Science & Environment

Aquarius satellite comes of age

Ocean salinity map (Nasa)

Nasa has released the first global map of ocean surface salinity acquired by the Aquarius/SAC-D satellite, which was launched in June this year.

Knowing the saltiness of seawater will improve scientists' understanding of some key climatic processes.

Variations in salinity help drive ocean circulation and their measurement can also reveal how freshwater is moving around the planet.

The mission is a joint venture with the space agency of Argentina (Conae).

The new map incorporates just the first two-and-a-half weeks of data since Aquarius became operational on 25 August.

Red and yellow colours denote areas of higher salinity; blues and purples represent areas of lower salinity. Areas coloured black represent gaps in the data.

The map picks up well-established, large-scale features such as the differences in salinity between the Atlantic, Pacific and Indian oceans.

Also apparent are the areas of lower salinity associated with rain belts near the equator, and the higher values associated with evaporation in the subtropics.

You can also see smaller features such as the freshwater outflow from the Amazon River, which tends to dilute the immediate surface waters of the Atlantic.

Ocean salinity is measured in parts per thousand (grams of salt per kilogram of sea water). The observed range is a small one - generally between about 32 and 37 parts per thousand out over the open seas.

The goal of the Aquarius mission is to retrieve salinity with a resolution of 0.2 parts per thousand. That is a concentration change equivalent to about one millilitre of salt in six litres of water.

Scientists have been able to measure ocean salinity for decades by lowering instruments from ships or by deploying robotic floats, but the technology to gather data from orbit is a recent innovation.

Aquarius carries three high-precision radio receivers that will record the natural microwave emissions coming up off the water's surface.

These emissions vary with the electrical conductivity of the water - a property directly related to how much dissolved salt it is carrying.

The Nasa-Conae spacecraft is not the first ocean salinity mission in orbit.

Europe already has a satellite in operation called Smos. This was launched in 2009, and produced the first-ever global maps of salinity built from space data.

The intention is to inter-calibrate and combine the Aquarius and Smos measurements.

Together, these spacecraft are now acquiring volumes of salinity data that dwarf all the information ever gathered in this field of study.

Nasa has provided the ocean salinity instrument; Conae has provided the main satellite bus and a number of other instruments. The spacecraft circles the Earth, travelling from pole to pole

Jonathan.Amos-INTERNET@bbc.co.uk

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