NASA says it is seeking proposals for scientific instruments for a future planned mission aimed at Jupiter's moon Europa to seek answers to central questions regarding the icy moon and a search for alien life beyond Earth.

In an Announcement of Opportunity this week, the space agency says it wants to encourage submissions of external proposals for kinds of instruments that could examine the moon from a spacecraft making multiple close fly-bys of Europa.

Central to the research would be an attempt to strengthen current evidence pointing to the existence of a liquid water ocean under the moon's icy surface that might harbor extraterrestrial life, NASA officials say.

"The possibility of life on Europa is a motivating force for scientists and engineers around the world," says John Grunsfeld, associate administrator for NASA's Science Mission Directorate in Washington. "This solicitation will select instruments which may provide a big leap in our search to answer the question: are we alone in the universe?"

After selecting around 20 proposals by April 2015, NASA will offer some $24 million for those selected for instrument design and development.

Once that is complete, NASA officials will choose approximately eight of the proposed instruments, which will then be constructed and undergo science and flight operational testing.

The instruments' main purposes will include analyzing the scope of Europa's hidden sea and its relation to both the moon's icy cover and its deep interior, NASA says.

Particular emphasis would be on determining the chemistry and composition of both the ice cover and the underlying ocean in terms of its possible habitability, the agency says.

Any instrument aboard a spacecraft sent to Europa would have to be capable of withstanding the harsh radiation environment existing there that will require considerable protection for both, NASA scientists say.

The final cost of any selected instruments would have to fall within the budget set for the Europa mission, which at slightly less than $1 billion does not cover the cost of the mission's launch vehicle, they say.

In a recommendation of planetary strategy given to NASA by the U.S. National Research Council, the council has ranked a mission centered on Europa among the efforts that should be given the highest priority.

The instruments would have to examine Europa either from orbit around the moon or during several fly-bys since NASA currently has insufficient data to be able to select safe landing sites on its icy surface, scientists say.