Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100

twins have liquid-water oceans and if there are any radio emissions from intelligent life-forms.
    EUROPA—OUTSIDE THE GOLDILOCKS ZONE
    There is also another tempting target for our probes within our solar system: Europa. For decades, it was believed that life in the solar system can exist only in the “Goldilocks zone” around the sun, where planets are not too hot or too cold to sustain life. The earth is blessed with liquid water because it orbits at the right distance from the sun. Liquid water will boil on a planet like Mercury, which is too close to the sun, and will freeze on a planet like Jupiter, which is too far. Since liquid water is probably the fluid in which DNA and proteins were first formed, it was long believed that life in the solar system can exist only on earth, or perhaps Mars.
    But astronomers were wrong. After the
Voyager
spacecraft sailed past the moons of Jupiter, it became apparent that there was another place for life to flourish: under the ice cover of the moons of Jupiter. Europa, one of the moons of Jupiter discovered by Galileo in 1610, soon caught the attention of astronomers. Although its surface is permanently covered with ice,beneath that ice there is a liquid ocean. Because the ocean is much deeper on Europa than on earth, the total volume of the Europan ocean is estimated to be twice the volume of earth’s oceans.
    This was a bit of a shock, realizing that there is an abundant energy source in the solar system other than the sun. Underneath the ice, the surface of Europa is continually heated by tidal forces. As Europa tumbles in its orbit around Jupiter, that massive planet’s gravity squeezes the moon in different directions, creating friction deep within its core. This friction creates heat, which in turn melts the ice and creates a stable ocean of liquid water.
    This discovery means that perhaps the moons of distant gas giants are more interesting than the planets themselves. (This is probably one reason James Cameron chose a moon of a Jupiter-size planet for the site of his 2009 movie,
Avatar.
) Life, which was once thought to be quite rare, might actually flourish in the blackness of space on the moons of distant gas giants. Suddenly, the number of places where life might flourish has exploded by many times.
    As a consequence of this remarkable discovery, the Europa Jupiter System Mission (EJSM) is tentatively scheduled for launch in 2020. It is designed to orbit Europa and possibly land on it. Beyond that, scientists have dreamed of exploring Europa by sending even more sophisticated machinery. Scientists have considered a variety of methods to search for life under the ice. One possibility is the Europa Ice Clipper Mission, which would drop spheres on the icy surface. The plume and debris cloud emerging from the impact site would then be carefully analyzed by a spacecraft flying through it. An even more ambitious program is to put a remote-control hydrobot submarine beneath the ice.
    Interest in Europa has also been stoked by new developments under the ocean here on earth. Until the 1970s, most scientists believed that the sun was the only energy source that could make life possible. But in 1977, the
Alvin
submarine found evidence of new life-forms flourishing where no one suspected before. Probing the Galapagos Rift, it found giant tube worms, mussels, crustaceans, clams, and other life-forms using the heat energy from volcano vents to survive. Where there is energy, there might be life; and these undersea volcano vents have provided a new source of energy in the inky blackness of the sea floor. In fact, some scientists have suggestedthat the first DNA was formed not in some tide pool on the earth’s coast but deep undersea near a volcano vent. Some of the most primitive forms of DNA (and perhaps the most ancient) have been found on the bottom of the ocean. If so, then perhaps volcano vents on Europa can provide the energy to get something like DNA off the ground.
    One can only speculate about the possible life-forms that might form under Europa’s ice. If they exist at all, they probably will be swimming creatures that use sonar, rather than light, for navigational purposes, so their view of the universe will be limited to living under the “sky” of ice.
    LISA—BEFORE THE BIG BANG
    Yet another space satellite that could create an upheaval in scientific knowledge is the Laser Interferometer Space Antenna (LISA) and its successors. These probes may be able to do