An outbound comet that provided a nice show for skywatchers late last year is the target of an ongoing investigation by NASA's Swift satellite. Formally designated C/2009 P1 (Garradd), the unusually dust-rich comet provides a novel opportunity to characterize how cometary activity changes at ever greater distance from the sun.

A comet is a clump of frozen gases mixed with dust. These "dirty snowballs" cast off gas and dust whenever they venture near the sun. What powers this activity is frozen water transforming from solid ice to gas, a process called sublimation. Jets powered by ice sublimation release dust, which reflects sunlight and brightens the comet. Typically, a comet's water content remains frozen until it comes within about three times Earth's distance to the sun, or 3 astronomical units (AU), so astronomers regard this as the solar system's "snow line."

"Comet Garradd was producing lots of dust and gas well before it reached the snow line, which tells us that the activity was powered by something other than water ice," said Dennis Bodewits, an assistant research scientist at the University of Maryland, College Park, and the study's lead investigator. "We plan to use Swift's unique capabilities to monitor Garradd as it moves beyond the snow line, where few comets are studied."

Comets are known to contain other frozen gases, such as carbon monoxide and dioxide (CO and CO2), which sublimate at colder temperatures and much farther from the sun. These are two of the leading candidates for driving cometary activity beyond the snow line, but phase transitions between different forms of water ice also may come into play.

C/2009 P1 was discovered by Gordon J. Garradd at Siding Spring Observatory, Australia, in August 2009. Astronomers say that the comet is "dynamically new," meaning that this is likely its first trip through the inner solar system since it arrived in the Oort cloud, the cometary cold-storage zone located thousands of AU beyond the sun.

Comet Garradd was closest to the sun on Dec. 23, 2011, and passed within 118 million miles (1.27 AU) of Earth on March 5, 2012. The comet remains observable in small telescopes this month as it moves south though the constellations Ursa Major and Lynx.

Although Swift's prime task is to detect and rapidly locate gamma-ray bursts in the distant universe, novel targets of opportunity allow the mission to show off its versatility. One of Swift's instruments, the Ultraviolet/Optical Telescope (UVOT) is ideally suited for studying comets.

For more information visit http://www.nasa.gov/mission_pages/swift/bursts/comet-garradd.html

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To highlight declines in the world's groundwater supplies, a new visualization of Earth's groundwater reserves, created in part with space data from the joint NASA/German Aerospace Center (DLR) Gravity Recovery and Climate Experiment (GRACE) mission, debuted on New York's Times Square on March 22, International World Water Day.

The 30-second animation, titled "Visualizing Seasonal and Long-term Changes in Groundwater Levels," will be on display several times each hour through April 22 on Times Square's massive Thomson Reuters and NASDAQ digital signboards. Viewers of the interactive animation are invited to use their mobile devices to submit their city and add a graph to the sign. The animation can be viewed at: http://vimeo.com/user10042778 .

Netherlands designer Richard Vijgen developed the animation using GRACE data analyzed by professor Jay Famiglietti, director of the UC Center for Hydrologic Modeling at the University of California, Irvine; and from United States Geological Survey data supplied by Leonard Konikow. Vijgen was the winning entry in an international design visualization competition sponsored by the organization HeadsUP!, in collaboration with Visualizing.org. Founded by digital media artist Peggy Weil, HeadsUp! challenges designers to visualize critical global issues and create a shared sign for the public square.

Groundwater is a critical, but often overlooked, natural resource. According to a U.N. report, more than 1.5 billion people around the world depend on groundwater for their drinking water. It comes from the natural percolation of precipitation and other surface waters down through Earth's soil and rock, accumulating in cavities and layers of porous rock, gravel, sand or clay. Groundwater levels respond slowly to changes in weather and can take months or years to replenish once pumped for irrigation or other uses.

Famiglietti's analyses show that groundwater is being depleted at alarming rates in many of the world's major aquifers. "The GRACE data set is exciting, because it gives us the first global pictures of Earth's changing freshwater," he said.

The twin GRACE satellites, which celebrated their 10th year in orbit this week, measure minute changes in Earth's gravity field by measuring micron-scale variations in the separation between the two spacecraft, flying in formation 137 miles (220 kilometers) apart in low Earth orbit. These variations in gravitational pull are caused by local changes in Earth's mass. Masses of water, ice, air and solid Earth can be moved by weather patterns, seasonal change, climate change and even tectonic events such as large earthquakes. GRACE was developed by NASA's Jet Propulsion Laboratory, Pasadena, Calif.

For more information visit http://www.nasa.gov/mission_pages/Grace/news/grace20120322.html

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