LONDON — Atoms of noble gases like helium, neon and argon — odorless, colorless, single-atom gases with low chemical reactivity — have long been known to exist in outer space. But molecules involving them have escaped detection, despite years of searching. Now, European astronomers say they have detected noble gas molecules in space for the first time, even though they were not looking for them. Researchers at University College London were using the European Space Agency's Herschel Space Observatory to measure the amount of dust in the remnants of various supernovas when they detected strong, mysterious emissions of light in the Crab nebula. Based on the frequency of the emissions, it was determined that they must be coming from spinning molecular ions of argon hydride. Beyond being a scientific first, the discovery helps confirm existing theories of how argon is formed. “We can tell from the frequency we observed which isotope of argon we're looking at,” said Michael J. Barlow, the lead author of the study, which was published in the journal Science, “and it was definitely argon-36. That's 18 neutrons and 18 protons.” Scientists had previously predicted that a supernova like the Crab nebula would produce argon-36 and not, say, argon-40, which is created by the radioactive decay of rocks on Earth. Most previous searches for noble gas molecules in space had focused on molecules containing helium, which is more abundant in the universe than other noble gases. Why they escaped detection for so many years remains a mystery, Dr. Barlow said. The molecule, argon hydride, was seen in the Crab Nebula, the remains of a star that exploded 1,000 years ago. Before the discovery, molecules of this kind have only been studied in laboratories on Earth. The noble gases, which include helium, argon, radon and krypton, usually do not react easily with other chemical elements, and are often found on their own. In the right circumstances, however, they can form molecules with other elements. Such chemical compounds have only ever been studied in laboratories on Earth, leading astronomers to assume the right conditions simply do not occur in space. "The Crab Nebula was only formed 1, 000 years ago when a massive star exploded," said Dr. Haley Gomez of Cardiff University's School of Physics and Astronomy. "Not only is it very young in astronomical terms, but also relatively close, at just 6,500 light years away, providing an excellent way to study what happens in these stellar explosions. Last year, we used the European Space Agency's Herschel Space Observatory to study the intricate network of gas filaments to show how exploding stars are creating huge amounts of space dust." Further measurements of the Crab Nebula were made using Herschel's SPIRE (Spectral and Photometric Imaging Receiver) instrument. Its development and operation was led by Professor Matt Griffin, from the School of Physics and Astronomy. As molecules spin in space, they emit light of very specific wavelengths, or colors, called "emission lines." The precise wavelength is dictated by the composition and structure of the molecule. Studying the emission lines observed by the SPIRE instrument allows astronomers to study the chemistry of outer space. The team did not set out to make the discovery, but stumbled upon it almost by accident. "We were really concentrating on studying the dust in the filaments with SPIRE, and out pops these two bright emission lines exactly where we see the dust shining," says Dr. Gomez. "The team had a hard time figuring out what these lines were from, as no-one had seen them before." It now seems the Crab Nebula provides exactly the right conditions to form such molecules. The argon was produced in the initial stellar explosion, and then ionized, or energized, with electrons stripped from the atoms in resulting intense radiation as shockwaves.
These shockwaves led to the formation of the network of cool filaments containing cold molecular hydrogen, made of two hydrogen atoms. The ionized argon then mixed with the cool gas to provide perfect conditions for noble gas compounds to form. The measurements allowed the team to gauge other properties in argon molecules. "Finding this kind of molecule allowed us to evaluate the type (or isotope) of argon we discovered in the Crab Nebula," said Dr. Gomez. "We now know that it is different from argon we see in rocks on the Earth. Future measurements will allow us to probe what exactly took place in the explosion 1000 years ago." "What a great detective story," added Griffin, from Cardiff University, and lead scientist of the team behind the SPIRE instrument. "Here we see the excellent performance of the Herschel-SPIRE spectrometer, the expertise of the instrument team in producing the highest quality data, and the tenacity and vision of the scientists analysing it, all coming together to make an intriguing new discovery." —Agencies
