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How big is Io?

3,600 kilometers!

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Io

Io is the innermost of Jupiter's four large moons, and it is about 3,640 kilometers (2,260 miles) across. That is its mean diameter, a little more than the width of the Moon. Io is the most volcanically active world in the Solar System.

Size

NASA gives Io's mean radius as 1,821.5 kilometers, so its mean diameter is about 3,640 kilometers. The mean diameter is the width of a perfect sphere with the same volume. Io is the third-largest of Jupiter's moons, after Ganymede and Callisto.

Io's surface is not fixed in place. Io orbits about 422,000 kilometers (262,000 miles) from Jupiter, whose gravity raises tides in Io's solid ground. The surface bulges up and down by as much as 100 meters (330 feet). On Earth, the largest ocean tides rise and fall about 18 meters (60 feet), and those are in water, not rock.

Size comparisons

  • Io is about 5% wider than the Moon, and has about 15% more volume.
  • Io is a little over a quarter as wide as Earth.
  • Callisto, the outermost of Jupiter's large moons, is about 4,820 kilometers across, about a third wider than Io.
  • If Jupiter were shrunk to the size of a basketball, Io would be a bead about 6 millimeters wide, circling about 72 centimeters (28 inches) from the ball's center.
  • Loki Patera, the largest known volcanic depression on Io as of 2026, is 202 kilometers (125 miles) wide, about one-eighteenth of Io's width.

Why Io has volcanoes

Io has hundreds of volcanoes, and some throw fountains of lava dozens of kilometers high. Their heat comes from a tug-of-war. Jupiter's huge gravity pulls on Io, and so do two moons farther out, Europa and Ganymede.

The three moons move in a regular rhythm: for every orbit Ganymede makes, Europa makes two and Io makes four. Their repeated pulls keep Io's orbit stretched, so its distance from Jupiter keeps changing. The changing tides heat Io's interior, and molten rock escapes to the surface. Lava fills in craters and spreads new plains, so Io's surface is constantly renewed.

How Io and its volcanoes were found

Galileo Galilei discovered Io on January 8, 1610, along with three other moons of Jupiter. They were the first moons found orbiting a planet other than Earth. The first signs of Io's volcanoes came in 1979, when NASA's Voyager 1 spacecraft caught one erupting.

Sources

Scale of the Universe > Io
Moon
The Moon, also known as "Luna” is the brightest object in the night sky. It orbits Earth every 27 days, 7 hours and 43 minutes. I like to imagine seeing several moons scattered across the night sky. That would be amazing!
Europa
Europa is the puniest of the Galilean moons. It is covered with ice. There are mysterious streaks that cover the moon's surface. Scientists think a water ocean might exist beneath the layer of ice, possibly even holding life.
Triton
Triton is Neptune's largest moon. It is also the coldest object we've visited in the Solar System, at -240 ° C. Triton is also a very windy place, with winds going at almost the speed of sound!
United States of America
Here is the U. S. A. Although it may seem to be larger than Pluto, Pluto has a back face, which means Pluto has more surface area!
Callisto
Callisto is the most distant of the Galilean moons. It orbits outside all the other Galilean moons and has the most meteor impact craters of them all.
Mercury
Mercury is the closest planet to the Sun. It has no atmosphere. During the day, temperatures reach 430 °C, but at night, the temperatures fall to -180 °C. One Mercurian year is 88 Earth days.

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