Feature Story
NASA's Galaxy Evolution Explorer Looks Beyond a Comet's Dusty Tail
Comets may be admired for their spectacular "tails," but in their "hearts" they carry important ingredients that could shed light on conditions in the early solar system and planet formation.
To gain some insights into these ingredients, NASA's Galaxy Evolution Explorer (GALEX) periodically breaks from observing distant galaxies located billions of light years away, and directs its wide ultraviolet eyes toward comets that swing by the inner solar system.
"The properties of GALEX that make it an excellent instrument for studies of external galaxies, also suit it to study comets. These properties include a wide field of view with nearly uniform sensitivity, and sensitivity to UV emissions," said Dr. Jeffrey Morgenthaler of the University of Washington, Seattle, Wash.
In early 2005, Morgenthaler led a GALEX guest investigator project to conduct ultraviolet observations of comet Machholz as it zipped by Earth.
Comets are icy dirtballs that were created around the same time as planets, and spend most of their time roaming the coldest reaches of our solar system. Many astronomers consider them to be "cosmic freezers" preserving the ingredients of our early solar system in pristine condition. Periodically, comets do travel to the inner solar system. As these freezers near the Sun, they start to defrost in a process called "sublimation," in which a solid converts directly to gas. The material released in the sublimation of a comet forms the object's signature tail, and the tenuous shell of gas surrounding its core, commonly known as a "coma." Except for the handful of comets that have been targets of spacecraft fly-by missions such as NASA's Deep Impact and Stardust, the "icy dirtball," or nucleus of a comet has been difficult to see.
According to Morgenthaler, until astronomers actually land on a comet and "taste it" which the European Space Agency's Rosetta mission is scheduled to do in 2014, their best insights into the composition of the ices that make up the nucleus comes from studies of a comet's coma.
This is where GALEX comes in handy. A comet's coma is often difficult to study with optical telescopes because the sunlight reflected by dust particles in the comet's tail outshines it. But, because reflecting sunlight does not shine as brightly in the ultraviolet, GALEX gives astronomers a better view of the coma. Using the spacecraft's "grism" observation mode, astronomers can spread the glow of a comet's coma into different colors to identify its atomic and molecular makeup.
"In principle, by studying the relative brightness of coma emission lines, we can tell what the relative concentrations of various materials in a comet nucleus are. In practice, this problem is complicated by the various processes the materials go through on their way out to the coma," said Morgenthaler. "Disentangling these effects is part of the fun of science."
The molecule hydroxyl, for instance, exists in a comet's coma, but isn't actually found in its nucleus. From previous observations, astronomers know that hydroxyl forms when harsh UV light from the Sun breaks apart sublimated water molecules in a process called "photochemistry." Through the same process, hydroxyl molecules are eventually broken down even further into oxygen and hydrogen as time goes on. By studying exactly how the hydroxyl emission from comet Machholz fades with increasing distance from the comet's nucleus, Morgenthaler and his team hope to understand the process of photochemistry better.
"As we improve our understanding of photochemistry, our ability to probe the composition of a comet nucleus using coma emissions improves, " said Morgenthaler. "Since we can't send spacecraft to every comet that swings by, coma observations are the only way we can study a large enough number of comets to figure out what went on in the early solar system and what has happened to comets since then."
On July 4, 2005, a team of guest investigators led by Dr. Paul Feldman of Johns Hopkins University, in Baltimore, Md., also used GALEX to study the coma of comet Tempel 1 when NASA's Deep Impact mission smashed an 820-pound (373-kilogram) impactor probe into the comet. Data taken from the mission's flyby imaging spacecraft, various observatories in space including GALEX, and ground-based telescopes worldwide, are currently being analyzed.
Based on the success of the Machholz and Tempel 1 observations, Morgenthaler's team used GALEX to observe comet Schwassmann-Wachmann 3 in early 2006. This comet put on a spectacular display for astronomers, splitting into multiple pieces as it came unusually close to the earth.
"It was like nature's version of Deep Impact," said Morgenthaler.
