Saturn atmosphere's winds with VLT/UVES Doppler velocimetry

Abstract

We present Doppler wind velocity final results of Saturn’s zonal flow at cloud level. Our aim is help to constrain the characterization of the equatorial jet at cloud level and the latitudinal variation of the zonal winds, to measure its spatial and temporal variability, to contribute to monitor the variability in order to achieve a better understanding of the dynamics of Saturn’s zonal winds (Sánchez-Lavega et al. 2003, 2007, 2016); Finally, the complementarity with Cassini, providing an independent set of observations.

Figure 1: (a) Raw echellogramme showing the spectral orders for one of the detectors. (b) Magnification of part of one order, where absorption lines (dark vertical bands) are visible. From each order, a stack of 61 spectra are extracted. (c) Set of 61 spectra, with each one corresponding to one pixel in the slit’s active window. (d) Each spectrum is divided into 16 orders in the MIT detector and 23 orders in the EEV detector. The plot shows an example of the 16 components of an MIT spectrum, each coming from one spectral order. (e) Example spectrum from one order and one location in the Venus disk. Machado et al. (2012).

The study of the planet’s global system of winds at the 0.7 bar region is based on high resolution spectra from the UV-Visual Echelle Spectrograph (UVES) instrument at ESO’s Very Large Telescope (VLT). Under the assumption of predominantly zonal flow, this method allows the simultaneous direct measurement of the zonal velocity across a range of latitudes and local times. The technique, based on long slit spectroscopy combined with the high spatial resolution provided by the VLT, has provided the first ground-based characterization of the latitudinal profile of zonal wind in the atmosphere of Saturn and the first zonal wind field map in the visible. It promises to improve the characterization of the equatorial jet and the latitudinal variation of the zonal winds, as well the measurement (and monitorization) of its spatial and temporal variability, achieving a better understanding of the dynamics of Saturn’s zonal winds (which Sánchez-Lavega have found to have changed in recent years). A complete characterization of the dynamical behaviour of Saturn atmosphere is crucial for understanding its driving mechanisms. Finally, the complementarity with Cassini, has provided an independent set of observations to compare with and help validate the method. The zonal wind profile retrieved is consistent with previous spacecraft measurements based on cloud tracking, but with non-negligible variability in local time (longitude) and in latitude.

Figure 2: Geometry of the slit positions at the observation days. Saturn’s diameter is 17.4", and the slit aperture is 0,3”x25” . The aperture offset between consecutive exposures is 1". The sub-terrestrial point is at -26.1ºS.

The UVES/VLT instrument has been used, which simultaneously achieves high spectral resolving power and high spatial resolution. The field has been derotated in order to have the aperture aligned perpendicularly to Saturn’s rotation axis. In this configuration, spatial information in the East-West direction is preserved in a set of spectra in the direction perpendicular to dispersion. Our Doppler velocimetry method is based on the technique of absolute accelerometry (Connes, 1985) which has been applied to the backscattered solar spectrum in order to determine the Doppler shift associated with the zonal circulation. Our measurements have been made in the wavelength range of 480-680 nm. Previously we successfully adapted and fine tuned this Doppler velocimetry technique for measuring winds at Venus cloud tops (Machado et al. 2012, 2014,2017, 2021; Gonçalves et al., 2020). In the present study we will show the adaptation of this method for Saturn’s case. We will use coordinated observations from the Cassini’s Visible and Infrared Mapping Spectrometer (VIMS), in order to compare with the Doppler winds obtained from the UVES/VLT high-resolution spectra.

The observations consisted of 4 blocks of 15 exposures of 90 sec, plus two shorter blocks of 9 exposures, totaling 7.3 hours of telescope time. In order to cover the whole disk the aperture has been offset by 1 arcsec in the North-South direction between consecutive exposures. Most of the northern hemisphere was covered by the rings. Saturn’s diameter was 17.4 arcsec, and the slit aperture was 0.3x25 arcsec. The aperture offset between consecutive exposures was 1 arcsec. Two shorter observations blocks of 9 exposures only covered the central part of the disk, and four others covered the whole disk. The sub-terrestrial point was at -26.1 S. The presence of the rings lead to severe order superposition. The dark region between the rings and the disk may or may not be present, depending on the slit position. On the other hand, defects in the response of the UVES slit in the upper part preclude its use for accurate Doppler measurements such as these. For these reasons only the central part of the aperture has been considered for the measurements.

It can be easily noticed that we were able to reproduce with a significant agreement the amplitudes of the wind velocities previously observed in a vast range of latitudes and that they are highly consistent with the cloud tracking measurements from almost simultaneous Cassini data.

Figure 3: Contour map of Saturn disk for the first night of observations. The wind velocities have units of m/s. Thecolor scale was arbitrary.

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