More on the dynamics of Venus’ South Polar Vortex from VIRTIS-VEx observations

Venus has one of today’s most unknown atmospheres with one of the unsolved mysteries being the origin and maintenance of its polar vortices. Although the current orbiting espacecraft, the Japanese Akatsuki, cannot adequately observe the planet's poles, the previous European Space Agency’s Venus Express mission studied the south polar vortex for more than 7 years. The VIRTIS-M imaging spectrometer observed this peculiar atmospheric structure and its constantly changing shape day after day from 2006 to 2014. This translates into an immense amount of data, which is still not fully exploited today.

Some time ago we obtained preliminary maps of the Ertel potential vorticity [1], a characteristic magnitude in the study of fluid dynamics as it is conserved following the full three-dimensional motions of an air parcel. However, the number of data-cubes we were able to analyse did not allow us to draw statistically strong conclusions. We now aim to improve these statistics in order to confirm or disprove previously observed trends, such as the annular shape of the potential vorticity at the upper cloud level or the anti-correlation between the structures visible in thermal infrared (the most characteristic structures of the Venus’ polar vortex) and the potential vorticity’s peaks [1].

To this end, here we will present new measurements of the wind field at two atmospheric levels (~42 km above the surface and ~62 km) using images at 1.74 microns and at 3.8 or 5.1 microns obtained by the VIRTIS instrument for about 10 different dates and morphologiess. We measured the wind velocity by cloud tracking in more than one pair of images per date.

We will also present new measurements of three-dimensional distribution of the air temperature. The VIRTIS instrument provides us with spectra for each pixel of the image in the range 1.0 - 5.1 microns. Using radiative transport and inversion techniques [2], we know how to calculate the spatial distribution of the temperature field for several atmospheric layers between 55 and 85 km height [3]. We intend to obtain temperature maps for the lower and upper cloud layers in as many pairs of images as possible, thus studying both the short and long term evolution.

And finally, with wind and temperature fields’ distribution at both cloud layers in our hands, we will try to construct and exhibit Ertel’s potential vorticity maps.

 

References

[1] I. Garate-Lopez, R. Hueso, A. Sánchez-Lavega, A. García Muñoz. Potential Vorticity of the South Polar Vortex of Venus. Journal of Geophysical Research: Planets 121, 574-593 (2016). https://doi.org/10.1002/2015JE004885

[2] D. Grassi, et al., 2008. Retrieval of air temperature profiles in the venusian mesosphere from VIRTIS-M data: Description and validation of algorithms. Journal of Geophysical Research: Planets 113, 1–12. https://doi.org/10.1029/2008JE003075

[3] I. Garate-Lopez, A. García Muñoz, R. Hueso, A. Sánchez-Lavega. Instantaneous three- dimensional thermal structure of the South Polar Vortex of Venus. Icarus 245, 16-31 (2015). https://doi.org/10.1016/j.icarus.2014.09.030