With its thick CO2 atmosphere, moonless skies, and proximity to the Sun, Venus is considered to be a close analog to common, presumably lifeless, rocky exoplanets. However, the recent suggestion of PH3 in the clouds of Venus (Greaves et al., 2020) has sparked renewed interest in the prospects for living organisms residing in the skies of Earth’s nearest planetary neighbor. As a disequlibrium species, PH3 is readily photolyzed and chemically reacts with H, OH and H2O. In addition, PH3 interacting with the ubiquitous H2SO4 cloud particles readily converts into phosphorous and phosphoric acids (H3PO3 and H3PO4, respectively). Together, these limit the mean lifetime of PH3 molecules in the Venusian clouds to < 10 hours. The possible discovery of ~1-20 ppb PH3 then means that this amount needs to be regenerated approximately every half Earth day. With no known natural photo- or thermo-chemical means to sufficiently generate PH3 from other phosphorus compounds, a working hypothesis is that PH3 is generated by microbial organisms, as occurs on Earth. Irrespective of whether PH3 is eventually confirmed by future observations, in-depth investigation of the present atmosphere of Venus is fundamentally important for understanding mysterious climate history of the planet, as well as the workings of exo-Venuses that are likely going to be the most observable type of exoplanets in the foreseeable future. As proposed by recent mission studies — both a large Flagship class mission (Gilmore et al., 2020) and a more narrowly focused New Frontiers class mission (Baines et al., 2020) — a balloon-based mission to the clouds of Venus would use in-situ measurements to directly investigate the chemistry, dynamics, and potentially biological processes within the cloud environment of our “exoplanet next door”. Utilizing the large (~80 m s-1) zonal winds that predominate at < 60o latitude, the aerobot mission concept would circle the planet more than a dozen times over a notional 100-Earth-day science phase as it likely wanders poleward from its deployment near 10o latitude, with an excellent chance of visiting high latitudes >50o. Onboard instrumentation would sample the environment over all times of day including the composition of the air and aerosols, including (1) phosphorous compounds potentially linked to life processes, (2) UV-absorbing materials which possibly are also linked to astrobiology, (3) the reactive sulfur-cycle gases that create the dominant H2SO4 aerosols, and (4) the noble gases, their isotopes and the isotopes of light gases — key to understanding the formation and evolution of the planet and its atmosphere. A digital holographic microscope would image particles in three dimensions at 0.7 micron-scale spatial resolution, searching for cellular morphologies. The balloon mission also directly and continuously measures the pressure/temperature structure, and, supported by balloon-tracking orbiter, winds in all three dimensions. The aerobot, capable of multiple 10-km-altitude traverses centered near 55-km (~0.5 bar, 25C), would enable 3-dimensional maps of these environmental characteristics as well as the dynamically/chemically influenced size distribution of aerosol particles via a nephelometer/particle-counter (Renard et al., 2020) testing, for example, the life cycle hypothesis of Seager et al (2020). These traverses also reveal the vertically-varying characteristics of atmospheric stability, gravity and planetary waves and Hadley cells, important for understanding the mechanisms that power and sustain the planet’s strong super-rotation. Such altitude excursions also enable measurements of radiative balance and solar energy deposition via a Net Flux Radiometer (Aslam et al., 2015), another key to understanding super-rotation.
References: Aslam, S., et al. (2015) EPSC Abstracts, Vol 10. EPSC2015-388. Baines, K. H. et al. (2020). New-Frontiers Class In-Situ Exploration of Venus: The Venus Climate and Geophysics Mission Concept. White paper submitted to Planetary Science Decadal Survey 2023-2032. Gilmore, M.S., Beauchamp. P. M., Lynch, R., Amato, M. J., et al. (2020). Venus Flagship Mission Decadal Study Final Report. https://www.lpi.usra.edu/vexag/reports/Venus-Flagship-Mission_FINAL.pdf Greaves JS., Richards MS., Bains W. et al. (2020) Phosphine in the cloud decks of Venus. Nature Astronomy doi.org/10.1038/s41550-020-1174-4. Renard, J.-B., Mousis, O., Rannou, P., Levasseur-Regourd, A. C., Berthet, G., Geffrin, J.-M., Hadamcik, E., Verdier, N., Millet, A.-L., and Daugeron, D. (2020) Counting and phase function measurements with the LONSCAPE instrument to determine physical properties of aerosols in ice giant planet atmospheres, Space Science Reviews, 206, 28. Seager S, Petkowski JJ, Gao P, et al. (2020) A proposed life cycle for persistence of the Venusian aerial biosphere. Astrobiology 2021, 21:2. DOI: 10.1089/ast.2020.2244