Most of NASA's efforts to learn if life exists beyond Earth have focused on Mars, the only other planet in the solar system that is properly positioned for liquid surface water. What happened to Mars' water - and if it stayed around long enough for life to evolve - remains a mystery.
Some scientists believe a richer target to look for present-day life is Europa, one of Jupiter's moons. Although Europa is far too cold for surface water, there is strong evidence of a liquid ocean locked beneath the moon's icy surface. And where there is water, there may be life.
PHOTO: Dazzling New View of Europa's Frozen Red Veins
NASA this week put out a call for proposals for science instruments to begin chipping away at the question.
"This solicitation will select instruments which may provide a big leap in our search to answer the question: are we alone in the universe?" NASA associate administrator for science John Grunsfeld said in a press release.
A review panel will mull the proposals, which are due Oct. 17, and in April 2015 select about 20 for preliminary studies.
Europa Report: An Epic Voyage to Jupiter's Moon (Review)
Ultimately, about eight instruments would be chosen to fly on a satellite that would either put itself into orbit around Europa or make multiple flybys. The mission, which NASA hopes to do for less than $1 billion plus the cost of a rocket ride, would launch sometime in the 2020s.
Top science objectives for the mission are to:
Characterize the extent of the ocean and its relation to the deeper interior;
Characterize the ice shell and any subsurface water, including their heterogeneity, and the nature of surface-ice-ocean exchange;
Determine global surface, compositions and chemistry, especially as related to habitability;
Understand the formation of surface features, including sites of recent or current activity, identify and characterize candidate sites for future detailed exploration; and Understand Europa's space environment and interaction with the magnetosphere.