Mars once seemed the best candidate for finding alien life in our solar system.
These days though, scientists think icy moons like Europa and Enceladus are more likely to harbor living creatures than the Red Planet.
That's because there's growing evidence suggesting some moons have vast oceans of water flowing under their icy surfaces.
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“I think we need to explore worlds where life might be alive today,” said Kevin Hand, a researcher at NASA's Jet Propulsion Laboratory in Pasadena.
For more than a decade, he's studied Jupiter's moon Europa.
Hand says the ocean on that moon appears to be billions of years old.
“And if the conditions are right... that ocean could be thriving with life,” he explained.
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Europa's not the only moon in our solar system that seems to have a hidden ocean, says CalTech planetary sciences professor David Stevenson.
Stevenson recently co-published a paper on Saturn's moon Enceladus which concludes it has a body of water the size of Lake Superior under its icy crust.
He says there's evidence suggesting Jupiter's moons Ganymede and Callisto, Saturn's moon Titan and possibly even Neptune's moon Triton have oceans under thick layers of ice.
Ingredients for life
Of course, water alone isn't enough to sustain life.
Stevenson says you also need organic material. Europa and Enceladus likely have that because their oceans sit on top of a rocky core.
“Then you can have a lot of chemistry taking place because water can react with rock,” Stevenson noted.
You also need energy, he said.
On Earth we get that from the sun, but for an ocean under miles of ice, that isn’t an option.
Stevenson says some of these moons seem to get energy by orbiting large planets.
They travel around their home world in an elliptical pattern, leading gravity to pull unevenly on them – contracting and expanding their cores creating heat that could be warming their oceans.
NASA scientist Robert Poppalardo says the effect is similar to volcanic heat vents deep underwater in Earth's oceans.
"Surrounding them are these bizarre ecosystems of giant tube worms and lobsters, strange critters and bacteria," he explained.
It's evidence that isolated ecosystems can thrive even without sunlight.
New possibilities for life
All of this research is helping to push our idea of where in the universe life could take hold, says Emily Lakdawalla, a science writer for the Planetary Society.
“I’ve even heard of a notion of life being possible within these planets that have been totally ejected from their solar system," she added.
"So free floating planets could have life within them.”
But Lakdawalla is quick to point out, all of this is just speculation. So far our only example of life is what we have observed here on Earth, she says.
NASA is in the planning stages of a mission to gather more data from Europa. However, it likely won't lunch until the mid 2020s.
For now, researchers are left to look at the these moons from a distance and wonder what secrets they are hiding.