Solar System Exploration

A discovery about the moon made in the 1960s is helping researchers unlock secrets about Earth's ocean today.

By applying a method of calculating gravity that was first developed for the moon to data from NASA's Gravity Recovery and Climate Experiment, known as Grace, JPL researchers have found a way to measure the pressure at the bottom of the ocean. Just as knowing atmospheric pressure allows meteorologists to predict winds and weather patterns, measurements of ocean bottom pressure provide oceanographers with fundamental information about currents and global ocean circulation. They also hold clues to questions about sea level and climate. Oceanographers have been measuring ocean bottom pressure for a long time, but the measurements have been limited to a few spots in a huge ocean for short periods of time, says JPL oceanographer Victor Zlotnicki.

Launched in 2002, the twin Grace Satellites map Earth's gravity field from orbit 500 kilometers (310 miles) above the surface. They respond to how mass is distributed in the Earth and on Earth's surface -the greater the mass in a given area, the stronger the pull of gravity from that area. The pressure at the bottom of the ocean is determined by the amount of mass above it. Ocean bottom pressure is the sum of the weight of the whole atmosphere and the whole ocean," says Zlotnicki. When winds move water on the surface, ocean bottom pressure changes. When glaciers melt and add water to the ocean, the ocean's mass increases and bottom pressure increases, either at one place or globally.

Measuring ocean bottom pressure was one of the things we said we wanted to do from the very beginning of the mission, says Grace Project scientist Michael Watkins, "but it has been a challenge. The signal is very small and hard to detect. Gravity changes over the ocean are miniscule compared to those over land. The ocean is a fluid. It yields to pressure and spreads the effect over a vast area. Nothing in the ocean gives as big a gravity signal as a flooding Amazon River or melting glaciers in Greenland or Alaska , changes that Grace can measure fairly easily, says Watkins. Those hydrology signals are huge in comparison, he says.

However, as the mission progressed, Watkins explains, the science team has found better ways to process Grace Data. And by turning to a technique developed for the lunar world, Grace Researchers are getting the precise measurements of ocean bottom pressure they were hoping for.

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