Water may be spouting from Jupiter's icy moon Europa - considered one of the best places to find alien life in the Solar System.
Images by the Hubble Space Telescope show surpluses of hydrogen and oxygen in the moon's southern hemisphere, say astronomers writing in Science journal.
If confirmed as water vapour plumes, it raises hopes that Europa's underground ocean can be accessed from its surface.
Future missions could probe these seas for signs of life.
Reddish spots and shallow pits peppering the enigmatic ridged surface of Jupiter's moon Europa. Photo: AFP
Nasa's planetary science chief Dr James Green told the BBC: "The presence of the water has led scientists to speculate that the Europa we know today harbours life.
"The plumes are incredibly exciting if they are there - they are bringing up material from the ocean. Perhaps there are organic molecules lying there on the surface of Europa."
The findings were reported at the American Geophysical Union (AGU) Fall Meeting in San Francisco, California.
Scientists discovered the enormous fountains in images taken by Hubble in November and December of last year, as well as older images from 1999.
They saw evidence of water being broken apart into hydrogen and oxygen over the south polar regions of Europa.
"They are consistent with two 200-km-high plumes of water vapour," said lead author Lorenz Roth, of Southwest Research Institute in San Antonio, Texas.
Every second, seven tonnes of material is ejected from the moon's surface.
Dr Kurt Retherford, also of the Southwest Research Institute, told the AGU meeting: "This is just an amazing amount.
"It is travelling at 700m a second... All of this gas comes out, and almost all falls back towards the surface - it doesn't escape out into space."