Io's Optical Airglow in Jovian Eclipse

Abstract:

Sublimation support of Io’s sulfur dioxide atmosphere is very sensitive to small variations in its surface temperature. As Io passes into Jupiter’s shadow each orbit, its sublimation-supported atmosphere rapidly collapses, leaving volcanic outgassing as the primary mechanism sustaining Io’s thin atmosphere. Using an optical echelle spectrograph at the Apache Point Observatory (APO) 3.5m telescope, we observe Io’s atomic emissions excited solely by electron impact or ion recombination while Io is in umbra. Red line oxygen 6300Å and the sodium D doublet are Io’s brightest emissions at several kiloRayleighs (Bouchez et al. 2000). We observed these emissions falling quickly in just several minutes following Io’s ingress, with sodium airglow declining more rapidly than oxygen. Although the atomic atmosphere is not in vapor pressure equilibrium like SO₂, Na and O airglow drops with similar timescales to a decline in SO₂ column density that Tsang et al. (2016) reported post-ingress. Interpretation of this behavior warrants care since airglow emissions depend both on photochemical pathways producing neutral atoms and on the plasma conditions: electron density and temperature.

While no new species are identified, several previously unreported emissions in atomic neutrals are observed in the far-red and near-infrared spectral range. Emissions here range from 0.5 up 4 kiloRayleighs when averaged over Io’s disk, and our measurements are insufficiently sensitive to record their temporal response to ingress. Co-added exposures reveal the potassium D doublet and a second doublet of neutral sodium near 8189Å. Laboratory experiments of electrons impacting SO₂ produce O I triplets at 7774Å and 8446Å, as well as S I at 9225Å at energy thresholds of 25eV or more (Ajello et al., 2008). Since this threshold is more energetic than the bulk ~5eV population within Io’s plasma torus, detection of these three triplets offers a new tracer for the superthermal plasma population local to the Io–torus interaction region. These spectra can now identify the emissions seen in the Cassini ISS near-infrared filters as it the passed near Io (Geissler et al. 2004), and bright spots near Io’s equator that ISS imaged reaffirm our assertion that Io’s near-IR emissions can be a tracer for energetic electrons. To the best of our knowledge, this is the first time that several of these emissions have been reported in a planetary atmosphere other than the Earth’s.

1. Technique:

Ground-based observations of Io in eclipse require precise timing and non-sidereal blind tracking with high accuracy. Successful blind tracking can be confirmed by evaluating pointing errors in offsets to adjacent satellites. Quadrature geometries far from opposition optimize the geometry required to see Io ingress or egress, whilst the moon is well separated from Jupiter’s bright limb. Optical spectra are still strongly contaminated by Jupiter’s scattered light even under optimal geometry. To mitigate this issue, Jupiter spectra are smoothed, aligned, fit, and subtracted from the eclipsed data. A residual airglow spectrum from the Earth and Io remain, which can be separated by Doppler shift. In regions where telluric absorption interferes with the data, such as O I 6300Å, telluric features are characterized and removed using blue fast rotator (BFR) and A0V stars. A Gaussian is then fit to Io’s line spread functions and integrated to give total brightness. Jupiter’s reflectance spectrum is used as a “standard candle” to calibrate absolute flux, and a correction is made for the fact that Io does not fill the slit aperture, so brightness levels reported here are effectively disk-averages.

2. Temporal response of the brightest emissions

Fig. 1 and 2 show the changes Jovian scatter, which is subtracted to isolate Io’s airglow. Fig. 3 shows the temporal evolution of Io’s emission Rayleighs, where the two sodium D lines are summed. It is clear that immediately after Io enters the Jovian umbra both Na D and O6300 lines sharply decrease, with Na falling more rapidly. On the timescales permitted by this observation it remains unclear if these emissions have reached a new steady state associated with an atmosphere solely supported by volcanism.

3. Detection of far-red and near infrared O and S airglow

Io’s eclipse behind Jupiter presents an opportunity to study fainter emissions that are usually swamped by bright surface reflectance. Large cross-sections for several near-IR emissions are known from laboratory spectra of electrons smashing sulfur dioxide (Ajello et al. 2008). To search for these faint emissions, we repeated the above procedure, but co-added all the residuals, thereby averaging any temporal behavior. This indeed reveals oxygen transitions in highly excited states near 11 eV. A sulfur triplet near ~9225Å also indicates transitions from 7.9 eV to 6.5 eV which are somewhat more intense by comparison, but still require a co-addition of exposures to readily identify.

While detections of these lines are marginal individually, together, the three triplets in Fig. 4-6 indicate dissociative excitation of SO₂ produces measureable near-IR emission at Io. This in turn constrains the dissociative excitation contribution to the FUV emissions, since the gas states cascade to produce the intense O I 1356Å, 1304Å and S I 1900Å multiplets, respectively. Similarly, near-IR sodium transitions cascade to the produce the intense D lines.

References

[1] Bouchez, A.H., Brown, M.E., Schneider, N.M., 2000. Eclipse spectroscopy of Io’s Atmosphere. Icarus 148, 316–319

[2] Constantine C. C. Tsang, John R. Spencer, Emmanuel Lellouch, Miguel A. Lopez‐Valverde, Matthew J. Richter, The collapse of Io's primary atmosphere in Jupiter eclipse, Journal of Geophysical Research: Planets, 10.1002/2016JE005025, 121, 8, (1400-1410), (2016).

[3] Geissler P., McEwen A., Phillips C., 2004. Surface changes on Io during the galileo mission. Icarus 169, 29–64

[4] Joseph M. Ajello, Alejandro Aguilar, Rao S. Mangina, Geoffrey K. James, Paul Geissler, Laurence Trafton, Middle UV to near‐IR spectrum of electron‐excited SO2, Journal of Geophysical Research: Planets, 10.1029/2007JE002921, 113, E3, (2008).

[5] Schmidt, C., Moullet, A., de Kleer, K., Schneider, N., Roth, L., Spencer, J "A Multi-Wavelength Study of Io's Atomic Oxygen and Sulfur Emission", American Geophysical Union, Dec 2019