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The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH₂CCH₂, an isomer of the well-measured propyne (also called methylacetylene) CH₃CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule.

The Cassini spacecraft explored the Saturn system from 2004 through 2017, during which it made 127 targeted flybys of Titan. On board the spacecraft were a suite of remote sensing instruments, including the Composite Infrared Spectrometer (CIRS).

The Texas Echelle Cross Echelle Spectrograph (TEXES) is a ground-based mid-IR spectrograph capable of achieving a resolving power of about 50 times higher resolution than CIRS.

This increased resolution has allowed Lombardo et al, as published in ApJ Letter Volume 881, Number 2, to differentiate the spectral signatures of these isomers, and to make the first unambiguous detection and measurement of the propadiene molecule on Titan, and in an extraterrestrial environment. The team discovered that there is about one-tenth as much propadiene as there is propyne in Titan’s middle atmosphere.

A detailed comparison of the amount of each molecule on Titan can help reveal the available hydrogen in its atmosphere and promote a better understanding of the chemistry of Saturn’s largest moon.