Plumes of water vapour 100 miles high may have been spotted bursting out of the icy surface of Jupiter's moon Europa.
Scientists believe they detected two vapour jets shooting into space for seven hours at a time.
Like "Old Faithful", the famous volcanic geyser in Yellowstone National Park, US, the plumes appear to erupt at regular intervals.
But while Old Faithful manages around 100 feet, Europa's vapour fountains are thought to reach heights of 124 miles.
If confirmed, the observations prove that water from the moon's ice-covered ocean can easily reach the surface, which has important implications for future space missions.
Many experts believe the hidden ocean surrounding Europa, warmed by powerful tidal forces caused by Jupiter's gravity, may have conditions favourable to life.
Scientists used images from the Hubble Space Telescope to identify unusual "spikes" of hydrogen and oxygen in two distinct regions of the moon's southern hemisphere.
They were observed for just seven hours at a time when Europa was at the furthest point of its orbit away from Jupiter, vanishing as the moon got closer.
The discovery suggests the presence of high jets of water vapour - water is made from hydrogen and oxygen - that may be triggered by tidal forces opening cracks in Europa's surface.
The team led by Dr Lorenz Roth, from Southwest Research Institute, San Antonio, Texas, US, wrote in the journal Science: "These emissions are persistently found in the same area over around seven hours.. They are consistent with two 200 kilometre-high plumes of water vapour."
Similar plumes have previously been seen on one of Saturn's moons, Enceladus.
Details of the discovery were also presented at the American Geophysical Union's autumn meeting in San Francisco.