On measuring planetary winds using high-resolution spectroscopy in visible wavelengths

¹ Research and Scientific Support Department, ESA/ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands e-mail: tciveit@rssd.esa.int
² Payloads and Advanced Concepts Office, ESA/ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
³ Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
⁴ Observatoire de Paris, LESIA, 5 place Jules Janssen, 92195 Meudon Cedex, France
⁵ Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁶ GEPI, UMR 8111 du CNRS, Observatoire de Paris-Meudon, 92195 Meudon Cedex, France

Received: 12 July 2004
Accepted: 11 October 2004

Abstract

We present a new method that uses high-resolution spectroscopy in the visible wavelength domain to measure planetary winds. A rotating atmosphere illuminated by the Sun induces a Doppler shift in the back-scattered solar light. Its analysis with a cross-dispersed echelle spectrometer allows the direct determination of both the wind speed and direction. We describe in this paper the image and data processing algorithms used in the method and implemented in a data reduction and analysis package. Since the velocity of planetary winds leads to Doppler shifts smaller than the width of the solar lines, accurate Doppler measurements are performed by running the algorithm proposed by [CITE], which is an optimum technique using the full available spectral information. We apply the method to Io as a test case, a small slowly-rotating body with no atmosphere, to measure its solid rotation. The observations span wavelengths from 414 to 621 nm and were carried out with the Ultraviolet and Visual Echelle Spectrograph (UVES) on the 8.2 m Kueyen unit at the Very Large Telescope (VLT – ESO). The results we obtain for Io validate the principle of the method by optimally measuring the well-known surface rotational velocity of this moon, with an uncertainty smaller than 2 m s^-1. However, the analysis of the set of observations shows that systematic errors are large and one needs to consider the retrieved velocity as a lower limit.