Observations from the ground or by space telescopes have provided important contributions to understand the Io-Jupiter system, not least because they allow to cover longer time-scales of many years or even decades. The observational possibilities and sensitivity of specific observational methods is continuously improved and remote observations might be key for addressing the issue of Io’s mass loss in the future.
Almost all parts of the system can be observed in some way remotely from Earth: Io’s volcanic hot spot emission, which is monitored frequently with decent spatial resolution since the availability of Adaptive Optics (Section 2.1). The atmosphere is observed with a variety of methods at various wavelengths (Section 2.2). On the contrary, it is relatively difficult to observe gas or dust plumes remotely, with most notable observations by the Hubble Space Telescope (HST) (Spencer et al., 2000; Section 2.1) or ALMA (Section 2.2). Even the plasma-interaction can be indirectly probed from Earth through UV observations of Io’s local aurora or the moon’s footprint in Jupiter’s aurora (Section 2.4). The neutral clouds and plasma torus are observable also primarily in the UV and thus from space-based telescopes, as, for example, monitored regularly in the last decade by the Hisaki satellite (Sections 2.5 and 2.6). Visible observations from the ground (or space) are a tool to monitor not only the trace species (primarily Na) near Io or in the extended nebulae but also sulfur ion torus emissions or even neutral oxygen emissions. And lastly, Jupiter’s UV aurora is regularly imaged with HST for more than three decades now (Section 2.7).
The advances in the capabilities of telescopes, e.g., enhanced spatial resolution to resolve Io, enabled new insights as for example the recent detection of SO IR emission at 1.7 μm directly above a volcanic hot spot (de Pater et al., 2023). More frequent observations over longer times similarly provided relevant insights like the apparent stability of the atmosphere or a more complete picture of hot spot variability (Sections 2.1 and 2.2).
There are currently several ongoing programs to observe Io. Some of them are in support of the close flybys of the Juno spacecraft in December 2023 and February 2024, in particular for providing constraints on the neutral atmosphere, which cannot and will not be measured with Juno’s instrumentation.
A large program with the Hubble Space Telescope and James Webb Space Telescope (HST GO 17470, PI K. Retherford) targets different observables in the system from surface composition through solar reflection, to hot spot activity, Io’s local aurora, and to the neutral clouds and plasma torus out to radial distance of Europa (~10 RJ). A tailored program with only JWST (GO 4078) aims to map the gas distribution on the dayside through the 7.3 micron SO2 band during the Juno flyby on February 3, 2024. The 7.3 micron band was successfully detected in an earlier JWST program (1373) but the work is not yet published. These mid-IR observations will provide additional information on the hot spot activity, if successful. Another longer-term program to regularly measure Io’s sayside SO2 abundance is currently carried out with the Submillimeter Array (SMA, PI W. Tseng). The observations are similar to those published by Moullet et al. (2010) and the program targeted Io 9 times during observing seasons in 2022-2023. The PI will continue to propose it in the following years.
Efforts in ground-based monitoring of the thermal IR emissions with the Keck telescope (e.g., de Kleer et al., 2016; 2019) and of the sodium cloud and nebula (Yoneda et al., 2013; Morgenthaler et al., 2019) are continued. The increasing number of observations and thus temporal coverage on the different parts in the systems might enable further tests of correlations and connections.
Two observations could be of particular interest: One is a sensitive observation of the SO2 atmosphere (density and also temperature) right at the onset of an increase in emissions from the neutral clouds. If Io triggers the transient event through an enhancement in the mass loss, the atmosphere should undergo some considerable change at least around the starting time of the enhancement in the neutral cloud. The other one would be an observation of the escaping molecular species from Io through e.g., spatially resolving exospheric layers, which is yet extremely challenging. None of the available telescope facilities and previously applied methods for the bulk SO2 atmosphere (from UV with HST, to IR from ground or now JWST, and sub-mm interferometry) provide the sensitivity to detect the expected SO2 abundances of the escaping population or in the neutral clouds.
Future telescopes – planned or under construction – might provide capabilities to address some aspects like direct measurements of escaping neutral gases. The currently constructed Extremely Large Telescope (ELT) with its ~40-m primary mirror has a nominal spatial resolution of 5 μarcsec, which corresponds to ~20 km on Io or ~200 pixels across Io’s diameter. With state-of-the-art high-resolution spectrographs it might provide high sensitivity for accurate SO2 measurements and thermal emissions (and mapping) at infrared wavelengths.
LAPYUTA (Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly) is a future UV space telescope, which is selected as a candidate for JAXA’s 6th M-class mission in 2023. Launch is planned for the early 2030s. LAPYUTA will perform spectroscopic and imaging observations in the far ultraviolet spectral range (110-190 nm) with a large effective area (>300 cm2 ) and a high spatial resolution (0.1 arcsec). LAPYUTA will have capabilities to monitor mass loss from Io’s SO2 atmosphere to Io’ neutral cloud and plasma torus as well as their effects on the magnetospheric dynamics, similar to but enhancing the successful observations of Hisaki (Section 2.6).
Powerful space telescopes in planning include the concept of the Habitable Worlds Observatory (HWO) for observations from UV to infrared wavelengths as part of the Great Observatory Maturation Program (GOMAP) recommendation of the Pathways to Discovery in Astronomy and Astrophysics for the 2020s (Astro2020) Astrophysics Decadal Survey (see also Large Ultraviolet Optical Infrared Surveyor final report, 2019). With currently discussed mirror diameters of 8 m or 15 m and being located at Lagrange Point L2 (continuous view and unaffected by the geocorona), such a space telescope would increase the sensitivity and spatial resolution in the UV as compared to HST by more than an order of magnitude and a factor of 3-6, respectively. Other telescopes built or planned by different agencies and organizations like the Giant Magellan Telescope (GMT) or the Thirty Meter Telescope (TMT) might also allow useful observational advances.
Authors:
(1) L. Roth, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and a Corresponding author;
(2) A. Blöcker, KTH Royal Institute of Technology, Space and Plasma Physics, Stockholm, Sweden and Department of Earth and Environmental Sciences, Ludwig Maximilian University of Munich, Munich, Germany;
(3) K. de Kleer, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA;
(4) D. Goldstein, Dept. Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin, Austin, TX USA;
(5) E. Lellouch, Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, Meudon, France;
(6) J. Saur, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany;
(7) C. Schmidt, Center for Space Physics, Boston University, Boston, MA, USA;
(8) D.F. Strobel, Departments of Earth & Planetary Science and Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA;
(9) C. Tao, National Institute of Information and Communications Technology, Koganei, Japan;
(10) F. Tsuchiya, Graduate School of Science, Tohoku University, Sendai, Japan;
(11) V. Dols, Institute for Space Astrophysics and Planetology, National Institute for Astrophysics, Italy;
(12) H. Huybrighs, School of Cosmic Physics, DIAS Dunsink Observatory, Dublin Institute for Advanced Studies, Dublin 15, Ireland, Space and Planetary Science Center, Khalifa University, Abu Dhabi, UAE and Department of Mathematics, Khalifa University, Abu Dhabi, UAE;
(13) A. Mura, XX;
(14) J. R. Szalay, Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA;
(15) S. V. Badman, Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK;
(16) I. de Pater, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA;
(17) A.-C. Dott, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany;
(18) M. Kagitani, Graduate School of Science, Tohoku University, Sendai, Japan;
(19) L. Klaiber, Physics Institute, University of Bern, 3012 Bern, Switzerland;
(20) R. Koga, Department of Earth and Planetary Sciences, Nagoya University, Nagoya, Aichi 464-8601, Japan;
(21) A. McEwen, Department of Astronomy and Department of Earth & Planetary Science, University of California, Berkeley, CA 94720, USA;
(22) Z. Milby, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 USA;
(23) K.D. Retherford, Southwest Research Institute, San Antonio, TX, USA and University of Texas at San Antonio, San Antonio, Texas, USA;
(24) S. Schlegel, Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany;
(25) N. Thomas, Physics Institute, University of Bern, 3012 Bern, Switzerland;
(26) W.L. Tseng, Department of Earth Sciences, National Taiwan Normal University, Taiwan;
(27) A. Vorburger, Physics Institute, University of Bern, 3012 Bern, Switzerland.
