Lessons from Earth Bioaerosol Sampling for Venus Habitability & Biosignature Science

Venus's clouds are often discussed as a potential habitat. Several high-priority science goals - cloud aerosol composition, internal radiative flux, and aerosol residence time and circulation models - will improve our understanding of Venus in an astrobiology context. Lessons from Earth's aerobiosphere can inform these requirements and interpretation of results.

A stable Venus aerobiosphere would need microorganisms to reproduce faster than they settle out due to gravity. This is a joint constraint of the potential nutrients and energy influx, bioenergetic costs such as desiccation and radiation damage, and aerosol dynamics. Though life cycles involving S- and Fe-based redox metabolism have been proposed, at an optimistic estimate of 75% H₂SO₄, Venus aerosol water activity (a_w) is still ~0.02, far below the observed microbial growth limit of ~0.6. Long-term desiccation with brief spurts of repair, growth and reproduction in response to transient water influx (perhaps volcanism) is the most likely model for Earth-like life on Venus - a `desert bloom' scenario.

In Earth's troposphere, clouds can carry 10³ - 10⁵ cells/mL, some metabolically active. A better analogue may be Earth's stratospheric sulfate aerosol layer: supercooled sulfuric acid aerosols (acid weight fraction 0.6 - 0.85, 0.1 - 1 µm diameter, 0.1 - 1 cm^-3) with little water activity, long residence times, high UV radiation, and only sporadic influx from surface particle sources. Viable cells isolated from this region are rare (~10² cells/m³), though `hot spots' at tropospheric mixing occur. Cells are primarily in inactive forms such as spores, and it is unclear if any are associated with sulfate aerosols or simply co-located.

Given the heterogeneities in Venus clouds and the model of a sparse, largely dormant ecosystem, a single transect on descent would most likely pass through a low-water, inactive region, missing potential signs of habitability or biosignatures. Even a passive aerial platform (balloon) may end up following a single air mass over its lifetime. A better strategy for Venus would target an aerial region with some upwelling from surface sources, and take multiple transects separated in time and space. Earth's stratospheric bioaerosol population could provide a starting point for estimating the required lower volume and detection limits.