When this reaction took place is another question-one that gets to the heart of comet science. The team found that 45P has a larger-than-average share of frozen methanol. But the researchers think the carbon monoxide might have reacted with hydrogen to form methanol. It’s possible that the methane is trapped inside other ice, making it more likely to stick around. 45P, however, is rich in methane and is one of the rare comets that contains more methane than carbon monoxide ice. But methane is almost as likely to escape, so an object lacking carbon monoxide should have little methane. By itself, this wouldn’t be too surprising, because carbon monoxide escapes into space easily when the Sun warms a comet. The results reveal that 45P is running so low on frozen carbon monoxide, that it is officially considered depleted. The findings helped fill in some gaps but also raised new questions. For five ices, including carbon monoxide and methane, the researchers compared levels on the sun-drenched side of the comet to the shaded side. While observing for two days in early January 2017-shortly after 45P’s closest approach to the Sun-the team made robust measurements of water, carbon monoxide, methane and six other native ices. “The combination of iSHELL’s high resolution and the ability to observe in the daytime at IRTF is ideal for studying comets, especially short-period comets,” said John Rayner, director of the IRTF, which is managed for NASA by the University of Hawai’i. This is particularly necessary in the cases of carbon monoxide and methane, because their fingerprints in comets tend to overlap with the same molecules in Earth’s atmosphere. ISHELL also has high enough resolving power to separate infrared fingerprints that fall close together in wavelength. The instrument covers wavelengths starting at 1.1 micrometers in the near-infrared (the range of night-vision goggles) up to 5.3 micrometers in the mid-infrared region. The spectral range of the instrument makes it possible to detect many vaporized ices at once, which reduces the uncertainty when comparing the amounts of different ices. With iSHELL, researchers can observe many comets that used to be considered too faint. DiSanti and colleagues conducted their studies using the iSHELL high-resolution spectrograph recently installed at IRTF on the summit of Maunakea. To identify native ices, astronomers look for chemical fingerprints in the infrared part of the spectrum, beyond visible light. Much less is known about native ices in this group than in the long-haul comets from the Oort Cloud. The comet-officially named 45P/Honda-Mrkos-Pajdušáková-belongs to the Jupiter family of comets, frequent orbiters that loop around the Sun about every five to seven years. “Comets retain a record of conditions from the early solar system, but astronomers think some comets might preserve that history more completely than others,” said Michael DiSanti, an astronomer at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the new study in the Astronomical Journal. These native ices are thought to hold clues to the comet’s history and how it has been aging. The gases all originate from the hodgepodge of ices, rock and dust that make up the nucleus.
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