Io, a satellite of Jupiter, is the most geologically active body in the solar system. Its spewing volcanoes were first sighted by Voyager 2 in 1979, but some scientists had earlier predicted such activity. They theorized that tides raised in Io by the nearness of Jupiter would produce so much frictional heat that Io would be partially molten and volcanically active. The satellite has an orangish, sulfurous crust with white and dark markings. Bright red patches are observed near its poles and in young deposits of volcanic ejecta, while near the equator the surface is yellow with sulfur deposits, or greenish due to unknown contaminants. Io's several volcanoes send dust and gas high above the surface, leaving a trail of ions and moleculesÑespecially of sodium, sulfur, and oxygenÑin the satellite's orbit. Calderas and flows of molten material are observed, as well as nonvolcanic mountains about 10 km (6 mi) high. A large eruption observed by the Galileo orbiter in 1997 apparently consists of silicate materials. No impact craters exist, because the surface is constantly being renewed. Io's surface appearance changed significantly between the Voyager and Galileo observations, and even over the course of months during the Galileo mission, due to spewing volcanic ejecta and ongoing surface flows. A trace atmosphere of sulfur dioxide and oxygen exists. Surface temperatures are in the range of ç148¡ C (ç235¡ F), but in the vicinity of eruptions they may reach 27¡ C (80¡ F). The Galileo orbiter determined that hot spots on Io may reach temperatures in excess of 1500¡ C (2700¡ F), visibly suggesting that hot magma rich in magnesium and iron may be glowing through cracks in the satellite's surface.
Silicate volcanism with lavas hotter than 1000¡ C is widespread. Of the four moons of Jupiter discovered by Galileo Galilei, Io lies closest to the planet, at an average distance of about 421,600 km (261,970 mi) from its center, although four small satellites have since been found to lie inside Io's orbit. Io completes one revolution of Jupiter in about 42 hours 27 minutes, keeping the same side facing toward the planet. Its orbit is locked in with that of another Galilean moon, Europa, with Io completing two orbits of Jupiter for a single one by Europa. Slightly smaller than Earth's Moon, with a diameter of 3,630 km (2,255 mi), Io has a high densityÑ3.55 times that of waterÑand a large metallic core extending fully halfway from the center to the surface. A magnetic field has been detected, but scientists do not yet know whether the field is induced by Jupiter or intrinsic to the satellite. Electric current induced as Io moves through Jupiter's magnetic field discharges at Io's magnetic "footprint," giving a visible glow on the surface of the giant planet.