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The text discusses the second workshop on the potential habitability of the Venus cloud layer, which was held by the Roscosmos/IKI-NASA Joint Science Definition Team for Russia's Venera-D mission. The workshop featured presentations and panel discussions with participants from 36 different countries. The text also mentions the delays in publishing a collection of Venus-relevant papers due to the global COVID pandemic, geopolitical developments, and budget pressures. It highlights the progress made in new discoveries and investigations, including the detection of phosphine in the venusian atmosphere and the observation of geologic signatures of active volcanism. The text concludes by mentioning upcoming missions to Venus and the potential for paradigm-shifting results in the field of astrobiology. [Extracted from the article]

We introduce the collection of papers from the first workshop on the habitability of the venusian cloud layer organized by the Roscosmos/IKI-NASA Joint Science Definition Team (JSDT) for Russia's Venera-D mission and hosted by the Space Research Institute in Moscow, Russia, during October 2–5, 2019. The collection also includes three papers that were developed independently of the workshop but are relevant to venusian cloud habitability. [ABSTRACT FROM AUTHOR]

We show that solar irradiances calculated across Venus' clouds support the potential for Earth-like phototrophy and that treatment of Venus' aerosols containing neutralized sulfuric acid favor a habitable zone. The phototrophic potential of Venus' atmosphere was assessed by calculating irradiances (200–2000 nm, 15° solar zenith angle, local noon) using a radiative transfer model that accounted for absorption and scattering by the major and minor atmospheric constituents. Comparisons to Earth's surface (46 W m−2, 280–400 nm) suggest that Venus' middle and lower clouds receive ∼87% less normalized UV flux (6–7 W m−2) across 200–400 nm, yet similar normalized photon flux densities (∼4400–6200 μmol m−2 s−1) across 350–1200 nm. Further, Venus' signature phototrophic windows and subwindows overlap with the absorption profiles of several photosynthetic pigments, especially bacteriochlorophyll b from intact cells and phycocyanin. Therefore, Venus' light, with limited UV flux in the middle and lower clouds, is likely quite favorable for phototrophy. We additionally present interpretations to refractive index and radio occultation measures for Venus' aerosols that suggest the presence of lower sulfuric abundances and/or neutralized forms of sulfuric acid, such as ammonium bisulfate. Under these considerations, the aerosols in Venus' middle clouds could harbor water activities (≥0.6) and buffered acidities (Hammett acidity factor, H0 -0.1 to -1.5) that lie within the limits of acidic cultivation (≥H0 -0.4) and are tantalizingly close to the limits of oxygenic photosynthesis (≥H0 0.1). Together, these photophysical and chemical considerations support a potential for phototrophy in Venus' clouds. [ABSTRACT FROM AUTHOR]

We present a case for the exploration of Venus as an astrobiology target—(1) investigations focused on the likelihood that liquid water existed on the surface in the past, leading to the potential for the origin and evolution of life, (2) investigations into the potential for habitable zones within Venus' present-day clouds and Venus-like exo atmospheres, (3) theoretical investigations into how active aerobiology may impact the radiative energy balance of Venus' clouds and Venus-like atmospheres, and (4) application of these investigative approaches toward better understanding the atmospheric dynamics and habitability of exoplanets. The proximity of Venus to Earth, guidance for exoplanet habitability investigations, and access to the potential cloud habitable layer and surface for prolonged in situ extended measurements together make the planet a very attractive target for near term astrobiological exploration. [ABSTRACT FROM AUTHOR]

The lower cloud layer of Venus (47.5–50.5 km) is an exceptional target for exploration due to the favorable conditions for microbial life, including moderate temperatures and pressures (∼60°C and 1 atm), and the presence of micron-sized sulfuric acid aerosols. Nearly a century after the ultraviolet (UV) contrasts of Venus' cloud layer were discovered with Earth-based photographs, the substances and mechanisms responsible for the changes in Venus' contrasts and albedo are still unknown. While current models include sulfur dioxide and iron chloride as the UV absorbers, the temporal and spatial changes in contrasts, and albedo, between 330 and 500 nm, remain to be fully explained. Within this context, we present a discussion regarding the potential for microorganisms to survive in Venus' lower clouds and contribute to the observed bulk spectra. In this article, we provide an overview of relevant Venus observations, compare the spectral and physical properties of Venus' clouds to terrestrial biological materials, review the potential for an iron- and sulfur-centered metabolism in the clouds, discuss conceivable mechanisms of transport from the surface toward a more habitable zone in the clouds, and identify spectral and biological experiments that could measure the habitability of Venus' clouds and terrestrial analogues. Together, our lines of reasoning suggest that particles in Venus' lower clouds contain sufficient mass balance to harbor microorganisms, water, and solutes, and potentially sufficient biomass to be detected by optical methods. As such, the comparisons presented in this article warrant further investigations into the prospect of biosignatures in Venus' clouds. [ABSTRACT FROM AUTHOR]

The Venus International Reference Atmosphere (VIRA) model contains tabulated values of temperature and number densities obtained by the experiments on the Venera entry probes, Pioneer Venus Orbiter and multi-probe missions in the 1980s. The instruments on the recent Venus Express orbiter mission generated a significant amount of new observational data on the vertical and horizontal structure of the Venus atmosphere from 40 km to about 180 km altitude from April 2006 to November 2014. Many ground based experiments have provided data on the upper atmosphere (90–130 km) temperature structure since the publication of VIRA in 1985. The "Thermal Structure of the Venus Atmosphere" Team was supported by the International Space Studies Institute (ISSI), Bern, Switzerland, from 2013 to 2015 in order to combine and compare the ground-based observations and the VEx observations of the thermal structure as a first step towards generating an updated VIRA model. Results of this comparison are presented in five latitude bins and three local time bins by assuming hemispheric symmetry. The intercomparison of the ground-based and VEx results provides for the first time a consistent picture of the temperature and density structure in the 40 km–180 km altitude range. The Venus Express observations have considerably increased our knowledge of the Venus atmospheric thermal structure above ∼40 km and provided new information above 100 km. There are, however, still observational gaps in latitude and local time above certain regions. Considerable variability in the temperatures and densities is seen above 100 km but certain features appear to be systematically present, such as a succession of warm and cool layers. Preliminary modeling studies support the existence of such layers in agreement with a global scale circulation. The intercomparison focuses on average profiles but some VEx experiments provide sufficient global coverage to identify solar thermal tidal components. The differences between the VEx temperature profiles and the VIRA below 0.1 mbar/95 km are small. There is, however, a clear discrepancy at high latitudes in the 10–30 mbar (70–80 km) range. The VEx observations will also allow the improvement of the empirical models (VTS3 by Hedin et al., 1983 and VIRA by Keating et al., 1985) above 0.03 mbar/100 km, in particular the 100–150 km region where a sufficient observational coverage was previously missing. The next steps in order to define the updated VIRA temperature structure up to 150 km altitude are (1) define the grid on which this database may be provided, (2) fill what is possible with the results of the data intercomparison, and (3) fill the observational gaps. An interpolation between the datasets may be performed by using available General Circulation Models as guidelines. An improved spatial coverage of observations is still necessary at all altitudes, in latitude–longitude and at all local solar times for a complete description of the atmospheric thermal structure, in particular on the dayside above 100 km. New in-situ observations in the atmosphere below 40 km are missing, an altitude region that cannot be accessed by occultation experiments. All these questions need to be addressed by future missions. [ABSTRACT FROM AUTHOR]

We present a significantly updated CO 2 altitude profile for Venus (64.2–0.9 km) and provide support for a potential deep lower atmospheric haze of particles (≤17 km). We extracted this information by developing a new analytical model for mass spectra obtained by the Pioneer Venus Large Probe (PVLP) Neutral Mass Spectrometer (LNMS). Our model accounts for changes in LNMS configuration and output during descent and enables the disentanglement of isobaric species via a data fitting routine that adjusts for mass-dependent changes in peak shape. The model yields CO 2 in units of density (kg m−3), isotope ratios for 13C/12C and 18O/16O, and 14 measures of CO 2 density across 55.4–0.9 km, which represents the most complete altitude profile for CO 2 at ≤60 km to date. The CO 2 density profile is also consistent with the pressure, temperature, and volumetric gas measurements from the PVLP and VeNeRa spacecraft. Nominal and low-noise operations for the LNMS mass analyzer are supported by the behaviors (e.g. , ionization yields, fragmentation yields, and peak shapes) of several internal standards (e.g. , CH 3 +, CH 4 +, 40Ar+, 136Xe2+, and 136Xe+), which were tracked across the descent. Lastly, our review of the CO 2 profile and LNMS spectra reveals hitherto unreported partial and rapidly clearing clogs of the inlet in the lower atmosphere, along with several ensuing data spikes at multiple masses. Together, these observations suggest that atmospheric intake was impacted by particles at ≤17 km and that rapid particle degradation at the inlet yielded a temporary influx of mass signals into the LNMS. • We obtained novel chemical information by re-analysis of archived mass spectra from the Pioneer Venus Large Probe. • Our analytical model accounts for the impacts of descent and enables the disentanglement of isobaric species. • We obtained the first and most complete CO 2 altitude profile in units of density (kg m-3) across ∼55-1 km. • Our values for CO 2 are consistent with associated other spacecraft measures. • Tracking of the CO 2 profile reveals hitherto unreported partial and rapidly clearing clogs of the LNMS at ≤17 km. • The partial clogs are suggestive of a deep lower atmospheric haze at ≤17 km. [ABSTRACT FROM AUTHOR]

Aerosol phase elements such as phosphorus (P), sulfur (S), and metals including iron (Fe) are essential nutrients that could help sustain potential biodiversity in the cloud deck of Venus. While the presence of S and Fe in the venusian cloud deck has been broadly discussed (Zasova et al.,1981; Krasnopolsky, 2012, 2013, 2016, 2017; Markiewicz et al.,2014), less attention has been given to the presence of P in the aerosols and its involvement in the multiphase chemistry of venusian clouds and potential sources of P deposition in the venusian atmosphere. A detailed characterization of phosphorus atmospheric chemistry in the cloud deck of Venus is crucial for understanding its solubility and bioavailability for potential venusian cloud microbiota (Schulze-Makuch et al.,2004; Grinspoon and Bullock, 2007; Limaye et al.,2018). We summarize our current understanding of the presence of P in the clouds of Venus and its role in a hypothetical atmospheric (bio)chemical cycle. The results of the VeGa lander measurements are put into perspective with regard to nutrient limitation for a potential biosphere in venusian clouds. Our work combines the results of the VeGa measurements and focuses on P as an inorganic nutrient component and its potential sources and chemical behavior as part of multiple transformations of atmospheric chemistry. The VeGa data indicate that a plentiful phosphorus layer exists within a layer that reaches into the lower venusian clouds and exceeds minimum P abundances for terrestrial microbial life. Extreme acidification of airborne phases in the atmosphere of Venus may facilitate P solubilization and its bioavailability for a potential ecosystem in venusian clouds. Further sampling and P abundance measurements in the atmosphere of Venus would improve our knowledge of P speciation and facilitate determination of a bioavailable fraction of P detected in venusian clouds. The previous results deserve further experimental and modeling analyses to diminish uncertainties and understand the rates of atmospheric deposition of P and its role in a potential venusian cloud ecosystem. [ABSTRACT FROM AUTHOR]

Orbits around the two close collinear Sun-Earth Lagrange points have been utilized in recent decades for many solar and astronomical missions to exploit the observational advantages. DSCOVR is the first satellite to observe Earth continuously from the vicinity of the L1 point and is filing a crucial gap in Earth observations with its vast collection of polar, geosynchronous and some retrograde low inclination orbits. Spacecraft orbiting Venus in the last four decades have provided us with a wealth of information and many unanswered questions, which cannot be addressed adequately by observing from polar or near equatorial orbits around the planet. The Sun-Venus collinear Lagrange points L1 (sunward) and L2 (behind the planet from the Sun) points are key vantage points located about a million km away from the planet along the direction to the Sun which enable continuous monitoring of the planet's day and night hemispheres. As spacecraft positions at L1 and L2 points are unstable, they can be inserted in orbits around them to observe Venus over a small range of phase angles unlike any Venus orbiter observations which cover 0–180° solar phase angle twice each orbit for a very long time with minimal station keeping costs. To help better understand Venus, we propose that monitoring Venus continuously at nearly the same phase angle from the vicinity of L1 and L2 Lagrange points is critical. Such Lagrange orbiters around Sun-Venus L1 and L2 points can provide crucial information continuously about the evolution and variability of: (i) reflectance of the global cloud cover, (ii) the night side cloud cover opacity, (iii) surface activity, and (iv) interaction of planet's atmosphere with the solar wind, and loss of atmosphere. In addition, the two Lagrange orbiters can provide a crucial continuous communications capability for relaying data from in-situ atmospheric or surface platforms. Well instrumented missions to L1 and L2 points of Venus would significantly improve our understanding Earth's perplexing neighbor by obtaining continuous record of data on the day and night hemispheres not available from Venus orbiting missions. • Prior Venus orbiters show a need for filling observational gaps that impede understanding the evolution of the global cloud cover. • Two closer collinear Sun-Venus Lagrange points offer continuous views of day and night hemispheres of Venus. • Orbiters at L1 and L2 can also measure incoming solar wind (L1) and the Venus tail to estimate atmospheric escape (L2). • Such orbiters can support continuous data relay from Venus surface and atmospheric platforms. • Significant science can be achieved with small to medium spacecraft with imagers, spectrometers and solar wind instruments. [ABSTRACT FROM AUTHOR]

Since insertion into orbit on December 7, 2015, the Akatsuki orbiter has returned global images of Venus from its four imaging cameras at eleven discrete wavelengths from ultraviolet (283 and 365 nm) and near infrared (0.9–2.3 µm), to the thermal infrared (8–12 µm) from a near-equatorial orbit. The Venus Express and Pioneer Venus Orbiter missions have also monitored the planet for long periods but from polar or near-polar orbits. The wavelength coverage and views of the planet also differ for all three missions. In reflected light, the images reveal features seen near the cloud tops (~ 70 km altitude), whereas in the near-infrared images of the nightside, features seen are at mid- to lower cloud levels (~ 48–60 km altitude). The dayside cloud cover imaged at the ultraviolet wavelengths shows morphologies similar to what was observed from Mariner 10, Pioneer Venus, Galileo, Venus Express and MESSENGER. The daytime images at 0.9 and 2.02 µm also reveal some interesting features which bear similarity to the ultraviolet images. The nighttime images at 1.74, 2.26 and 2.32 µm and at 8–12 µm reveal features not seen before and show new details of the nightside including narrow wavy ribbons, curved string-like features, long-scale waves, long dark streaks, isolated bright spots, sharp boundaries and even mesoscale vortices. Some features previously seen such as circum-equatorial belts (CEBs) and occasional areal brightenings at ultraviolet (seen in Venus Express observations) of the cloud cover at ultraviolet wavelengths have not been observed thus far. Evidence for the hemispheric vortex organization of the global circulation can be seen at all wavelengths on the day- and nightsides. Akatsuki images reveal new and puzzling morphology of the complex nightside cloud cover. The cloud morphologies provide some clues to the processes occurring in the atmosphere and are thus, a key diagnostic tool when quantitative dynamical analysis is not feasible due to insufficient information. [ABSTRACT FROM AUTHOR]

A lightweight, low-power instrument package to measure, in situ, both (1) the local gaseous environment and (2) the composition and microphysical properties of attendant venusian aerosols is presented. This Aerosol-Sampling Instrument Package (ASIP) would be used to explore cloud chemical and possibly biotic processes on future aerial missions such as multiweek balloon missions and on short-duration (<1 h) probes on Venus and potentially on other cloudy worlds such as Titan, the Ice Giants, and Saturn. A quadrupole ion-trap mass spectrometer (QITMS; Madzunkov and Nikolić, J Am Soc Mass Spectrom 25:1841–1852, 2014) fed alternately by (1) an aerosol separator that injects only aerosols into a vaporizer and mass spectrometer and (2) the pure aerosol-filtered atmosphere, achieves the compositional measurements. Aerosols vaporized <600°C are measured over atomic mass ranges from 2 to 300 AMU at <0.02 AMU resolution, sufficient to measure trace materials, their isotopic ratios, and potential biogenic materials embedded within H2SO4 aerosols, to better than 20% in <300 s for H2SO4 -relative abundances of 2 × 10−9. An integrated lightweight, compact nephelometer/particle-counter determines the number density and particle sizes of the sampled aerosols. [ABSTRACT FROM AUTHOR]

Venus has a thick atmosphere that rotates 60 times as fast as the surface, a phenomenon known as super-rotation. We use data obtained from the orbiting Akatsuki spacecraft to investigate how the super-rotation is maintained in the cloud layer, where the rotation speed is highest. A thermally induced latitudinal-vertical circulation acts to homogenize the distribution of the angular momentum around the rotational axis. Maintaining the super-rotation requires this to be counteracted by atmospheric waves and turbulence. Among those effects, thermal tides transport the angular momentum, which maintains the rotation peak, near the cloud top at low latitudes. Other planetary-scale waves and large-scale turbulence act in the opposite direction. We suggest that hydrodynamic instabilities adjust the angular-momentum distribution at mid-latitudes. [ABSTRACT FROM AUTHOR]

Abstract: Since the discovery of ultraviolet markings on Venus, their observations have been a powerful tool to study the morphology, motions and dynamical state at the cloud top level. Here we present the results of investigation of the cloud top morphology performed by the Venus Monitoring Camera (VMC) during more than 3years of the Venus Express mission. The camera acquires images in four narrow-band filters centered at 365, 513, 965 and 1010nm with spatial resolution from 50km at apocentre to a few hundred of meters at pericentre. The VMC experiment provides a significant improvement in the Venus imaging as compared to the capabilities of the earlier missions. The camera discovered new cloud features like bright “lace clouds” and cloud columns at the low latitudes, dark polar oval and narrow circular and spiral “grooves” in the polar regions, different types of waves at the high latitudes. The VMC observations revealed detailed structure of the sub-solar region and the afternoon convective wake, the bow-shape features and convective cells, the mid-latitude transition region and the “polar cap”. The polar orbit of the satellite enables for the first time nadir viewing of the Southern polar regions and an opportunity to zoom in on the planet. The experiment returned numerous images of the Venus limb and documented global and local brightening events. VMC provided almost continuous monitoring of the planet with high temporal resolution that allowed one to follow changes in the cloud morphology at various scales. We present the in-flight performance of the instrument and focus in particular on the data from the ultraviolet channel, centered at the characteristic wavelength of the unknown UV absorber that yields the highest contrasts on the cloud top. Low latitudes are dominated by relatively dark clouds that have mottled and fragmented appearance clearly indicating convective activity in the sub-solar region. At ∼50° latitude this pattern gives way to streaky clouds suggesting that horizontal, almost laminar, flow prevails here. Poleward from about 60°S the planet is covered by almost featureless bright polar hood sometimes crossed by dark narrow (∼300km) spiral or circular structures. This global cloud pattern can change on time scales of a few days resulting in global and local “brightening events” when the bright haze can extend far into low latitudes and/or increase its brightness by 30%. Close-up snapshots reveal plenty of morphological details like convective cells, cloud streaks, cumulus-like columns, wave trains. Different kinds of small scale waves are frequently observed at the cloud top. The wave activity is mainly observed in the 65–80° latitude band and is in particular concentrated in the region of Ishtar Terra that suggests their possible orographic origin. The VMC observations have important implications for the problems of the unknown UV absorber, microphysical processes, dynamics and radiative energy balance at the cloud tops. They are only briefly discussed in the paper, but each of them will be the subject of a dedicated study. [Copyright &y& Elsevier]

Abstract: With its comprehensive suite of near-infrared instruments, Venus Express will perform the first detailed global exploration of the depths of the thick Venusian atmosphere. Through the near-daily acquisition of Visible and Infrared maps and spectra, three infrared-sensing instruments—the Planetary Fourier Spectrometer (PFS), the Venus Monitoring Camera (VMC), and the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)—will comprehensively investigate the Thermal structure, meteorology, dynamics, chemistry, and stability of the deep Venus atmosphere. For the surface, these instruments will provide clues to the emissivity of surface materials and provide direct evidence of active volcanism. In so doing, ESA''s Venus Express Mission directly addresses numerous high-priority Venus science objectives advanced by America''s National Research Council (2003) decadal survey of planetary science. [Copyright &y& Elsevier]