For circulating energy, for distributing essential chemistry, and as a fundamental requirement for most biological processes, water defines Earth's dynamic identity. The more than seventy percent of our planet covered by water is in many ways the reason life has survived and thrived for so long.
A simple trip to the ocean's edge highlights how water constantly moves. But water sloshing back in forth in ocean basins only begins to describe the complex processes of its circulation on Earth.
NASA takes the water cycle as not merely an academic exercise but as a vital area for exploration. Satellites can examine aspects of the global water cycle that in situ measurements and observations can only dream about seeing. The TRMM spacecraft is the world's most advanced precipitation measuring system to date, gathering vital information about tropical precipitation and other features every day. Other sensors, like the AMSR and AIRS instruments on the AQUA spacecraft take profiles of the planet's atmosphere, examine water vapor concentrations and distribution, among other things.
A number of instruments look at water at or below the surface. MODIS makes sea surface temperature measurements that provide essential information about how oceans work and how they're changing over time. GRACE keeps track of elusive, yet massive, quantities of water both underground and in the oceans by making precise gravitational measurements. And the planned Aquarius mission, scheduled for launch in just a few years, will make unprecedented measurements of ocean salinity, a vital characteristic for describing a wide variety of phenomena, from life to physical processes that govern global circulation patterns.