Abstract Venus is the planet in the Solar System most similar to Earth in terms of size and (probably) bulk composition. Until the mid-20th century, scientists thought that Venus was a verdant world—inspiring science-fictional stories of heroes battling megafauna in sprawling jungles. At the start of the Space Age, people learned that Venus actually has a hellish surface, baked by the greenhouse effect under a thick, CO 2 -rich atmosphere. In popular culture, Venus was demoted from a jungly playground to (at best) a metaphor for the redemptive potential of extreme adversity. However, whether Venus was much different in the past than it is today remains unknown. In this review, we show how now-popular models for the evolution of Venus mirror how the scientific understanding of modern Venus has changed over time. Billions of years ago, Venus could have had a clement surface with water oceans. Venus perhaps then underwent at least one dramatic transition in atmospheric, surface, and interior conditions before present day. This review kicks off a topical collection about all aspects of Venus’s evolution and how understanding Venus can teach us about other planets, including exoplanets. Here we provide the general background and motivation required to delve into the other manuscripts in this collection. Finally, we discuss how our ignorance about the evolution of Venus motivated the prioritization of new spacecraft missions that will rediscover Earth’s nearest planetary neighbor—beginning a new age of Venus exploration. Until the mid-20th century, scientists thought that Venus was a verdant world—inspiring science-fictional stories of heroes battling megafauna in sprawling jungles. At the start of the Space Age, people learned that Venus actually has a hellish surface, baked by the greenhouse effect under a thick, CO 2 -rich atmosphere. In popular culture, Venus was demoted from a jungly playground to (at best) a metaphor for the redemptive potential of extreme adversity. However, whether Venus was much different in the past than it is today remains unknown. 2021 ; Way et al. 2022a ). This transition may have been rapid in geological terms but slow relative to biological generations. Life might have migrated from the increasingly inhospitable surface to the possibly last habitable niche in the clouds (e.g., Limaye et al. 2018 , 2021 ; Seager et al. 2021 ). New missions can search for atmospheric signatures of a clement past, geological traces of ancient oceans, and evidence of active biology. Fig. 1 Full size image Venus and Earth perhaps resembled each other after their accretion but set off on divergent evolutionary paths after a few billion years. Today, we think that Venus was habitable in the past—but is advocacy of this idea only a coping mechanism for the disappointing discoveries of the Space Age? Sect. 3 presents the fundamental properties of Venus, especially those related to the evolution of its atmosphere, surface, and interior. This section also advertises the chapters in this topical collection that are most relevant to each aspect of Venus. Finally, Sect. 4 shows how the idea that Venus evolved over time motivates strategies for planetary exploration. Tectonics only become critical if there was a flourishing (or, eventually, flailing) biosphere on the planet that needed to access renewable resources. Nevertheless, a better understanding of the physical mechanisms responsible for Venus’s geologic history will greatly advance our understanding of what makes a rocky planet habitable and, ultimately, life emerge. 3 Fundamental Properties of Venus Relevant to Its Evolution Understanding the evolution of Venus is, by definition, a more complex task than making direct observations of its modern properties. A detailed catalog of the fundamental properties of Venus is the foundation of attempts to study its past. Intuitively, preserving crustal magnetism on Venus might seem difficult because Venus has the hottest surface on average of any terrestrial planet in the Solar System. However, temperatures in the top few kilometers of the crust should be low enough for common minerals such as magnetite and hematite to retain thermal remanent magnetism from a past dynamo for billions of years (O’Rourke et al. 2019a , 2019b and references therein) unless the surface was much hotter in the past (e.g., Bullock and Grinspoon 1996 ). 4.1 Importance of Venus’s Evolution to ESA ESA’s planetary exploration in the early 21st century has been based largely on its “Cosmic Vision” strategy (ESA 2005 ). This poses four big questions, the first two of which are “What are the conditions for planet formation and the emergence of life?” and “How does the Solar System work?” The first of these themes explicitly calls for a study of planetary formation and evolution, and the emergence of habitable environments, and then of life itself. https://doi.org/10.1029/JA085iA13p08007 Article ADS Google Scholar Seager S, Petkowski JJ, Gao P, Bains W, Bryan NC, Ranjan S, Greaves J (2021) The Venusian lower atmosphere haze as a depot for desiccated microbial life: a proposed life cycle for persistence of the Venusian aerial biosphere. Astrobiology 21:1206–1223. https://doi.org/10.1089/ast.2020.2244 Article ADS Google Scholar Seiff A, Schofield JT, Kliore AJ, Taylor FW, Limaye SS, Revercomb HE, Sromovsky LA, Kerzhanovich VV, Moroz VI, Marov MY (1985) Models of the structure of the atmosphere of Venus from the surface to 100 kilometers altitude. Adv Space Res 5:3–58. https://doi.org/10.1016/j.gsf.2017.03.001 Article Google Scholar Westall F, Hickman-Lewis K, Hinman N, Gautret P, Campbell KA, Bréhéret JG, Foucher F, Hubert A, Sorieul S, Dass AV, Kee TP, Georgelin T, Brack A (2018) A hydrothermal-sedimentary context for the origin of life. Astrobiology 18:259–293. https://doi.org/10.1089/ast.2017.1680 Article ADS Google Scholar Westall F, Way MJ, Izenberg NR, Helbert J, Gilmore MS, Weller MB, Carter L, Gillmann C,

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∘C\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$200~^{\circ}\text{C}$\end{document}), 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 and Paleoarchean, we attempt to review 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 on the global environment in 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 tra 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. 2016 ), there is little chance of observing cratered remains of any such ancient impactors. If such an impactor did collide with the planet in Venus’ past, it may be possible to detect it isotopically if it was sufficiently different from the bulk composition of Venus, but measuring this would be challenging. To paraphrase Way and Del Genio ( 2020 ) “it is clear that Brasser et al. ( 2016 ) and Mojzsis et al. ( 2019 ) prefer the hypothesis that the Earth’s late veneer was mainly delivered by a single Charon- or Ceres-sized impactor. To characterize the present cloud-level environment including searching for molecular biosignatures of past or present-day life. This can be partially addressed by descent probes, but a more comprehensive investigation would require sustained presence in the clouds as from a balloon platform. 1. Studying the past habitability of Venus It is very difficult The search for biosignatures in Venusian clouds If Venus was habitable in the past (meaning, it had liquid water on its surface, the other ingredients of life being a given on a rocky planet such as Venus, and similar to early Earth), and life emerged, could it have survived to the present day in atmospheric aerosols? With regard to the habitability of the Venusian cloud deck, high priority in situ investigations include the structure of the atmosphere and variables, such as temperature, pressure, pH, UV radiation flux (cf. Grinspoon and Bullock 2007 ; Dartnell et al. Astrobiology 21(1):1–38 Article ADS Google Scholar Dodd MS, Papineau D, Grenne T, Slack JF, Rittner M, Pirajno F, O’Neil J, Little CT (2017) Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature 543(7643):60–64 Article ADS Google Scholar Donahue T, Hodges R Jr (1992) Past and present water budget of Venus. J Geophys Res, Planets 97(E4):6083–6091 Article Google Scholar Donahue TM, Hodges RR Jr (1993) Venus methane and water. Geophys Res Lett 20(7):591–594 Article ADS Google Scholar Donahue T, Hoffman J, Hodges R, Watson A (1982) Venus was wet: a measurement of the ratio of deuterium to hydrogen. S Afr J Geol 124(1):225–252 Article Google Scholar Hickman-Lewis K, Cavalazzi B, Foucher F, Westall F (2018a) Most ancient evidence for life in the Barberton greenstone belt: microbial mats and biofabrics of the 3.47 Ga Middle Marker horizon. Precambrian Res 312:45–67 Article ADS Google Scholar Hickman-Lewis K, Westall F, Cavalazzi B (2018b). Trace of early life in the Barberton greenstone belt Hickman-Lewis K, Cavalazzi B, Sorieul S, Gautret P, Foucher F, Whitehouse MJ, Jeon H, Georgelin T, Cockell CS, Westall F (2020a) Metallomics in deep time and the influence of ocean chemistry on the metabolic landscapes of Earth’s earliest ecosystems. In: Origin and evolution of planetary and satellite atmospheres, pp 450–483 Chapter Google Scholar Schuerger AC, Smith DJ, Griffin DW, Jaffe DA, Wawrik B, Burrows SM, Christner BC, Gonzalez-Martin C, Lipp EK, Schmale DG III, Yu H (2018) Science questions and knowledge gaps to study microbial transport and survival in Asian and African dust plumes reaching North America. Aerobiologia 34(4):425–435. https://doi.org/10.1007/s10453-018-9541-7 Article Google Scholar Schulze-Makuch D, Irwin LN (2002) Reassessing the possibility of life on Venus: proposal for an astrobiology mission. Astrobiology 2(2):197–202.

Venus is not generally at the forefront when considering extraterrestrial life. Yet, based on the physical similarities and proximity to Earth and with the little knowledge of its evolutionary history, there is a possibility that Venus may have hosted life in the past on the surface if Venus had liquid water and perhaps even has water present in the clouds today. While the early suggestions during the beginning of the space exploration about life on Venus were mostly speculative due to limited data, recent interest has arisen from realizations: (i) the unexplained ultraviolet absorption spectrum of Venus resembles many organics, (ii) there is chemical disequilibria in the cloud layer, (iii) the cloud aerosols likely contain significant abundances of hydrated iron and magnesium sulfates, and (iv) the solar radiation received in the cloud layer contains the appropriate wavelengths and flux to support phototrophy. Considering the extreme environmental survival of many terrestrial microorganisms, the possibility remains that any extant life on Venus in the past could have adapted to survival in the cloud layer far above the surface where energy and nutrients are available, but the precise compositions of the cloud particles and water availability are still uncertain. The key to solving the mystery of life on Venus is to determine if Venus had liquid water on the surface in its past and to measure the precise chemical composition of the Venus atmosphere and the cloud particles. Missions which will be launched in the next few years will provide much needed data that should provide some answers we seek and will surely raise more questions. This perspective reviews recent developments.