FIG. 9.

A schematic representation summarizing the ideas presented in this hypothesis paper regarding the potential for microorganisms to survive in Venus' lower clouds and contribute to the observed bulk spectra. In this scheme, the approximate altitude and temperatures are shown on the left axis, the approximate pressure on the right axis, while the surface topography represents an exaggerated perspective view of Venus. The cloud layer is depicted by a yellow-tinted hazy region between an altitude of ∼47 and 72 km, where the varying opacities and thicknesses represent differences in mass loading. The black dots within the cloud layer depict the sulfuric acid aerosols with diameters ranging from 0.2 μm (which are found as high as 90 km) to 2.5 μm and to as large as 36 μm (in smaller quantities) near the bottom of the cloud layer (Knollenberg and Hunten, 1980); aerosols below the cloud base have also been reported by the Venera probes. The hypothetical microbial contents of particles from the lower cloud layer are depicted in a magnified view using the dashed-line callout bubble, which shows differing possible microbial morphologies. These microorganisms may potentially survive by fixation of carbon dioxide (CO₂) through the phototrophic or chemolithotrophic oxidation of iron and sulfur compounds, and by a coupled iron-sulfur metabolism (depicted by the blue reaction scheme). The cloud-based microbial communities may remain afloat through gravity waves (red wavy line), which propagate up, and are triggered by westward ambient flows over the elevated topographies; gravity waves have been detected at the cloud tops in thermal infrared in the Akatsuki data (Fukuhara et al., 2017). Additionally, the convective activity of the lower cloud region may persist on the night side, thereby leading to opacity variations and differing thermal emissions through the cloud layer, as is observed in the near infrared in the Akatsuki and Venus Express data. Consequently, the spectra of Venus may include contributions from the cloud-based microorganisms, as is depicted by the dashed-line callout originating from the magnified view of the particles; the inset spectral plot shows the albedo of Venus compiled from differing observations (red) and the sunlight absorption estimated by a singular measurement on the dayside (at one location), as calculated from the difference between the VIRA cloud model and the MESSENGER spectra (Perez-Hoyos et al., 2017). The absorption of sunlight may actually extend to much longer wavelengths based on muted contrasts observed by the Akatsuki orbiter (Limaye et al., 2018), which is consistent with the albedo variation with wavelength.