A close pass of Comet Wirtanen in 2018 offered researchers an unprecedented opportunity.
Comets are full of surprises. Not only do they often under- or very occasionally over- perform versus expectations, but they also offer a glimpse of the remnants of the very early solar system. In December 2018, astronomers had an unprecedented opportunity to study one of these relics of the early solar system up close as Comet 46P/Wirtanen sped by Earth just 30 times the Earth-Moon distance (7.1 million miles away) on its closest passage for this century.
Discovered by astronomer Carl A. Wirtanen in 1948, short period Comet 46P Wirtanen orbits the Sun every 5.4 years, on a path that takes it from a perihelion 1.06 AU from the Sun to an aphelion of 5.13 AU, just outside the perihelion of Jupiter.
The 2018 approach past Earth for the comet was an especially favorable one, and this time, astronomers at the W.M. Keck Observatory on Maunakea, Hawai’i were ready. Keck’s Near Infrared Spectrograph (NIRSPEC) just received a major upgrade, featuring more pixels and higher sensitivity, an upgrade that would see first light obtaining spectra of the comet.
And the results, recently published in The Planetary Science Journal were a spectacular success. Not only did the team classify a list of key compounds seen out-gassing from Comet Wirtanen, but they discovered a high alcohol ratio for the comet, along with an anomalous heating mechanism at play.
“46P/Wirtanen has one of the highest alcohol-to-aldehyde ratios measured in any comet to date,” says Neil Dello Russo (JHU/APL) in a recent press release. “This tells us information about how carbon, oxygen, and hydrogen molecules were distributed in the early solar system where Wirtanen formed.”
The Keck study also noticed a continuous heating of cometary material sublimating through the coma, the familiar wreath of gas and dust surrounding the nucleus of a comet. The amount of heating is thought to decrease with distance, and was more than what could be explained by simple incoming solar radiation.
“Interestingly, we found that the temperature measured for water gas in the coma did not decrease significantly with the distance from the nucleus, which implies a heating mechanism,” says Erika Gibb (University of Missouri—St Louis) in the recent press release.
One possibility is ionization via sunlight close to the nucleus. “Another possibility is there may be solid chunks of ice flying off of Wirtanen.” Says Gibb. This has been documented in the past. Most notably during NASA’s EPOXI mission to Comet Hartley 2. “Those ice chunks tumble away from the nucleus and sublimate, releasing energy farther out in the coma.”
This copious release release of water is consistent with a young, hyperactive comet such as 46P/Wirtanen. Like many periodic comets, Wirtanen was probably captured in the inner solar system in the last few million years. Ancient comets are thought to be one of the possible sources of Earth’s primordial oceans.
Up until now, it has been difficult to capture action in the inner coma where icy grains are releasing water, versus the sublimation of ethane, hydrogen cyanide and acetylene in the outer coma; missions such as ESA’s Rosetta to comet 67P/Churyumov-Gerasimenko only gave us a glimpse of this process, and the Earth’s own atmosphere makes it difficult to study this process.
The Keck team used the infrared NIRSPEC instrument to target specific water transition wavelengths, allowing analysis of volatile distributions throughout the coma. This capability represents a significant first for a ground-based telescope.
It’s also worth studying Comet Wirtanen, as it has been on the short list for exploration proposals in the past: NASA’s Comet Hopper mission would have sent a nuclear-powered lander to the comet, and Wirtanen was the original target for the Rosetta mission.
One day, we may see Comet Wirtanen up close. For now, raise a glass to Wirtanen, and NIRSPEC and remember that comets may well be the source of the ice in that next refreshing drink.