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Venus Atmosphere Anomalies

UV Absorber

While Venus appears relatively bland and featureless at visible wavelengths, observers starting in the 1920s noticed unusual high-contrast features in the ultraviolet. These features move with the ~4-day super-rotation of Venus’ upper cloud deck, yet also display great variability on a wide range of temporal and spatial scales. The unknown absorber is remarkably efficient, capturing more than 50% of the solar energy reaching Venus, with consequent effects on atmospheric structure and dynamics. Much effort has gone into attempting to identify the substances responsible for the unknown absorption, but no proposed candidate satisfies all of the observational constraints.

Image showing visible wavelengths of ultraviolet light in Venus's atmosphere

Image of Venus at ultraviolet and visible wavelength. The dark stripes are caused by an unknown ultraviolet absorber.  Image credit: NASA

Depleted Sulfur Dioxide And Water Vapor In The Venus Cloud Layers

The atmospheric vertical abundance profiles of sulfur dioxide (SO₂) in the Venus cloud layers and water vapor (H₂O) in and above the clouds remain unexplained. The presence of SO₂ is expected in the atmosphere of Venus and in the clouds. SO₂ is a common volcanically produced gas. However, the observed abundance of SO₂ ascending through the Venus cloud layers drops from an average of ~150 ppm below the clouds to sub-ppm levels above the clouds. This depletion cannot be currently explained by known atmospheric chemistry including cloud formation. There is, therefore, a missing atmospheric process of some kind that awaits discovery.

Presence Of Parts-Per-Million Oxygen

In situ detections of oxygen (O₂) gas in the Venusian lower clouds and below the clouds have been reported by at least two probes at the 10s of ppm level: the US Pioneer Venus and USSR's Venera 14. In both cases O₂ has been detected by on board gas chromatographs, at similar altitudes and in similar abundance. The O₂ in situ detections have been dismissed as artifactual either because of the difficulties in reconciliation with the remote observations from Earth — which do not support the presence of O₂ — or lack of known physical or chemical processes that could maintain 10s of ppm O₂ levels in the hot, reactive lower atmosphere of Venus. However, the multiple, consistent in situ detections would suggest that O₂ is indeed present at ~10s ppm in the clouds and the lower haze below the clouds.

Tentative Detection of Ammonia
 

Ammonia (NH₃) gas has been tentatively detected by two separate probes. In 1972, the Venera 8 descent probe reported the presence of NH₃ below the clouds using a chemical sensor. The recent re-assessment of the Pioneer Venus Large Probe Neutral Mass Spectrometer (LNMS) has also provided suggestive, although not conclusive, evidence for the presence of NH₃ in the Venus cloud layers. Similarly to O₂, the tentative in situ detections await reconciliation with the remote observations from Earth and orbiters which do not support the presence of NH₃. These observations above the clouds are difficult to reconcile with the tentative in situ observations, unless the NH₃ loss in the upper atmosphere is balanced by a constant production that is localized to the clouds and the stagnant haze layer below.

Unexplained and unexplored Venus’ atmospheric observations. The molecule models show individual detections and altitudes for the atmospheric observations (e.g. NH₃ has been tentatively observed three times, twice, at two different altitudes by Venera 8, at ~32 and ~45 km, and once by Pioneer Venus at ~51 km). The SO₂ and H₂O molecule models do not represent every individual observation but rather denote the anomalous abundance profiles for SO₂ and H₂O in the atmosphere). Most of the unexplained atmospheric observations have been recorded within the clouds (48-70 km) and in the stagnant haze layer below the clouds (31-47 km).  For a review of Venus atmosphere anomalies, see Petkowski et al. 2024. 

Chart showing Anomalies Phosphine molecules

Large “Mode 3” Particles That Are Not Pure Concentrated Sulfuric Acid

The existence of a population of large (>7 µm in diameter), non-spherical particles in the lower cloud decks is a hotly debated topic since their initial discovery by the Pioneer Venus probe more than 40 years ago. The nature and composition of the Mode 3 particles is unknown, but if they do exist they cannot be made of pure concentrated liquid sulfuric acid, leaving room, albeit speculative, for unknown chemistry or maybe even life.

Presence Of Non-Volatile Elements In The Cloud Particles

Both the Vega balloons’ and Venera probes’ in situ measurements of the elemental composition of the cloud particles suggest that non-volatile elements relevant for cloud habitability are present. The data collected by Soviet's probes indicates the presence of sulfur, chlorine, iron and phosphorus in the lower clouds. Recent preliminary re-analysis of data from Pioneer Venus mass spectrometer supports the Venera and VeGa measurements and shows evidence of a non-homogenous composition of cloud and haze particles. The particles could contain many chemicals dissolved in concentrated sulfuric acid, for example, various metal ions, salts, silica, and even "insoluble organics".

The Controversial Report Of Phosphine Gas In The Venus Cloud Layers

Phosphine (PH₃) gas in the Venus atmosphere has been reported at a few ppb levels in 2020 and followed up by evidence of PH₃ from a data reanalysis of NASA’s 1978 Pioneer Venus Probe’s mass spectrometer. The tentative detection of PH₃ in Venus clouds is still controversial and a subject of a heated debate more than four years since the original report. This continuous scientific debate on the topic of Venusian PH₃ is summarized in the “Phosphine back and forth” section. 

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