Acquarius:Overview: Sea Surface Salinity (SSS)

A defining characteristic of ocean water is its relatively high concentration of dissolved salts or salinity. Understanding why the sea is salty begins with knowing how water cycles among its physical states: liquid, vapor, and ice. As a liquid, water dissolves rocks and sediments, and reacts with emissions from volcanoes and hydrothermal vents: this creates a complex solution of mineral salts in our ocean basins. In other parts of the cycle, however, water and salt are incompatible: water vapor and ice are essentially salt free.

Through Earth's history, certain processes - including weathering of rocks, evaporation of ocean water, formation of sea ice - have served to make the ocean salty. These "salinity raising" factors are continually counterbalanced by processes that decrease salinity such as the continuous input of fresh water from rivers, precipitation of rain and snow, and melting of ice.


Seafarers through history have discovered that Sea Surface Salinity (SSS) varies from place-to-place: SSS in the open ocean generally ranges between 32 and 37 psu, but may be much lower near fresh water sources or as high as 42 in the Red Sea. Between 300 and 600 AD, awareness of changes in salinity, temperature, and smell helped Polynesians explore the southern Pacific Ocean.

In the 1870s, scientists aboard H.M.S. Challenger systematically measured salinity, temperature, and water density in the world's oceans. Over the years techniques for measuring such ocean water properties have changed drastically in method and accuracy. Aquarius is a NASA Pathfinder mission that will provide a new type of SSS measurement through an innovative use of technology.

Although everyone knows that seawater is salty, few know that even small variations in SSS can have dramatic effects on the water cycle and ocean circulation . This is why long-term, accurate, global maps of SSS - such as those to be delivered by Aquarius -- are crucial to climate studies.

Glossary

atmosphere: Gaseous layer surrounding a planet; the whole mass of air surrounding the earth. climate: The prevailing or normal pattern of weather at a place, or in a region, averaged over a long period of time; in contrast to weather, which is the state of the atmosphere at a particular time. conductivity: A measure of the ability of a material to conduct or transmit an electric charge. density: Mass per unit volume of a substance. Usually expressed as grams per cubic centimeter. For ocean water with a salinity of 35 at 0°C, the density is 1.028 grams per cubic centimeter. evaporation: The physical process of converting a liquid to a gas. Commonly considered to occur at a temperature below the boiling point of the liquid. fresh water: Non-saline water. hydrothermal vents: On the ocean floor, fissures, cracks and vents can result from volcanic activity and crustal movement. Surrounding some vents may be biological communities that are supported by geothermal ("earth-heated") energy rather than by solar energy. practical salinity unit (psu): Used to describe the concentration of dissolved salts in water, the UNESCO Practical Salinity Scale of 1978 (PSS78) defines salinity in terms of a conductivity ratio, so it is dimensionless. Salinity was formerly expressed in terms of parts per thousand (ppt) or by weight (parts per thousand or 0/00). That is, a salinity of 35 ppt meant 35 pounds of salt per 1,000 pounds of seawater. Open ocean salinities are generally in the range between 32 and 37. solar energy: Thermal and electromagnetic energy from the sun. salinity: A measure of the quantity of dissolved solids in ocean water. In general, salinity reflects the total amount of dissolved solids in ocean water in parts per thousand by weight after all carbonate has been converted to oxide, the bromide and iodide to chloride, and all the organic matter oxidized. Salinity is now measured as pratical salinity units (psu). weathering: Any of the mechanical chemical processes by which rocks explosed to weather decay to soil.