# Atmosphere of Io
The atmosphere of Io is the extremely tenuous envelope of gas, dominated by sulfur dioxide (SO₂), that surrounds Jupiter's innermost large moon. Its surface pressure is only 3 to 40 nbar, a few billionths of Earth's atmospheric pressure, and it is concentrated within about 40 degrees of the equator[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485). The gas is supplied by two processes, sublimation of SO₂ frost from the surface and direct outgassing from Io's active volcanoes, and it collapses and reforms on a daily cycle as Jupiter's shadow alternately chills and warms the surface[2](https://www.nasa.gov/solar-system/new-research-reveals-fluctuating-atmosphere-of-jupiters-volcanic-moon/). Material escaping from the atmosphere, about one ton per second, feeds the ring of plasma around Jupiter known as the Io plasma torus[3](https://science.nasa.gov/missions/hubble/nasas-hubble-space-telescope-explores-the-volcanic-moon-io/).
| Key fact | Detail |
|---|---|
| Main gas | Sulfur dioxide, with sulfur monoxide (SO), potassium chloride (KCl), and atomic sulfur, oxygen, sodium, potassium, and chlorine as minor components[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485) • [4](https://www.almaobservatory.org/en/press-releases/alma-shows-volcanic-impact-on-ios-atmosphere/) |
| Surface pressure | 3 to 40 nbar, collisionally thick, concentrated within ±40° of the equator[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485) |
| Column density | Equatorial vertical column of 1.5 × 1016 to 15 × 1016 cm−2; density above the poles is about 2% of the equatorial value[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485) |
| Sources | Frost sublimation dominates; active volcanoes directly produce 30–50% of the atmosphere[4](https://www.almaobservatory.org/en/press-releases/alma-shows-volcanic-impact-on-ios-atmosphere/) |
| Daily cycle | The atmosphere collapses in Jupiter's shadow as SO₂ freezes onto the surface and is restored by sublimation within about 10 minutes of returning to sunlight[2](https://www.nasa.gov/solar-system/new-research-reveals-fluctuating-atmosphere-of-jupiters-volcanic-moon/) • [5](https://arxiv.org/abs/2009.07729) |
| Ionosphere | Pioneer 10 found a peak electron density of about 60,000 cm−3 near 100 km altitude; Galileo occultations in 1997 found peaks up to about 250,000 cm−3 • [1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485) |
| Escape | About one ton of material per second leaves Io to supply the plasma torus[3](https://science.nasa.gov/missions/hubble/nasas-hubble-space-telescope-explores-the-volcanic-moon-io/) |
## Discovery and early measurements
The first evidence came from radio rather than direct gas detection. [The Pioneer](https://www.edgepedia.org/the-pioneer) 10 S-band occultation experiment in 1973–1974 revealed an ionosphere on Io with an electron density peak of about 6 × 104 cm−3 at an altitude of roughly 60 to 140 km, implying a surface number density of about 1010 to 1012 cm−3 and a surface pressure between 10−8 and 10−10 bar[6](https://pubmed.ncbi.nlm.nih.gov/17821099/). From that point Io was known to possess a tenuous atmosphere[7](https://www.nature.com/articles/346639a0).
The gas itself was identified in 1979, when the Voyager 1 Infrared Interferometric Spectrometer (IRIS) detected gaseous sulfur dioxide associated with a major volcanic plume near Loki. The detection worked only because the underlying surface was warm enough, above 210 K, to be observed near 7 µm, and the estimated local column abundance was about 0.2 cm atm (column abundance: gas layer thickness at standard pressure)[8](https://www.nature.com/articles/285308a0). A model of an SO₂ atmosphere in equilibrium with surface frost predicted a much smaller global average column of 0.032 cm atm, and ultraviolet reflectivity measurements from Earth orbit showed the hemispheric average was far below the local value near Loki, indicating a strongly non-uniform atmosphere[8](https://www.nature.com/articles/285308a0). The first microwave detection of SO₂ followed in 1992, and reanalysis of the Voyager IRIS data with a non-local-thermodynamic-equilibrium transfer model gave temperature and pressure conditions at Loki consistent with the millimeter-wave results[7](https://www.nature.com/articles/346639a0) • [9](https://www.sciencedirect.com/science/article/pii/001910359290095O).
## Composition and structure
[Sulfur dioxide](https://www.edgepedia.org/sulfur-dioxide) dominates because it is both vented by the volcanoes and frozen as frost across the surface, the bright white patches seen in visible images[3](https://science.nasa.gov/missions/hubble/nasas-hubble-space-telescope-explores-the-volcanic-moon-io/). Sulfur monoxide is a significant minor species, and its spatial distribution, whether global or localized, remained uncertain in the major review of the mid-1990s[10](https://www.sciencedirect.com/science/article/pii/S0019103596901864). ALMA observations resolved plumes of SO₂ and SO rising from volcanoes for the first time and also detected potassium chloride (KCl) over volcanic regions where neither SO₂ nor SO was seen, evidence that the magma reservoirs beneath different volcanoes differ in composition[4](https://www.almaobservatory.org/en/press-releases/alma-shows-volcanic-impact-on-ios-atmosphere/). [Sodium chloride](https://www.edgepedia.org/sodium-chloride) and potassium chloride gases have also been used to measure chlorine isotopes, and submillimeter spectra of SO₂ and SO provided the sulfur isotope measurements that indicate long-lived volcanism[11](https://ntrs.nasa.gov/api/citations/20240005851/downloads/MandtIsotopicSTI.pdf).
Structurally, the atmosphere is best described as collisionally thick, with number densities of 1011 to 1012 cm−3 and pressures of 3 to 40 nbar, but covering only a limited fraction, 5 to 20 percent, of Io's surface[9](https://www.sciencedirect.com/science/article/pii/001910359290095O). [Hubble Space Telescope](https://www.edgepedia.org/hubble-space-telescope) spectroscopy of sulfur and oxygen emissions showed the emitting atmosphere is patchy, with dense regions having about one thousand times the pressure of adjoining low-density regions[3](https://science.nasa.gov/missions/hubble/nasas-hubble-space-telescope-explores-the-volcanic-moon-io/). The gas is strongly tied to latitude and longitude: the vertical column at the equator ranges from 1.5 × 1016 cm−2 at sub-Jovian longitudes to 15 × 1016 cm−2 at anti-Jovian longitudes, and the density above the poles is only about 2% of the equatorial value, a distribution attributed mainly to the equatorial concentration of surface frost and to low-latitude volcanoes[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485).
## Sublimation versus volcanism
Three mechanisms have been proposed for generating the atmosphere: sublimation of SO₂ frost, sputtering of the surface by impinging particles, and active volcanism. Both sublimation and volcanism clearly contribute, while sputtering matters only if atmospheric densities drop to very low values[12](https://www.lpi.usra.edu/science/moses/io.shtml). The two processes can be separated because sublimation depends on sunlight and volcanism does not[13](https://arxiv.org/abs/2405.19253).
**Eclipse collapse.** When Io passes into Jupiter's shadow, the surface cools from about −235 °F in sunlight to about −270 °F, and most of the SO₂ gas freezes onto the surface as frost; the atmosphere redevelops when the moon returns to sunlight. Eclipse occupies about two hours of each Io day of 1.7 Earth days. John Spencer, a co-author of the Gemini North/TEXES study at the [Southwest Research Institute](https://www.edgepedia.org/southwest-research-institute), described the result as an atmosphere in a constant state of collapse and repair, with a large fraction supported by sublimation of SO₂ ice[2](https://www.nasa.gov/solar-system/new-research-reveals-fluctuating-atmosphere-of-jupiters-volcanic-moon/). Ultraviolet measurements found the atmospheric column drops by a factor of 5 ± 2 after 40 minutes of eclipse[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485).
**The volcanic share.** ALMA snapshots taken as Io entered and left eclipse showed that during eclipse only volcanically sourced SO₂ remains visible. Comparing in-sunlight and in-eclipse fluxes, disk-integrated SO₂ in sunlight is about 2 to 3 times the in-eclipse value, implying that active volcanoes directly produce 30 to 50 percent of the atmosphere[4](https://www.almaobservatory.org/en/press-releases/alma-shows-volcanic-impact-on-ios-atmosphere/) • [5](https://arxiv.org/abs/2009.07729). After sunlight returns, the atmosphere reforms linearly within about 10 minutes, and the low-level SO₂ still present during eclipse may come from stealth volcanism, volcanic gas that reaches the atmosphere without a visible plume, or from a layer of non-condensible gases preventing complete collapse[5](https://arxiv.org/abs/2009.07729). Atmospheric observations indicate a dominant sublimation-supported component and reinforce the presence of stealth volcanism[14](https://www.annualreviews.org/doi/10.1146/annurev-earth-082420-095244).
**Seasonal and hemispheric patterns.** Twenty-two years of mid-infrared observations with the TEXES spectrograph at NASA's Infrared Telescope Facility, spanning nearly two Jovian years, show that the anti-Jovian atmospheric density follows a repeating seasonal cycle, with a minimum near aphelion in February 2017 and a maximum near perihelion in January 2023. The pattern is modeled as a seasonally varying frost-sublimation component plus a constant component assumed to be volcanic. The SO₂ column on the Jupiter-facing hemisphere is 5 to 8 times lower than on the anti-Jovian hemisphere, plausibly because daily Jupiter eclipses cool the surface there and reduce the sublimation-driven component[13](https://arxiv.org/abs/2405.19253).
## The ionosphere and the plasma torus
Because the atmosphere is immersed in Jupiter's magnetosphere, its interaction with plasma shapes both the moon and its surroundings. Radio occultations by [Pioneer 10](https://www.edgepedia.org/pioneer-10) and Galileo supported the existence of a substantial atmosphere: Pioneer 10 revealed a dense dayside ionosphere with a peak density of about 60,000 cm−3 near 100 km altitude, comparable to the ionospheres of Mars and Venus, while the nightside ionosphere appeared less dense[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485). Galileo performed six occultations in 1997, finding peak densities of at least 50,000 cm−3 and a maximum of about 250,000 cm−3 in one occultation, with the plasma density depending strongly on longitude[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485). During its close flybys of Io in December 2023 and February 2024, the Juno spacecraft measured electron densities of 20,500 to 27,000 cm−3 and 15,300 to 31,000 cm−3 in the Alfvén wing, the magnetic structure connecting Io to Jupiter[15](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GL111568).
The atmosphere is also the intermediary for Io's mass loss. Sulfur from the volcanic plumes cannot escape directly into space; instead, sulfur and oxygen are stripped from the atmosphere in the interaction with the plasma torus, at a rate of about one ton per second[3](https://science.nasa.gov/missions/hubble/nasas-hubble-space-telescope-explores-the-volcanic-moon-io/). Modeling of the [Voyager 1](https://www.edgepedia.org/voyager-1) torus observations found that an SO₂ atmosphere is most likely needed as an intermediate buffer to transport sulfur and oxygen to the torus, with a minimum atmosphere of 1.4 × 10−10 bar, an SO₂ column of about 1016 cm−2, and a surface density of about 1.2 × 1010 cm−3 • [16](https://ntrs.nasa.gov/citations/19840065473). Juno's JEDI instrument also detected energetic neutral atoms of oxygen and sulfur above 150 keV coming from Io's mostly nightside atmosphere during the 30 December 2023 encounter, a new way of diagnosing the extended atmosphere and corona of SO₂, SO, O, and S[17](https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL112668).
## Open quantitative limits
Even after decades of study, several basic quantities remain loosely bounded. A mid-1990s review noted that the surface pressure and areal extent of the atmosphere were known only to within factors of about 10 and 5 respectively, the vertical structure was extremely poorly characterized, and it was not known for certain whether the atmosphere is in hydrostatic equilibrium or rapidly ejected by volcanic venting[10](https://www.sciencedirect.com/science/article/pii/S0019103596901864). A 2020 review similarly states that Io's atmosphere is still surprisingly poorly known, with properties deduced from ground-based, Earth-orbit, and spacecraft observations together with plasma modeling[1](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485). The relative roles of sublimation and volcanism are likewise not yet fully understood, although the evidence for significant sublimation support is now very strong[13](https://arxiv.org/abs/2405.19253).
## References
1. [F. Bagenal et al. (2020). The Space Environment of Io and Europa. Journal of Geophysical Research: Space Physics.](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JA027485)
2. [New Research Reveals Fluctuating Atmosphere of Jupiter's Volcanic Moon, NASA](https://www.nasa.gov/solar-system/new-research-reveals-fluctuating-atmosphere-of-jupiters-volcanic-moon/)
3. [NASA's Hubble Space Telescope Explores the Volcanic Moon Io, NASA Science](https://science.nasa.gov/missions/hubble/nasas-hubble-space-telescope-explores-the-volcanic-moon-io/)
4. [ALMA Shows Volcanic Impact on Io's Atmosphere, ALMA Observatory](https://www.almaobservatory.org/en/press-releases/alma-shows-volcanic-impact-on-ios-atmosphere/)
5. [I. de Pater et al. (2020). ALMA Observations of Io Going into and Coming out of Eclipse. arXiv.](https://arxiv.org/abs/2009.07729)
6. [Preliminary results on the atmospheres of Io and Jupiter from the Pioneer 10 S-band occultation experiment, PubMed](https://pubmed.ncbi.nlm.nih.gov/17821099/)
7. [Io's atmosphere from microwave detection of SO2, Nature](https://www.nature.com/articles/346639a0)
8. [An upper limit to the global SO2 abundance on Io, Nature](https://www.nature.com/articles/285308a0)
9. [The structure, stability, and global distribution of Io's atmosphere, Icarus](https://www.sciencedirect.com/science/article/pii/001910359290095O)
10. [Urey Prize Lecture. Io's Atmosphere: Not Yet Understood, Icarus](https://www.sciencedirect.com/science/article/pii/S0019103596901864)
11. [Isotopic evidence of long-lived volcanism on Io, NASA NTRS](https://ntrs.nasa.gov/api/citations/20240005851/downloads/MandtIsotopicSTI.pdf)
12. [Io's Atmosphere: Direct Volcanic Outgassing, Lunar and Planetary Institute (J. Moses)](https://www.lpi.usra.edu/science/moses/io.shtml)
13. [Seasonal and longitudinal variability in Io's SO2 atmosphere from 22 years of IRTF/TEXES observations, arXiv](https://arxiv.org/abs/2405.19253)
14. [A 2020 Observational Perspective of Io, Annual Review of Earth and Planetary Sciences](https://www.annualreviews.org/doi/10.1146/annurev-earth-082420-095244)
15. [D. Buccino et al. (2025). Electron Density in Io's Alfvén Wing Observed Via Radio Occultation With Juno. Geophysical Research Letters.](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GL111568)
16. [S. Kumar (1984). Sulfur and oxygen escape from Io and a lower limit to atmospheric SO2 at Voyager 1 encounter. NASA NTRS.](https://ntrs.nasa.gov/citations/19840065473)
17. [B. Mauk et al. (2025). Diagnosing the Atmosphere of Jupiter's Volcanic Moon Io Using Energetic Neutral Atoms. Geophysical Research Letters.](https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL112668)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Solar System › Solar System phenomena and dynamics › Transits and occultations › Occultations › Occultations by planets and satellites*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license