Without hesitation, NASA’s chief scientist Ellen Stofan told lawmakers that Mars is her top candidate for finding life beyond Earth. “We now know that Mars was once a water world, much like Earth, with clouds and a water cycle and indeed some running water currently on the surface. For hundreds of millions of years about half the northern hemisphere of Mars had an ocean possibly a mile deep in places,” Stofan said. “Life as we know it requires liquid water that has been stable on the surface of a planet for a very long time. That’s why Mars is our primary destination in our search for the life in the solar system,” she added. NASA’s next rover, scheduled to launch in 2020, will be outfitted with instruments to look for ancient microbial life, though Stofan, a geologist by training, believes it will take astronauts on Mars, cracking opening rocks and running experiments, to make the definitive discovery.

The Jupiter moon Europa is roughly the size of Earth’s moon, yet it hosts a salty ocean that has twice as much water as Earth’s oceans. The Europa sea contacts a rocky core, which presents suitable conditions for life to brew. The moon also has abundant sources of energy. That leaves one big question in the search for life: Does it have organics? A mission targeted for launch in the 2020s will attempt to find organics that have welled up from the sea into cracks on the moon’s icy surface. It also will search for a mysterious plume that may be behind a 2012 Hubble Space Telescope detection of water vapor above Europa’s southern polar region. Scientists also want to know how deeply the ocean is buried beneath Europa’s frozen crust. “That will be important for coming up with a strategy to search for life there,” Cornell University planetary scientist Jonathan Lunine told the House Committee on Space, Science and Technology. “There’s a lot of groundwork that has to be done on Europa … if there are fresh organics in the cracks, that’s a good place to go,” he said.