The Solar System Wiki

Io spacepedia.png
Name of satellite Io
Planet of origin Jupiter
Discovered by Galileo Galilei
Date of discovery January 8, 1610
Surface color Light yellow
Mass 8.93 x 10^22 kg
Apogee 2.65 million miles
4.26 million km
Perigee 2.63 million miles
4.2 million km
Apoapsis 2.65 million miles
4.26 million km
Alternate name(s) Jupiter I
Named geographical features Tvashtar, Tohil Mons, Pele, Euboea Montes, Haemus Mons, Prometheus

Io, also known is the innermost of the Galilean moons belonging to the outer planet of Jupiter. It is also the 4th largest moon in the solar system. Io is a moon of active volcanoes and lava flowing all around the surface of the planet. Io constantly has erupting volcanoes. In fact, Io is the most active object in the Solar System because of its volcanoes. In fact, each black spot on the picture you see is a volcano. This moon was discovered by the astronomer Galileo Galilei on January 8, 1610. It is the fourth-largest natural satellite.


Io is full of active volcanoes. Some of these volcanoes are dormant, or extinct volcanoes. The active volcanoes are erupting constantly and can fire ash and cinders 300 km above the surrounding surface. One of these active volcanoes is named Prometheus, not to be confused with the moon of Saturn with the same name.

Mountains are also populous around the planet's surface as well. In fact, Io is home to more than one hundred fifty mountains. One of those mountains includes Tohil Mons, meaning "Mount Tohil". This mountain reaches about five to six kilometres in height. (3.1 miles)


The atmosphere of Io consists of mostly making sulfur dioxide, making up about 95% of the atmosphere. The rest is other carbon-based gases such as carbon dioxide.


As astronomers do not know the exact formation of Io, astronomers do believe Io was a stray meteor located in the nearby Asteroid belt. In this belt, Io was not an asteroid of unknown classification, yet it did orbit the Sun like the rest of the asteroids. As it began to attract different asteroids, heat fused the asteroids together, giving more mass to Io, which is why Io maintains hydrostatic equilibrium. Once it reached this classification, Io drifted off into space and was captured by Jupiter's gravitational pull.