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Aquarius to Study the Power of Sea Salt

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June 7, 2011: A new observatory is about to leave Earth to map a powerful compound of global importance: Common everyday sea salt.

Aquarius (spacecraft, 200px)
An artist's concept of Aquarius/ SAC-D in orbit. [more]

Researchers suspect that the salinity of Earth's oceans has far-reaching effects on climate, much as the salt levels within our bodies influence our own delicate internal balance. An international team of scientists from NASA and the Space Agency of Argentina, or CONAE, will investigate this possibility with the aid of a satellite named "Aquarius/SAC-D," scheduled to launch on June 9th.

"Based on decades of historical data gathered from ocean areas by ships and buoys, we know the salinity has changed over the last 40 years," says Aquarius principal investigator Gary Lagerloef. "This tells us there's something fundamental going on in the water cycle."

Salinity is increasing in some ocean regions, like the subtropical Atlantic, which means more fresh water is being lost through evaporation at the sea surface. But no one knows why this is happening; nor can anyone pinpoint why other areas are experiencing more rainfall and lower salinity. To solve these mysteries, scientists need a comprehensive look at global salinity.

Within a few months, Aquarius will collect as many sea surface salinity measurements as the entire 125-year historical record from ships and buoys.

Aquarius (seashore, 550px)
A NASA video explains the role of ocean salinity in Earth's water cycle.

"Salinity, along with temperature, governs the density of seawater," says Lagerloef. "The saltier the water, the denser it is, and density drives the currents that determine how the ocean moves heat around the planet. For example, the Gulf Stream carries heat to higher latitudes and moderates the climate. When these currents are diverted by density variations, weather patterns such as rainfall and temperature change."

Scientists have gathered an ensemble of measurements over the ocean--e.g., wind speed and direction, sea surface heights and temperatures, and rainfall. But these data do not provide a complete picture.

"We've been missing a key element – salinity," says Lagerloef. "A better understanding of ocean salinity will give us a clearer picture of how the sea is tied to the water cycle and help us improve the accuracy of models predicting future climate."

Aquarius (radiometer, 200px)
A pre-launch view of the Aquarius radiometer. [more]

Aquarius is one of the most sensitive microwave radiometers ever built, and the first NASA sensor to track ocean salinity from space.

"It can detect as little as 0.2 parts salt to 1,000 parts water -- about the same as a dash of salt in a gallon of water. A human couldn't taste such a low concentration of salt, yet Aquarius manages to detect it while orbiting 408 miles above the Earth."

The Aquarius radiometer gets some help from other instruments onboard the satellite. One of them helps sort out the distortions of the choppy sea. CONAE's Sandra Torrusio, principal investigator for the Argentine and other international instruments onboard, explains:

"One of our Argentine instruments is another microwave radiometer in a different frequency band that will measure sea surface winds, rainfall, sea ice, and any other 'noise' that could distort the Aquarius salinity measurement. We'll subtract all of that out and retrieve the target signal."

Torrusio is excited about the mission.

"I've met so many new people, not only from Argentina, but from the US and NASA! It's been a great experience to work with them and exchange ideas. We may come from different places, but we all talk the same language. And it isn't English – it's science."

Working together, these international "people of science" will tell us more about the ocean's role in our planet's balance – and in our own – no matter where we live.

For whatever we lose (like a you or a me),
It's always our self we find in the sea.
(e.e. cummings)"

Author: Dauna Coulter | Editor: Dr. Tony Phillips | Credit: Science@NASA

More Information

Aquarius/SAC-D: This multipurpose observatory continues the long-standing partnership between NASA and the Argentine Comisión Nacional de Actividades Espaciales, or CONAE. NASA provided launch vehicles and science instruments, while CONAE contributes the spacecraft, mission operations, and science instruments for their national space program. The NASA Aquarius instrument to measure ocean salinity is the prime instrument on the Aquarius/SAC-D mission. JPL will manage the NASA Aquarius implementation through its commissioning phase and archive mission data. Goddard will manage Aquarius instrument operations and process science data. NASA's Launch Services Program at the agency's Kennedy Space Center in Florida is managing the launch. CONAE is providing the SAC-D spacecraft, an optical camera, a thermal camera in collaboration with Canada, a microwave radiometer; sensors from various Argentine institutions and the mission operations center there. France and Italy are contributing instruments.

SAC stands for Satélite de Aplicaciones Científicas. This is "D" in this series of four science application satellites Argentina has built in collaboration with NASA.

A radiometer is essentially a sensitive radio receiver, which, in this case, detects natural microwave emissions given off by the ocean's surface. The Aquarius radiometer scans the sea surface to measure the emitted power in a certain frequency band (1400-1430 MHz) that is proportional to the water's salt content.

There are an average of 35 parts per thousand of salt in the ocean (this ratio varies from 32-37 in open ocean areas). That is, the ocean is 3 1/2 percent salt, and in 1 kilogram seawater there's about 35 grams of salt. Since salinity levels in the open ocean vary by only about five parts per thousand, the instrument must be very sensitive.