A close pass by Comet Wirtanen in 2018 offered researchers an unprecedented opportunity.
Comets are full of surprises. Not only do they often perform below or occasionally above expectations, but they also offer a glimpse of what remains of the very early solar system. In December 2018, astronomers had the unprecedented opportunity to study one of these relics of the early solar system up close when Comet 46P / Wirtanen passed Earth only 30 times the Earth-Moon distance (7.1 million miles away ) flew by in the century.
The orbit of comet 46P / Wirtanen. NASA / JPL
The short-period comet 46P Wirtanen was discovered in 1948 by the astronomer Carl A. Wirtanen and orbits the Sun every 5.4 years on an orbit that takes it from a perihelion of 1.06 AU from the Sun to an aphelion of 5.13 AU directly outside of the perihelion of Jupiter.
The 2018 approach to Earth was particularly favorable for the comet, and this time astronomers at the WM Keck Observatory in Maunakea, Hawaii were ready. Keck’s near-infrared spectrograph (NIRSPEC) just received a major upgrade with more pixels and higher sensitivity, an upgrade that would receive the comet’s first light spectra.
Instruments also need hugs. Dr. Emily Martin with the newly upgraded NIRSPEC instrument. WM-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 observed in the outgassing of Comet Wirtanen, they also discovered a high alcohol ratio for the comet along with an abnormal warming mechanism.
“46P / Wirtanen has one of the highest alcohol-aldehyde ratios measured to date of all comets,” says Neil Dello Russo (JHU / APL) in a current press release. “This gives us information about how carbon, oxygen and hydrogen molecules were distributed in the early solar system in which Wirtanen originated.”
Findings from the most recent NIRSPEC upgrade. Keck Observatory Astronomy Talk Video
The Keck study also found continuous warming of cometary material that sublimes through coma, the well-known ring of gas and dust that surrounds a comet’s core. The amount of heat is believed to decrease with distance and was more than what could be explained by simple incident solar radiation.
“Interestingly, we found that the temperature measured for water gas in the coma did not decrease significantly with distance from the nucleus, which suggests a warming mechanism,” says Erika Gibb (University of Missouri-St. Louis) in the current press release.
One possibility is ionization by sunlight close to the core. “Another possibility is that solid chunks of ice fly away from Wirtanen.” Says Gibb. This has been documented in the past. Especially during NASA’s EPOXI mission to Comet Hartley 2. “These chunks of ice fall away from the core and sublimate and release energy further out in the coma.”
This copious release of water is consistent with a young, hyperactive comet like 46P / Wirtanen. Like many periodic comets, Wirtanen was likely trapped in the inner solar system over the past million years. Ancient comets are considered to be one of the possible sources of the earth’s primordial seas.
An Arecibo radar image of Comet 46P captured during the 2018 pass near Earth. Photo credit: Arecibo / NASA / NSF
So far it has been difficult to capture the effects in the internal coma, where icy grains release water, in contrast to the sublimation of ethane, hydrogen cyanide and acetylene in the external coma; Missions like ESA’s Rosetta to Comet 67P / Churyumov-Gerasimenko only gave us a glimpse into this process, and the Earth’s atmosphere makes this process difficult to study.
The Keck Observatory Complex. Photo credit: WM Keck Observatory
The Keck team used the infrared NIRSPEC instrument to determine specific wavelengths of water transition, which made it possible to analyze the distribution of volatiles throughout the coma. This capability is a major innovation for a ground-based telescope.
Comet Wirtanen is also worth studying, as it has been shortlisted 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 destination of the Rosetta mission.
One day we can see Comet Wirtanen up close. For now, raise a glass to Wirtanen and NIRSPEC and keep in mind that comets could well be the source of the ice in this next refreshing drink.