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.

The orbit of Comet 46P/Wirtanen. NASA/JPL

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.

Instruments need hugs, too. Dr. Emily Martin with the newly upgraded NIRSPEC instrument. W.M. Keck Observatory.

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.”

Findings from the recent NIRSPEC upgrade. Keck Observatory Astronomy Talk Video

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