Venus today is inhospitable at the surface, its average temperature of 750 K being incompatible to the existence of life as we know it. However, the potential for past surface habitability and upper atmosphere (cloud) habitability at the present day is hotly debated, as the ongoing discussion regarding a possible phosphine signature coming from the clouds shows. We review current understanding about the evolution of Venus with special attention to scenarios where the planet may have been capable of hosting microbial life. We compare the possibility of past habitability on Venus to the case of Earth by reviewing the various hypotheses put forth concerning the origin of habitable conditions and the emergence and evolution of plate tectonics on both planets. Life emerged on Earth during the Hadean when the planet was dominated by higher mantle temperatures (by about 200$^\circ$C), an uncertain tectonic regime that likely included squishy lid/plume-lid and plate tectonics, and proto continents. Despite the lack of well-preserved crust dating from the Hadean-Paleoarchean eons, we attempt to resume current understanding of the environmental conditions during this critical period based on zircon crystals and geochemical signatures from this period, as well as studies of younger, relatively well-preserved rocks from the Paleoarchean. For these early, primitive life forms, the tectonic regime was not critical but it became an important means of nutrient recycling, with possible consequences to the global environment on the long-term, that was essential to the continuation of habitability and the evolution of life. For early Venus, the question of stable surface water is closely related to tectonics. We discuss potential transitions between stagnant lid and (episodic) tectonics with crustal recycling, as well as consequences for volatile cycling between Venus’ interior and atmosphere. In particular, we review insights into Venus’ early climate and examine critical questions about early rotation speed, reflective clouds, and silicate weathering, and summarize implications for Venus’ long-term habitability. Finally, the state of knowledge of the venusian clouds and the proposed detection of phosphine is covered.

A three-color, extremely high signal to noise, precisely pointed, sub-arcsecond photometer would enable us to distinguish Earth-like exoplanets from other rocky, gassy, or icy worlds - if we had the right three wavelengths, and the ability to block out the primary star’s glare. Color-color discrimination of Earth-like planets has been posited for quite some time. Broadband filters are not seen to be precise diagnostics of planets with life, but rather broad but critical similarity or difference to the one habitable planet we know. Visible wavelengths (Vis) are advantageous due to relatively greater abundance of photons of that wavelength range from planets around Sun-like stars. Near-ultraviolet (UV) wavelengths may be advantageous due to ozone and scattering properties in Earth’s atmosphere that make Earth truly stand out from other known and modeled planets. Near infrared wavelengths (IR) help discriminate potential biological color contributions. We conducted an optimization exercise to arrive at three broadband filters that reliably separate modeled Earth-like, nominally habitable planets from other possible exoplanets. Criteria we use included: - Atmosphere/clouds of habitable worlds - Sea/Land proportions (ocean, granite, basalt, other surface material) - Vegetation (chlorophyll spectra and fictional other photosynthetic materials depending on wavelengths of parent stars etc.) - Other factors that can be spectrally modeled (planet-covering cities, world oceans, etc.) The optimized bands resemble previous work for exoplanets and the solar system, but underscore the advantage of UV wavelengths and indicate their potential utility for exoplanet identification and/or discrimination in concert with other exoplanet observations. An exoplanet survey that could quickly identify such planets could then be followed up by more detailed, longer term study by more intensive campaigns with more capable, but more resource or time constrained.

Does life currently exist, or did life once exist, on other worlds in our solar system? The proximity of the rocky planets of our solar system, Venus and Mars, make them obvious targets for the first attempts to answer these questions via direct exploration, with concomitant implications for, and input to, how we think of exoplanets. Given the limited resources we have to explore our neighbors in space, an ecological assessment (based on terrestrial ecosystem principles) might help us target our search and methodology. Studies of extreme life on Earth consistently reveal adaptability. Mars has been the target of many life-related investigations [1, many others]. Venus has not, yet there may be compelling reasons to think about extant life on the second planet [2], and lessons to learn there about searching for life elsewhere in the solar system and beyond. The Venus Life Equation: Venus may have been habitable for billions of years its history and may still be habitable today. Our current state of knowledge of the past climate of Venus suggests that the planet may have had an extended period – perhaps 1-2 billion years – where a water ocean and a land ocean interface could have existed on the surface, in conditions possibly resembling those of Archaean Earth [3]. At present, Venus’ surface is not hospitable to life as we know it, but there is a zone of the Venus middle atmosphere, ~55 km altitude, just above the sulfuric acid cloud layer, where the combination of pressure, temperature, and gas-mix are more Earth-like than anywhere else in the solar system [2, 4]. 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. Here we approach the question of present-day life on Venus in a manner analogous to the Drake Equation [5], treating the possibility of current Venus life as an exercise in informal probability – seeking qualitatively the likelihood or chance of the answer being nonzero.The working version of the Venus Life Equation is expressed as: L = O * R * A where L is the likelihood (zero to 1) of there being life on Venus in the present-day, O (origination) is the chance life ever began and “broke out” on Venus, R (robustness) is the potential current and historical size of diversity of the Venus biosphere, A (acceptability) is the chance that conditions amenable to live persisted spatially and temporally to the present. The Venus Life Equation is a work-in-progress as a pre-decadal White Paper [6] and its variables are currently being refined. [1] McKay 1997, Springer, Dordrecht, 1997. 263-289. [2] Limaye et al. 2018 Astrobiology, 18(9), 1181-1198. [3] Way et al. 2016 JGR 43(16) 8376-8383. [4] Schulz-Makuch et al. 2004 Astrobiology 4, 11-18. [5] Burchell 2006, Int. J. Astrobio, 5(3) 243-250. [6] Izenberg et al. 2020, https://is.gd/vd4JE7 (location of Latest version of Venus Life Equation White Paper).