The evidence comes from instrument data and from close-up images of a 500-mile-long mountain on the moon captured by NASA's Cassini spacecraft orbiting the ringed planet, they said.
The spacecraft's magnetometer has detected a faint particle stream coming from the moon, and images showed evidence for a possible liquid or slushy layer under its rock-hard ice crust, NASA's Jet Propulsion Laboratory in Pasadena, Calif., reported Wednesday.
Cassini images have revealed ancient, inactive fractures at Dione similar to those seen at on Saturn's moon Enceladus that currently spray water ice and organic particles.
"A picture is emerging that suggests Dione could be a fossil of the wondrous activity Cassini discovered spraying from Saturn's geyser moon Enceladus or perhaps a weaker copycat Enceladus," Cassini science team leader Bonnie Buratti at JPL said. "There may turn out to be many more active worlds with water out there than we previously thought."
The presence of a subsurface ocean at Dione would boost the astrobiological potential of this once-boring iceball, the researchers said.
Other bodies in the solar system thought to have a subsurface ocean -- including Saturn's moons Enceladus and Titan and Jupiter's moon Europa -- are among the most geologically active worlds in our solar system, they said.