Atmospheric processes on Venus are driven by the interaction of water and sulphur dioxide. There molecules combine to form a sulphuric acid cloud layer, which covers the surface of the planet. These clouds are thought to produce sulphuric acid rain which falls towards the surface, but usually evaporates due to the planet’s heat before reaching the ground, disassociating into water and sulphur dioxide again as it does so. Below the cloud layer water and sulphur dioxide are present in the atmosphere at rates of about 30 and 130 parts per million respectively, while above the cloud layer their presence drops to about 1-3 parts per million for water, and 10-1000 parts per billion for sulphur dioxide.
The atmosphere of Venus has been monitored by a series of spacecraft, including Pioneer Venus, Venera, Venus Express, and Akatsuki, which have given us a good general understanding of the composition of the Venusian atmosphere, though their proximity to the planet has hampered the ability of these probes to study the whole disk of Venus, preventing the study of wider scale atmospheric behaviour. To this end the Texas Echelon Cross-Echelle Spectrograph (TEXES) at the NASA InfraRed telescope Facility at Mauna Kea Observatory has been used to monitor the behaviour of sulphur dioxide and water on Venus since January 2012. These studies have shown that the distribution of water in the upper atmosphere of Venus tends to be even, and remains more-or-less constant, while sulphur dioxide levels vary a great deal with what appear to be plumes of the gas appearing and disappearing over the space of a few hours.
In a paper published in the journal Astronomy & Astrophysics on 7 March 2019, Therese Encrenaz of the Observatoire de Paris at Université Sorbonne Paris Cité, Thomas Greathouse of the Southwest Research Institute, Emmanuel Marcq of the Laboratoire atmosphères, milieux, observations spatiales, Hideo Sagawa of Kyoto Sangyo University, Thomas Widemann, Bruno Bézard, and Thierry Fouchet, also of the Observatoire de Paris at Université Sorbonne Paris Cité, Franck Lefèvre, also of the Laboratoire atmosphères, milieux, observations spatiales, Sébastien Lebonnois of the Ecole Polytechnique and the University Paris Saclay, Sushil Atreya of the Planetary Science Laboratory at the University of Michigan, Yeon Joo Lee of the University of Tokyo, Rohini Giles of the Jet Propulsion Laboratory, and Shigeto Watanabe of Hokkaido Information University, describe the results of a long term study of the movement of sulphur dioxide within the atmosphere of Venus, made using the Texas Echelon Cross-Echelle Spectrograph at the NASA InfraRed telescope Facility at Mauna Kea Observatory between January 2016 and September 2018.
Geometrical configurations of the disk of Venus during the six TEXES runs of 2016, 2017, and 2018. The terminator is indicated with a black line and the subsolar point with a black dot. The January 2016 and July 2017 runs correspond to the morning terminator; the four other runs correspond to the evening terminator. Encrenaz et al. (2019).
As had previously been noted, Encrenaz et al. found that sulphur dioxide plumes appeared and disappeared over the course of a few hours. However, not all the plumes behaved in the same way, with some plumes being very localised, with sulphur dioxide levels reaching as much as four times as high as in the rest of the atmosphere at the same altitude over a very narrow area, while others were more diffuse, occurring over a wide longitudinal (east-west) range. Plumes typically reached maximum intensity within two hours, then dissipated over the course of another two hours, with the sulphur dioxide dispersing in the same direction as the movement of the clouds.
Almost all the plumes appeared close to the equator, at latitudes of between 30° north and 30° south, although it would have been hard to detect plumes close to the poles, so the presence of polar plumes cannot be ruled out. Two regions within this equatorial zone produced significantly less plumes; one of these corresponds to longitudes between 100 and150 east (which is roughly the longitudinal extent of the Aphrodite Terra highland region) and the other centres on a longitude of about 300 east.
Almost all the plumes appeared close to the equator, at latitudes of between 30° north and 30° south, although it would have been hard to detect plumes close to the poles, so the presence of polar plumes cannot be ruled out. Two regions within this equatorial zone produced significantly less plumes; one of these corresponds to longitudes between 100 and150 east (which is roughly the longitudinal extent of the Aphrodite Terra highland region) and the other centres on a longitude of about 300 east.
Maps of the line depth ratio of a weak sulphur dioxide multiplet (around 1345.1 cm⁻¹) to the CO2 transition at 1345.22 cm⁻¹. The subsolar point is shown as a white dot. The scale is not the same for the six maps; the maximum sulphur dioxide abundance is observed in July 2018. Encrenaz et al. (2019).
Finally, Encrenaz et al. compared the data from the TEXES instrument to that from the UV Imager on the Akatsuki spacecraft and the SPICAV UV spectrometer on the Venus Express spacecraft, finding that both instruments produced corelating data, with a particularly good match between data from TEXES and that from Akatsuki.
(Left panel) UV albedo map derived from the Akatsuki UV Imager data recorded on 21 January 2017, at 01.46 GMT. Dashed lines represent the equator and the evening terminator. (Right panel) TEXES map of the sulphur dioxide volume mixing ratio at the cloud top, inferred from the sulphur dioxide /carbon dioxide line depth ratio at 7.4 μm on 21 January 2017, at 03.43–04.18 GMT. Encrenaz et al. (2019).
Plumes were also significantly less likely at noon local time anywhere on the planet, i.e. when the Sun is directly overhead. Encrenaz et al. speculate that this might be caused by photochemical processes breaking down the sulphur dioxide, or solar energy disrupting convection currents within the cloud layer.
Finally, Encrenaz et al. compared the data from the TEXES instrument to that from the UV Imager on the Akatsuki spacecraft and the SPICAV UV spectrometer on the Venus Express spacecraft, finding that both instruments produced corelating data, with a particularly good match between data from TEXES and that from Akatsuki.
(Left panel) UV albedo map derived from the Akatsuki UV Imager data recorded on 21 January 2017, at 01.46 GMT. Dashed lines represent the equator and the evening terminator. (Right panel) TEXES map of the sulphur dioxide volume mixing ratio at the cloud top, inferred from the sulphur dioxide /carbon dioxide line depth ratio at 7.4 μm on 21 January 2017, at 03.43–04.18 GMT. Encrenaz et al. (2019).
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