Io

Identity and discovery

Io is one of the four Galilean moons. NASA dates its separate identification from Europa to 8 January 1610, following Galileo’s observation the previous day. Its NAIF code is 501; interactive catalogue keys are internal identifiers, not scientific designations.

Size and measurements

JPL’s physical table gives a mean radius of 1,821.49 ± 0.50 km and a mean density of 3.5276 ± 0.0029 g/cm³. The radius describes a sphere of equivalent volume. JPL lists GM = 5,959.91547 ± 0.00135 km³/s²: this is the gravitational parameter, not a mass in kilograms. Density is calculated from volume and the mass derived from GM.

Orbit and heating

JPL’s JUP365 mean solution lists a semimajor axis of 421,800 km and a period of 1.762732 days, in the Laplace plane at epoch 2000-01-01.5 TDB. These are mean elements, not a current position or precise ephemerides. NASA uses an approximate distance of 422,000 km and explains that Io keeps the same side facing Jupiter. Perturbations from Europa and Ganymede maintain a noncircular orbit; tidal deformation generates interior heat.

Surface and atmosphere

NASA describes hundreds of volcanoes, some producing lava fountains tens of kilometres high. Erupted material renews the surface and covers craters. Sulfur dioxide is the main constituent of its thin atmosphere. Volcanism and intense radiation make life as we know it unlikely; this is not an experimental detection or exclusion of every possible form of life.

Interaction with Jupiter

Material originating from Io becomes ionized in the Jovian magnetic environment and forms a plasma torus. Some ions follow field lines toward Jupiter and contribute to its auroras. This concerns Jupiter’s field and magnetosphere; it is not presented here as proof of Io’s own magnetic field.

Observation and limitations

The NASA page includes a Juno image from 30 December 2023. An image documents the appearance during that encounter but does not by itself establish the complete internal structure. JPL quantities and NASA descriptions retain their attribution: approximate figures do not become precision measurements, and the historical orbital solution is not a prediction of today’s position.