First disk-resolved millimeter observations of Io's surface and SO₂atmosphere

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 SO 2 atmosphere at 1.4 mm obtained with the IRAM Plateau de Bure Interferometer in January−February 2005. With a synthesized beam of 0.5 × 1.5 , 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 SO 2 216.643 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 SO 2 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 SO 2 emission is small.

Multi-wavelength simulations of atmospheric radiation from Io with a 3-D spherical-shell backward Monte Carlo radiative transfer model

“…The IRAM-30 m observations obtained in 1999 were reanalyzed by Moullet et al (2008) based on SO 2 spatial distributions of Feaga et al (2009) and the latitude-dependent model of Spencer et al (2005). For hydrostatic and isothermal atmospheric model, the line emission data were fit with gas temperatures lower than 200 K, which required scaling the SO 2 column density distribution of Feaga et al (2009) by factors of $3 and 0.7 on the leading and trailing sides, respectively, and scaling the SO 2 column density distribution of Spencer et al (2005) by the factor of 0.2 on the trailing side.…”

“…The unknown millimeter-wave surface emittance of Io was estimated by requiring that the brightness temperature of the 1.4 mm continuum emission equals 93 and 99 K for the leading and trailing sided, respectively, as determined by Moullet et al (2008). Assuming a uniform surface emittance and using the surface temperature distribution model of Walker et al (2009), one finds surface emittances of 0.838 and 0.892 on the leading and trailing sides, respectively, in order to explain the observed continuum brightness temperatures.…”

“…The simulated line profiles also appear to be too narrow for both models when compared to the IRAM-30 m data. Following Moullet et al (2008) we examined the contribution of atmospheric dynamics toward broadening of the 216.643 GHz line. The DSMC gas dynamics model includes two effects contributing to the atmospheric motion: day-to-night flows resulting from pressure gradients generated by sublimating gas and volcanic plume expansion.…”

“…The main feature of the measured Doppler shift map is the limb-to-limb velocity difference of 330 ± 100 m/s produced by strong 160 ± 80 m/s red shifts on the western limb and equally strong 170 ± 80 m/s blue shifts on the eastern limb. Provided that the equatorial linear velocity for Io is 75 m/s, Moullet et al (2008) suggested that the atmosphere may undergo ''super-rotation" in the prograde direction with an equatorial zonal velocity of 170-300 m/s. Such strong circumplanetary flows, however, are not predicted either by the current DSMC model or by any previous sublimation-driven models (Ingersoll et al, 1985;Ingersoll, 1989).…”

“…In Section 4, the simulations of disk-resolved images at Lyman-a are presented and their comparison to the HST/STIS data (Roesler et al, 1999;Feaga et al, 2009) are discussed. Section 5 presents the comparison of simulated and observed emission line profiles (Moullet et al, 2008) in the millimeter range and their implications for understanding Io's vertical thermal structure and atmospheric dynamics. The results of the paper are summarized in Section 6.…”