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Astronomers believe the rough, icy surface of Jupiter's moon, Europa, is the most likely place in the solar system to harbour alien life.

Now Nasa has set aside £14.6 million ($25 million) to design probes that could reveal whether Europa is, in fact, habitable.

The agency yesterday asked scientists to come forward with potential experiments for a Europa probe that could be launched in the 2020s and arrive at the icy satellite within three years of take-off.

Compiled from Nasa's Galileo spacecraft data, this surface image of Europa shows the blue-white terrains which indicate relatively pure water ice. Scientists are very interested in these features because they may offer a way to investigate the habitability of the moon's interior ocean

It said that next year, about 20 proposals will be chosen to receive shares of the £14.6 million ($25 million) for further study.

'The possibility of life on Europa is a motivating force for scientists and engineers around the world,' said John Grunsfeld, associate administrator for Nasa's Science Mission Directorate.

'This solicitation will select instruments which may provide a big leap in our search to answer the question: Are we alone in the universe?'

In April, Nasa sent out a request for information to interested participants on the mission itself, which is expected to cost less than £580 million ($1 billion).

Habitable? Scientists believe that there could be life on Europa, pictured, a moon of Jupiter. Now Nasa has set aside £14.6 million ($25 million) to design probes that could find out if life could thrive on the moon

Water plumes seen spouting from the surface provide evidence for the existence of a vast ocean beneath Europa. It is thought the moon holds more water than all the oceans and sea on Earth combined, making it one of the most attractive science destinations in the solar system

The mission is designed to characterise the extent of Europa's ocean and its relation to the deeper interior.

It will also look at the ice shell and determine global surface, compositions and chemistry, especially related to whether life could survive in that environment.

Congress appropriated £47 million ($80 million) for this work in the Nasa's last budget, while the most recent budget proposal requests an additional £9 million ($15 million).

Previous scientific findings point to the existence of a liquid water ocean located under the moon's icy crust.

This ocean covers Europa entirely and contains more liquid water than all of Earth's oceans combined.

Although Europa and Jupiter's other moons have been visited by other spacecraft, they were each limited to a single distant flyby of these satellites.

Nasa's Galileo spacecraft, launched in 1989 by the space shuttle, was the only mission to make repeated visits to Europa, passing close by the moon fewer than a dozen times.

There have been various proposals to explore Europa over the years, such as the Cryobot (pictured) which would be taken underground by a larger drill before it was then released into the ocean, searching for signs of past or present alien life

In December 2013, Nasa's Hubble Space Telescope observed water vapour above the moon's frigid south polar region.

This provided the first strong evidence of water plumes erupting off the moon's surface, and a spacecraft could even be designed to fly through the plumes to make measurements of the composition of the underground ocean.

Any mission to Europa must take into account the harsh radiation environment that would require unique protection of the spacecraft and instruments.

In addition, spacecraft must meet planetary protection requirements intended to protect Europa's potentially habitable ocean.

These requirements are very strict and involve ensuring that a viable Earth organism is not introduced into the Europa ocean.

EUROPA - THE 'MOST LIKELY PLACE IN THE SOLAR SYSTEM TO SUPPORT LIFE'

Europa has very few craters on its surface

Europa is the sixth closest moon to Jupiter and the smallest of the four Galilean satellites that belong to the planet.

It was discovered by Galileo Galilei in January 1610 and is named after a Phoenician noblewoman in Greek mythology who was courted by Zeus and became the Queen of Crete.

Europa orbits Jupiter in around three-and-a-half days with an orbital radius of 670,900km.

It is slightly smaller than the Earth's moon, but at 3,100km in diameter it is the sixth largest moon and 15th largest object in the entire solar system.

It is likely to have an outer layer of water, some 100km thick.

The outer layer of the water is believed to be frozen with a liquid ocean underneath.

It is one of the smoothest objects in the solar system with few craters on its surface, which is tectonically active and relatively young.