Why Study Gravity to Learn About Water?

The force of gravity not only keeps us from floating away, it lets NASA study Earth’s water & ice from space. Using a pair of twin satellites named GRACE, we can monitor where our planet’s water is going, even when it is underground. Gravity is an attraction between two objects—such as a human and the Earth—and its strength varies depending on how much mass those objects have and how far apart they are. For example, the tug of gravity is weaker for a person on the Moon than on the Earth because Earth is more massive, bigger and more dense.

The Earth’s surface always changes, but for the most part, the variations occur over long periods of time. A mountain tends to stay where it is, and the mass of the mountain itself remains largely unchanged. This means that the gravity influence of larger features is relatively constant and is known as the mean (or long-term average) gravity field. However, water on Earth’s surface is almost always in motion. Though it is distributed over and below the landscape as well as in the oceans, water has mass; the greater the mass, the greater the gravitational attraction. Its mass variations occur on smaller time scales from days to months and multiple years, mostly due to water being cycled between the atmosphere, oceans, continents, glaciers and polar ice caps. These shorter-term mass fluctuations contribute to what is known as the time variable gravity field.

NASA's Gravity Recovery and Climate Experiment (GRACE) mission measures variations in gravity over Earth's surface, producing a new map of the gravity field every 30 days. The twin satellites of GRACE (launched in March 2002) show how the planet's gravity differs not only from one location to another, but also from one period of time to another. “Time-variable gravity is the thing you want to measure,” Science Team Lead Mike Watkins said. That's because gravitational differences over time are due largely to movement of Earth's water, both in liquid and ice forms. “The changes that we want to observe in the Earth system, like sea-level rise, polar ice-cap mass loss in Greenland and Antarctica, or large-scale water storage,” he said, “are actually quite different from year to year and we want to keep tracking that.”

How it Works

GRACE, GRACE-Follow-on and the lunar GRAIL mission all use the same method to map gravitational fields. Each mission consists of two nearly identical satellites. One follows the other along the same orbit as both continually measure the distance changes between them using microwave ranging instruments. GRACE doesn’t carry a suite of scientific instruments; the twin satellites are the instrument.

As the leading satellite approaches a region of greater gravity (for example, a mountain or--in the case of Earth--a large mass of ice or aquifer water underground), it is pulled a little bit farther ahead of the trailing satellite, slightly increasing the distance between them. Then, as the lead satellite flies past the high-gravity area, it gets pulled slightly back while the trailing satellite--which is now approaching the gravitational mass--is pulled slightly ahead, narrowing the gap. Scientists are able to interpret the changes in satellite separation distance to map the gravity field. Though the two GRACE satellites are about 137 miles (220 km) apart, they are able to measure their separation distance to within one micron, about the diameter of a blood cell, enabling them to sense very subtle differences in Earth's gravity field. Flying at an altitude of about 300 miles (500 km), they are able to detect gravitational differences on the planet's surface equivalent to that of a 300-km disk of water only 1 centimeter thick.

When a mountain is covered in snow, it has more mass. GRACE detects that change when the pull on the leading satellite is slightly stronger than normal, changing the distance between the two satellites. GRACE detects loss of mass, too. When groundwater supplies vanished in the Southwest drought, GRACE was able to track the loss of mass.

The unique comprehensive measurements from GRACE have allowed the GRACE Science Team to track changes in the mass of the Colorado River Basin related to changes in water amount on and below the surface. Monthly measurements of the change in water mass from December 2004 to November 2013 revealed the basin lost nearly 53 million acre feet (65 cubic kilometers) of freshwater, almost double the volume of the nation's largest reservoir, Nevada's Lake Mead. More than three-quarters of the total - about 41 million acre feet (50 cubic kilometers) - was from groundwater. “There's only one way to put together a very large-area study like this, and that is with satellites,” said senior author Jay Famiglietti, senior water cycle scientist at JPL. The Colorado River is the only major river in the southwestern United States. Its basin supplies water to about 40 million people in seven states, as well as irrigating 4 million acres of farmland.

Mind the Gap

GRACE-Follow-on is scheduled for launch in 2017. How long the original GRACE spacecraft pair will remain operational is unknown, but it is hoped that they will continue to function at least through 2015. “We are doing the best we can to extend the life of GRACE and get GRACE Follow-on up," said Watkins, who was Project Scientist for both missions and now leads the GRACE-FO Science Team. If GRACE does not last until its follow-on mission takes over, he said, "I think it's safe to say that it will certainly leave a gap in our ability to track large-scale changes in polar ice mass and water storage.”

The two GRACE-FO satellites will use the same kind of microwave ranging system as GRACE, and are expected to achieve a similar level of precision. They will also test an experimental instrument using lasers instead of microwaves, which promises to make the measurement of their separation distance at least 20 times more precise.

GRACE-FO, like the original GRACE, is a partnership between NASA and the German Research Centre for Geosciences (GFZ).

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