On Jupiter's moon Europa, which has a fractured outer surface of ice (under which may lie an ocean), volcanism may still be operating, although probes have yet to detect the tell-tale signs of a volcanic eruption. "Although it is pure speculation, there could be silicate volcanism transmitted through the water," McEwen says. Another possibility, McEwen says, is ice volcanism. "A plume of water vapor or a water flow in the surface is considered a volcanic fluid if it is heated and warmed up by the internal heat of the moon," McEwen says. Some researchers have also suggested that the same sort of ice volcanism may be present on Saturn's moon, Enceladus. Another moon of Saturn's, Titan, may also be volcanic (which would explain the satellite's dense atmosphere), as may be Neptune's moon, Triton.
 

Io's calderas (craters) are remarkably hot. "Our temperature measurements indicate hotter temperatures than any active volcanism on Earth," McEwen says. "It implies that the magma is magnesium-rich -- comparable to the mantle composition of the Earth. This is what happened on Earth during the Archean era, 2.5 billion years ago, when the Earth was hotter. That's interesting, because it suggests that we might learn something about ancient volcanoes on Earth by studying Io today."