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Emission from Volcanic SO Gas on Io at High Spectral Resolution

de Kleer, Katherine and de Pater, Imke and Ádámkovics, Máté (2019) Emission from Volcanic SO Gas on Io at High Spectral Resolution. Icarus, 317 . pp. 104-120. ISSN 0019-1035.

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Jupiter’s moon Io hosts a dynamic atmosphere that is continually stripped off and replenished through frost sublimation and volcanic outgassing. We observed an emission band at 1.707 µm thought to be produced by hot SO molecules directly ejected from a volcanic vent; the observations were made with the NIRSPEC instrument on the Keck II telescope while Io was in eclipse by Jupiter on three nights in 2012–2016, and included two observations with 10 × higher spectral resolution than all prior observations of this band. These high-resolution spectra permit more complex and realistic modeling, and reveal a contribution to the SO emission from gas reservoirs at both high and low rotational temperatures. The scenario preferred by de Pater et al. (2002) for the source of the SO gas – direct volcanic emission of SO in the excited state – is consistent with these two temperature components if the local gas density is high enough that rotational energy can be lost collisionally before the excited electronic state spontaneously decays. Under this scenario, the required bulk atmospheric gas density and surface pressure are n ∼ 10^(11) cm^(−3) and 1–3 nbar, consistent with observations and modeling of Io’s dayside atmosphere at altitudes below 10 km (Lellouch et al., 2007; Walker et al., 2010). These densities and pressures would be too high for the nightside density if the atmospheric density drops by an order of magnitude or more at night (as predicted by sublimation-supported models), but recent results have shown a drop in SO_2 gas density of only a factor of 5  ±  2 (Tsang et al., 2016). While our observations taken immediately post-ingress and pre-egress (on different dates) prefer models with only a factor of 1.5 change in gas density, a factor of 5 change is still well within uncertainties. In addition, our derived gas densities are for the total bulk atmosphere, while Tsang et al. (2016) specifically measured SO_2. The low-temperature gas component is warmer for observations in the first 20 min of eclipse (in Dec 2015) than after Io had been in shadow for 1.5 h (in May 2016), suggesting cooling of the atmosphere during eclipse. However, individual spectra during the first  ∼ 30 min of eclipse do not show a systematic cooling, indicating that such a cooling would have to take place on a longer timescale than the  ∼ 10 min for cooling of the surface (Tsang et al., 2016). Excess emission is consistently observed at 1.69 µm, which cannot be matched by two-temperature gas models but can be matched by models that over-populate high rotational states. However, a detailed assessment of disequilibrium conditions will require high-resolution spectra that cover both the center of the band and the wing at 1.69 µm. Finally, a comparison of the total band strengths observed across eight dates from 1999 to 2016 reveals no significant dependence on thermal hot spot activity (including Loki Patera), on the time since Io has been in shadow, nor on the phase of Io’s orbit at the time of observation.

Item Type:Article
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de Kleer, Katherine0000-0002-9068-3428
de Pater, Imke0000-0002-4278-3168
Additional Information:© 2018 Elsevier Inc. Received 10 February 2018, Revised 12 June 2018, Accepted 13 July 2018, Available online 17 July 2018. Supplementary material associated with this article can be found, in the online version, at 10.1016/j.combustflame.2015.11.022. K. de Kleer is supported by the Heising-Simons Foundation 51 Pegasi b postdoctoral fellowship; this work was also partially supported by the National Science Foundation grant AST-1313485 to UC Berkeley. This work made use of the JPL Solar System Dynamics high-precision ephemerides through the HORIZONS system. Data were obtained with the W.M. Keck Observatory, which is operated by the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Data were also obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil). The authors extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented would have been possible.
Funding AgencyGrant Number
Heising-Simons FoundationUNSPECIFIED
W. M. Keck FoundationUNSPECIFIED
Subject Keywords:Io; Volcanism; Satellites; Atmospheres; Spectroscopy; Infrared Observations
Record Number:CaltechAUTHORS:20180717-143037489
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Official Citation:Katherine de Kleer, Imke de Pater, Máté Ádámkovics, Emission from volcanic SO gas on Io at high spectral resolution, Icarus, Volume 317, 2019, Pages 104-120, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:87933
Deposited By: Tony Diaz
Deposited On:17 Jul 2018 21:39
Last Modified:13 Aug 2018 17:25

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