"But we do now have the last piece of evidence needed to demonstrate that life is possible there".
Now, the American space agency has announced that Saturn's moon Enceladus is a habitable world because it has almost all the ingredients to support life.
The Cassini spacecraft discovered hydrogen molecules in water plumes rising from the the stripe fractures in Enceladus' icy surface.
The vapor or gas contains hydrogen, one of the essential components of life.
Observations by Cassini suggest "hydrogen gas, which could potentially provide a chemical energy source for life, is pouring into the subsurface ocean of Enceladus from hydrothermal activity on the seafloor".
Thomas Zurbuchen, associate administrator of the agency's Science Mission Directorate, started the news conference off by saying "we can't answer it now, hopefully in the near future, but is there life elsewhere?"
"It really represents a capstone finding for the mission because we now know that Enceladus has nearly all of the ingredients that you would need to support life as we know it on Earth", said Linda Spilker, Cassini project scientist who works on the Cassini project that made this discovery.
NASA unveiled evidence of "ocean worlds" on moons of Saturn and Jupiter today. This new finding is therefore an independent line of evidence supporting the theory of hydrothermal activity taking place in the ocean of Enceladus.
The Cassini mission ends this September, and no other spacecraft is on the books to visit Enceladus.
The hydrogen detection resulted from Cassini's October 2015 deep dive close to the surface of Enceladus.
While some ingredients for life were found on Enceladus, the scientists made clear that the discovery didn't confirm life on the planet, but merely confirmed favorable conditions. "It would be like a candy store for microbes", said Hunter Waite, lead author of the Cassini study. The Hubble Space Telescope has observed what looks to be plumes emanating from Europa. Cassini passed close enough to Enceladus - less than 30 miles from the moon's surface - to cross through one of these geysers. "So money for the moment, is still on Europa - but it could be on any of these moons".
NASA says the discovery will help it to better equip the Europa Clipper mission set for the 2020's when a probe will visit the moon to examine the plumes.
"We're pushing the frontiers - we're finding new environments", said Jim Green, director of the Planetary Science Division at NASA Headquarters. The hydrogen may be the result of deep-sea chemical reactions between water and rock, which could spark microscopic life, scientists announced Thursday.