In April 2011, NASA plans to launch the Aquarius satellite into orbit with a mission of answering a critical question about the Earth below: how is the salinity of Earth's ocean changing?

A remote-sensing sea surface salinity satellite (SSS), Aquarius will record how much salt ocean waters contain across the globe. 

Measuring the amount of salty ocean seasoning might seem like a trivial venture considering the cost and scale of the project, which has been in development for around a decade. But ocean salinity is about a lot more than just salt.

“Salinity, along with temperature, determines the density of the seawater, which will drive the 3-D ocean circulation that plays a key role in modulating the climate,” said Yi Chao, project scientist on the Aquarius mission.

The more salt is dissolved in ocean water, the greater its density and, in turn, the slower the ocean circulates.

“Once Aquarius is launched next year, we can integrate the Aquarius data into the existing Earth observing system including both in situ and satellite data so that we can start addressing the question (of whether the ocean is getting saltier),” Chao said.

According to the Smithsonian Institute, salt makes up about 3.5 percent of the ocean. Rain droplets pick up mineral ions as they wash over rock formations on their way out to sea. When ocean water evaporates, it leaves the ions behind, where they have accumulated for millions of years.

“Salt ions themselves aren’t added or removed from the ocean, but water molecules are freely gained or lost through the processes of evaporation and precipitation, or freezing and melting of ice,” said Ruth Curry, a senior research specialist at the Woods Hole Oceanographic Institution.

Consequently, ocean salinity is a barometer for change in the global water cycle — the rate of precipitation versus evaporation — it can also lend clues as to the rates at which water temperatures are rising and ice sheets are melting.

“It isn’t the salinity change itself that’s cause for concern, but rather that it’s evidence that the global hydrologic cycle is shifting to a new balance,” Curry explained. “As greenhouse warming continues, parts of the planet will become drier, other parts will become wetter, with greater examples like floods and droughts.”

As ocean temperature has risen in recent decades and ice sheets have melted at faster rates, studies have indicated that, on average, seawater is becoming fresher. Yet, site-specific salinity surveys also note that subtropical regions (25 to 30 degrees latitude) in particular have actually gotten saltier. 

Here’s where Aquarius comes into the picture.

For the first time, Aquarius will provide satellite measurements that give a definitive look at salinity shifts.  Oceanographers and other scientists can use that data to improve their models of ocean circulation and climate change.

“The salinity measurement is important for climate research,” said Gary Lagerloaf, lead scientist on the Aquarius mission and president of Earth and Science Research Institute in Seattle. “Part of it is to really understand how ocean circulation, changes in the water cycle, and climate variability interact.”

The European Space Agency has already launched a similar SSS satellite, but Aquarius will provide more fine-tuned salinity data by detecting microwave radiation from the top centimeter of ocean water.

“The (ocean) surface is really important to understanding climate dynamics because that’s where the interaction with the atmosphere takes place,” Lagerloaf told Discovery News.

Within that delicate environmental interface, minute changes in salt ion concentration can have powerful ripple effects, underscoring the significance of the Aquarius mission.

“Even though the salinity of the ocean is relatively constant, it varies enough to matter from one part of the ocean to next,” Lagerloaf said. “Small changes in ocean salinity can have significant impacts on ocean circulation because salinity helps determine the density of seawater, and the density of seawater is important to driving the ocean currents.”