That measurement has been missing from the array of clues -- rainfall, atmospheric chemistry, humidity and temperature -- used by scientists to predict change in the local and global climate. Using soil moisture, they can calculate evaporation -- the process that links the water, energy and carbon cycles -- giving them a better understanding of global change.

"Soil moisture has been one of the Holy Grails. The community of Earth-system scientists has been trying to measure it for a long, long time, but couldn't because it's so expensive," said Dara Entekhabi, a hydrologist and professor of civil and environmental engineering at MIT. "We have a measurement for rainfall, atmospheric chemistry, humidity and temperature, but surface soil moisture has been missing."

The Hydrosphere State mission (Hydros), selected by NASA from 18 competitors, would be launched upon mission approval by NASA in 2007. Hydros would measure soil moisture globally from a satellite in near-Earth orbit.

The satellite will be equipped with a reflector antenna (the same type used to send cell phone signals), weighing less than 40 pounds and measuring six meters in diameter.

The antenna will rotate at 15rpm to scan the Earth, measuring how much water is contained in the soil on which we walk, farm and build our houses.

The project will cost $218 million to design, build and launch by a team of scientists from MIT's Department of Civil and Environmental Engineering and Center for Space Research, NASA's Jet Propulsion Laboratory and Goddard Space Flight Center, the Canadian and Italian space agencies, and the University of Montana. The Department of Defense, which could use soil moisture data to determine if terrain is passable and to make weather predictions, is a funding partner as well. Entakhabi is principal investigator on the project.

Not Just A Drop In The Bucket
Soil moisture gives important clues to weather, because it affects climate in a big way. If pushed just a little one way or the other, it can have a profound effect. Take the drought in the Midwest in 1988. That started with a small area over the Midwest: the soil became dry and there was less evaporation, which meant less precipitation. Soon, crops and cattle were dying as the drought spread. The opposite happened in 1993, when the Mississippi River flooded for weeks on end.

"Soil moisture tells us the state of the surface land. In the same way that temperature tells the state of the surface oceans, soil moisture controls the rate of the water cycle, which affects weather and climate and how much evaporation takes place," said Entekhabi.

Kyoto Requirements
In addition to measuring soil moisture, Hydros would also tell scientists whether the surface soil moisture is frozen or not. In forests this can help scientists determine the length of the growing season, telling them whether a forest is a net source or net sink of carbon. During the growing season, carbon is sequestered in forests through the process of photosynthesis, when plants use carbon dioxide, sunlight and water to create biomass.

The Kyoto Protocol of 1997 called for each participating country to have a quota for carbon emission, a system that presumes all atmospheric carbon has been accounted for. If industry is a carbon source, a forest could be a carbon sink, offsetting the carbon released by industry. Without proper measurements, the Kyoto agreement won't be able to balance the books.

"It's like having a bank account and not knowing whether someone is depositing into it or withdrawing from it. You can't balance your bank statement," said Entekhabi. "We need this if we're going to stabilize the amount of carbon in the atmosphere."

Hydros is part of NASA's Earth System Science Pathfinder (ESSP) program, characterized by medium-sized missions capable of being built, tested and launched in short time intervals. Hydros is one of three ESSP finalists, at least two of which are expected to be approved for flight next year. The others propose to measure the salinity of the ocean and create an atmospheric carbon dioxide profile.

MIT scientists have been developing the algorithms required for the operation over the past 10 years. Two years ago, when Entekhabi heard about NASA's ESSP, he organized the group of about 30 collaborators to run Hydros through the program. The data will be processed at MIT, the University of Montana, Goddard and the Canadian Space Agency.

Components of the satellite will be provided by industrial firms, such as Spectra-Astro; the satellite will be launched on a NASA-owned Taurus rocket.

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