The discovery of subsurface life on Earth, surviving independently from surface life, refuted the belief that biological processes require not only liquid water but sunlight as well, thus greatly enhancing the possibility of life beyond Earth. Take Jupiter's moon Europa. Space probes show a body covered with a thick layer of ice. As Europa orbits its planet, however, it flexes due to the gravitational tug-of-war between it, its sister moons, and Jupiter. Through friction, this flexing produces heat in the moon's interior capable of melting ice. Indeed, observations suggest liquid water exists beneath Europa's icy crust. Photosynthetic life is impossible there because sunlight is completely absent, but life such as the microbes that flourish deep within earth may still be possible.