You don't have to fly to Mars or to the moons of Jupiter to find the stuff that solar-system explorers are most excited about these days. Just twist the tap in your kitchen.
The water trickling out isn't only a convenience, it's a necessity.
And humans aren't alone in this. Liquid water is a necessity for every form of life known, with the possible exception of some plants or fungi that may get by on water vapor. With this in mind, scientists are eagerly searching for liquid water in places other than Earth. If found, these places would be the most compelling locations to seek an answer to the question of whether life exists beyond our home planet.
Water in its various forms pervades the solar system, from traces of water vapor on the Sun itself to water ice in the likely composition of Pluto and the Kuiper Belt objects beyond it. Even without its link to the question of extraterrestrial life, water would be an important theme in exploration of the solar system, as a possible resource and as a substance that's geologically intriguing in its own right. Water ice on the Moon and Mars might help supply future human explorers. Some of the water on Earth probably came from comets crashing into the planet. Icy comets and the ice-rich rings of Saturn hold clues to how solar systems evolve.
Liquid ice, however, holds its special, life-enabling allure. In recent years, NASA spacecraft managed by JPL have found a tantalizing sprinkle of clues supporting both the possibility that liquid water may persist below the dry surface of Mars and the icy surfaces of three large moons circling Jupiter. With NASA's strategy to "follow the water" in the search for life, Mars and Jupiter's moon Europa are priority targets for future robotic missions.
The ice-rich crust of Europa, imaged by the Galileo spacecraft.
Whatever discoveries future missions make will be powerful. If some worlds have conditions that could support life, but are lifeless, our dear Earth looks even more like a special case. If other worlds do have life, we're not alone.