29 July 2015
By Larry O’Hanlon
Images of an unusually dusty comet have revealed strange streaming clumps that could hold the secrets to how comets create their beautiful, sweeping, striated tails.
Comet C/2011 L4 barged into the research of solar physicist Nour E. Raouafi when he was studying the sun using images from the SECCHI/HI-1 telescope aboard the solar-observing spacecraft STEREO-B.
“I was looking at CMEs (coronal mass ejections) and stumbled into this,” said Raouafi, who conducts his research at Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Maryland. “This comet came into the field of view and it was amazing.”
C/2011 L4 (Pan-STARRS) is likely a new comet on its first trip to the inner solar system from the Oort cloud, where comets in the solar system originate, Raouafi said. Studying the comet provides scientists a rare opportunity to monitor the composition and dynamics of a relatively pristine comet, he said.
Not only was the tail of the comet spectacularly dusty, but Raouafi noticed strange clumps that appeared to be blowing in the solar wind straight off the nucleus of the comet. Not being a comet researcher, he asked for advice from JHUAPL astrophysicist Casey Lisse, who was impressed with what he saw.
“This is about one of the dustiest comets we’ve ever seen,” said Lisse. “It’s really bright. It has really given us a show.”
The images were put together to make a movie that shows the creation of the comet tail and reveals the dynamic clumps pouring off the nucleus like smoke from a blown-out candle. The dustiness make it an unprecedented laboratory for studying how comets grow their tails, said the researchers, who are authors of a new study detailing the findings in Journal of Geophysical Research-Space Physics, a publication of the American Geophysical Union.
Adding to the value of the video was the lucky position of the STEREO-B spacecraft, which enabled the scientists to look straight down on the dusty path of the comet. With the movie, the researchers have been able to work out the speed and size of the clumps, which helps them figure out what they might be.
“The angle we’re seeing it from lets us see it in great detail,” said Raouafi. “These small blobs get ejected by the comet and carried away by the solar wind.”
Since the clumps are moving directly away from sun, it appears they are being driven by the solar wind. The clumps are flowing past the comet at typical speeds of 250 to 500 kilometers per second (about 155 to 310 miles per second). This is about the same speed as the solar wind, suggesting the clumps are very small, lightweight particles or gas, according to the new study.
The fact that the clumps do not change in size as they blow away suggests they are being held together by the magnetic field of the solar wind. This means they are likely charged particles, or ionized gas, which are sensitive to magnetic fields. Likely candidates are ionized potassium and sodium that have been roasted out of the rocks on the comet as it passes close to the sun, according to the researchers.
The other, more exciting, possibility is that the clumps are evidence of dust exploding, said Lisse. Models that have been developed to try and explain why comet tails have lines, or straie, that make them appear like they have been combed. These models assume that an important part of the process is explosion of comet dust, he said.
“Those clumps could be the signature of the explosion,” said Lisse.
Raouafi and Lisse hope the new paper will spark interest in the clumps by other researchers and lead to some new insights into how comets grow their glorious, awe-inspiring tails.
“We want people to help us tell the story and find out what’s going on,” said Lisse.
“These observations are a foretaste of what future missions that will fly very close to the sun, like NASA’s Solar Probe Plus and ESA’s Solar Orbiter, may teach us about sun-grazing comets,” said Raouafi.
– Freelance science journalist Larry O’Hanlon acts as AGU’s blogs manager and social media coordinator.