SAN FRANCISCO — Oceans cover more than 70 percent of the Earth’s surface, yet salinity, one of the key variables that influence ocean circulation, remains poorly understood. NASA officials hope to help remedy that situation with the scheduled June 9 launch of the joint U.S.-Argentine Aquarius sea surface salinity mission.
Scientists have been eager to develop a space-based instrument to measure variations in the distribution and concentration of salt in the world’s oceans for decades. Until recently, however, the technological challenges of making the measurements needed were simply too great, said Gary Lagerloef, Aquarius principal investigator and president of Earth and Space Research, a nonprofit institute in Seattle.
On average, a kilogram of sea water holds 35 grams of salt. Globally, sea surface salinity levels vary from a high of approximately 37 grams per kilogram of water to a low of 32 grams per kilogram, Lagerloef said. To detect those small but important variations in salinity levels, scientists developed “the most accurate radiometer ever used for Earth observation,” Lagerloef said. That radiometer, built by engineers at NASA’s Goddard Space Flight Center in Greenbelt, Md., and Jet Propulsion Laboratory (JPL) in Pasadena, Calif., will measure thermal microwave emissions on the sea surface to reveal minute variations in the water’s salt content — variations as small as one-fifth of a gram of salt in a kilogram of water, he said.
By obtaining information on varying salinity levels, scientists hope to improve understanding of ocean circulation and the global water cycle, said Yi Chao, Aquarius project scientist at JPL. Once scientists know the salinity level of water, they can combine that information with sea surface temperature data to find the water’s density. Density drives ocean circulation models, which forecast the way water travels from one place to another.
As the water circulates, it also transports heat throughout the water as well as between oceans and the atmosphere. Many of the variables needed to model this complex chain of events already are being measured from space, including sea surface temperature, wind speed and rainfall. The missing piece of the puzzle is salinity, Lagerloef said.
By tracing changes in salinity levels, scientists will be able to observe the impact of precipitation, melting ice, river runoff and evaporation. Each of those events has a signature in salinity, Chao said. Melting ice adds fresh water to the ocean, decreasing its salinity level, while evaporation draws out fresh water and leaves high salinity levels in its wake, he said.
The Aquarius instrument has a design life of three years, but mission scientists said they hope it continues to gather salinity data for much longer. In addition to observing average salinity levels, scientists want to watch salinity levels change from “month to month, season to season and year to year,” Lagerloef said.
Argentina’s CONAE space agency built the spacecraft for the Aquarius mission, which is scheduled for launch June 9 into a 657-kilometer polar orbit aboard a United Launch Alliance Delta 2 rocket from Vandenberg Air Force Base in California.
In addition to providing the spacecraft, Argentina is supplying an optical camera, a microwave radiometer, sensors and a thermal camera built in collaboration with the Canadian Space Agency. The spacecraft also will be equipped with a radio occultation receiver built by Thales Alenia Space Italia as well as a radiation monitoring instrument and an orbital particle detector supplied by the French space agency CNES.
Aquarius is not the first space-based instrument to measure sea surface salinity levels. In November 2009, the European Space Agency achieved that goal with the Soil Moisture Ocean Salinity (SMOS) mission. The SMOS instrument, a Microwave Imaging Radiometer using Aperture Synthesis built by EADS Casa Espacio of Spain, is designed to provide a complete set of global data with 35-kilometer spatial resolution every three days.
In contrast, the Aquarius instrument integrates a scatterometer with a radiometer to map worldwide salinity levels every seven days and enable NASA officials to produce monthly salinity maps with a spatial resolution of 150 kilometers. The scatterometer is designed to measure sea surface roughness, which can reduce the accuracy of the radiometer’s measurements, Lagerhoef said.
The Aquarius and SMOS missions are complementary, Chao said, because SMOS is designed to offer higher spatial resolution and Aquarius is designed to offer better accuracy. Together, the two satellites “will help us understand how to measure salinity from space,” he said.
While the Aquarius team hopes to one day combine the information derived from its sensors with SMOS data, the team’s initial goal is to launch the spacecraft and test its instruments. Mission officials plan to spend approximately one month after launch evaluating the on-orbit status of the spacecraft, deploying antennas and turning instruments on, Chao said. Then, it will take anywhere from “a few months to no more than 12 months” to calibrate the instruments.
Aquarius mission data will be collected by a ground station in Argentina and processed at NASA Goddard. To fulfill mission requirements, the Aquarius team must produce validated salinity data within one year, Lagerloef said.
In the past, salinity information was gathered in the water by ships, buoys and autonomous underwater vehicles. Those techniques provided salinity data for locations where maritime traffic was heavy, but there was a lack of information in areas such as the South Pole where rough weather and strong currents inhibit travel, Chao said. Thousands of ships and unmanned vehicles would be needed to provide the type of comprehensive salinity data that can be obtained by satellite, he added.
The scientific community has been pushing for space-based measurements of salinity for a long time, Chao said. “I hope we will fly satellites from now on to monitor salinity on a continuous basis,” he said.
