Mapping the Salinity of the Ocean

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More than seventy percent of the Earth is made up of extensive oceans yet we still know so little about them and how they affect the rest of the planet. Scientists from Europe and the United States are trying to change that, however, and expand our understanding of the ocean especially when it comes to how the ocean’s salinity affects its dynamics. Both the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) are doing their part to map the salinity of the ocean in order to try to understand the role the ocean plays in the vital recycling of our planet’s water resources and our climate.

Scientists are paying attention to the salinity of the ocean for several reasons. Most of them have to do with the ocean’s impact upon the Earth’s water cycle, the continuous recycling of water through evaporation, condensation, and precipitation. There is a constant exchange of water between the land and the ocean, and the majority of global evaporation and precipitation occur over saltwater. The salinity of the ocean also determines the density of seawater as well and that plays a large role in global ocean circulation and sea surface winds and temperature. Measuring sea surface salinity (SSS) is crucial for understanding how the input and output of fresh water affects the dynamics of the ocean. These dynamics in turn affect the weather patterns and climate of the Earth.

The ESA and NASA hope to map the ocean’s salinity through the use of two satellites. Both of them have been equipped with ground-breaking technology that allows a mapping of different ocean measurements never undertaken before. The ESA launched the Soil Moisture and Ocean Salinity (SMOS) satellite as part of its Earth Explorers missions. These missions are designed to help improve our understanding of the Earth’s processes through the use of innovative mapping technologies. The SMOS is specifically known as the ESA’s water mission for its ability to map the salinity of the ocean. Scientists are hopeful that the SMOS will lead to improvements in weather and climate modeling plus the management of water resources.

The SMOS satellite is a unique device in that it is outfitted with only one instrument to measure the ocean’s salinity. This instrument is an L-band radiometer that can measure microwave radiation around the frequency of 1.4 GHz. There is a difference in the electromagnetic properties of both pure and salt water, and the instrument can detect these differences to get an extremely accurate reading of how salty ocean water is. The SMOS can detect 0.1 grams of salt in a liter of water. The maps that the SMOS provides cover areas of approximately 200 x 200 kilometers.

To make the SMOS possible, scientists developed an instrument known as MIRAS (the Microwave Imaging Radiometer with Aperture Synthesis). Instead of employing one large antenna to pick up the wavelengths, MIRAS has sixty-nine smaller antennas that are dispersed over the three arms of the satellite and the satellite’s central hub. This technology is known as aperture synthesis or interferometry. Each of the antenna elements, called LICEFs, measure L-band radiation emitted by the surface of the ocean and are then turned into an image or map of sea surface salinity.

Another similar but different device being used to map the ocean’s salinity is NASA’s Aquarius satellite. It was launched in 2011 with the purpose of providing information on the salinity of the ocean so that scientists can better understand the how the ocean affects the water cycle and climate. In order to do this, Aquarius provides monthly maps of changes in the sea surface salinity for at least three years repeating its global flyby patterns every seven days. The maps that it produces have a resolution of 150 kilometers (ninety-three miles), and scientists will be able to track the long-term changes in salinity from month-to-month and even year-by-year.

Unlike the SMOS which employs only one L-band radiometer, Aquarius maps the salinity of the ocean using several radiometers. Aquarius is equipped with three passive microwave radiometers that detect the electromagnetic emissions coming from the ocean’s surface. These radiometers operate at about 1.4 GHz over a 390 kilometer swath. Aquarius is also outfitted with an active scatterometer so that it can measure the ocean waves that have an impact upon measurements of salinity. The scatterometer operates at 1.2 GHz and uses both a real aperture and a 2.5 meter reflector antenna. Aquarius’ sensor can measure changes in the salinity of the ocean down to 0.2 psu which is equal to one-eighth of a teaspoon of salt in a gallon of water.

Global salinity maps from SMOS (top) and Aquarius (bottom). Source: IFREMER/ESR/ESA/NASA

Global salinity maps from SMOS (top) and Aquarius (bottom). Source: IFREMER/ESR/ESA/NASA

Aquarius provides monthly maps of sea surface salinity with a resolution of 150 kilometers. Like the SMOS, the Aquarius was developed in order to enhance our understanding of the water cycle, climate, and the circulation of the ocean. Sea salinity has not previously been mapped from space and certainly not to the degree that both the SMOS and Aquarius deliver. Past attempts to measure the ocean salinity have been done with buoys and ocean-going vessels. Needless to say, those measurements have not in any way come close to providing the amount of data that the SMOS and Aquarius can. The vast expanse of the ocean has always been on obstacle to mapping salinity, but these obstacles are being overcome through the use of space mapping technology.

Both the SMOS and Aquarius are helping scientists explore and understand a part of the Earth that has remained a mystery up until now. The ocean plays a vital role in the natural processes of our world, especially the water cycle and weather patterns, yet our knowledge has been limited. Then again, the SMOS from the ESA and Aquarius from NASA along with their innovative instruments are doing their part to expand our knowledge of the ocean. The data and maps they provide will hopefully expand our understanding about the crucial impact the ocean has on the daily life of the planet.

References

“Taking Two Bites at Ocean Salinity” ESA.int. European Space Agency, 22 Apr. 2013. Web. 17 June 2013.

“Mapping Soil Moisture and Ocean Salinity” ESA.int. European Space Agency, n.d. Web. 17 June 2013.

“NASA: Aquarius Mission Web Site.” Http://aquarius.nasa.gov. NASA, 4 June 2013. Web. 17 June 2013.

 

 




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