Venus looks different from day to night across wavelengths: morphology from Akatsuki multispectral images

In situ bio/chemical characterization of Venus cloud particles using Life-signature Detection Microscope for Venus (Venus LDM)

Much of the information about the size and shape of aerosols forming haze and the cloud layer of Venus is obtained from indirect inferences from nephelometers on probes and from analysis of the variation of polarization with the phase angle and the glory feature from images of Venus. Microscopic imaging of Venus' aerosols has been advocated recently. Direct measurements from a fluorescence microscope can provide information on the morphology, density, and biochemical characteristics of the particles; thus, the fluorescence microscope is attractive for the in situ particle characterization of Venus' cloud layer. Fluorescence imaging of Venus' cloud particles presents several challenges due to the sulfuric acid composition and the corrosive effects. In this article, we identify the challenges and describe our approach to overcoming them for a fluorescence microscope based on an in situ bio/chemical and physical characterization instrument for use in the clouds of Venus from a suitable aerial platform. We report that a pH adjustment using alkali was effective for obtaining fluorescence images, and that fluorescence attenuation was observed after the adjustment, even when the acidophile suspension in the concentrated sulfuric acid was used as a sample.

Amateur Observers Witness the Return of Venus’ Cloud Discontinuity

Firstly identified in images from JAXA’s orbiter Akatsuki, the cloud discontinuity of Venus is a planetary-scale phenomenon known to be recurrent since, at least, the 1980s. Interpreted as a new type of Kelvin wave, this disruption is associated to dramatic changes in the clouds’ opacity and distribution of aerosols, and it may constitute a critical piece for our understanding of the thermal balance and atmospheric circulation of Venus. Here, we report its reappearance on the dayside middle clouds four years after its last detection with Akatsuki/IR1, and for the first time, we characterize its main properties using exclusively near-infrared images from amateur observations. In agreement with previous reports, the discontinuity exhibited temporal variations in its zonal speed, orientation, length, and its effect over the clouds’ albedo during the 2019/2020 eastern elongation. Finally, a comparison with simultaneous observations by Akatsuki UVI and LIR confirmed that the discontinuity is not visible on the upper clouds’ albedo or thermal emission, while zonal speeds are slower than winds at the clouds’ top and faster than at the middle clouds, evidencing that this Kelvin wave might be transporting momentum up to upper clouds.

Venus’ Cloud-Tracked Winds Using Ground- and Space-Based Observations with TNG/NICS and VEx/VIRTIS

Characterizing the wind speeds of Venus and their variability at multiple vertical levels is essential for a better understanding of the atmospheric superrotation, constraining the role of large-scale planetary waves in the maintenance of this superrotation, and in studying how the wind field affects clouds’ distribution. Here, we present cloud-tracked wind results of the Venus nightside, obtained with unprecedented quality using ground-based observations during July 2012 with the near-infrared camera and spectrograph (NICS) of the Telescopio Nazionale Galileo (TNG) in La Palma. These observations were performed during 3 consecutive days for periods of 2.5 h starting just before dawn, sensing the nightside lower clouds of Venus close to 48 km of altitude with images taken at continuum K filter at 2.28 μm. Our observations cover a period of time when ESA’s Venus Express was not able to observe these deeper clouds of Venus due to a failure in the infrared channel of its imaging spectrometer, VIRTIS-M, and the dates were chosen to coordinate these ground-based observations with Venus Express’ observations of the dayside cloud tops (at about 70 km) with images at 380 nm acquired with the imaging spectrometer VIRTIS-M. Thanks to the quality and spatial resolution of TNG/NICS images and the use of an accurate technique of template matching to perform cloud tracking, we present the most detailed and complete profile of wind speeds ever performed using ground-based observations of Venus. The vertical shear of the wind was also obtained for the first time, obtained by the combination of ground-based and space-based observations, during the Venus Express mission since the year 2008, when the infrared channel of VIRTIS-M stopped working. Our observations exhibit day-to-day changes in the nightside lower clouds, the probable manifestation of the cloud discontinuity, no relevant variations in the zonal winds, and an accurate characterization of their decay towards the poles, along with the meridional circulation. Finally, we also present the latitudinal profiles of zonal winds, meridional winds, and vertical shear of the zonal wind between the upper clouds’ top and lower clouds, confirming previous findings by Venus Express.

Venus, an Astrobiology Target

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.

The nightside cloud-top circulation of the atmosphere of Venus

Although Venus is a terrestrial planet similar to Earth, its atmospheric circulation is much different and poorly characterized¹. Winds at the cloud top have been measured predominantly on the dayside. Prominent poleward drifts have been observed with dayside cloud tracking and interpreted to be caused by thermal tides and a Hadley circulation^2-4; however, the lack of nightside measurements over broad latitudes has prevented the unambiguous characterization of these components. Here we obtain cloud-tracked winds at all local times using thermal infrared images taken by the Venus orbiter Akatsuki, which is sensitive to an altitude of about 65 kilometres⁵. Prominent equatorward flows are found on the nightside, resulting in null meridional velocities when these are zonally averaged. The velocity structure of the thermal tides was determined without the influence of the Hadley circulation. The semidiurnal tide was found to have an amplitude large enough to contribute to the maintenance of the atmospheric superrotation. The weakness of the mean meridional flow at the cloud top implies that the poleward branch of the Hadley circulation exists above the cloud top and that the equatorward branch exists in the clouds. Our results should shed light on atmospheric superrotation in other celestial bodies.

Planetary-scale streak structure reproduced in high-resolution simulations of the Venus atmosphere with a low-stability layer

Cloud patterns are important clues for revealing the atmospheric circulation of Venus. Recently, a planetary-scale streak structure has been discovered in middle- and lower-cloud images of Venus' night-side taken by IR2, the 2-μm camera, on board the Akatsuki orbiter. However, its formation mechanism has not been investigated. Here we succeed, for the first time, in reproducing the patterns of the observed streak structure, as regions of strong downward flows that develop in high-resolution global simulations of the Venus atmosphere. The streaks are formed in both hemispheres with equatorial symmetry, which is caused by equatorial Rossby-like and Kelvin-like waves with zonal wavenumber one. The low-stability layer that has been suggested by past observations is essential for reproducing the streak structure. The streaks of downward flow result from the interaction of the meridionally tilted phase lines of the Rossby-like waves and the characteristics of baroclinic instability produced around the low-stability layer.

Venus' Spectral Signatures and the Potential for Life in the Clouds

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.

Ultraviolet imager on Venus orbiter Akatsuki and its initial results

The ultraviolet imager (UVI) has been developed for the Akatsuki spacecraft (Venus Climate Orbiter mission). The UVI takes ultraviolet (UV) images of the solar radiation reflected by the Venusian clouds with narrow bandpass filters centered at the 283 and 365 nm wavelengths. There are absorption bands of SO2 and unknown absorbers in these wavelength regions. The UV images provide the spatial distribution of SO2 and the unknown absorber around cloud top altitudes. The images also allow us to understand the cloud top morphologies and haze properties. Nominal sequential images with 2-h intervals are used to understand the dynamics of the Venusian atmosphere by estimating the wind vectors at the cloud top altitude, as well as the mass transportation of UV absorbers. The UVI is equipped with off-axial catadioptric optics, two bandpass filters, a diffuser installed in a filter wheel moving with a step motor, and a high sensitivity charge-coupled device with UV coating. The UVI images have spatial resolutions ranging from 200 m to 86 km at sub-spacecraft points. The UVI has been kept in good condition during the extended interplanetary cruise by carefully designed operations that have maintained its temperature maintenance and avoided solar radiation damage. The images have signal-to-noise ratios of over 100 after onboard desmear processing.