giovedì 8 settembre 2011
How Salty the Ocean
On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every kilogram (roughly one liter by volume) of seawater has approximately 35 grams of dissolved salts (predominantly sodium chloride. The average density of seawater at the ocean surface is 1.025 g/ml. Seawater is denser than both fresh water and pure water because the dissolved salts add mass without contributing significantly to the volume. The freezing point of seawater decreases as salt concentration increases. NASA's Aquarius satellite has successfully completed its commissioning phase and is now tasting the saltiness of Earth's ocean surface, making measurements from its perch in near-polar orbit. Aquarius will make NASA's first space observations of the salinity, or concentration of salt, at the ocean surface, a key variable in satellite studies of Earth. Variations in salinity influence the ocean's deep circulation, outline the path freshwater takes around our planet and help drive Earth's climate. The Aquarius/SAC-D (Satélite de Aplicaciones Científicas) observatory, a collaboration between NASA and Argentina's space agency, Comisión Nacional de Actividades Espaciales, launched from California's Vandenberg Air Force Base on June 10 aboard a United Launch Alliance Delta II rocket and was placed in its proper initial orbit. Ground controllers at the SAC-D Mission Operations Center in Teófilo Tabanera Space Center in Cordoba, Argentina, then began a complete in-orbit checkout of all SAC-D spacecraft systems. With all observatory systems confirmed to be healthy, SAC-D spacecraft commissioning activities were completed on July 24. The spacecraft's propulsion system then underwent a series of tests, and preliminary orbit adjustments were performed in preparation for turning on the observatory's eight science instruments. Ocean salinity has been stable for billions of years, most likely as a consequence of a chemical/tectonic system which removes as much salt as is deposited; for instance, sodium and chloride sinks include evaporite deposits, pore water burial, and reactions with seafloor basalts. Since the ocean's formation, sodium no longer leaches from the ocean floor, but instead is captured in sedimentary layers covering the ocean bed. One theory is that plate tectonics forces salt under the continental land masses, where it slowly leaches again to the surface. Ocean currents greatly affect the Earth's climate by transferring heat from the tropics to the polar regions, and transferring warm or cold air and precipitation to coastal regions, where winds may carry them inland. Surface heat and freshwater fluxes create global density gradients that drive the thermohaline circulation part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in sea ice regulation. Changes in the thermohaline circulation are thought to have significant impacts on the Earth's radiation budget. Insofar as the thermohaline circulation governs the rate at which deep waters reach the surface, it may also significantly influence atmospheric carbon dioxide concentrations. The term thermohaline circulation refers to the part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water.