Planetary scientists once thought Earth was an oasis in a dry solar system, as early missions to our neighbors revealed desert-like conditions on the Moon, Mars, and Mercury. In contrast, our Earth is literally covered with water.
Missions in recent years have overturned our view of a dry solar system, returning mounting evidence of ample water from a vast array of locations.
Comets from the remote corners of our solar system are made of water and other ices. Orbiters, landers, and rovers reveal Mars as a watery world in the distant past -- a world that today may contain entire underground oceans of frozen water.
The Moon, once thought dry-as-a-bone, has a water cycle -- with small amounts of water moving across its surface -- and millions of tons of water ice locked into frozen crater floors at its poles. Rings of ice orbit the gas giants, and several moons of these distant worlds have immense oceans of liquid water beneath their frozen crusts.
Even Mercury has ice in the dark craters at its poles, as revealed by the ongoing MESSENGER mission.
Water is critical to life and to future human forays into space. While we now know that Earth is not the only place with water, it is the only oasis that contains life. We should remember to take care of our water resources on our home planet, even as we are discovering water almost everywhere in our solar system!
There are a variety of websites with details on water in our solar system, and the role of water on Earth.
Water in the Solar System Background
Water Cycle by NASA Science: Earth
Why is Water a Priority for Mars Exploration? What Is So Special About Water?
Ice on Mercury
Article: Mars Rover Scientists Wring Water Story from Rocks
Article: Three Flavors of Water on the Moon
Water on Enceladus
Given the critical role of water in supporting life, it is not surprising that almost all planetary missions are examining water in the solar system. These include, but are not limited to:
NASA Oceanography Missions
Part of NASA's mission is to develop an understanding of the total Earth system; our oceans play a major role in influencing changes in the world's climate and weather. Today there are several ocean-observing satellite missions and an extensive scientific research community studying these data.
Lunar Reconnaissance Orbiter
The instruments on this mission are studying and mapping the Moon globally; one of its key objectives is to map the locations and amounts of water on the Moon. It has discovered the widespread presence of water ice in large areas of the lunar south pole.
Mars Exploration Rovers
The big science question for the Mars Exploration Rovers is how past water activity on Mars has influenced the red planet's environment over time. Major discoveries of evidence of past liquid water on the Martian surface include a stratified pattern and cross bedding in Martian rocks, and minerals that form in the presence of water.
Mars Reconnaissance Orbiter
Mars Reconnaissance Orbiter is designed to track changes in the water and dust in Mars' atmosphere, look for more evidence of ancient seas and hot springs and peer into past Martian climate changes by studying surface minerals and layering. Among the mission's major findings is that the action of water on and near the surface of Mars occurred for hundreds of millions of years.
Cassini launched to Saturn in October 1997, and has been in orbit around the ringed planet since 2004. Among other things, it has extensively studied the water on Saturn's moons, including Enceladus and Titan. Its mission to study Saturn, its moons and rings has been extended to 2017.