Venus Atmospheric Dynamics: Akatsuki UVI and TNG HARPS-N observations

As the closest planet to Earth, it should be expected Venus to be the most Earth-like planet we know. Both Earth and Venus share almost the same radius, mass and density and were formed from the same available ingredients, at the same time and location in the Solar System. Yet, Venus has undoubtedly ended up with an extreme climate, with a dense carbon dioxide dominated atmosphere responsible for a runaway greenhouse effect and high surface temperatures. Because of these similarities and differences, Venus is a key planet in the understanding of planetary evolution to which the study of the atmospheric dynamics is indispensable. For Venus, the better understanding of cloud circulation can yield important results such as the possibility to explain and describe one of its most fascinating characteristics: the superrotation of Venus’ atmosphere.

To accurately describe the atmospheric circulation of Venus, this work employed the use of two distinct methods (described below) to obtain wind velocities on specific layers of the Venusian atmosphere:

The Doppler velocimetry for fibre-fed spectrographs was initially developed by Thomas Widemann (Widemann et al., 2008) and was later evolved by Pedro Machado who also developed and fine-tuned a Doppler velocimetry method for long slit spectrographs (Machado et al., 2012, 2014).  This technique is based on solar light scattered on Venus’ dayside and provides instantaneous wind velocities measurements of its atmosphere. The sunlight is absorbed by cloud particles in Venus’ top clouds and then re-emitted in Earth’s direction in a single back-scatter approximation (Machado et al., 2012, 2014, 2017).

The cloud-tracking method consists of a simple analysis of a pair of navigated and processed images, provided that the time interval between both is known. It is possible to analyse the motion of cloud features between the initial and second image, either by matching specific points or areas in both images. This matching process allows us to measure displacements and velocities of cloud features and deduct an average velocity for a certain cloud layer of the atmosphere, selected in the wavelength range of the observations (Peralta et al. 2018).

The use of an evolved tool of cloud tracking based on phase correlation between images and other softwares (Hueso et al. 2010) allows to explore Venus' atmospheric dynamics based on space and ground observations including data from Akatsuki UVI instrument and TNG/HARPS-N. The images used were navigated and processed for optimal identification of cloud features which help with the processes described above.

In short, the main goal of this work was to build wind profiles in different wavelengths which allow us to analyse several layers of the Venusian atmosphere. Some results of this study are presented following the works of Sánchez-Lavega et al. 2008, Hueso et al. 2013 and Horinouchi et al. 2018.

Another goal of this study is connected to the detection and characterisation of atmospheric gravity waves also using Akatsuki/UVI images. These waves are oscillatory disturbances on an atmospheric layer in which buoyancy acts as the restoring force. They can only exist in stably stratified atmospheres, that is, a fluid in which density varies mostly vertically (Silva et al. 2021). It is possible that the exploration of these waves can lead to a better understanding of the mechanisms that drive the state of superrotation of the Venusian atmosphere.

 

References

[1] Hueso et al., The Planetary Laboratory for Image Analysis (PLIA). Advances in Space Research, 46(9):1120–1138, 2010. 

[2] Sánchez-Lavega et al., Variable winds on Venus mapped in three dimensions. Geophysical Research Letters, 35 (13), 2008

[3] Hueso et al., Venus winds from ultraviolet, visible and near infrared images from the VIRTIS instrument on Venus Express.  2013.

[4] Horinouchi et al., Mean winds at the cloud top of venus obtained from two-wavelength UV imaging by Akatsuki. Earth, Planets and Space, 70:10, 2018.

[7] Machado et al., Characterizing the atmospheric dynamics of Venus from ground-based Doppler velocimetry, Icarus, Volume 221, p.248-261, 2012.

[6] Machado et al., Wind circulation regimes at Venus’ cloud tops: Ground-based Doppler velocimetry using CFHT/ESPaDOnS and comparison with simultaneous cloud tracking measurements using VEx/VIRTIS in February 2011, Icarus, 2014.

[7] Machado et al., Venus Atmospheric Dynamics at Two Altitudes: Akatsuki and Venus Express Cloud Tracking, Ground-Based Doppler Observations and Comparison with Modelling. Atmosphere 2021, 12, 506.

[8] Machado et al., Venus cloud-tracked and Doppler velocimetry winds from CFHT/ESPaDOnS and Venus Express/VIRTIS in April 2014. Icarus, vol. 285, p. 8-26, 2017.

[9] Peralta et al., Nightside Winds at the Lower Clouds of Venus with Akatsuki/IR2: Longitudinal, Local Time, and Decadal Variations from Comparison with Previous Measurements. The American Astronomical Society. The Astrophysical Journal Supplement Series, Volume 239, Number 2, 2018

[10] Widemann et al., Venus Doppler winds at cloud tops observed with ESPaDOnS at CFHT, Planetary and Space Science, Volume 56, p. 1320-1334, 2008.

[11] Silva et al., Characterising atmospheric gravity waves on the nightside lower clouds of Venus: a systematic analysis, A&A 649 A34, 2021.

 

Acknowledgements

We thank the JAXA’s Akatsuki team for support with coordinated observations. We gratefully acknowledge the collaboration of the TNG staff at La Palma (Canary Islands, Spain) - the observations were made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. We acknowledge support from the Portuguese Fundação Para a Ciência e a Tecnologia (ref. PTDC/FIS-AST/29942/2017) through national funds and by FEDER through COMPETE 2020 (ref. POCI-01-0145 FEDER-007672) and through a grant of reference 2020.06389.BD.