1. Science & Technology Corp., Boulder, CO, United States.
The hypothesis that Venus clouds harbor life is not ruled out by the currently available remote sensing and in situ data. Several authors have suggested that unidentified UV and blue absorbers in the Venus clouds may due to microbes that metabolize sulfur or iron compounds1,2. Aside from pH, the physical condition in Venus lower clouds are Earth-like. Microbes in terrestrial water cloud aerosols have been shown to complete entire lifecycles within the aerosols. If thermo-acidophilic organisms live within the Venus clouds, what is their potential biomass? The number of microbes in each lower cloud aerosol, nB, can be calculated by solving a rate equation with growth and loss terms. Microbes grow at an exponential rate characterized by the generational life time, tB, and are lost by the evaporation of cloud aerosols at the bottom of the cloud, characterized by the aerosol residence time, tS. Microbial growth is limited by the available nutrients in the cloud aerosol, and total biomass MB is assumed to not exceed a fraction f of the aerosol mass MP. The maximum number of microbes in an aerosol is nB* = f(MP/MB) . The rate equation is: The steady state solution is: The residence time of mode 3 (r=3.6 _m) aerosols in Venus lower cloud is about 2 days3 The rate equation shows that if the microbial lifetime is shorter than this, a steady state population can exist within the aerosols. The column mass of mode 3 particles is 7.85 x 10-3 g/cm2 (Esposito et al., 1983). The global surface area of the clouds is approximately 4.6 x 1018 cm2. If microbes grow to represent one tenth of the aerosol mass, the total biomass in Venus clouds would be 3.6 billion tonnes, or about the biomass of the Earths oceans. 1Schulze-Makuch, et al., Astrobiology 4, 11-18. 2Limaye, S. S., et al, 2018. Astrobiology, 18. 3James, E. P., et al, 1997. Icarus. 129, 147-171. 4Esposito, L. W., et al, Venus D. M. Hunten et al, eds,University of Arizona Press, Tucson, 1983, pp. 484-564.