Atmosphere of Io

SO₂ is the main constituent, comprising 90% of the atmospheric pressure. About 3%–10% is SO. The atmospheric pressure varies from 0.033 to 0.3 mPa or 0.33 to 3 nbar,^{[disagrees with figures above]} seen on Io's anti-Jupiter hemisphere and along the equator, and temporally in the early afternoon when the temperature of surface frost peaks. On the night side,^{[clarification needed]} SO₂ freezes, decreasing the atmospheric pressure to 0.1 × 10⁻⁷ to 1 × 10⁻⁷ Pa (0.0001 to 0.001 nbar).^[4] Some studies suggest that the night side atmosphere consists of non-condensable gases like atomic O and SO. The atmosphere on the side facing away from Jupiter is not just denser but also extends over a greater range of latitudes than the side facing Jupiter. The vertical column density at the equator ranges from 1.5 × 10¹⁶ cm⁻² at sub-Jovian longitudes to 15 × 10¹⁶ cm⁻² at anti-Jovian longitudes.^[5]

On the surface, sulfur dioxide is in vapor pressure equilibrium with frost. The temperatures increases to 1,800 K at higher altitudes where the lower atmospheric density permits heating from plasma in the Io plasma torus and from Joule heating from the Io flux tube. The day-side atmosphere is mostly confined to within 40° of the equator, where the surface is warmest and most active volcanic plumes are found.^[6] The polar atmospheric pressure is only 2% of the equatorial atmospheric pressure. At about ±40° latitude, the atmospheric pressure will be half of that at the equator. The atmospheric density increases the closer Io gets to the Sun.^[7]

Farther away from the surface, higher the concentration of O and S₂^{[clarification needed]} gets. This is because of the lower mass of oxygen and sulphur atoms compared to others. The O/SO₂ ratio is estimated to be between 10% and 20% in the upper atmosphere. These gases exist up to a distance of 10 times the radius of Io.

Io has a sodium tail similar to the sodium tail of the Moon. Io also has an ionosphere with a density of 2.8 × 10¹⁰ m⁻³ at 80 km altitude, comparable to the ionospheres of Mars and Venus. Occultation studies by Pioneer 10 revealed that the night-side ionosphere is significantly less dense for the first time.^{[clarification needed]} Based on the six occultations conducted by the Galileo probe in 1997, the ionosphere is asymmetrical: the plasma density varies by longitude. The interpretation of the observations assumes that the increased plasma density is distributed in a spherically symmetrical bound ionosphere with a dense downstream wake. Depending on the location, peak densities of about 5 × 10¹⁰ m⁻³ were found, reaching a maximum of about 2.5 × 10¹¹ m⁻³ in one of the occultations.

Due to its thinness, Io's atmosphere does not cause that much effect on the surface, other than moving SO₂ ice around and expanding the size of plume deposit rings when plume material re-enters the denser dayside atmosphere. Every second, almost one tonne of gases escape from Io's atmosphere into outer space due to Jupiter's magnetosphere. Due to this,^{[clarification needed]} the atmosphere should be constantly replenished. These gases orbit Jupiter along with Io, creating a Io plasma torus.

Io's atmospheric density is directly related to surface temperature. When Io falls into the shadow of Jupiter during an eclipse, the temperature falls. This causes deposition of the SO₂, and results in an 80% decrease in the atmospheric pressure.^[8] This increases the albedo of Io; thus Io appears brighter when covered with frost immediately after an eclipse. After about 15 minutes the brightness returns to normal, presumably because the frost has disappeared through sublimation. Post-ecliptic brightening can be observed with ground telescopes. Cassini spacecraft captured post-eclipse brightening in near-infrared wavelengths.^[9] Further evidence for this theory came in 2013 when the Gemini Observatory was used to directly measure the collapse of Io's SO₂ atmosphere during, and its reformation after, eclipse by Jupiter.^[10]

Io hosts aurora events, even though the atmosphere is extremely thin. Unlike other celestial bodies where an aurora occurs at the North and South poles, aurora on Io occurs near the equator. This is because aurorae on other bodies are caused by the interactions of the body's magnetosphere with the solar wind. In contrast, Io has no magnetic field of its own. Instead of solar wind, charged particles from Jupiter's magnetosphere interact with Io's atmosphere, creating aurora.^[11]

Sodium atoms cause a green glow in the aurora. Here blue glows caused by SO₂ are nearer to the surface than red glows caused by oxygen. This is because SO₂ is heavier than oxygen, and as a result will be more gravitationally bound to the surface. Due to this, red glows reach up to a height of 900 km (560 miles). The aurora moves across Io, as it changes its orientation with respect to Jupiter's magnetosphere as it orbits the planet.