The Venus Life Equation

Ancient Venus and Earth may have been similar in crucial ways for the development of life, such as liquid water oceans, land-ocean interfaces, favorable chemical ingredients and energy pathways. If life ever developed on, or was transported to, early Venus from elsewhere, it might have thrived, expanded and then survived the changes that have led to an inhospitable surface on Venus today. The Venus cloud layer may provide a refugium for extant life that persisted from an earlier more habitable surface environment. We introduce the Venus Life Equation - a theory and evidence-based approach to calculate the probability of extant life on Venus, L, using three primary factors of life: Origination, Robustness, and Continuity, or L = O x R x C. We evaluate each of these factors using our current understanding of Earth and Venus environmental conditions from the Archaean to the present. We find that the probability of origination of life on Venus would be similar to that of the Earth and argue that the other factors should be nonzero, comparable to other promising astrobiological targets in the solar system. The Venus Life Equation also identifies poorly understood aspects of Venus that can be addressed by direct observations with future exploration missions.


Introduction:
One of the biggest motivators for exploring the solar system beyond Earth is to determine whether extant life currently exists, or now-extinct life once existed, on worlds beyond ours. Current knowledge about the past and present climate of Venus suggests it once had an extended period -perhaps 2 billion years -where a water ocean and land-ocean interfaces could have existed on the surface, under conditions resembling those of Archaean Earth (Way et al., 2016;Way and Del Genio, 2020). Although today the Venus surface (450 °C, 92 bars) is not hospitable to life as we know it, there is a zone of the Venus middle atmosphere, at around 55 km altitude just above the sulfuric acid middle cloud layer, where some conditions are Earth-like ( Figure 1) (Cavicchioli, 2002;Titov et al., 2007). Further interest stems from energetic and chemical observations, not explained by current models, that have similarities to known biological phenomena: the presence of phosphine  and regions of strong UV absorption (Limaye et al., 2018). The question of whether life could have -or could still -exist on the Earth's closest neighbor is more open today than it's ever been (Morowitz and Sagan, 1967;Limaye et al., 2018;Seager et al., 2020).
This paper approaches the question of extant life on Venus in a similar manner to the strategy of the Drake Equation established for estimating probabilities of extraterrestrial intelligent life (Drake, 1965;Burchel, 2006). We approach the question of whether life exists currently "on" Venus (we include the planet's atmosphere in this definition) as an exercise in informal probability -seeking the qualitative likelihood (i.e. not the statistical likelihood function) of the answer being nonzero. The fundamental goal of the Venus Life Equation (VLE) is to provide a scaffolding for estimating the chance of extant life based on factors that can be constrained or quantified through observation, experiment, and data-based modeling. Here we offer a framework in which these estimations can be made.

The Venus Life Equation:
The Venus Life Equation (Figure 2) is expressed as: where L is the likelihood (zero to 1 representing "no chance", to "a certainty") of there being current Venus life, O (origination) is the chance life ever began and became established on Venus, R (robustness) is the potential size and diversity of the Venus biosphere past or present, and C (continuity) is the chance that conditions amenable to life persisted spatially and temporally until now. Subfactors are similarly treated as likelihoods (zero to 1).
The VLE is intentionally agnostic about the scale and type of life that might be present on Venus. Simple or complex, microbial or multicellular; any or all (e.g., Ganti, 2003) would mean L > 0. Our focus in this paper is applying this systematic framework to evaluate the current dominant hypothesis for extant Venus life: a global, persistent airborne ecosystem consisting of organisms small enough to be suspended in or among the cloud aerosols ( Figure 3). Therefore, we examine the constraints and unknowns relevant to the global history and current state of the Venus clouds. However, the same framework could be applied to develop a plausible range for the likelihood of past Venus life, and/or of life in a different niche. Other suggestions that have been put forward for Venus include surface life adapted to use supercritical carbon dioxide as a solvent (Budisa and Schulze-Makuch, 2014), and subsurface microbes in refugia of highly pressurized water (Schulze-Makuch et al., 2005).
The following sections describe each variable of the VLE. The purposefully global scale of Eq. 1, and the major subfactors exemplified by Eqs. 2 and 3, are intended to present an argument for pursuing deeper investigations in those high-level areas (i.e., prioritizing future research efforts). Subvalues of O, R, and C could be broken out to far finer levels of detail than is possible 5 to cover in a single paper -eventually to scales ( Figure 4) appropriate to specific scientific investigations in the lab, analogue field sites on Earth, or in-situ measurements on Venus.
Origination: Life on a planet can start via independent abiogenesis, or importation from elsewhere (panspermia), where: where OA is the likelihood of origin by abiogenesis and OP is the likelihood of origin by conversely, if abiogenesis on Earth was a rare event or if early Venus was more different from early Earth than current models constrain, OA may be 0.1 or less.
6 OP in our solar system may be nonzero from possible transportation of life due to impacts ejecting material from Earth, known to have occurred semi-regularly (Nicholson, 2009;Beech et al., 2018). The potential for panspermia in the solar system has been investigated in some detail (von Hegner et al., 2020a;, and highlights the importance of assessing overlapping periods of habitability between worlds transporting viable life, as well as the ability of life to survive the transportation process (González-Toril et al., 2005). The subfactor OP could thus be broken down into multiple subfactors of its own. For our purposes, it is sufficient to note that because of its relative proximity and size, Venus is the most likely potentially habitable body to receive viable life dislodged from Earth by large impacts (Gladman et al., 1996), and thus, over geologic time is the most likely body in the solar system outside Earth to have an OP >0.  , Harrington et al., 2004), and further exploration of potential abodes and possible life cycles on Venus in particular (Seager et al., 2016;, will also help constrain the range of values of the Origination factor. Why O is not zero for Venus: Current models suggest that early Venus conditions paralleled those of early Earth during the period in which Earth life arose (Way et al., 2016;Way and Del Genio, 2020), which, absent other information, supports an Earth-like value of OA = 0.9 ~ 1.
Regardless of a potential independent biogenesis on Venus, we know that lithopanspermia subfactors outlined in OP should have sent endolithic terrestrial microbes towards Venus throughout its habitable history (Beech et al., 2018), implying that OP could be greater than 0.
Breakout, also assuming habitable conditions paralleling early Earth as our model, is also estimable at greater than zero. With at least one of OA and OP nonzero, and OB nonzero, it thus follows that O > 0 for Venus.

Robustness:
Life on Earth arose relatively early in the planet's history, and persisting through catastrophic asteroid impacts, global glaciation events, the oxygenation of the atmosphere, and many other challenges severe enough to cause mass extinctions (Hoffman et al., 1998(Hoffman et al., , 2002Melezhik, 2006). Life spread so widely, quickly, and with such variety and quantity that the resulting biosphere was robust enough to not be completely eradicated in the face of both acute and gradual environmental changes, due both to external forcings and internal planetary changes.
An estimation of this robustness may be expressed with: where RB is a measure of potential biomass supported over time, and RD is a similar measure of potential biodiversity supported over time. The lower the value of R, the smaller and more fragile the biosphere is to endure the losses or threats captured in the final term of the VLE, Continuity (see below).
At its most abstract, R can be considered to represent the "best case" for a planetary biosphere at a given time. On Earth, our only example of a biosphere, the value of R has been sufficiently high to allow survival through dramatic climate events, near-global mass extinctions, and other regional changes delivering stress or pressure on ecosystems. We can estimate the lower end of an Earth-like planet's R by extrapolating from studies of life in extreme environments, comparative ecology, paleoclimatology, etc., as reference points for less-favorable global conditions. However, it is much more difficult to make conservative estimates about a planet theoretically more habitable than Earth. To account for these limits, we define R here as a fraction of REarth, and let REarth = 1. This makes our bias explicit, and allows a straightforward recalculation if one wishes to make different assumptions about Earth's relative habitability. 9 Conservatively, REarth should represent Earth's total biosphere. However, a less stringent approach might use the metrics of Earth's closest analogue environment as the divisor instead.
This would be justified in cases where (1) the potentially habitable environments of the target planet are relatively uniform (as is the case for Venus's aerosol layer, at least compared to the Venus surface and subsurface conditions), and (2) the corresponding Earth analogue environment is well-isolated, such as a subglacial lake, or has persisted as a habitat over long periods of time without re-seeding from other habitats, such as Earth's ocean.
The first subfactor of R is RB, a measure of the amount of life present on a planet. In keeping with our strategy of normalizing these terms to REarth, we will first discuss past and present estimates of Earth's biomass, and then move on to possible past and present Venus biomass.
Biomass on Earth, on a planetary scale, is usually quantified as organically bound carbon measured in gigatons (Gt) C. This definition assumes both a biosphere based on carbon and that the majority of organic carbon is biogenic. Earth's total biomass also includes large contributions from multicellular organisms, particularly plants. None of these are necessarily true for other potential biospheres (NRC, 2007). However, part of the astrobiological appeal of Venus is its similarity to Earth during the early period in which life may have arisen, implying a similar potential biochemistry. We therefore use estimates of Earth's microbial RB in Gt C as our divisor.
In the modern era, bacteria and archaea on Earth are the second largest global biomass component (~77 Gt C), the vast majority of which reside in subsurface environments (Bar-On et al., 2018).
Although much remains unknown about Earth's Archaean biosphere, which was entirely microbial and aquatic, it has been suggested that the warmer oceans and higher CO2 levels supported similar or even higher levels of microbial biomass (Franck et al., 2005).
For early Venus, consistent with our approach for the O factor, let us assume that its planetary history was relatively similar to early Earth's, at least in the initial 1-2 Ga, and that a potentially similar biochemistry developed. The maximum size of an early Venus biosphere modelled on Earth's Archaean biosphere would then be constrained by the size of potential Venus aquatic habitats (oceans, lakes, groundwater) among other factors such as temperature, atmospheric composition, and levels of biologically important solutes such as nitrates and phosphates. These factors are not currently well constrained; for example, models of early Venus have used a water presence ranging from a global ocean covering ~60% of the surface --not dissimilar to Earth's 70% --to sparse groundwater --less than 0.05% (Way & Del Genio, 2020).
In the absence of information on other factors, we use early Earth as a template, yielding a range of values of RB for early Venus of 0.0005 -0.85.
For the aerial biosphere hypothesis on modern-day Venus, we can place an order-ofmagnitude upper bound on RB with a thought experiment that assumes that all the particles in Venus's cloud layer larger than 0.2 µm (the lower end of terrestrial microbes' size range) are microorganisms. A quick calculation using particle concentrations summarized from in-situ measurements (Esposito et al., 1984) yields a count of 5×10 24 potential organisms. By comparison, the estimated number of prokaryotes on Earth currently is in the range of 4×10 30 (Whitman et al., 1998). If we assume an optimistic range of 10 -75 fg C cell -1 (Kallmeyer 2012, Cermack 2017), the present day Venus clouds yield a biomass range of 0.00005 -0.0004 Gt C, or an RB of 7×10 -7 -5×10 -6 . These assumptions yield a very low, but still nonzero, upper limit on the current RB of Venus. The small total volume of Venus's aerosols, as derived from the sizes and distributions measured to date, is a significant constraint for this scenario. The quantity, nature, and variation of these aerosols (Figure 3) could be further constrained by in-situ atmospheric observation.
The less conservative approach of comparing the potential habitat of Venus's clouds to Earth's closest analogue environment is complicated by our limited understanding of Earth's aerobiosphere. Airborne transit of microbes occurs at a global scale , and prior sampling efforts indicate that the total number of viable cells in the atmosphere may be on the order of 5×10 20 , with ~90% in fog and cloud water (Fuzzi et al., 1997;Harris et al., 2002;Amato et al., 2005;Smith et al., 2018;Bryan et al., 2019). However, traces of metabolic activity have been observed so far only in warm, wet cloud droplets near the surface (Amato et al., 2019), and reproduction while airborne has not yet been observed in-situ; at higher-altitude regions more isolated from surface sources, recovered cells are desiccated and dormant (Bryan et al., 2019). If we assume that Earth does have a persistent, if low-level, aerobiosphere, then using its biomass as the divisor for the same upper bound on Venus biomass calculated above yields a value three orders of magnitude larger than 1. (This is expected, as an upper bound on Earth's atmospheric biomass based on Earth's water vapor volume would similarly exceed the actual estimated value).
This alternate, non-conservative approach to RB would therefore give a final value of 1.
The second subfactor of R is RD, a measure of the diversity of life present. In keeping with our strategy of normalizing these terms to REarth, we will first discuss past and present estimates of Earth's biodiversity, and then move on to possible past and present Venus biodiversity.
Life on Earth is incredibly diverse, with nearly every liquid and solid surface colonized with a detectable microbial population. Taxonomic classification of bacteria and archaea is a rapidly evolving field, and by some estimates 99.99% of microbes in most field samples belong to unknown taxa regardless of which diversity index is used to catalog species; estimates of the number of microbial species distributed across Earth vary, with conservative estimates on the order of 10 12 (e.g., Locey and Lennon, 2016). This estimate does not include the number of microbial species that may have arisen and gone extinct over Earth's history, and we know even less about the biodiversity of Archaean Earth.
Quantitative metrics of taxonomic diversity, such as abundance measures, are therefore unlikely to be helpful in modeling other theoretical biospheres. Instead, functional diversity, which reflects how many distinct niches life occupies in a given habitat --e.g., "apex predator (obligate heterotroph)" or "primary producer (sulfur-reducing chemolithotroph)" --is probably the most intuitively applicable to a theoretical, non-terrestrial biosphere. As with biomass, the metric chosen should reflect constrainable similarities between Earth and Venus. For hundreds of millions of years, early Earth lacked several major functional niches present today (Nisbet, 1995), including oxygenic photosynthesis and all, or nearly all, land-based ecology. Conversely, we currently have no way of knowing how many historical niches may once have existed --e.g., chemolithotrophs utilizing minerals only stable in a reducing atmosphere --but are now lost.
We know even less, of course, about early Venus. However, since R is meant to represent a best-case, and we know that any extant life on Venus is almost certainly a relic of a more thriving era, we might look to an early Earth-like range for RD and use this as our estimate for early Venus as well. The same caveats regarding factors needed to better understand RB on Venus apply.
Modern-day Venus is a more tractable case for RD. Although Earth has no direct analogue environment resembling Venus's clouds (Figure 1), we can estimate an upper bound based on partial analogues. Chemically speaking, terrestrial acid hot springs have been proposed as the inhabited environments that most closely reproduce Venus cloud temperature (97 to -45 ºC) and pH (less than -1.3 to 0.35; Grinspoon and Bullock, 2007;Krasnopolsky, 2019). Terrestrial deserts or concentrated brines may best represent the low water activity in Venus aerosols (~0.02 at a relatively optimistic assumption of 75% H2SO4 and 25% H2O; Deno and Taft, 1954;Hansen and 13 Hovenier, 1974;Kieft, 2003;Bolhuis et al., 2006). Each of these environments show significantly less diversity than more typical mesophilic environments. At pH levels at or below 1, terrestrial life is limited to a few lineages of archaea (Barrie Johnson and Hallberg, 2008). Brines at water activities below 0.75 are similarly limited to other lineages of archaea (Grant, 2004;Oren, 2011).
Life forms in both environments rely upon "narrow" metabolic pathways (Barrie Johnson and Hallberg, 2008;Oren, 2011), that likely emerged via horizontal gene transfer with bacteria (Fütterer et al., 2004;Sorokin et al., 2017); and although these modern archaea are generally aerobic heterotrophs (not useful for analogies to Venus), Earth has been oxygenated for a sufficiently long time to make aerobic metabolisms evolutionarily favorable. Only the most extreme deserts on Earth approach water activities below 0.1, and here, though more taxonomically diverse, life is primarily phototrophic, adapted to long periods of inactive desiccation followed by brief spurts of activity during sporadic water influx. To give a sense of the possible range of values, the smallest possible non-zero value of RD is equivalent to the apparently natural monoculture found in a terrestrial deep subsurface fracture (Lin et al., 2006); its long-term isolation from other habitats and energy/nutrient limitations have some similarities to the suggested relict Venus ecosystem. At the high end of the range, it has been suggested that Earth's total airborne biodiversity might be equivalent to that of soil (Brodie et al., 2007), approaching an RD of 1; this is possible due to the long tail of rare species, despite the much lower biomass. The latter value is less representative of a Venus scenario, as it includes a large contribution from short-term bioaerosols kicked up from the well-populated surface and quickly deposited again. We would therefore expect RD for a Venus aerobiosphere to be significantly lower than RD for Earth, but still nonzero.

Planetary and Astrobiology Study of Robustness:
The similarity between Venus and Earth's early history, and the drastic divergence between their current states, is significant in the study of rocky planets. Many of the questions surrounding the potential characteristics of an early Venus biosphere are the same as those we still seek to answer for early Earth: nutrient and energy sources and cycling, ocean breadth and depth, radiation flux, transport of atmospheric gases and particulates. The global imaging provided by Magellan show us that regions of tessera terrain are stratigraphically older than the bulk of the Venus surface and represent an extinct tectonic and possible mineralogic and/or weathering regime (e.g., Gilmore et al., 2017). Additional imaging, spectroscopy, mineralogy and chemistry of the tesserae, in-situ measurement of noble gases to assess volatile inventory and assessment of current and past geologic activity are all necessary steps to constrain the volatile history of ancient Venus.
The case of modern-day Venus is also complicated by Earth's lack of a habitat directly analogous to Venus's cloud layer. The closest regime, in terms of chemistry and isolation from surface nutrient and water sources, is probably the stratospheric sulfate layer (Gentry and Dahlgren, 2019), where the longest-enduring microbe-sized aerosols may have residence times of many months. However, under stratospheric conditions, significantly less dense than the "gas ocean" of Venus, it is likely impossible for terrestrial microorganisms to metabolize, grow, or reproduce. Survivors recovered at extreme heights above the Earth's surface tend to be dormant, resilient, endospore-forming bacteria enduring harsh irradiation until dropping out by gravitational settling (Bryan et al., 2019). At lower altitudes, terrestrial aerobiologists are exploring whether short-lived airborne ecosystems exist within Earth clouds where environmental conditions are more favorable, including water and nutrient availability (Amato et al., 2019).
A better understanding of Earth's aerobiosphere, and whether it has been as consistent as Earth's surface or subsurface biospheres, might lead to an alternative value for REarth. More generally, further exploration and characterization of the biomass limitations and functional diversity of those partial Venus analogues that are more accessible on Earth will continue to help inform our estimates of RB and RD for both Earth and Venus.
The VLE is inherently a whole planet question, based on what we know, can know, and must surmise about the planet as an integrated whole. Factors like R, and subfactors like RD and RB can be further subdivided into smaller and smaller components, down to individual biomes or niches ( Figure 4). While doing this many times across the Earth would, in aggregate, improve an understanding and estimate of global R for Earth, and concomitantly Venus, delving deeply into any one example at this high level can in fact be counterproductive to a global estimation. Thus,

detailed analysis and interrelationships of individual niches (on Earth and potentially on Venus)
can be part of the greater solution, but is beyond the scope of this overview.
Why R Is Not Zero for Venus: Because R represents a "best-case" biosphere, it could only be zero for a target environment which meets no known criteria for habitability -for example, the sun, dry lunar regolith, or the exposed surface of an asteroid. While the potentially supportable biosphere on modern Venus may be quite low or limited by terrestrial standards, the relative clemency of early Venus, and its similarity to the empirically inhabited early Earth, allows R to be greater than zero.
Continuity: This factor reflects the necessity of continuous existence of habitats over time and space; or, equivalently, the lack of global extinction-level events. This might naturally lead to subfactors for C of CT and CS, for temporal and spatial continuity respectively, and further 16 subdivision to individual biosignatures or disequilibrium chemistry (e.g., Wogan and Catling, 2020) but our current state of knowledge of Venus is not sufficient to allow us to mathematically relate them with confidence (conveyed in Figure 3). For life to exist on Venus into the present, both CT and CS must be nonzero -that is, from the Origin point for life, there must be an unbroken continuity of habitable condition in both time and space. require conditions to evolve contiguously and continuously from a marine-land interface (e.g., one of the likely 'breakout' environments for Earth), to a globe-spanning biosphere, to eventual adaptation towards complete airborne life cycles and an aerobiosphere maintainable solely in the clouds. The identification and confirmation of potential biosignatures in the atmosphere of a planet (e.g., Kaltenegger, 2017;Wogan and Catling, 2020), would argue for a larger value for continuity.
The recent discovery of phosphine in the Venus atmosphere , if verified as a biologically produced gas, would set C =1 for Venus.
However, complications for continuity are rooted in both understanding of terrestrial biology and lack of understanding of Venus' geologic history and present conditions. If one assumes a terrestrial-like biochemistry, neither the trace composition of Venus's aerosols nor current conditions such as atmospheric circulation of dust are understood well enough to determine water activity or the presence of bioavailable forms of nitrogen and phosphorus, let alone the enzymatically important heavy elements like Fe, Zn, Pb, Cu, Sn, V, Cd, Ni, Se, Mn, Co, Cr, As, Mo and W that must be available at low but consistent levels to terrestrial life. On Earth these are quite scarce in the atmosphere. However, in situ detection of both phosphorus and iron were reported by the Vega X-ray fluorescence spectrometers (Andreychikov et al., 1987), and a comprehensive trace elemental assay of the Venus clouds, with sensitive 21 st century instruments, has not been performed. Non-chemical requirements are also critical. Energy pathways such as photosynthesis or chemosynthesis need to have been established and maintained or evolved to in a similar contiguous and continuous manner. For example, although Venus receives more photonic energy at the top of its atmosphere than Earth, the thick atmosphere and haze layer reflect or block larger fractions of it (Titov et al., 2007) thus the zonally averaged incoming flux at the Venus cloud tops is ~1.5× less than that for Earth. This is particularly important for a potential atmospheric ecosystem, as the residence time of potential aerosol habitats imposes a particular constraint 18 . Many terrestrial microbes in extremely harsh or nutrient-limited environments have very long generation times of weeks to months, potentially in combination with long periods of inactivity. Seager et al. (2020) suggest that updrafts and/ or gravity waves on Venus may be sufficient to return desiccated life in the haze layer to conditions where they can thrive and continue their life cycle.
One of the major sub-factors of C specific to Venus (affecting continuity in both space and time) is the timeline of Venus's water loss and cloud formation. Although life is capable of very rapid adaptation and diversification in some circumstances, major habitat transitions on Earth such as colonization of land took at least hundreds of millions of years. The shorter the periods of overlap between origination of life in the oceans, the evaporation of the oceans into a water-cloudshrouded planet, and the formation of the modern-day habitat of persistent sulfuric cloud cover, the lower the likelihood of colonization; and if any two did not overlap at all, it might be negligible at best. What matters to estimating C is knowing the history of the planet; its geological and climate evolution. For example, Venus' water history timeline is not currently well constrained, and some models include the possibility of a gap of 10s Myr to 100s Myr between the end of Venus' surface oceans and the current state of a persistent cloud deck of sulfuric acid aerosols (Bullock and Grinspoon, 2001). The duration of that gap (if it was present), or its absence, could significantly affect the estimation of C.

Planetary and Astrobiology Study of Continuity:
Continuity is hard to estimate given how little we know about both Venus' history and current potential habitats and the potential biases the N=1 of Earth engenders. For example, what is observed to be an "essential" element in the terrestrial biota is also the product of an opportunistic evolutionary process which might well have found "work arounds" in other planetary environments with a different complement of available elements. More exotic proposed biochemistries (such as direct use of sulfuric acid as an alternative polar solvent) are even less constrained in terms of energy and chemistry requirements (Schulze-Makuch and Irwin, 2008;Cockell and Nixon, 2016).
However, of the three factors of the VLE, Continuity is the one we can do the most to improve quantification through direct study of Venus. Most of the areas delineated in the Goals, Objectives, and Investigations for Venus Exploration (VEXAG, 2019) document will result in direct quantitative improvement of the estimate for C. For example, determining the presence and extent of silicic igneous rocks constrains the history of possible early Venus oceans and crustal evolution. Measuring isotopic ratios of noble gases, oxygen, hydrogen in the atmosphere will constrain the history of water, and possibly biological or prebiotic effects on global chemistry. If microbe-bearing aerosols, on average, settle out (as on Earth) or fall to an altitude at which they dry out or boil off (as on Venus) faster than the microbes can reproduce, an aerosol-based biosphere without periodic injections from other reservoir habitats will not be stable over the long term, even if short-term conditions are otherwise favorable. Deep dynamics will constrain the possibility of circulation of materials from near the surface through the lower atmosphere, and geologic history and activity will determine the present and past supply of chemicals to different parts of potential Venus ecosystems. The entirety of VEXAG Goal 1, in fact, prioritizes the understanding of Venus' early history and potential habitability. And certainly, verification of the presence and a biogenic source for phosphine  would result in C=1.

Why C Is Not
These numbers are simply example calculations that can and should be refined by others. The only terrestrial life that might endure conditions in Venus aerosols are extreme acidophiles, which have not been observed to survive long periods of time airborne and are unlikely to be spacecraft assembly facility contaminants (Smith et al., 2017). This is true even if one assumes that putative extant Venus microbes rely on substantially different metabolic inputs and outputs from possible transported terrestrial life to the point that the "potentially habitable region" for each does not overlap. This implies that the types of possible terrestrial (bio)chemical contamination that could survive exposure to Venus atmosphere/aerosols are unlikely to cause false positives in experiments looking for Venus life. The NAS study did not recommend any scientific investigations for the specific purpose of reducing uncertainty with respect to planetary protection issues. Thus, the Category II classification of the NAS study remains as yet unchallenged by a nonzero value for L. Like any life detection experiment, however, any in-situ instrumentation will need to invoke a high level of sterilization and cleanliness to ensure accurate measurement. water availability and ultraviolet radiation flux in this altitude range (see also Arking, 1996). The "habitable range" shown here for Venus is thus conservatively based on the "inhabited" range for Earth; the limits of terrestrial organisms may or may not reflect the possibilities for Venus. Unidentified group of complex shapes adhered to an aerosol particle, 4) Objects that resemble Earth bacteria or archaea, and 5) volcanic ash particles.