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Perspective

Life on Venus?

Space Science and Engineering Center, University of Wisconsin-Madison, Madison, WI 53706, USA
Life 2025, 15(5), 717; https://doi.org/10.3390/life15050717
Submission received: 3 April 2025 / Revised: 21 April 2025 / Accepted: 23 April 2025 / Published: 29 April 2025
(This article belongs to the Section Astrobiology)

Abstract

:
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.

1. Introduction

More than a century ago when few properties of the Venus atmosphere were known, Arrhenius [1] speculated that Venus had plenty of liquid water with forests and flora and fauna. In the following five decades more was learned about the atmosphere and surface through ground-based and balloon telescopes and first space missions. These led to the first suggestion of life in the clouds of Venus [2] based on the speculation that the cloud particles were water based.
Measurements from atmospheric entry probes, fly-by spacecraft, orbiters, and balloons floating in the Venus atmosphere (1967–1985) revealed that the mostly carbon dioxide atmosphere and the surface were quite warm and devoid of liquid water and harbored clouds of concentrated sulfuric acid [3]. The realization that the surface and perhaps even the clouds were far different from early speculations quickly dispelled any notions of extant life.
The early discoveries about Venus and its atmospheric superrotation [3,4,5,6,7] included some mysterious ones that could be interpreted as possible presence of life. Boyer [8] following a suggestion by his friend Paul Malaval, invoked microorganisms, or “biotes” to explain the acceleration of the atmospheric flow from morning to noon through solar energy absorption. Others included chemical disequilibrium [9,10,11], absorptive properties [12,13], and the possible past presence of liquid water [14]. Specifically, the possible presence of ammonia [9] and molecular oxygen was puzzling. Discovery of ammonia was quickly dismissed as it could be present under equilibrium conditions [15], which may have hindered future measurements and progress. One major mystery that remains today is the nature and identity of the ultraviolet absorbers in the clouds [16,17,18,19,20,21]. The persisting mysteries led to a reconsideration of life in 1997 when the mysteries about Venus—past presence of water and the identity of the ultraviolet absorber in the clouds responsible for the dominant deposition of solar energy in the atmosphere—led to new speculations about life in the clouds [22,23,24]. The suggestion that some terrestrial bacteria and proteins show absorption features similar to Venus led to further renewed interest in the possibility of life in the clouds [25,26].
The report of phosphine in the Venus atmosphere [27] has led to more interest and investigations due to it being suggested as a bio-signature [28]. The debate following detection atmosphere has distracted from the primary reasons for suggestions of life on Venus. Recently, Schulze-Makuch [29] present the issues involved, Westall et al. [30] discuss the habitability of Venus [31], and Widemann et al. [32] present a review of planned future missions and the questions they will address. This perspective focuses on the question of possibility of past and present life on Venus with a brief review of the historical context and new findings in the last five years or so.

The Phosphine Conundrum

With growing interest in habitable exoplanets, Sousa-Silva et al. [28] suggested phosphine as a biogenic signature in search of life on exoplanets. This led Jane Greaves and colleagues to observe Venus to seek any signature of phosphine [27] in the spectra obtained in the mm range using ground-based telescopes. Phosphine was not expected to be present in the Venus atmosphere based on our current (and incomplete) knowledge of the Venus atmospheric chemistry. It is detectable in Earth’s atmosphere due to industrial production as well as natural processes [33]. The detection of phosphine in the atmosphere of Venus [27] has thus been covered in media in the context of a possibility of life on Venus. Such coverage has overshadowed the reasoning behind the original suggestions of origins and survival of life on the surface of Venus in the past and possible evolution to survive in the cloud cover that shrouds the planet today.
The initial detection was swiftly disputed by many [34,35,36,37] with the objections originating in the data analysis procedures used to detect the very low spectral signal of the gas from a bright, extended object such as Venus. This created problems not posed by the deep space astronomical objects that the James Clark Maxwell Telescope (JCMT) and the Atacama Large Millimeter Array (ALMA) commonly observe. The brightness of Venus poses particular difficulties for the ALMA data, as each antenna in the array has a somewhat different characteristic. The single-dish JCMT data presented fewer challenges, and the results were not disputed as vigorously or largely ignored.
The debate about phosphine in the Venus atmosphere involves several issues: (i) is the detection real, (ii) is the detection confused with another species (SO2), (iii) what is the altitude level of the reported phosphine, (iv) is the phosphine abiotic or not, and (v) can biology produce phosphine. Although the investigators did not attribute the source of phosphine to life in the Venus atmosphere, the prevailing knowledge of Venus’ atmospheric chemistry appeared to exclude abiotic sources. This has also proved to be controversial, with some [38,39,40] arguing that volcanic sources could produce it, and others arguing just the opposite [41]. Lost in the intense reactions towards the spectroscopic detection and the source of the phosphine was the strong hint of the presence of phosphine in the cloud layer, along with H2S, supported by the fractionation product (PH+2) in the data collected by the PV LNMS [42].
The three main questions—(i) presence of phosphine in the atmosphere at different altitudes, (ii) its origins, and (iii) any extant life—will be settled as new observations are collected from terrestrial telescopes and with future space missions that will need to sample the cloud cover at different altitudes as well as trace constituent profiles in the atmosphere.

2. Past Habitability of Venus

The arguments for the possibility of life in the past on Venus have been discussed previously [43,44]. In brief, life requires water, although other solvents have been proposed, including sulfuric acid [45]. We do not know whether early oceans or large water bodies on Earth were acidic (present mean value is 8.2, but turning lower [46] due to increasing atmospheric CO2). The current research suggests that over the last 20 million years, the oceans have been slightly alkaline (pH ≈ 8 (https://news-oceanacidification-icc.org/2021/03/19/ocean-acidification-its-lasting-impacts/, accessed on 22 April 2025)). The primary suggestion for past presence of liquid water on Venus comes from the enhanced ratio of deuterium to hydrogen (D/H) in the Venus atmosphere compared with the terrestrial mean ocean water composition [14]. Other mechanisms have also been proposed [47] but have generally not received much support. Secondary suggestions come from the modeling of early climates [48,49], which indicated that liquid water could have survived until as recently as one or two billion years ago, assuming that water condensed onto the surface, inferences of possible minerals that formed under aqueous conditions [50], and geophysical modeling of mantle convection [51,52]. Krissansen-Totton et al. [53], Way et al. [49], Way and Del Genio [54], Warren and Kite [55], and Westall et al. [31] also looked into past habitability from different perspectives and modeling and found Venus habitable in the past under the assumptions and initial conditions used. On the other hand, Constantinou et al. [56] and Turbet et al. [57] found the planet to be too dry with no condensable water and hence inhabitable. Needless to say, there are few data to constrain the modeling assumptions.

2.1. Unknowns About Early Venus

Unlike on Earth where fossils and core samples provide some information about past conditions on Earth, Venus presents no such avenue. Thus, only physical similarities with Earth and proximity provide some guidance regarding some of the properties. These include the early rotation rate, nature and extent of the clouds, and planetary albedo. Few means are available to assess the validity of assumed values.

2.1.1. Presence of Liquid Water on Surface

Donahue et al. [14] concluded from the enhanced D/H ratio compared to the Mean Ocean Water composition that Venus had a shallow global ocean, perhaps a few meters thick, by modeling the excess loss of hydrogen to space compared to the heaver deuterium. However, from a modeling study of the evolution of the climate from about a few hundred million years after formation of the crust to present, Turbet et al. [57] arrived at a different conclusion. They argue that Venus never cooled enough to condense water from a steam atmosphere onto the surface to form an ocean [57], as the atmosphere remained “clear” during the day and cloudy during the night, blocking loss of heat and thereby keeping the atmosphere warm. Constantinou et al. [56] also reach a similar conclusion but based on backward modeling of atmospheric chemistry using a general circulation climate model. Not addressed by these studies is the key question of the origin of the water itself—where did it come from?
Water loss on Venus has been estimated from Venus Express data [58,59]. The recent determination of a large variation in the D/H ratio with altitude in the atmosphere [58] provides information about the loss rates on Venus and may shed some light on the likelihood of a liquid water ocean in the past until new measurements of isotopic ratios of noble gases in the Venus atmosphere from the DAVINCI probe [32,59]. It is interesting to note that the Pioneer Venus mass spectrometer data which yielded the high D/H ratio were obtained at an altitude of about 51.3 km [60] only. At higher altitudes, the D/H ratio increases rapidly as determined from the Solar Occultation Infrared Radiometer (SOIR) on Venus Express [58], indicating the complex atmospheric processes active on Venus. With the recent indications that Venus is volcanically active at present [61], it is possible that some water vapor is being emitted along with sulfur dioxide, which may explain the measured altitude profiles of those gases.
Global topography variations on early Venus on regional or smaller scales to allow for lakes and tidal pools should have been present. Such surface pools of water along with hydrothermal vents have been suggested to be the locations for life origins on Earth [62,63].

2.1.2. Rotation Rate

A connection between life and the rotation period is found on Earth through the circadian rhythm [64]. A planet’s rotation period affects the distribution of absorbed solar energy in the atmosphere and surface along with the thickness of the atmosphere and its composition. Lunar tidal effects have slowed the rotation rate of Earth from approximately 6 h to the present-day value of about 24 h. Without a moon of its own, we cannot assess whether Venus also had a much faster rotation period in its infancy.
The rotation rate of Venus (243.0212 ± 0.0006 d) has been measured from Earth-based radar observations during 1988–2017 [65] and differs from the value derived (243.0185 ± 0.0001 d) from the Magellan orbiter radar (1992–1995). Length of day (LoD) variations of as much as 20 min between 2006 and 2020 with a mean rotation period of 243.0226 ± 0.0013 d suggest changes in atmospheric angular momentum with at least ~4% through transfer to the solid planet [66]. Such short-term changes in the rotation rate may be indicative of a far different length of day when Venus formed a crust, if they are assumed as quasi-secular over long periods. However, that is contingent upon atmospheric circulation (surface pressure, wind direction) and surface topography, which have been subject to modification due to volcanic activity and lava flows.
It is possible that the early rotation rate was as short as a few hours, as in the case of Earth. It has been suggested that regardless of the initial rotation rate (and direction) of Venus, it would eventually end up in its present state [67,68,69] of a slow rotation rate with a direction opposite to the orbital motion (i.e., the orbital motion and self-rotation axes are in nearly opposite directions).
With an orbital period of 224.7 days and a rotation period of 243 days, the length of the solar day on Venus (at the surface) is 116.7 days. If the rotation period were much shorter in the past than the present-day value, the solar day would also have been shorter, allowing for larger day–night thermal contrasts (supporting dry and wet conditions on the surface) in a thinner (than present) atmosphere. In a deep atmosphere with zonal circulation, the solar day would vary with altitude depending on the strength and of the flow.
Thus, any life that may have existed on early Venus would have had to adapt to the changes in the rotation rate/solar day. Laboratory experiments with bacteria have shown that such adaptations can take place quickly [70,71]. Microorganism life forms on Venus may have undergone such adaptation and migration from the surface to the habitable clouds in the past. Once the surface became hostile, life forms may have survived in the clouds with a life cycle [26]. If the present-day atmospheric conditions persist on Venus, the settling time for the microorganisms would be long, and the atmospheric circulation could support their presence in the cloud layer [25,26].

3. Present Habitability of Venus Atmosphere

Any discussion of life in the clement conditions found in the present-day cloud layer (48–60 km) is contingent upon life having originated or survived on Venus and adapting to the changing conditions and migration to the habitable layer in the clouds [25,26,72]. Origins of life on Earth are still being explored with little consensus, and the possibility that life could have originated on Venus relies on early conditions being similar to those on Earth. The question of origin of life is not addressed here, only whether life could have survived and evolved/adapted is implicit in the discussion.
Possibility of life in the present-day clouds has been discussed previously [22,25,26,44,73,74]. Mogul et al. [75] found that phototrophy is possible in Venus’ clouds. Although the temperatures in the cloud layer (48–70) are within the thermal range within which terrestrial organisms can survive, there are other environmental challenges that any life must overcome. Lacking any means for direct detection of life in the Venus cloud layer, only indirect indicators of life are available. These include chemical disequilibria in the atmosphere and properties of the cloud cover. These and survival challenges are discussed briefly below.

3.1. Chemical Dis-Equilibria

Ammonia [15,42,73,76,77,78], molecular oxygen [11,79], methane [80], and phosphine [27] have been reported to be present in the Venus atmosphere below the cloud tops, which indicates chemical disequilibrium based on our current knowledge and available data [53,81,82]. Both biological and abiotic processes can produce these gases and disequilibrium conditions. After the initial dismissal of the ammonia detection, no attempts were made to specifically look for it by the subsequent Venera and Pioneer Venus entry probes. The Pioneer Venus Large Probe Neutral Mass Spectrometer (PV LNMS) data did indicate possible presence of ammonia based on new analysis of the data [42]. Recently, millimeter wave spectral data have been obtained for Venus, and those data also reveal presence of ammonia in the atmosphere as well as phosphine [83]. Following the suggestion that Venus cloud aerosols may contain hydrated ammonia salts, thus lowering the pH, Bains et al. [73] argue that production of ammonia in the Venus clouds makes them habitable.
Phosphine was suggested as a bio-signature of life on exoplanets with hydrogen-rich atmospheres [28], and this led to the search for phosphine on Venus [27], despite its present atmosphere not being rich in hydrogen. Anaerobic bacteria have been found to produce phosphine and methane from sewage sludge through a yet unknown biochemical pathway [84]. It is possible that Venus’ past atmosphere could have been abundant in methane, similar to Earth’s past atmosphere [82,85,86], but not much has been investigated about phosphine on Earth in its past.
Phosphine is not expected to last very long in the present-day Venus atmosphere, so a source is required. Even though the authors did not point to life as a source of the gas, the report faced wide skepticism [34,35,36,37,38,87,88,89,90,91,92,93] and controversy, unfortunately. Subsequent investigations bolster the presence of phosphine without any claims regarding its origins. A few claimed it can be produced geologically [38], while others argued it could not [93,94]. Since then, the new data collected from earth based telescopes by the original authors confirm the presence of phosphine [83].
The presence of methane in the Venus atmosphere remains unconfirmed. While the PV LNMS showed the presence of methane [80], a gas chromatograph (PV GC), the companion instrument on the same probe, did not detect it [11]. New analyses of the PV LNMS may yield more information, but confirmation awaits new measurements to be obtained by the DAVINCI probe in 2031 [59].
Molecular oxygen remains to be confirmed as well, as the PV GC remains the only instrument that has detected it in situ. Above the cloud tops, the upper limit for molecular oxygen is (O2/CO2) is 10−7 [95]. No satisfactory explanation for the simultaneous presence of ammonia and molecular oxygen in the habitable layer has been presented to date.

3.2. Environmental Challenges

The main survival challenges to life in the cloud layer (48–72 km) are (i) ionizing radiation, (ii) harmful UV radiation, (iii) water availability, (iv) energy availability, and (v) nutrient availability. Regarding ionizing radiation, a common misperception is that lacking an intrinsic magnetic field, cosmic rays and solar wind will be damaging to life. Unlike on Mars, the thicker atmosphere around Venus leads to an induced magnetosphere, and in the cloud layer, the ionizing and harmful UV radiation are survivable for life [25,43,96,97,98,99,100].
The water availability in the acidic cloud droplets thought to be limiting for life [101] may not be as low as believed according to some new investigations [102,103]. Further, the belief that water may be the only solvent to support life has been challenged by the demonstration that some amino acids and nucleic acids can also survive in concentrated sulfuric acid [45,74,104,105]. The survival of lipids and peptide nucleic acids in concentrated sulfuric acid has been reported as well [106,107], which advances the possibility of life in acidic conditions.

4. Life Detection Strategies—Bio-Signatures?

The thinking and approach to detection of life on other worlds has evolved since the first attempts with Viking missions to Mars in 1970s. The two Viking landers each carried three “Life Detection Experiments” and collected data [108]. Of the three, the Labeled Release experiment produced results that have been argued as having detected signatures of life [109] but have not been accepted by the community [110]. With the pause on further data on Mars until the claim that ancient life [111] in the ALH84001 meteorite determined to have come from Mars [112], there were no attempts to investigate the possibility of life on Mars or elsewhere. While the ancient life report was immediately controversial, it shifted the paradigm for search for life from direct detection to finding evidence of liquid water, an essential ingredient for life.
The lessons learned from the search for life have been useful in the approach to detection of life in the clouds of Venus. Various “bio-signatures” have been proposed as evidence for life on other worlds [113,114,115] and have been discussed in “An Astrobiology Strategy for the Search for Life in the Universe” [116]. Schweiterman et al. [117] review remotely detectable signs of life, and Kiang et al. [118] present a summary of the progress in challenges and issues in bio-signature use in detection of life stemming from the community workshop “The Exoplanet Biosignatures Workshop Without Walls” (EBWWW) held in 2016.
Vickers et al. [119] discuss the challenge of determining confidence in the detection of life against a universe of unconceived alternatives and suggest using the framework of developing uncertainties. Smith and Mathis [120] point out the problems with using bio-signatures as a proxy for life: “astrobiology aims to determine the distribution and diversity of life in the universe. But as the word “bio-signature” suggests, what will be detected is not life itself, but an observation implicating living systems. Our limited access to other worlds suggests this observation is more likely to reflect out-of-equilibrium gasses than a writhing octopus. Yet, anything short of a writhing octopus will raise skepticism about what has been detected. Resolving that skepticism requires a theory to delineate processes due to life and those due to abiotic mechanisms. This poses an existential question for life detection: How do astrobiologists plan to detect life on exoplanets via features shared between non-living and living systems?”
Des Marais et al. [113] identify, but do not limit to, the following: (i) Cellular and extracellular morphologies, (ii) Biogenic fabrics in rocks, (iii) Bio-organic molecular structures, (iv) Chirality, (v) Biogenic minerals, (vi) Biogenic stable isotope patterns in minerals and organic compounds, (vii) Atmospheric gases, (viii) Remotely detectable features on planetary surfaces, and (ix) Temporal changes in global planetary properties. Of these, only a few are presently practical for Venus regarding past or present life—stable isotope patterns, atmospheric gases, and temporal changes in global planetary properties.

4.1. Atmospheric Composition

The clues to the possible existence of liquid water on Venus in the past may be present in the present composition of its atmosphere. Wordsworth et al. [121] discussed the past N2 contents on Earth and Venus and the challenges involved in understanding its past amount even on Earth. The general belief that planetary tropospheres are well mixed in their bulk components is contradicted by Venus, which shows a curious vertical gradient in the abundances of the two main constituents (CO2 and N2) [122,123]. The high temperatures and pressures in the lower atmosphere near the surface render both CO2 and N2 into supercritical states. Whether this leads to density separation among the main constituents is uncertain [124,125]. It may also impact the 15N/14N ratio with altitude, complicating clues about past water.
Beyond ascertaining the make-up of the atmosphere in its bulk constituents (CO2 and N2) as a function of altitude below 120 km, the abundances of minor constituents (water vapor, sulfur dioxide, sulfuric acid vapor) and trace species (carbon monoxide, ammonia, phosphine, methane, molecular oxygen, hydrogen sulfide, carbonyl sulfide among others), as well as noble gases, are essential for a better understanding of the Venus atmospheric chemistry and supportive environment for life. The planned DAVINCI mission to Venus will deploy a well-instrumented probe into the atmosphere that will make many detailed and more precise measurements below the 70 km altitude than have been obtained to date [59]. While these will improve the data coverage, the global distribution of the constituents will need to be obtained by other future missions.

4.1.1. Minor Constituents: Water Vapor, Sulfur Dioxide, Sulfuric Acid Vapor

The minor constituents of the Venus atmosphere have been sampled only by a few entry probes below about 64 km with low precision [9,11,126,127]. Water vapor, sulfur dioxide, and sulfuric acid vapor have been measured by different instruments at different times, and all show variations with altitude, latitude, and longitude over measurement periods. Greaves et al. suggest some variation in phosphine with latitude and local time from further analysis and new data.
Not much is known about trace constituents (noble gas isotopes, CO, halides, metal oxides, hydrates, and salts in vapor form) abundances and their vertical profiles likely to be present in the Venus atmosphere due to past and present volcanism and meteoritic activity. Many of these are essential for life, e.g., phosphorus [128].

4.1.2. Noble Gas Isotopic Ratios

If there was a liquid water reservoir on the surface, when that was lost either through evaporation and escape to space after hydrolysis or through impacts is an open question. Assuming that the terrestrial planets were formed with similar proportion of nitrogen, then the 15N/14N isotopic ratio may provide an important constraint of when the primordial ocean may have been lost. The lighter 14N would have escaped to space at a higher rate than the heavier 15N, thus enriching the atmosphere with 15N when the atmosphere was relatively less massive, i.e., before the onset of the runaway greenhouse [129,130]. There is some reason to be cautious about this, as the present-day atmosphere shows a distinct vertical gradient in the bulk composition below about 60 km—the nitrogen content is found to decrease towards the surface from about 5% to 3.5% [11,103,123,131].
Another caution is the role of biology in the nitrogen cycle due to life in hydrothermal vents, which show variations in the nitrogen isotope intake [132] on Earth. Whether this could have affected the atmospheric isotopic ratio is unknown along with whether this could have happened on Venus in its ancient past.

4.2. Ultraviolet and Other Absorbers

Ever since the contrasts noticed in visual and photographic observations of Venus in the early twentieth century, the identity of the major absorbers responsible for producing the contrasts through inhomogeneous abundances has been a mystery [21,133,134]. The global cloud cover shows maximum contrasts in reflected sunlight between about 320–400 nm, peaking (40%) at about 365 nm, and weaker contrasts are seen at shorter and longer wavelengths [25,133]. SO2 is the only confirmed absorber (below 300 nm) found in the atmosphere, and the identity of other absorbers remains unknown [135,136,137,138,139,140,141].
Several authors have proposed OSSO and its dimers as a possible ultraviolet absorber [142,143,144], but its spectrum and abundances in the Venus atmosphere are not sufficient to explain the observed Venus spectrum [145,146]. As of now, proteins and acidithiobacillus spectra [25] appear to show as good or better match for the Venus spectrum [141] in the 200–500 nm range than any other proposed inorganic absorbers.
While the focus about absorbers has been on the sunlight absorbers, it is worth noting that the near-infrared images of Venus also show contrasts [147,148]. Nightside images of Venus cloud cover (0.8–5.5 μm) also show contrasts in the many CO2 windows [149]. The observed structures have been attributed to oxygen airglow (near 1.27 μm, [150,151,152]) and inhomogeneous cloud opacities at other wavelengths. No absorbers have been proposed or detected but some contribution from unknown absorbers cannot be ruled out [25].

4.3. Temporal Changes in Global Planetary Properties

One key property of even microbial life is the presence of a circadian rhythm [64]. Should any life exist in the cloud cover, then it should exhibit such a cycle. In the Venus atmosphere, the length of the solar day varies with altitude due to the dominant atmospheric zonal flow. Figure 1 shows the length of the solar day on present day Venus as a function of altitude.
Monitoring the global cloud cover with multi-spectral imaging from Venus orbiting spacecraft is one means to detect changes in the cloud cover properties. However, reflected solar light observations vary with phase angle, and it can be challenging to separate the temporal variations from phase angle variations. Only in an average sense can these be separated [141]. Spacecrafts in orbit around Venus–Sun L2 Lagrange point offer a solution to monitor the temporal variations in cloud cover with minimal phase angle variations [154]. Akatsuki and Venus Express orbiters have acquired many years of images at multiple wavelengths, and these enable looking for any such periodicities in the reflectivity of the cloud cover as well as determining global scale contrast.
Figure 2 shows the periodicities in the global contrast determined as the root mean square (RMS) deviation from the brightness of the planet predicted by the Minnaert law from 365 nm Akatsuki images [155] obtained during 2016–2018. The strongest peaks reveal the phase angle dependence at the Venus orbital, solar day, and Akatsuki orbiter periods (225, 116, and 10–11 days, respectively), while the other peaks reveal periodicities that correspond to periodic changes in the UV contrasts, to which life forms may contribute with periods matching the length solar day in the cloud layer. It is likely that the atmospheric rotation is also responsible for the periods near 4 days. The existence of such anticipated is of course not an indicator of life, but only corroborative. A short period equatorial orbit is not ideal for determining the circadian period due to the rotation of the atmosphere. A near constant view of the dayside can be obtained from the L2 Lagrange point of the Sun–Venus system, and an orbit around this point would be ideal for continuous day side imaging of Venus [154] to search for the circadian period.
The average cloud top rotation period from tracking 365 nm features varies with latitude (2.5 days at 45 degrees to 5 days at equator) but the dominant contrasts are at low latitudes. The low latitude rotation period over a roughly 12 year period has been found to vary between 4 and 5 days [156] corresponding to a mean zonal speed of 98.6 ms−1 and an amplitude of 10 ms−1. It is reassuring that the short periods in the contrast periodogram are comparable to the atmospheric rotation period which determines the expected circadian period. But this is not an affirmation of biological circadian activity responsible for the contrasts.

4.4. Bio-Organic Molecular Structures

There have been no explicit attempts to detect organic molecules on Venus explicitly, and no organics have been identified in the spectral observations to date. Pathways to organics in the Venus atmosphere were proposed earlier [157], but it is only now that a search will be undertaken. The privately funded Morning Star mission [158] now expected to be launched in 2026 by Rocket Lab (https://www.rocketlabusa.com/missions/upcoming-missions/first-private-mission-to-venus/, accessed on 22 April 2025) will deploy a mini-probe carrying a single instrument to detect the presence of organic molecules [159] using auto-fluorescence. The probe will sample the cloud layer for about three minutes during its rapid descent to the surface. However, the UV contrasts observed in images reveal spatial contrasts, suggesting non-uniform mixing of the absorbers horizontally. If the organics are ubiquitous, there is a possibility that they may be sampled by the probe. The successful return of data from this mission is eagerly anticipated. A negative result in itself may not be indicative of absence of life. The Venus Life finder team has proposed follow on missions after the mini probe mission [160]. Ultimately, cloud aerosol samples may need to be collected for deeper investigation of their composition [161,162,163], but they will need investments in technology for platforms and instruments.

5. Remarks

The need for new and improved observations of Venus and its atmosphere/cloud cover are forthcoming. After the anticipated launch of Morning Star mini-probe in 2026, other missions in development may be launched. New missions to be launched by NASA (DAVINCI and VERITAS), ESA (Envision), and India by 2031 will obtain additional new data on Venus and its atmosphere. There may be additional missions from other space agencies and Rocket Lab. Although none of them except the Rocket Lab missions have astrobiology as a science goal, they will nonetheless provide corroborative data on the past and present habitability of Venus. Only the DAVINCI atmospheric entry probe carrying many instruments will make detailed atmospheric measurements below 70 km altitude every 200 m, as it descends to the surface. Two instruments—Venus Mass Spectrometer (VMS) with a wider mass range and higher mass resolution and the Venus Tunable Laser Spectrometer (VTLS)—will sample the atmosphere along the descent path to measure the chemical composition.
The VERITAS, Envision, and ISRO Venus orbiters will map the surface with synthetic aperture radar with an order-of-magnitude better resolution than is available from the Magellan and Venera 15/16 orbiters. They also carry imagers and spectrometers, which have the potential to shed more light on the variable nature of the Venus atmosphere.
The search for life beyond the Earth is motivated by the urge to learn if we are alone in the universe. As many planets around other stars are being discovered, each with its own characteristic atmospheric and stellar environment, with some Earth-like planets deemed habitable, while many others lie in the Venus zone [164] of indeterminate habitability, Venus presents an accessible planet for the search. After the initial excitement of identifying bio-signatures, an era of a cautious approach in the search for life is beginning.
Venus may or may not have had liquid water in its past. If it did, the possibility of life is viable. It is important that we understand our neighbor to assess the possibility of life on Venus like exoplanets. Venus has guarded its mysteries under a thick cloud cover and sampling the clouds is essential to solve this puzzle. With focused, open minded unbiased investigations we may hope to find the answers soon.

Funding

This research received no external funding.

Conflicts of Interest

The author declares no conflict of interest.

References

  1. Arrhenius, S. The Destinies of Stars; G. P. Putnam and Sons, The Knickerbocker Press: New York, NY, USA; London, UK, 1918. [Google Scholar]
  2. Morowitz, H.; Sagan, C. Life on the Surface of Venus? Nature 1967, 216, 1198–1199. [Google Scholar] [CrossRef]
  3. Suomi, V. Cloud Motions on Venus. In NASA Special Publication; NASA: Washington, DC, USA, 1975; p. 42. [Google Scholar]
  4. Suomi, V.E.; Limaye, S.S. Venus: Further Evidence of Vortex Circulation. Science 1978, 201, 1009–1011. [Google Scholar] [CrossRef]
  5. Limaye, S.S.; Suomi, V.E. Cloud motions on Venus—Global structure and organization. J. Atmos. Sci. 1981, 38, 1220–1235. [Google Scholar] [CrossRef]
  6. Boyer, C.; Guérin, P. Étude de la Rotation Rétrograde, en 4 Jours, de la Couche Extérieure Nuageuse de Vénus. Icarus 1969, 11, 338. [Google Scholar] [CrossRef]
  7. Boyer, C. The 4-day rotation of the upper atmosphere of venus. Planet. Space Sci. 1973, 21, 1559. [Google Scholar] [CrossRef]
  8. Boyer, C. La haute atmosphère de Vénus: Tentative d’explication de sa rotation. Astronomie 1986, 100, 77. [Google Scholar]
  9. Surkov, Y.A.; Andrejchikov, B.M.; Kalinkina, O.M. On the content of ammonia in the Venus atmosphere based on data obtained from Venera 8 automatic station. Akad. Nauk SSSR Dokl. 1973, 213, 296–299. [Google Scholar]
  10. von Zahn, U.; Moroz, V.I. Composition of the Venus atmosphere below 100 km altitude. Adv. Space Res. 1985, 5, 173. [Google Scholar] [CrossRef]
  11. Oyama, V.I.; Carle, G.C.; Woeller, F.; Pollack, J.B.; Reynolds, R.T.; Craig, R.A. Pioneer Venus gas chromatography of the lower atmosphere of Venus. J. Geophys. Res. 1980, 85, 7891. [Google Scholar] [CrossRef]
  12. Kuiper, G.P. Identification of the Venus cloud layers. Commun. Lunar Planet. Lab. 1969, 6, 229. [Google Scholar]
  13. Travis, L.D. On the origin of ultraviolet contrasts on Venus. J. Atmos. Sci. 1975, 32, 1190. [Google Scholar] [CrossRef]
  14. Donahue, T.M.; Hoffman, J.H.; Hodges, R.R.; Watson, A.J. Venus was wet—A measurement of the ratio of deuterium to hydrogen. Science 1982, 216, 630–633. [Google Scholar] [CrossRef]
  15. Goettel, K.A.; Lewis, J.S. Ammonia in the Atmosphere of Venus. J. Atmos. Sci. 1974, 31, 828–830. [Google Scholar] [CrossRef]
  16. Shimizu, M. Ultraviolet absorbers in the Venus clouds. Astrophys. Space Sci. 1977, 51, 497. [Google Scholar] [CrossRef]
  17. Pollack, J.B.; Ragent, B.; Boese, R.W.; Tomasko, M.G.; Blamont, J.; Knollenberg, R.G.; Esposito, L.W.; Stewart, A.I.; Travis, L. Nature of the ultraviolet absorber in the Venus clouds—Inferences based on Pioneer Venus data. Science 1979, 205, 76–79. [Google Scholar] [CrossRef]
  18. Pollack, J.B.; Toon, O.B.; Whitten, R.C.; Boese, R.; Ragent, B.; Tomasko, M.; Esposito, L.; Travis, L.; Wiedman, D. Distribution and source of the UV absorption in Venus’ atmosphere. J. Geophys. Res. Space Phys. 1980, 85, 8141–8150. [Google Scholar] [CrossRef]
  19. Bertaux, J.-L.; Ehkonomov, A.P.; Mege, B.; Moroz, V.I.; Abergel, A.; Gnedykh, V.I.; Grigor’ev, A.V.; Moshkin, B.E.; Hauchecorne, A.; Pommereau, J.-P.; et al. Investigation of UV absorption in the Venus atmosphere with the Vega 1 and Vega 2 reentry vehicles. Kosm. Issled. 1987, 25, 691. [Google Scholar]
  20. Markiewicz, W.J.; Petrova, E.; Shalygina, O.; Almeida, M.; Titov, D.V.; Limaye, S.S.; Ignatiev, N.; Roatsch, T.; Matz, K.D. Glory on Venus cloud tops and the unknown UV absorber. Icarus 2014, 234, 200–203. [Google Scholar] [CrossRef]
  21. Titov, D.V.; Ignatiev, N.I.; McGouldrick, K.; Wilquet, V.; Wilson, C.F. Clouds and Hazes of Venus. Space Sci. Rev. 2018, 214, 126. [Google Scholar] [CrossRef]
  22. Grinspoon, D.H. Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet; Perseus Books Group: New York, NY, USA, 1997. [Google Scholar]
  23. Schulze-Makuch, D.; Grinspoon, D.H.; Abbas, O.; Irwin, L.N.; Bullock, M.A. A Sulfur-Based Survival Strategy for Putative Phototrophic Life in the Venusian Atmosphere. Astrobiology 2004, 4, 11–18. [Google Scholar] [CrossRef]
  24. Grinspoon, D.; Bullock, M. Astrobiology and Venus Exploration. In Exploring Venus as a Terrestrial Planet; John Wiley & Sons: Hoboken, NJ, USA, 2007; pp. 191–206. [Google Scholar]
  25. Limaye, S.S.; Mogul, R.; Smith, D.J.; Ansari, A.H.; Słowik, G.; Vaishampayan, P. Venus’ Spectral Signatures and the Potential for Life in the Clouds. Astrobiology 2018, 18, 1181–1198. [Google Scholar] [CrossRef] [PubMed]
  26. Seager, S.; Petkowski, J.J.; Gao, P.; Bains, W.; Bryan, N.C.; Ranjan, S.; Greaves, J. The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere. Astrobiology 2020. [Google Scholar] [CrossRef] [PubMed]
  27. Greaves, J.S.; Richards, A.M.S.; Bains, W.; Rimmer, P.B.; Sagawa, H.; Clements, D.L.; Seager, S.; Petkowski, J.J.; Sousa-Silva, C.; Ranjan, S.; et al. Phosphine gas in the cloud decks of Venus. Nat. Astron. 2020, 5, 655–664. [Google Scholar] [CrossRef]
  28. Sousa-Silva, C.; Seager, S.; Ranjan, S.; Petkowski, J.J.; Zhan, Z.; Hu, R.; Bains, W. Phosphine as a Biosignature Gas in Exoplanet Atmospheres. Astrobiology 2020, 20, 235–268. [Google Scholar] [CrossRef]
  29. Schulze-Makuch, D. The Case (or Not) for Life in the Venusian Clouds. Life 2021, 11, 255. [Google Scholar] [CrossRef] [PubMed]
  30. Westall, F.; de Vries, S.T.; Nijman, W.; Rouchon, V.; Orberger, B.; Pearson, V.; Watson, J.; Verchovsky, A.; Wright, I.; Rouzaud, J.-N.l.; et al. The 3.466 Ga “Kitty’s Gap Chert,” an early Archean microbial ecosystem. In Processes on the Early Earth; Geological Society of America: Boulder, CO, USA, 2006; Volume 405. [Google Scholar]
  31. Westall, F.; Höning, D.; Avice, G.; Gentry, D.; Gerya, T.; Gillmann, C.; Izenberg, N.; Way, M.J.; Wilson, C. The Habitability of Venus. Space Sci. Rev. 2023, 219, 17. [Google Scholar] [CrossRef]
  32. Widemann, T.; Smrekar, S.E.; Garvin, J.B.; Straume-Lindner, A.G.; Ocampo, A.C.; Schulte, M.D.; Voirin, T.; Hensley, S.; Dyar, M.D.; Whitten, J.L.; et al. Venus Evolution Through Time: Key Science Questions, Selected Mission Concepts and Future Investigations. Space Sci. Rev. 2023, 219, 56. [Google Scholar] [CrossRef]
  33. Geng, J.; Zhang, R.; Wang, X. Chemical Origin of Phosphine in Nature. In Proceedings of the 2010 4th International Conference on Bioinformatics and Biomedical Engineering, Chengdu, China, 18–20 June 2010; pp. 1–12. [Google Scholar]
  34. Villanueva, G.; Cordiner, M.; Irwin, P.; de Pater, I.; Butler, B.; Gurwell, M.; Milam, S.; Nixon, C.; Luszcz-Cook, S.; Wilson, C.; et al. No phosphine in the atmosphere of Venus. arXiv 2020, arXiv:2010.14305. [Google Scholar]
  35. Snellen, I.A.G.; Guzman-Ramirez, L.; Hogerheijde, M.R.; Hygate, A.P.S.; van der Tak, F.F.S. Re-analysis of the 267 GHz ALMA observations of Venus. No statistically significant detection of phosphine. Astron. Astrophys. 2020, 644, L2. [Google Scholar] [CrossRef]
  36. Akins, A.B.; Lincowski, A.P.; Meadows, V.S.; Steffes, P.G. Complications in the ALMA Detection of Phosphine at Venus. Astrophys. J. 2021, 907, L27. [Google Scholar] [CrossRef]
  37. Villanueva, G.L.; Cordiner, M.; Irwin, P.G.J.; de Pater, I.; Butler, B.; Gurwell, M.; Milam, S.N.; Nixon, C.A.; Luszcz-Cook, S.H.; Wilson, C.F.; et al. No evidence of phosphine in the atmosphere of Venus from independent analyses. Nat. Astron. 2021, 5, 631–635. [Google Scholar] [CrossRef]
  38. Truong, N.; Lunine, J.I. Volcanically extruded phosphides as an abiotic source of Venusian phosphine. Proc. Natl. Acad. Sci. USA 2021, 118, e2021689118. [Google Scholar] [CrossRef]
  39. Mráziková, K.; Knížek, A.; Saeidfirozeh, H.; Petera, L.; Civiš, S.; Saija, F.; Cassone, G.; Rimmer, P.B.; Ferus, M. A Novel Abiotic Pathway for Phosphine Synthesis over Acidic Dust in Venus’ Atmosphere. Astrobiology 2024, 24, 407–422. [Google Scholar] [CrossRef]
  40. Glindemann, D.; Edwards, M.; Morgenstern, P. Phosphine from Rocks: Mechanically Driven Phosphate Reduction? Environ. Sci. Technol. 2005, 39, 8295–8299. [Google Scholar] [CrossRef] [PubMed]
  41. Bains, W.; Petkowski, J.J.; Seager, S.; Ranjan, S.; Sousa-Silva, C.; Rimmer, P.B.; Zhan, Z.; Greaves, J.S.; Richards, A.M.S. Phosphine on Venus Cannot be Explained by Conventional Processes. arXiv 2020, arXiv:2009.06499v1. [Google Scholar] [CrossRef]
  42. Mogul, R.; Limaye, S.S.; Way, M.J.; Cordova, J.A. Venus’ Mass Spectra Show Signs of Disequilibria in the Middle Clouds. Geophys. Res. Lett. 2021, 48, e2020GL091327. [Google Scholar] [CrossRef] [PubMed]
  43. Limaye, S.S.; Mogul, R.; Baines, K.H.; Bullock, M.A.; Cockell, C.; Cutts, J.A.; Gentry, D.M.; Grinspoon, D.H.; Head, J.W.; Jessup, K.-L.; et al. Venus, an astrobiology target. Astrobiology 2021, 21, 23. [Google Scholar] [CrossRef]
  44. Cockell, C.S. Life on Venus. Planet. Space Sci. 1999, 47, 1487–1501. [Google Scholar] [CrossRef]
  45. Bains, W.; Petkowski, J.J.; Zhan, Z.; Seager, S. Evaluating Alternatives to Water as Solvents for Life: The Example of Sulfuric Acid. Life 2021, 11, 400. [Google Scholar] [CrossRef]
  46. Jiang, L.-Q.; Carter, B.R.; Feely, R.A.; Lauvset, S.K.; Olsen, A. Surface ocean pH and buffer capacity: Past, present and future. Sci. Rep. 2019, 9, 18624. [Google Scholar] [CrossRef]
  47. Grinspoon, D.H. Was Venus wet? Deuterium reconsidered. Science 1987, 238, 1702–1704. [Google Scholar] [CrossRef]
  48. Way, M.J.; Del Genio, A.; Amundsen, D.S. Modeling Venus-Like Worlds Through Time. arXiv 2018, arXiv:1802.05434. [Google Scholar]
  49. Way, M.J.; Del Genio, A.D.; Kiang, N.Y.; Sohl, L.E.; Grinspoon, D.H.; Aleinov, I.; Kelley, M.; Clune, T. Was Venus the first habitable world of our solar system? Geophys. Res. Lett. 2016, 43, 8376–8383. [Google Scholar] [CrossRef] [PubMed]
  50. Mueller, N.; Helbert, J.; Hashimoto, G.L.; Tsang, C.C.C.; Erard, S.; Piccioni, G.; Drossart, P. Venus surface thermal emission at 1 μm in VIRTIS imaging observations: Evidence for variation of crust and mantle differentiation conditions. J. Geophys. Res. (Planets) 2008, 113, E00B17. [Google Scholar] [CrossRef]
  51. Kiefer, W.S.; Weller, M.B. From Mobile Lid to Stagnant Lid Mantle Convection: Variations in Solar Luminosity, Climate Evolution, and the Temporal Variability in Tectonic Style on Venus. In Proceedings of the Exoplanets in Our Backyard: Solar System and Exoplanet Synergies on Planetary Formation, Evolution, and Habitability Conference, Houston, TX, USA, 5–7 February 2020; p. 3020. [Google Scholar]
  52. Weller, M.B.; Kiefer, W.S. The Physics of Changing Tectonic Regimes: Implications for the Temporal Evolution of Mantle Convection and the Thermal History of Venus. J. Geophys. Res. (Planets) 2020, 125, e05960. [Google Scholar] [CrossRef]
  53. Krissansen-Totton, J.; Fortney, J.J.; Nimmo, F. Was Venus Ever Habitable? Constraints from a Coupled Interior–Atmosphere–Redox Evolution Model. Planet. Sci. J. 2021, 2, 216. [Google Scholar] [CrossRef]
  54. Way, M.J.; Del Genio, A.D. Venusian Habitable Climate Scenarios: Modeling Venus Through Time and Applications to Slowly Rotating Venus-Like Exoplanets. J. Geophys. Res. Planets 2020, 125, e2019JE006276. [Google Scholar] [CrossRef]
  55. Warren, A.O.; Kite, E.S. Narrow range of early habitable Venus scenarios permitted by modeling of oxygen loss and radiogenic argon degassing. Proc. Natl. Acad. Sci. USA 2023, 120, e2209751120. [Google Scholar] [CrossRef]
  56. Constantinou, T.; Shorttle, O.; Rimmer, P.B. A dry Venusian interior constrained by atmospheric chemistry. Nat. Astron. 2025, 9, 189–198. [Google Scholar] [CrossRef]
  57. Turbet, M.; Bolmont, E.; Chaverot, G.; Ehrenreich, D.; Leconte, J.; Marcq, E. Day–night cloud asymmetry prevents early oceans on Venus but not on Earth. Nature 2021, 598, 276–280. [Google Scholar] [CrossRef]
  58. Mahieux, A.; Viscardy, S.; Yelle, R.V.; Karyu, H.; Chamberlain, S.; Robert, S.; Piccialli, A.; Trompet, L.; Erwin, J.T.; Ubukata, S.; et al. Unexpected increase of the deuterium to hydrogen ratio in the Venus mesosphere. Proc. Natl. Acad. Sci. USA 2024, 121, e2401638121. [Google Scholar] [CrossRef] [PubMed]
  59. Garvin, J.B.; Getty, S.A.; Arney, G.N.; Johnson, N.M.; Kohler, E.; Schwer, K.O.; Sekerak, M.; Bartels, A.; Saylor, R.S.; Elliott, V.E.; et al. Revealing the Mysteries of Venus: The DAVINCI Mission. Planet. Sci. J. 2022, 3, 117. [Google Scholar] [CrossRef]
  60. Donahue, T.M.; Hodges, R.R., Jr. Past and present water budget of Venus. J. Geophys. Res. 1992, 97, 6083–6091. [Google Scholar] [CrossRef]
  61. Sulcanese, D.; Mitri, G.; Mastrogiuseppe, M. Evidence of ongoing volcanic activity on Venus revealed by Magellan radar. Nat. Astron. 2024, 8, 973–982. [Google Scholar] [CrossRef]
  62. Deamer, D.W.; Damer, B.F.; Van Kranendonk, M.J.; Djokic, T. An Origin of Life in Cycling Hot Spring Pools: Emerging Evidence from Chemistry, Geology and Computational Studies. In Proceedings of the XVIIIth International Conference on the Origin of Life, San Diego, CA, USA, 16–21 July 2017; p. 4130. [Google Scholar]
  63. Damer, B.; Deamer, D. The Hot Spring Hypothesis for an Origin of Life. Astrobiology 2019, 20, 429–452. [Google Scholar] [CrossRef]
  64. Paranjpe, D.A.; Sharma, V.K. Evolution of temporal order in living organisms. J. Circadian Rhythm. 2005, 3, 7. [Google Scholar] [CrossRef] [PubMed]
  65. Campbell, B.A.; Campbell, D.B.; Carter, L.M.; Chandler, J.F.; Giorgini, J.D.; Margot, J.-L.; Morgan, G.A.; Nolan, M.C.; Perillat, P.J.; Whitten, J.L. The mean rotation rate of Venus from 29 years of Earth-based radar observations. Icarus 2019, 332, 19–23. [Google Scholar] [CrossRef]
  66. Margot, J.-L.; Campbell, D.B.; Giorgini, J.D.; Jao, J.S.; Snedeker, L.G.; Ghigo, F.D.; Bonsall, A. Spin state and moment of inertia of Venus. Nat. Astron. 2021, 5, 676–683. [Google Scholar] [CrossRef]
  67. Correia, A.C.M.; Laskar, J. The four final rotation states of Venus. Nature 2001, 411, 767–770. [Google Scholar] [CrossRef]
  68. Correia, A.C.M.; Laskar, J. Long-term evolution of the spin of Venus. II. Numerical simulations. Icarus 2003, 163, 24–45. [Google Scholar] [CrossRef]
  69. Correia, A.C.M.; Laskar, J.; de Surgy, O.N. Long-term evolution of the spin of Venus. I. Theory. Icarus 2003, 163, 1–23. [Google Scholar] [CrossRef]
  70. Chavan, A.G.; Swan, J.A.; Heisler, J.; Sancar, C.; Ernst, D.C.; Fang, M.; Palacios, J.G.; Spangler, R.K.; Bagshaw, C.R.; Tripathi, S.; et al. Reconstitution of an intact clock reveals mechanisms of circadian timekeeping. Science 2021, 374, eabd4453. [Google Scholar] [CrossRef]
  71. Kondo, T.; Tsinoremas, N.F.; Golden, S.S.; Hirschie Johnson, C.; Kutsuna, S.; Ishiura, M. Circadian Clock Mutants of Cyanobacteria. Science 1994, 266, 1233. [Google Scholar] [CrossRef] [PubMed]
  72. Kotsyurbenko, O.R.; Kompanichenko, V.N.; Brouchkov, A.V.; Khrunyk, Y.Y.; Karlov, S.P.; Sorokin, V.V.; Skladnev, D.A. Different Scenarios for the Origin and the Subsequent Succession of a Hypothetical Microbial Community in the Cloud Layer of Venus. Astrobiology 2024, 24, 423–441. [Google Scholar] [CrossRef]
  73. Bains, W.; Petkowski, J.J.; Rimmer, P.B.; Seager, S. Production of ammonia makes Venusian clouds habitable and explains observed cloud-level chemical anomalies. Proc. Natl. Acad. Sci. USA 2021, 118, e2110889118. [Google Scholar] [CrossRef]
  74. Bains, W.; Petkowski, J.J.; Seager, S. Venus’ Atmospheric Chemistry and Cloud Characteristics Are Compatible with Venusian Life. Astrobiology 2023, 24, 371–385. [Google Scholar] [CrossRef]
  75. Mogul, R.; Limaye, S.S.; Lee, Y.J.; Pasillas, M. Potential for Phototrophy in Venus’ Clouds. Astrobiology 2021. [Google Scholar] [CrossRef] [PubMed]
  76. Smirnova, T.V.; Kuz’min, A.D. Water-vapor and ammonia abundance in the lower atmosphere of Venus estimated from radar measurements. Sov. Astron. 1974, 18, 357. [Google Scholar]
  77. Wildt, R. Photochemistry of Planetary Atmospheres. Astrophys. J. 1937, 86, 321. [Google Scholar] [CrossRef]
  78. Surkov, Y.A.; Andreichikov, B.M.; Kalinkina, O.M. Ammonia content in the atmosphere of Venus according to data from the Venus-8 probe. Sov. Phys. Dokl. 1974, 18, 679. [Google Scholar]
  79. Cleland, C.E.; Rimmer, P.B. Ammonia and Phosphine in the Clouds of Venus as Potentially Biological Anomalies. Aerospace 2022, 9, 752. [Google Scholar] [CrossRef]
  80. Donahue, T.M.; Hodges, R.R. Venus methane and water. Geophys. Res. Lett. 1993, 20, 591–594. [Google Scholar] [CrossRef]
  81. Krissansen-Totton, J.; Bergsman, D.S.; Catling, D.C. On Detecting Biospheres from Chemical Thermodynamic Disequilibrium in Planetary Atmospheres. Astrobiology 2016, 16, 39–67. [Google Scholar] [CrossRef]
  82. Krissansen-Totton, J.; Olson, S.; Catling, D.C. Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life. Sci. Adv. 2018, 4, eaao5747. [Google Scholar] [CrossRef]
  83. Clements, D.L. Venus Phosphine: Updates and lessons learned. arXiv 2024, arXiv:2409.13438. [Google Scholar] [CrossRef]
  84. Cao, J.; Zhang, C.; Rong, H.; Zhao, M.; Wei, W.; Zhao, L. Study on Effects of Electron Donors on Phosphine Production from Anaerobic Activated Sludge. Water 2017, 9, 563. [Google Scholar] [CrossRef]
  85. Zahnle, K.; Schaefer, L.; Fegley, B. Earth’s earliest atmospheres. Cold Spring Harb Perspect Biol 2010, 2, a004895. [Google Scholar] [CrossRef] [PubMed]
  86. Olson, S.L.; Reinhard, C.T.; Lyons, T.W. Limited role for methane in the mid-Proterozoic greenhouse. Proc. Natl. Acad. Sci. USA 2016, 113, 11447–11452. [Google Scholar] [CrossRef]
  87. Encrenaz, T.; Greathouse, T.K.; Marcq, E.; Widemann, T.; Bézard, B.; Fouchet, T.; Giles, R.; Sagawa, H.; Greaves, J.; Sousa-Silva, C. A stringent upper limit of the PH3 abundance at the cloud top of Venus. Astron. Astrophys. 2020, 643, L5. [Google Scholar] [CrossRef]
  88. Greaves, J.S.; Richards, A.M.S.; Bains, W.; Rimmer, P.B.; Clements, D.L.; Seager, S.; Petkowski, J.J.; Sousa-Silva, C.; Ranjan, S.; Fraser, H.J. Re-analysis of Phosphine in Venus’ Clouds. arXiv 2020, arXiv:2011.08176. [Google Scholar]
  89. Greaves, J.S.; Bains, W.; Petkowski, J.J.; Seager, S.; Sousa-Silva, C.; Ranjan, S.; Clements, D.L.; Rimmer, P.B.; Fraser, H.J.; Mairs, S.; et al. On the Robustness of Phosphine Signatures in Venus’ Clouds. arXiv 2020, arXiv:2012.05844. [Google Scholar]
  90. Greaves, J.S.; Richards, A.M.S.; Bains, W.; Rimmer, P.B.; Clements, D.L.; Seager, S.; Petkowski, J.J.; Sousa-Silva, C.; Ranjan, S.; Fraser, H.J. Recovery of Spectra of Phosphine in Venus’ Clouds. arXiv 2021, arXiv:2104.09285. [Google Scholar]
  91. Cockell, C.S.; McMahon, S.; Biddle, J.F. When is Life a Viable Hypothesis? The Case of Venusian Phosphine. Astrobiology 2021, 21, 261. [Google Scholar] [CrossRef] [PubMed]
  92. Greaves, J.S.; Richards, A.M.S.; Bains, W.; Rimmer, P.B.; Clements, D.L.; Seager, S.; Petkowski, J.J.; Sousa-Silva, C.; Ranjan, S.; Fraser, H.J. Reply to: No evidence of phosphine in the atmosphere of Venus from independent analyses. Nat. Astron. 2021, 5, 636–639. [Google Scholar] [CrossRef]
  93. Bains, W.; Petkowski, J.J.; Seager, S.; Ranjan, S.; Sousa-Silva, C.; Rimmer, P.B.; Zhan, Z.; Greaves, J.S.; Richards, A.M.S. Phosphine on Venus Cannot Be Explained by Conventional Processes. Astrobiology 2021, 21. [Google Scholar] [CrossRef] [PubMed]
  94. Bains, W.; Petkowski, J.J.; Seager, S.; Ranjan, S.; Sousa-Silva, C.; Rimmer, P.B.; Zhan, Z.; Greaves, J.S.; Richards, A.M.S. Venusian phosphine: A ‘wow!’ signal in chemistry? In Phosphorus, Sulfur, and Silicon and the Related Elements; Taylor & Francis: Abingdon, UK, 2021; pp. 1–6. [Google Scholar] [CrossRef]
  95. Trauger, J.T.; Lunine, J.I. Spectroscopy of molecular oxygen in the Atmospheres of Venus and Mars. Icarus 1983, 55, 272–281. [Google Scholar] [CrossRef]
  96. Nordheim, T.A.; Dartnell, L.R.; Desorgher, L.; Coates, A.J.; Jones, G.H. Ionization of the venusian atmosphere from solar and galactic cosmic rays. Icarus 2015, 245, 80–86. [Google Scholar] [CrossRef]
  97. Dartnell, L.R.; Nordheim, T.A.; Patel, M.R.; Mason, J.P.; Coates, A.J.; Jones, G.H. Constraints on a potential aerial biosphere on Venus: I. Cosmic rays. Icarus 2015, 257, 396–405. [Google Scholar] [CrossRef]
  98. Herbst, K.; Banjac, S.; Nordheim, T.A. Revisiting the cosmic-ray induced Venusian ionization with the Atmospheric Radiation Interaction Simulator (AtRIS). Astron. Astrophys. 2019, 624, A124. [Google Scholar] [CrossRef]
  99. Patel, M.R.; Mason, J.P.; Nordheim, T.A.; Dartnell, L.R. Constraints on a potential aerial biosphere on Venus: II. Ultraviolet radiation. Icarus 2022, 373, 114796. [Google Scholar] [CrossRef]
  100. Kotsyurbenko, O.R. Searching for Life on Venus: History of the Problem and Basic Concepts. Sol. Syst. Res. 2023, 57, 221–235. [Google Scholar] [CrossRef]
  101. Hallsworth, J.E.; Koop, T.; Dallas, T.D.; Zorzano, M.-P.; Burkhardt, J.; Golyshina, O.V.; Martín-Torres, J.; Dymond, M.K.; Ball, P.; McKay, C.P. Water activity in Venus’s uninhabitable clouds and other planetary atmospheres. Nat. Astron. 2021, 5, 665–675. [Google Scholar] [CrossRef]
  102. Rimmer, P.B.; Jordan, S.; Constantinou, T.; Woitke, P.; Shorttle, O.; Hobbs, R.; Paschodimas, A. Hydroxide Salts in the Clouds of Venus: Their Effect on the Sulfur Cycle and Cloud Droplet pH. Planet. Sci. J. 2021, 2, 133. [Google Scholar] [CrossRef]
  103. Mogul, R.; Limaye, S.S.; Way, M.J. The CO2 profile and analytical model for the Pioneer Venus Large Probe neutral mass spectrometer. Icarus 2023, 392, 115374. [Google Scholar] [CrossRef]
  104. Seager, S.; Petkowski, J.J.; Seager, M.D.; Grimes, J.H.; Zinsli, Z.; Vollmer-Snarr, H.R.; Abd El-Rahman, M.K.; Wishart, D.S.; Lee, B.L.; Gautam, V.; et al. Stability of nucleic acid bases in concentrated sulfuric acid: Implications for the habitability of Venus’ clouds. Proc. Natl. Acad. Sci. USA 2023, 120, e2220007120. [Google Scholar] [CrossRef]
  105. Seager, M.D.; Seager, S.; Bains, W.; Petkowski, J.J. Stability of 20 Biogenic Amino Acids in Concentrated Sulfuric Acid: Implications for the Habitability of Venus’ Clouds. Astrobiology 2024, 24. [Google Scholar] [CrossRef] [PubMed]
  106. Duzdevich, D.; Nisler, C.; Petkowski, J.J.; Bains, W.; Kaminsky, C.K.; Szostak, J.W.; Seager, S. Simple Lipids Form Stable Higher-Order Structures in Concentrated Sulfuric Acid. Astrobiology 2025, 25. [Google Scholar] [CrossRef]
  107. Petkowski, J.J.; Seager, S.; Seager, M.D.; Bains, W.; Marinus, N.; Poizat, M.; Plumet, C.; van Wiltenburg, J.; Visser, T.; Poelert, M. Astrobiological implications of the stability and reactivity of peptide nucleic acid (PNA) in concentrated sulfuric acid. Sci. Adv. 2025, 11, eadr0006. [Google Scholar] [CrossRef]
  108. Klein, H.P.; Lederberg, J.; Rich, A. Biological experiments: The viking Mars lander. Icarus 1972, 16, 139–146. [Google Scholar] [CrossRef]
  109. Levin, G.V.; Straat, P.A. The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment. Astrobiology 2016, 16, 798–810. [Google Scholar] [CrossRef]
  110. Klein, H.P.; Oyama, V.I.; Berdahl, B.J.; Carle, G.C.; Johnson, R.D.; Hobby, G.L.; Horowitz, N.H.; Straat, P.A.; Levin, G.V.; Lederberg, J.; et al. 2—The Viking Biological Investigation: Preliminary Results. In Comparative Planetology; Ponnamperuma, C., Ed.; Academic Press: Cambridge, MA, USA, 1978; pp. 7–26. [Google Scholar]
  111. McKay, D.S.; Gibson, E.K.; Thomas-Keprta, K.L.; Vali, H.; Romanek, C.S.; Clemett, S.J.; Chillier, X.D.F.; Maechling, C.R.; Zare, R.N. Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001. Science 1996, 273, 924–930. [Google Scholar] [CrossRef] [PubMed]
  112. Mittlefehldt, D.W. ALH 84001: A Cumulate Orthopyroxenite Member of the Martian Meteorite Clan. Meteoritics 1994, 29, 214. [Google Scholar] [CrossRef]
  113. Des Marais, D.J.; Nuth, J.A.; Allamandola, L.J.; Boss, A.P.; Farmer, J.D.; Hoehler, T.M.; Jakosky, B.M.; Meadows, V.S.; Pohorille, A.; Runnegar, B.; et al. The NASA Astrobiology Roadmap. Astrobiology 2008, 8, 715–730. [Google Scholar] [CrossRef]
  114. Barge, L.M.; Rodriguez, L.E.; Weber, J.M.; Theiling, B.P. Determining the “Biosignature Threshold” for Life Detection on Biotic, Abiotic, or Prebiotic Worlds. Astrobiology 2021, 22. [Google Scholar] [CrossRef] [PubMed]
  115. Seager, S.; Schrenk, M.; Bains, W. An Astrophysical View of Earth-Based Metabolic Biosignature Gases. Astrobiology 2012, 12, 61–82. [Google Scholar] [CrossRef]
  116. National Academies of Sciences, Medicine, Division on Engineering, Physical Sciences, Space Studies Board; Committee on Astrobiology Science Strategy for the Search for Life in the Universe. An Astrobiology Strategy for the Search for Life in the Universe; The National Academies Press: Washington, DC, USA, 2019; p. 188. [Google Scholar]
  117. Schwieterman, E.W.; Kiang, N.Y.; Parenteau, M.N.; Harman, C.E.; DasSarma, S.; Fisher, T.M.; Arney, G.N.; Hartnett, H.E.; Reinhard, C.T.; Olson, S.L.; et al. Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life. Astrobiology 2018, 18, 663–708. [Google Scholar] [CrossRef]
  118. Kiang, N.Y.; Domagal-Goldman, S.; Parenteau, M.N.; Catling, D.C.; Fujii, Y.; Meadows, V.S.; Schwieterman, E.W.; Walker, S.I. Exoplanet Biosignatures: At the Dawn of a New Era of Planetary Observations. Astrobiology 2018, 18, 619–629. [Google Scholar] [CrossRef]
  119. Vickers, P.; Cowie, C.; Dick, S.J.; Gillen, C.; Jeancolas, C.; Rothschild, L.J.; McMahon, S. Confidence of Life Detection: The Problem of Unconceived Alternatives. Astrobiology 2023, 23, 1202–1212. [Google Scholar] [CrossRef]
  120. Smith, H.B.; Mathis, C. Life detection in a universe of false positives. Bioessays 2023, 45, 2300050. [Google Scholar] [CrossRef]
  121. Wordsworth, R.D.; Kerber, L.; Pierrehumbert, R.T.; Forget, F.; Head, J.W. Comparison of “warm and wet” and “cold and icy” scenarios for early Mars in a 3-D climate model. J. Geophys. Res. Planets 2015, 120, 1201–1219. [Google Scholar] [CrossRef]
  122. Limaye, S.S.; Grassi, D.; Mahieux, A.; Migliorini, A.; Tellmann, S.; Titov, D. Venus Atmospheric Thermal Structure and Radiative Balance. Space Sci. Rev. 2018, 214, 102. [Google Scholar] [CrossRef]
  123. Peplowski, P.N.; Lawrence, D.J.; Wilson, J.T. Chemically distinct regions of Venus’s atmosphere revealed by measured N2 concentrations. Nat. Astron. 2020, 4, 947. [Google Scholar] [CrossRef]
  124. Lebonnois, S.; Schubert, G. The deep atmosphere of Venus and the possible role of density-driven separation of CO2 and N2. Nat. Geosci. 2017, 10, 473–477. [Google Scholar] [CrossRef]
  125. Lebonnois, S.; Schubert, G.; Kremic, T.; Nakley, L.M.; Phillips, K.G.; Bellan, J.; Cordier, D. An experimental study of the mixing of CO2 and N2 under conditions found at the surface of Venus. Icarus 2020, 338, 113550. [Google Scholar] [CrossRef]
  126. Hoffman, J.H.; Hodges, R.R.; Donahue, T.M.; McElroy, M.M. Composition of the Venus lower atmosphere from the Pioneer Venus mass spectrometer. J. Geophys. Res. 1980, 85, 7882–7890. [Google Scholar] [CrossRef]
  127. Surkov, I.A.; Andreichikov, B.M.; Kalinkina, O.M. Composition and structure of the cloud layer of Venus. In Proceedings of the Space Research XIV 1974, Sao Paolo, Brazil, June 1974; p. 673. [Google Scholar]
  128. Milojevic, T.; Treiman, A.H.; Limaye, S.S. Phosphorus in the Clouds of Venus: Potential for Bioavailability. Astrobiology 2021, 21, 1250–1263. [Google Scholar] [CrossRef]
  129. Avice, G.; Marty, B. Perspectives on Atmospheric Evolution from Noble Gas and Nitrogen Isotopes on Earth, Mars & Venus. Space Sci. Rev. 2020, 216, 36. [Google Scholar] [CrossRef]
  130. Gillmann, C.; Way, M.J.; Avice, G.; Breuer, D.; Golabek, G.J.; Höning, D.; Krissansen-Totton, J.; Lammer, H.; O’Rourke, J.G.; Persson, M.; et al. The Long-Term Evolution of the Atmosphere of Venus: Processes and Feedback Mechanisms. Space Sci. Rev. 2022, 218, 56. [Google Scholar] [CrossRef]
  131. Mogul, R.; Zolotov, M.Y.; Way, M.J.; Limaye, S.S.; Avice, G. Reassessing Venus’ Vertical Profiles for SO2, N2, and H2O Using Pioneer Venus Mass Spectra. In Proceedings of the Venus as a System, Venus, Albuquerque, NM, USA, 1–3 November 2023; p. 8040. [Google Scholar]
  132. Martin, A.N.; Stüeken, E.E.; Gehringer, M.M.; Markowska, M.; Vonhof, H.; Weyer, S.; Hofmann, A. Anomalous δ15N values in the Neoarchean associated with an abundant supply of hydrothermal ammonium. Nat. Commun. 2025, 16, 1873. [Google Scholar] [CrossRef]
  133. Limaye, S.S.; Watanabe, S.; Yamazaki, A.; Yamada, M.; Satoh, T.; Sato, T.M.; Nakamura, M.; Taguchi, M.; Fukuhara, T.; Imamura, T.; et al. Venus looks different from day to night across wavelengths: Morphology from Akatsuki multispectral images. Earth Planets Space 2018, 70, 24. [Google Scholar] [CrossRef]
  134. Esposito, L.W.; Knollenberg, R.G.; Marov, M.I.; Toon, O.B.; Turco, R.P.; Colin, L.; Donahue, T.M.; Moroz, V.I. The clouds and hazes of Venus. In Venus; Hunten, D.M., Ed.; University of Arizona Press: Tucson, AZ, USA, 1983; pp. 484–564. [Google Scholar]
  135. Jiang, C.Z.; Rimmer, P.B.; Lozano, G.G.; Tosca, N.J.; Kufner, C.L.; Sasselov, D.D.; Thompson, S.J. Iron-sulfur chemistry can explain the ultraviolet absorber in the clouds of Venus. Sci. Adv. 2024, 10, eadg8826. [Google Scholar] [CrossRef] [PubMed]
  136. Petkowski, J.J.; Seager, S.; Grinspoon, D.H.; Bains, W.; Ranjan, S.; Rimmer, P.B.; Buchanan, W.P.; Agrawal, R.; Mogul, R.; Carr, C.E. Astrobiological Potential of Venus Atmosphere Chemical Anomalies and Other Unexplained Cloud Properties. Astrobiology 2024, 24, 343–370. [Google Scholar] [CrossRef] [PubMed]
  137. Jessup, K.-L.; Marcq, E.; Mills, F.; Bertaux, J.-L.; Lee, Y.J.; Limaye, S.; Roman, A.; Yung, Y. Hubble’s Role in Studies of Venus’ Clouds, Climate and Habitability. In American Astronomical Society Meeting Abstracts# 232; American Astronomical Society: Washington, DC, USA, 2018; Volume 232, pp. 405–406. [Google Scholar]
  138. Marcq, E.; Lea Jessup, K.; Baggio, L.; Encrenaz, T.; Lee, Y.J.; Montmessin, F.; Belyaev, D.; Korablev, O.; Bertaux, J.-L. Climatology of SO2 and UV absorber at Venus’ cloud top from SPICAV-UV nadir dataset. Icarus 2020, 335, 113368. [Google Scholar] [CrossRef]
  139. Jessup, K.-L.; Marcq, E.; Bertaux, J.-L.; Mills, F.P.; Limaye, S.; Roman, A. On Venus’ cloud top chemistry, convective activity and topography: A perspective from HST. Icarus 2020, 335, 113372. [Google Scholar] [CrossRef]
  140. Lee, Y.J.; García Muñoz, A.; Yamazaki, A.; Yamada, M.; Watanabe, S.; Encrenaz, T. Investigation of UV Absorbers on Venus Using the 283 and 365 nm Phase Curves Obtained from Akatsuki. Geophys. Res. Lett. 2021, 48, e2020GL090577. [Google Scholar] [CrossRef]
  141. Lee, Y.J.; Muñoz, A.G.; Yamazaki, A.; Quémerais, E.; Mottola, S.; Hellmich, S.; Granzer, T.; Bergond, G.; Roth, M.; Gallego-Cano, E.; et al. Reflectivity of Venus’s Dayside Disk During the 2020 Observation Campaign: Outcomes and Future Perspectives. Planet. Sci. J. 2022, 3, 209. [Google Scholar] [CrossRef]
  142. Frandsen, B.N.; Wennberg, P.O.; Kjaergaard, H.G. Identification of OSSO as a near-UV absorber in the Venusian atmosphere. Geophys. Res. Lett. 2016, 43, 11. [Google Scholar] [CrossRef]
  143. Wu, Z.; Wan, H.; Xu, J.; Lu, B.; Lu, Y.; Eckhardt, A.K.; Schreiner, P.R.; Xie, C.; Guo, H.; Zeng, X. The near-UV absorber OSSO and its isomers. Chem. Commun. 2018, 54, 4517–4520. [Google Scholar] [CrossRef]
  144. Frandsen, B.N.; Farahani, S.; Vogt, E.; Lane, J.R.; Kjaergaard, H.G. Spectroscopy of OSSO and Other Sulfur Compounds Thought to be Present in the Venus Atmosphere. J. Phys. Chem. A 2020, 124, 7047–7059. [Google Scholar] [CrossRef]
  145. Egan, J.V.; Feng, W.; James, A.D.; Manners, J.; Marsh, D.R.; Lebonnois, S.; Lefèvre, F.; Stolzenbach, A.; Plane, J.M.C. Is OSSO a Significant Contributor to the Unknown UV Absorber in Venus’ Atmosphere? Geophys. Res. Lett. 2025, 52, e2024GL113090. [Google Scholar] [CrossRef]
  146. Pérez-Hoyos, S.; Sánchez-Lavega, A.; García-Muñoz, A.; Irwin, P.G.J.; Peralta, J.; Holsclaw, G.; McClintock, W.M.; Sanz-Requena, J.F. Venus Upper Clouds and the UV Absorber From MESSENGER/MASCS Observations. J. Geophys. Res. (Planets) 2018, 123, 145. [Google Scholar] [CrossRef]
  147. Allen, D.; Crawford, J. Cloud structure on the dark side of Venus. Nature 1984, 307, 222–224. [Google Scholar] [CrossRef]
  148. Satoh, T.; Sato, T.M.; Nakamura, M.; Kasaba, Y.; Ueno, M.; Suzuki, M.; Hashimoto, G.L.; Horinouchi, T.; Imamura, T.; Yamazaki, A.; et al. Performance of Akatsuki/IR2 in Venus orbit: The first year. Earth Planets Space 2017, 69, 154. [Google Scholar] [CrossRef]
  149. Pollack, J.B.; Dalton, J.B.; Grinspoon, D.; Wattson, R.B.; Freedman, R.; Crisp, D.; Allen, D.A.; Bezard, B.; de Bergh, C.; Giver, L.P.; et al. Near-Infrared Light from Venus’ Nightside: A Spectroscopic Analysis. Icarus 1993, 103, 1. [Google Scholar] [CrossRef]
  150. Crisp, D.; Allen, D.; Grinspoon, D.; Pollack, J. The dark side of Venus: Near-infrared images and spectra from the Anglo-Australian observatory. Science 1991, 253, 1263–1266. [Google Scholar] [CrossRef] [PubMed]
  151. Allen, D.; Crisp, D.; Meadows, V. Variable oxygen airglow on Venus as a probe of atmospheric dynamics. Nature 1992, 359, 516–519. [Google Scholar] [CrossRef]
  152. Gérard, J.-C.; Cox, C.; Saglam, A.; Bertaux, J.-L.; Villard, E.; Nehmé, C. Limb observations of the ultraviolet nitric oxide nightglow with SPICAV on board Venus Express. J. Geophys. Res. (Planets) 2008, 113. [Google Scholar] [CrossRef]
  153. Counselman, C.C.; Gourevitch, S.A.; King, R.W.; Loriot, G.B.; Ginsberg, E.S. Zonal and meridional circulation of the lower atmosphere of Venus determined by radio interferometry. J. Geophys. Res. 1980, 85, 8026. [Google Scholar] [CrossRef]
  154. Limaye, S.S.; Kovalenko, I.D. Monitoring Venus and communications relay from Lagrange Points. Planet. Space Sci. 2019, 179, 104710. [Google Scholar] [CrossRef]
  155. Yamazaki, A.; Yamada, M.; Lee, Y.J.; Watanabe, S.; Horinouchi, T.; Murakami, S.-y.; Kouyama, T.; Ogohara, K.; Imamura, T.; Sato, T.M.; et al. Ultraviolet imager on Venus orbiter Akatsuki and its initial results. Earth Planets Space 2018, 70, 23. [Google Scholar] [CrossRef]
  156. Khatuntsev, I.V.; Patsaeva, M.V.; Titov, D.V.; Zasova, L.V.; Ignatiev, N.I.; Gorinov, D.A. Twelve-Year Cycle in the Cloud Top Winds Derived from VMC/Venus Express and UVI/Akatsuki Imaging. Atmosphere 2022, 13, 2023. [Google Scholar] [CrossRef]
  157. Otroshchenko, V.A.; Surkov, Y.A. The possibility of organic molecule formation in the Venus atmosphere. Orig. Life 1974, 5, 487–490. [Google Scholar] [CrossRef] [PubMed]
  158. French, R.; Mandy, C.; Hunter, R.; Mosleh, E.; Sinclair, D.; Beck, P.; Seager, S.; Petkowski, J.J.; Carr, C.E.; Grinspoon, D.H.; et al. Rocket Lab Mission to Venus. Aerospace 2022, 9, 445. [Google Scholar] [CrossRef]
  159. Baumgardner, D.; Fisher, T.; Newton, R.; Roden, C.; Zmarzly, P.; Seager, S.; Petkowski, J.J.; Carr, C.E.; Špaček, J.; Benner, S.A.; et al. Deducing the Composition of Venus Cloud Particles with the Autofluorescence Nephelometer (AFN). Aerospace 2022, 9, 492. [Google Scholar] [CrossRef]
  160. Seager, S.; Petkowski, J.J.; Carr, C.E.; Grinspoon, D.H.; Ehlmann, B.L.; Saikia, S.J.; Agrawal, R.; Buchanan, W.P.; Weber, M.U.; French, R.; et al. Venus Life Finder Missions Motivation and Summary. Aerospace 2022, 9, 385. [Google Scholar] [CrossRef]
  161. Schulze-Makuch, D.; Irwin, L.N.; Irwin, T. Proposed Missions to Collect Samples for Analyzing Evidence of Life in the Venusian Atmosphere. Astrobiology 2024, 24, 397–406. [Google Scholar] [CrossRef]
  162. Polidan, R.S.; Lee, G.; Ross, F.; Sokol, D.; Bolisay, L. Venus Atmospheric Maneuverable Platform Science Mission. In Proceedings of the AAS/Division for Planetary Sciences Meeting Abstracts, Washington, DC, USA, 8–13 November 2015. [Google Scholar]
  163. Agrawal, R.; Seager, S.; Petkowski, J.J.; Carr, C.E.; Buchanan, W.P.; Bywaters, K.; de Jong, M.; Grinspoon, D.H.; Iakubivskyi, I.; King, I.R.; et al. Venus cloud sample return concept for astrobiology. Adv. Space Res. 2024, 74, 490–504. [Google Scholar] [CrossRef]
  164. Kane, S.; Kopparapu, R.; Domagal-Goldman, S.D. The Venus Zone: Seeking the Twin of Earth’s Twin. In Proceedings of the AGU Fall Meeting Abstracts, New Orleans, LA, USA, 11–15 December 2017; p. P52A-01. [Google Scholar]
Figure 1. Length of solar day (in Earth days) in the Venus atmosphere from the zonal wind profiles obtained by tracking the Pioneer Venus probes [153]. The cloud top rotation rate determined by following clouds varies between 2.5 days (high latitudes) and ~5 days at the equator and somewhat variable over short and long periods. Tracking of contrasts in near-infrared images (believed to represent ~55–60 km altitude winds) have a period around 5–6 days.
Figure 1. Length of solar day (in Earth days) in the Venus atmosphere from the zonal wind profiles obtained by tracking the Pioneer Venus probes [153]. The cloud top rotation rate determined by following clouds varies between 2.5 days (high latitudes) and ~5 days at the equator and somewhat variable over short and long periods. Tracking of contrasts in near-infrared images (believed to represent ~55–60 km altitude winds) have a period around 5–6 days.
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Figure 2. Periodogram contrast (defined as the RMS deviation from Minnaert law for points within ±30° latitude) in the 365 nm images collected by the Akatsuki orbiter during 7 December 2015–30 September 2021 shown as spectral power vs. period. The two dominant peaks (99% confidence) found at 226.5 and 114.7 days correspond to the Venus orbital period and solar day, respectively, and the next group between 8- and 12-day periods (>95% confidence) likely correspond to the Akatsuki orbital period (~10.5 days). The next group includes periods of ~2.1, 2.7, 3.4, and 5.2 days representing the apparent day-night cycle and harmonics in the average contrast in the 365 nm images. The contrasts at 365 nm are believed to be due to absorption in the clouds (48–72 km altitude), and the shorter periods are consistent with the solar day in the cloud layer. Since the images were acquired from a near equatorial orbit with a period varying between 10 and 11 days over the observing duration, it is not possible to separate the orbital variation and a true circadian period which can be possible through imaging from a L2 Lagrange point orbiter [154].
Figure 2. Periodogram contrast (defined as the RMS deviation from Minnaert law for points within ±30° latitude) in the 365 nm images collected by the Akatsuki orbiter during 7 December 2015–30 September 2021 shown as spectral power vs. period. The two dominant peaks (99% confidence) found at 226.5 and 114.7 days correspond to the Venus orbital period and solar day, respectively, and the next group between 8- and 12-day periods (>95% confidence) likely correspond to the Akatsuki orbital period (~10.5 days). The next group includes periods of ~2.1, 2.7, 3.4, and 5.2 days representing the apparent day-night cycle and harmonics in the average contrast in the 365 nm images. The contrasts at 365 nm are believed to be due to absorption in the clouds (48–72 km altitude), and the shorter periods are consistent with the solar day in the cloud layer. Since the images were acquired from a near equatorial orbit with a period varying between 10 and 11 days over the observing duration, it is not possible to separate the orbital variation and a true circadian period which can be possible through imaging from a L2 Lagrange point orbiter [154].
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