NASA's astronomy picture of the day is nothing especial today. At least for more people. For me it is special because I like to look at the four moons that tantalize Earth scientists with the possibility of life—four moons that we should explore soon, shown here to scale. So beautiful, they never get old.

The Galileo mission to Jupiter discovered Europa's global subsurface ocean of liquid water and indications of Ganymede's interior seas. At Saturn, the Cassini probe detected erupting fountains of water ice from Enceladus indicating warmer subsurface water on even that small moon, while finding surface lakes of frigid but still liquid hydrocarbons beneath the dense atmosphere of large moon Titan. Now looking beyond the Solar System, new research suggests that sizable exomoons, could actually outnumber exoplanets in stellar habitable zones. That would make moons the most common type of habitable world in the Universe.

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