The Space Environment of Io and Europa

Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the Solar System, Io, we describe how sodium (Na I) and potassium (K I) $\textit{gas}$ could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing $\sim$ a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal $\mathcal{Q}_p \lesssim 10^{11}$. This tidal energy is focused into the satellite driving a $\sim 10^{5 \pm 2}$ higher mass loss rate than Io's supply to Jupiter's Na exosphere, based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are on average $\textit{more than sufficient}$ to provide the $\sim$ 10$^{10 \pm 1}$ Na cm$^{-2}$ required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended ($\sim 1000$ R$_J$) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP 49-b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.

Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets

Jupiter's great sodium nebula is a gas cloud that, were it visible to the naked eye, would be one of the largest objects seen in the Solar System. The sodium is thought to derive from Jupiter's volcanically active moon Io. Mendillo et al. have used a new observation technique to pinpoint the exact positions of the sodium source or sources. High-resolution spectra reveal two areas of sodium escape. One is at the wake of Io's interaction with the ring of plasma — ejected from Io itself — that shares the moon's orbit. The second is a spherical outflow region around Io, a result of atmospheric sputtering. The extent of the ionospheric wake, six Io radii, is more than twice the estimate from Galileo spacecraft data, implying considerable variability. On Jupiter's moon Io, volcanic hot gases form an atmosphere that is then ionized and distributed around its orbit, forming a torus. Although it is known that the plasma leaks away, the leakage locations were unknown. Mendillo et al. report that sodium is ejected from the wake of the Io-torus interaction, together with a spherically symmetrical outflow region. The spatial extent of the ionospheric wake is more than twice that observed by the Galileo spacecraft, implying considerable variability. On Jupiter’s moon Io, volcanic plumes and evaporating lava flows provide hot gases to form an atmosphere that is subsequently ionized. Some of Io’s plasma is captured by the planet’s strong magnetic field to form a co-rotating torus at Io’s distance; the remaining ions and electrons form Io’s ionosphere. The torus and ionosphere are also depleted by three time-variable processes that produce a banana-shaped cloud orbiting with Io1, a giant nebula extending out to about 500 Jupiter radii2,3,4,5, and a jet close to Io6,7,8,9. No spatial constraints exist for the sources of the first two; they have been inferred only from modelling the patterns seen in the trace gas sodium observed far from Io. Here we report observations that reveal a spatially confined stream that ejects sodium only from the wake of the Io–torus interaction, together with a visually distinct, spherically symmetrical outflow region arising from atmospheric sputtering. The spatial extent of the ionospheric wake that feeds the stream is more than twice that observed by the Galileo spacecraft and modelled successfully. This implies considerable variability, and therefore the need for additional modelling of volcanically-driven, episodic states of the great jovian nebula.

The sources of sodium escaping from Io revealed by spectral high definition imaging

We report ground-based spectroscopic observations of the Io sodium cloud before and after eclipse by Jupiter, in a search for possible effects of sunlight on the total atomic sodium content in Io's exosphere. These observations test the importance of the sun in maintaining the supply of atoms from the thicker atmosphere, which lies close to the surface, to the thinner, more extended, sodium cloud. We performed ground-based spectroscopy of the sodium doublet at 589 nm with a high-resolution echelle spectrograph on the Italian National Telescope Galileo (TNG) on La Palma Island. We find that the total atomic sodium content immediately after eclipse falls by a factor of ~4 during eclipse and recovers to pre-eclipse values on a timescale of ~5 hours. We evaluate potential causes of the precipitous drop including condensation of the atmosphere, collapse of the ionosphere, and changes in atmospheric chemistry. We conclude that most plausible cause of the drop in atomic sodium production is the interruption of photodissociation of sodium-bearing molecules during eclipse. We discuss further implications of this change in atmospheric chemistry.

https://hal.archives-ouvertes.fr/hal-00952745/document

Solar control of sodium escape from Io

/pdf/io-s-neutral-clouds-from-the-atmosphere-to-the-plasma-torus-50n2ukrur4.pdf

Io's neutral clouds: From the atmosphere to the plasma torus

We have measured the column density profile of Io's sodium corona using 10 mutual eclipses between the Galilean satellites. This approach circumvents the problem of spatially resolving Io's corona directly from Io's bright continuum in the presence of atmospheric seeing and telescopic scattering. The primary goal is to investigate the spatial and temporal variations of Io's corona. Spectra from the Keck Observatory and McDonald Observatory from 1997 reveal a corona that is only approximately spherically symmetric around Io. Comparing the globally averaged radial sodium column density profile in the corona with profiles measured in 1991 and 1985, we find that there has been no significant variation. However, there appears to be a previously undetected asymmetry: the corona above Io's sub-Jupiter hemisphere is consistently more dense than above the anti-Jupiter hemisphere.

/pdf/mutual-event-observations-of-io-s-sodium-corona-17zmprdtn8.pdf

Mutual Event Observations of Io’s Sodium Corona

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Mutual Event Observations of Io’s Sodium Corona