For Immediate Release
April 13, 2012
Contacts: Lee Tune, 301 405 4679 or ltune@umd.edu

UMD Team Monitors Pristine Comet Garradd To Watch It Twice Cross 'Snow Line'

Comet Garradd on Aug. 1. 2011 as seen from Australia. Credit: Peter Lake.

COLLEGE PARK, Md. -- Scientists consider comets to be relatively pristine leftovers from the early days of our Solar System. Now a University of Maryland team of scientists is tracking one named Garrard that offers a unique opportunity to study a comet both long before and long after it reaches is closet point to the Sun.

"This likely is this comet's first ever journey into the inner solar system, said team leader Dennis Bodewits, an assistant research scientist at the University of Maryland. "Thus it provides us a novel opportunity to see how comet material frozen since the dawn of Solar System will change and evolve as a result of being warmed by its first passage relatively close to the Sun."

Until a few years ago, Garradd hung out in the Siberia of the Solar System, a frozen region known as the Oort Cloud, where it orbited thousands of times farther from the Sun than does Earth. Then some gravitational perturbation, probably caused by a passing star, drastically changed Comet Garradd's orbit, sending it on a very elongated elliptical path that now takes it from the Oort Cloud nearly to Earth's inner solar system neighborhood and then back out again.

The team began following Comet Garradd, formally designated C/2009 P1, a year ago when it was inbound toward the Sun, but still some 320 million miles away, a distance four times the distance between the Earth and the Sun. The comet passed its closest point to the Sun, called its perihelion, in December and was at its closest to Earth in March. Using NASA's Swift satellite, the team plans to study this unusually dust-rich comet, for an equal distance on the outward-bound leg of its new orbit.

Frequently referred to as "dirty snowballs," comets consist of varying amounts of dust and ice particles. The ices in a comet are both frozen gases and frozen water. Comets warm up and give off gas and dust whenever they venture near the sun. According to current scientific understanding, 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 (3AU), so astronomers regard this as the solar system's "snow line."

But why do we see comet activity beyond the snow line where water can no longer sublimate? That is the question we're addressing here.

"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 Bodewits. "Exactly why we saw this comet activity beyond the snowline is what we want to figure out.

"We plan to do this using Swift's unique capabilities to monitor Garradd as it moves away from the Sun and beyond the snow line, where few comets are studied."

Comets are known to contain frozen gases, such as carbon monoxide and dioxide (CO and CO₂), which sublimate at colder temperatures and much farther from the sun than does water ice. 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.

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.

The instrument includes a prism-like device called a grism, which separates incoming light by its wavelength. While Swift's UVOT cannot detect water directly, the molecule quickly breaks up into hydrogen atoms and hydroxyl (OH) molecules when exposed to ultraviolet sunlight. The UVOT detects light emitted by hydroxyl and other important molecular fragments - such as cyanide (CN), carbon monosulfide (CS) and diatomic and triatomic carbon (C₂ and C₃, respectively) - as well as the sunlight reflected off of cometary dust.

"Tracking the comet's water and dust production and watching its chemistry change as it moves deeper into the solar system will help us better understand how comets work and where they formed," said Stefan Immler, a researcher and Swift team member at NASA's Goddard Space Flight Center in Greenbelt, Md.

Thanks to Garradd's brightness and the UVOT's sensitivity and resolution, the researchers can monitor the comet when it is beyond the grasp of most ground-based observatories. Plans call for observations at eight different distances from the sun out to about 5.5 AU, which the comet will reach in April 2013.

Garradd Research Team Features Leading Comet Scientists
Principle Investigator Bodewits is a member of the EPOXI/Deep Impact and Stardust-Next science teams. The Garradd research team also includes top comet scientists Michael A'Hearn and Tony Farnham of the University of Maryland and NASA Goddard's Wayne Landsman. UMD's A'Hearn, winner of 2008 Kuiper Prize in recognition of his seminal contributions to, and leadership of, the study of comets, led the spectacularly successful NASA comet missions Deep Impact and EPOXI. He also is a mission scientist on a proposed new mission known as Comet Hopper.

Media Contact
Lee Tune
Associate Director
Office of Public Affairs
University of Maryland
301-405-4679
ltune@umd.edu

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