How a single spacecraft measured the world's salt concentration from space

Space · Published 18 October 2026 Swipe more like this →

In short: Before the Aquarius mission, our understanding of ocean salinity was largely limited to summer observations in shipping lanes. This international mission changed everything, collecting more data in its first few months than ships and sensors had amassed in the previous 125 years.

The Aquarius/SAC-D mission, a joint venture between the U.S. and Argentina, launched on June 10, 2011. Its primary goal was to provide the first space-based global measurements of ocean surface salinity. By monitoring the concentration of dissolved salt, scientists could better understand the complex relationship between the water cycle and ocean circulation.

Salinity is a fundamental driver of Earth's climate. Together with temperature, it determines the density and buoyancy of seawater, which influences how water masses form and how currents move heat from the tropics to the poles. The mission tracked how freshwater enters the ocean through river runoff and melting ice, and how it leaves through evaporation—a process that accounts for over 85 percent of global evaporation.

The spacecraft operated in a sun-synchronous orbit 657 kilometers above Earth. Using three passive microwave radiometers and an active scatterometer, it could detect salinity changes as small as 0.2 psu—roughly the amount of salt in a 1/8 teaspoon pinch per gallon of water. While open ocean salinity typically ranges from 32 to 37 psu, the mission provided monthly maps at a 150-kilometer resolution.

Though the mission's primary three-year objective was met in November 2014, the spacecraft continued to operate until June 8, 2015, when a failure in the power and attitude control system ended the mission. The data gathered remains vital for improving the accuracy of global computer climate models.

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