assess their impact. Left, are the complete profiles, and Right, profiles are zoomed-in the cloud area where NO is depleted. The solid black line shows the baseline result with the full chemical network included. Grey shaded regions indicate observational constraints: NO measurements from the TEXES spectrograph KRASNOPOLSKY200680, and upper limits from non-detections by VEx MAHIEUX2024115862. Recombination reaction N + O <-> NO was not removed, but instead its reaction rate was replaced with that of Krasnopolsky2012 for comparison ( replacement rate coefficient is listed in Table \ref{['table:kr12reactions']}).
• Figure 3: Vertical profiles of the most abundant nitrogen-containing species as a function of altitude in our model. Left, Nominal eddy diffusion profile used in rimmer2021hydroxide, which overestimates NO abundances relative to observational constraints. Right, Modified eddy profile with reduced in-cloud diffusion (700 cm$^{2}$ s$^{-1}$), consistent with NO upper limits from Venus Express (VEx) MAHIEUX2024115862.
• Figure 4: Lightning energy and rate values consistent with the required lightning power for Venus. The best-fit curve (best agreement with observational constraints on atmospheric abundance of NO, orange line) is plotted as derived from our atmospheric photochemical-kinetic model, with in-cloud $K_{zz}$=700 cm$^{2}$ s$^{-1}$. The broader possible power range, as determined from the minimum depletion flux from observational constraints and alternative in-cloud $K_{zz}$ values, extends to higher energies and rates than the plotted range (orange area filled-in; Section \ref{['sec:results']}). For reference, also plotted is the Earth's average and energy per lightning strike Maggio2008 with the observed lightning rate Christian2003, and the equivalent rate if the entire surface was only made up of land, without any oceans 2016Hodos (lower and upper green circles on the left, respectively), and the average rate with the maximum energy per lightning strike Maggio2008 (right-most green circle). Also indicated are the global lightning flash rate of up to 320 s$^{-1}$ inferred from whistler mode waves 2022Hart, and minimum optical energy limit of $1.2\,\times\,10^{9}$ J from on ground-based observations of optical flashes on Venus KRASNOPOLSKY200680 (neither are confirmed to be linked to lightning).
• Figure 5: Vertical eddy diffusion profiles used as a function of altitude in Venus’s atmosphere. The nominal profile (turquoise) follows that of rimmer2021hydroxide and 2007Krasnopolsky. Modified profiles vary the in-cloud eddy diffusion coefficient within the cloud layer (45–65 km altitude), adopting values of 100, 500, 700, and 1000 cm$^{2}$ s$^{-1}$ (orange, purple, and green respectively). Below and above the cloud layer, all profiles follow the same nominal structure.