The Cassini spacecraft did a flyby of Jupiter on its way to Saturn. During that pass by the gas giant, it took this picture of the moon Io. The planet and the moon appear deceptively close in this image, but Io is orbiting 217,000 miles above the planet.
In Greek mythology Io was a priestess of Hera (Zeus’ wife) and a nymph who was seduced by Zeus. He changed her into a heifer to escape detection. Io is also the name of the innermost of the four Galilean moons of the planet Jupiter. The most volcanic body in the Solar System, Io is 3,600 kilometers in diameter, about the size of planet Earth’s moon.
While cruising past Jupiter at the turn of the millennium, the Cassini spacecraft captured this view of Io with Jupiter as a backdrop–offering an impressive demonstration of the ruling planet’s relative size. (An astronomer from another star system would probably describe our solar system as having one main planet and assorted debris.) Although Io appears to be located just in front of the swirling Jovian clouds, Io is about 350,000 km above Jupiter’s cloud tops. That’s roughly the same as the distance between Earth and Moon. The Cassini spacecraft itself was about 10 million km from Jupiter when this picture was taken.
Jupiter’s moon Io is the most volcanically active world in the solar system,with hundreds of erupting volcanoes blasting lava up to 400 km high. While the New Horizons spacecraft was flying by Jupiter for a gravity assist on its way to Pluto, it took these pictures of an eruption on Io.
On 24 January, the Hubble Space Telescope captured the rare occurrence of three of Jupiter’s largest moons moving across the face of the gas-giant planet: Europa, Callisto, and Io.
They are three of the four Galilean moons, named after the 17th-century scientist Galileo Galilei who discovered them with a small telescope. Their orbital periods around Jupiter range from 2 days to 17 days. The Galilean moons can often be seen transiting the face of Jupiter and casting shadows onto its cloud tops., but seeing three of them transiting the face of Jupiter at the same time is rare, occurring only once every five or ten years.
On the left, the moons Callisto and Io are above Jupiter’s cloud tops. The shadows from Europa, Callisto, and Io are strung out from left to right. Europa is not visible in this image. Approximately 42 minutes later (right-side image), Europa has entered the frame at lower left. Slower-moving Callisto is above and to the right of Europa. Fastest-moving Io is approaching the eastern limb of the planet; its shadow is no longer visible on Jupiter. Europa’s shadow is toward the left side of the image, and Callisto’s shadow to the right. The moons’ orbital velocities are proportionally slower with increasing distance from the planet. Ganymede, the other Galilean moon, was outside Hubble‘s field of view and too far from Jupiter to be part of this conjunction.
Each of these moons has a distinctive color. The cratered surface of Callisto is brown; the smooth icy surface of Europa is yellow-white; and the volcanic, sulfur-dioxide surface of Io is orange. The fuzziness of each depends on each moon’s distance from Jupiter. The farther away the moon, the softer the shadow because it is more spread out across the disk.
Jupiter’s moon Io is one of the weirdest in the Solar System. It’s bright yellow, and this picture is an attempt to show how Io would appear to the average human eye. Io’s colors derive from sulfur and molten silicate rock. The moon’s is constantly being refreshed by a system of active volcanoes. Tides caused by Jupiter’s gravity stretch Io, and the resulting friction greatly heats Io’s interior, causing molten rock to explode through the surface. Io’s volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io’s volcanic lava is so hot it glows in the dark.
This false color image shows Jupiter’s aurorae., The northern ring (yellow/orange) and the polar haze (blue) are particularly prominent. The white dot on the right is the moon Io. That’s its shadow is on the right edge of the planet.
Jupiter’s moon Io’s surface is constantly under construction. Io holds the distinction of being the Solar System’s most volcanically active body; its weird surface is continuously remade by lava flows. This high resolution composite picture was put together from images taken by the Galileo spacecraft back in 1996. It shows the side of Io that always faces away from Jupiter. The picture has been processed to bring out the moon’s surface brightness and color variations and shows details small as 2.5 km across. There aren’t many impact craters on Io which suggests that the entire surface gets covered with new volcanic deposits more rapidly than craters are created. The likely energy source for the vulcanism is the changing gravitational tides caused by Jupiter and the other three Galilean moons (Callisto, Ganymede, and Europa) as Io orbits the massive planet. The tides would heat the interior of the moon and generate its sulfurous volcanic activity.
Loki Patera is the largest volcanic depression on Jupiter’s moon Io, 202 km in diameter. It contains an active lava lake, with an episodically overturning crust. Voyager 1 took this composite picture of Io showing an active plume from Loki on the moon’s limb. The images that make up this mosaic were taken from an average distance of approximately 490,000 km.
A plume rises from a volcano over 300 km above Jupiter’s moon Io in this image from the New Horizons spacecraft. The volcano Tvashtar is marked by the bright glow (about 1 o’clock) at the moon’s edge, beyond the day/night shadow line. The shadow of Io cuts across the plume itself. The image was recorded when the spacecraft was 2.3 million kilometers from Io. The New Horizons spacecraft is to encounter Pluto in 2015.