First disk-resolved millimeter observations of Io's surface and SO₂ atmosphere^*

¹ LESIA-Observatoire de Paris, 5 place J. Janssen, 92195 Meudon, France e-mail: arielle.moullet@obspm.fr
² Harvard-Smithsonian Center for Astrophysics, Cambrige, MA 02138, USA
³ The University of Texas at Austin, TX 78712, USA

Received: 18 September 2007
Accepted: 3 December 2007

Abstract

Aims. In spite of considerable progress in the last two decades, Io's atmosphere remains poorly understood. The goal of this work is to improve our understanding of its spatial distribution, temperature and dynamics.

Methods. We present millimeter observations of Io's surface and SO₂ atmosphere at 1.4 mm obtained with the IRAM Plateau de Bure Interferometer in January-February 2005. With a synthesized beam of 0.51.5'', these observations resolve Io's ~1.0'' disk in the longitudinal / local time direction, and sample the leading and trailing hemispheres of Io.

Results. The measured continuum total flux and visibilities show that continuum radiation originates from a depth of at least 1 cm in Io's subsurface. On both the leading and trailing sides, emission in the SO₂ 216.643 GHz line appears spatially narrower than the continuum, and suggests that the atmosphere covers ~80% of the surface on the leading side and ~60% on the trailing. On the leading side, disk-resolved spectra yield Doppler shift measurements, indicating a beam-integrated limb-to-limb velocity difference of 330 ± 100 m/s in the prograde direction. Such a flow allows an improved fit of disk-averaged SO₂ spectra, but its origin remains to be understood. Mean gas temperatures are in the range 130-180 K, in agreement with estimates from IR measurements, and with a tendency for higher trailing vs leading side gas temperatures. On the basis of realistic plume models, we find that the contribution of isolated volcanic plumes to the SO₂ emission is small.