The greenness of comets

The 2022 hit movie Don’t Look Up (Netflix) portrays the government and industry attempting to acquire the valuable rare minerals in a newly discovered comet on a fatal collision course with earth.  Trillions of dollars of wealth and massive job opportunities are seen, blinding the authorities from seeing the impending disaster from the collision that the astronomers (played by Leonardo DiCaprio and Jennifer Lawrence) calculate from the data.  This is a fitting allegory for climate change and human denial, yet in a literal sense the color of comets really is green.  Why is that?

The images of comet Leonard, C/2021 A1 (Solar System Gallery) are from my home observatory in NJ using a 12.5” telescope and ZWO ASI071 camera, taken on Dec 8 just before morning twilight.  The green glow is quite apparent in these images which are carefully color-balanced.  The stars are trailing because the mount is tracking the comet (13x2min subframes, left panel;  6x2min subframes, right panel).  Known as Comet Leonard after its discoverer at the University of Arizona, it makes a good stand-in actor for the movie Don’t Look Up.  Although it won’t collide with the earth, Comet Leonard was found using Catalina Sky Survey’s 1.5m infrared telescope on Mt Lemmon.  Just like the movie, the Catalina survey is supported by NASA and the Near Earth Object Observation Program under the Planetary Defense Coordination Office.

From an astrochemistry view the emission of green or blue-green color from a comet’s nucleus but not the tail is an intriguing puzzle, only recently solved.  It is not the same photochemistry mechanism as others in astronomy, for example the fluorescent blue-green emission from ionized oxygen seen in planetary nebulae in the telescope. The nucleus of a comet is an agglomeration of rock, dust, and frozen gases.  As it gets closer to the sun, increasing heat causes the gases to sublimate and form a nebulous envelope around the nucleus known as the coma.  The tail of a comet is an extension of the coma drawn out by the solar wind.  Yet the green around the nucleus disappears in the tail which instead displays a reddish brown color. 

It has been thought for years that a comet’s green comes from the breakdown of the reactive molecule dicarbon (C2).  Dicarbon is an abundant molecule in the universe although not on earth, and multiple valence electronic states in its chemistry give it a rich spectroscopy.  The famous British scientist Wollaston reported the emission spectra of blue-green flames as early as 1802, the first glimpse of dicarbon.  Now a new study has solved the question of green in comets.  In this laboratory work, dicarbon chloride (C2Cl4) was irradiated by UV-laser, a way to generate dicarbon for spectral analysis (Borsovszky et al., Photodissociation of dicarbon: How nature breaks an unusual multiple bond. Proc Natl Acad Sci USA 2021, Vol 118, No 52).  Further irradiation at longer wavelengths generates a metastable state of the C2 molecule (a radical) which decays and radiates a characteristic greenish photon.  The emission spectrum of dicarbon is known as the Swan band, after the Scot physicist William Swan in the 1850s.  Swan bands are a characteristic of the spectra of carbon stars and some nebulae as well as comets.  Dicarbon photoemission in the Swan band requires two “forbidden” electron transitions which are favored in the environment of space but not on earth.  The spectral pattern (color) is a sensitive probe of local environment.  In their 2021 paper, Borsovszky et al. determined that the half-life of the C2 radical is a little under 2 days under the conditions of a comet at ~1 AU distance from the Sun.  This is the first solid explanation of why the head of a comet but not the tail glows green, because the dicarbon radical with its short half-life is dissipated as material streams out to the tail.