Comet C 2022 E3 (ZTF) was discovered by astronomers using the Wide Field Survey Camera at the Zwicky Transient Facility this year in early March. (Photo: Dan Bartlett/NASA)
“It’s important to pay attention to celestial events like this because they’re often how we learn about our world,” said Darryl Janzen, Ph.D., an astronomy expert and instructor in the College’s Department of Physics and Engineering Physics. in USask Arts and Science. “Science itself was essentially invented because people noticed that the planets followed wandering paths through the stars and wanted to explain that.”
Janzen also coordinates public activities for the USask observatory, which is open for public viewing of celestial objects on the first and third Saturdays of each month.
Like many other comets, Comet 2022 E3 (ZTF) emits a distinct green color from its bright head as it streaks across the sky. The puzzle of the exact cause of this green coloration was solved in recent years, thanks in part to Herzberg.
Herzberg, winner of the 1971 Nobel Prize in Chemistry and a pioneering researcher who did much of his seminal work at USask from 1935-1945, spent his career studying the structure and geometry of molecules using spectroscopy, studying the the absorption and emission of light and radiation by matter. His research led to innovative understandings of how molecules and atoms function and interact, and formed the basis of many advances in astronomy, health care, chemistry and physics.
Herzberg began studying the diatomic molecule C2 as early as 1937 while at USask. C2 is formed because carbon (C) is a relatively unstable element and tries to stabilize itself by bonding to a second carbon molecule. His work eventually evolved into an analysis of how C2 might have interstellar significance. A prediction made by Herzberg while analyzing the spectroscopy of C2 laid the foundation for our current understanding of why colors appear in cometary comas, the cloud of dust and gas that surrounds the comet’s head.
According to work published in 1939 in The Astrophysical Journal, Herzberg speculated that the cause of the comet’s green hue may be due to sunlight causing C2 to reach a high vibrational level, causing the two molecules to break their bond and fall apart Rupture of these molecules was thought to release energy emitted as a green color.
This prediction remained unconfirmed for nearly a century due to the difficulty of testing such a scenario. In December 2021, a team from the University of New South Wales published a research paper in the Proceedings of the National Academy of Sciences that tested Herzberg’s theory for the first time. Through a laboratory experiment, the research team showed that C2 molecules decay at high vibrational levels and cause the emission of green light from the comet’s coma. This discovery provided scientific proof of what Herzberg had suspected in 1939.
“Comet comas are usually green, and as Herzberg predicted, the color comes from the photodissociation of diatomic carbon, which is an abundant molecule in comets,” Janzen said. “This recent study confirmed that C2 has a lifetime of about two days until sunlight breaks the molecule apart and emits a green photon. Not only the color, but also this short time frame explains why the green light only comes from the coma and not the tail of the comet.”
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