A Wind Study of Venus’s Cloud Top: New Doppler Velocimetry Observations

Ruben Gonçalves,José Ribeiro,Thomas Widemann,Francisco Brasil,Pedro Machado

doi:10.3390/atmos12010002

Abstract

At Venus’s cloud top, the circulation is dominated by the superroration, where zonal wind speed peaks at ∼100 ms−1, in the low-to-middle latitudes. The constraining of zonal and meridional circulations is essential to understanding the mechanisms driving the superrotation of Venus’s atmosphere, which are still poorly understood. We present new Doppler velocimetry measurements of horizontal wind velocities at Venus’s cloud top, around 70 km altitude. These results were based on March 2015 observations at the Canada–France–Hawaii Telescope (CFHT, Mauna Kea, Hawaii), using ESPaDOnS. The Doppler velocimetry method used has already successfully provided zonal and meridional results in previous works led by P. Machado and R. Gonçalves, proving to be a good reference ground-based technique in the study of the dynamics of Venus’s atmosphere. These observations were carried out between 27 and 29 March 2015, using the Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) which provides simultaneous visible-near IR spectra from 370 to 1050 nm, with a spectral resolution of 81000 allowing wind field characterization in the scattered Franuhofer solar lines by Venus’s cloud top on the dayside. The zonal velocities are consistent with previous results while also showing evidence of spatial variability, along planetocentric latitude and longitude (local-time). The meridional wind circulation presents a notably constant latitudinal structure with null velocities at lower latitudes, below 10∘ N–S, and peak velocities of ∼30 ms−1, centered around 35∘ N–S. The uncertainty of the meridional wind results from ground observations is of the same order as the uncertainty of meridional wind retrieved by space-based observations using cloud-tracking, as also shown by previous work led by R. Gonçalves and published in 2020. These March 2015 measurements present a unique and valuable contribution to the study of horizontal wind at the cloud top, from a period when Doppler velocimetry was the only available method to do so, since no space mission was orbiting Venus between Venus Express ending in January 2015 and Akatsuki’s orbit insertion in December 2015. These results from new observations provide (1) constraints on zonal wind temporal and spatial variability (latitude and local time), (2) constraints on the meridional wind latitudinal profile, (3) additional evidence of zonal and meridional wind stability for the period between 2011 and 2015 (along previous Doppler results) (4) further evidence of the consistency and robustness of our Doppler velocimetry method.

Highlights

Venus is usually referred to as Earth’s twin due to its similarities, namely, the mass, radius, density and bulk chemical composition [1]
The atmospheric circulation is dominated by a zonal wind that peaks at the top of the upper clouds (70–75 km)
We applied the theoretical models for each wind circulation—(1) zonal wind, under the assumption of a pure zonal wind system for all data points with latitudes between 45◦ S-45◦ N, (2) and the same procedure for meridional wind, under the assumption of a pure meridional wind system for all points located at the half-phase angle (HPA) meridian ([φ − φE ] = −25◦ ), using the same method as described by Machado et al [8,10], Gonçalves et al [12]

Summary

Introduction

Venus is usually referred to as Earth’s twin due to its similarities, namely, the mass, radius, density and bulk chemical composition [1]. The superrotation of the Venusian atmosphere, only seen in slow rotating planets, challenges the current understanding of geophysical fluid dynamics inherited from non-superrotating Earth [2,3,4,5]. Venus’s middle and lower atmosphere are in a state of retrograde superrotation where the atmosphere rotates about 50 times faster than the solid planet. The mechanisms maintaining the superrotation of Venus’s atmosphere are still poorly understood [2,3,13]. Both the source and maintenance of a superrotating atmosphere in a slow rotating planet constitute a long-standing problem in planetary atmospheric dynamics [14,15]

Results

Discussion

Conclusion