cubes by shaking it violently. Io is scarred by fresh eruptions of molten sulfur and other minerals, which completely repave the moon's surface every few dozen years.

The next moon outward, Europa , also is heated by Jupiter's tides, but less vigorously. It may be warmed enough to maintain a deep ocean of liquid water beneath its icy cracked crust. This is tantalizing, for many biologists believe that life on Earth began in the oceans. If the right conditions exist, Europa 's dark seas could even now be nurturing extraterrestrial life--without the Sun as a source of energy.

Some of nature's most spectacular displays of cosmic mayhem arise from tidal interactions on a much larger scale. Entire galaxies can collide, drawn together by their mutual gravitational pulls. The gravity of each galaxy tugs much more strongly on stars in the nearer side of the other galaxy. These tidal forces stretch the original structures into sweeping arcs and rings of stars. After hundreds of millions of years, the two galaxies may merge into a disturbed and disrupted blob. This scenario may be in our future. Our Milky Way and its neighbor, the Andromeda galaxy, are moving toward each other and could meet in about 5 to 7 billion years. By that time it won't matter for Earth because our Sun will have run out of its nuclear fuel.

Individual stars almost never smash together when galaxies collide. They are simply too far apart and too small compared with the vast distances between them. If there were four snails running loose in the continental United States for a billion years, two of them would be more likely to bump into each other than would two stars during a galactic collision. However, the oversized clouds of gas and dust in galaxies certainly do interact. The gravitational turmoil stirred by this process sets off new bursts of star formation, lighting up galaxies like a holiday display. We often see the brilliant blue flares of newborn stars along graceful "tidal tails" flung into space during galactic collisions.

On the largest scale of all, the combined gravitational pulls of every object in the universe act to slow down the expansion of space-time that Edwin Hubble discovered in 1929. For years astronomers debated whether the universe contained enough mass to slow its own expansion to a halt. If so, gravity would pull everything together in the ultimate "big squeeze." But it now appears that won't happen. Our universe probably is destined to expand forever, providing more than enough time for all the stars to burn out.