The Intriguing Behavior of Ultraviolet Photodesorption and Color Temperature of Astrophysical CO Ice Analogs: A Possible Indication of Molecular Disorder

Ejection of molecules from ice-covered dust exposed to ultraviolet photons, known as photodesorption, is summoned to account for gas abundances in cold regions. Carbon monoxide (CO) is the second most abundant ice component in interstellar clouds. Photon energy transfer between CO molecules can eventually excite molecules on the ice surface, allowing desorption. CO photodesorption drops linearly by a factor of 4 for increasing ice deposition temperature between 7 and 20 K. Because translation of molecules leading to crystallization occurs above 20 K, amorphous CO ice must hold another property that depends on deposition temperature and has an impact on photodesorption. CO ice molecules act as tiny dipoles weakly bonded to each other. We used colorimetry to trace variations of the relative orientations between CO molecules, a process not observed using other techniques. Color variations seen by eye at different deposition temperatures range from brown to translucent, or transparent when CO ice is deposited above 20 K and is nearly crystalline. As expected, apolar N2 ice did not display a visual color. Color temperature measurements allowed quantification of this phenomenon. The behavior of color temperature matches the photodesorption, allowing a better understanding of this process, and provides a method to monitor structural changes in molecular solids. An implication in astrophysics is that the albedo of icy bodies can be low in CO patches. A brownish color is not only indicative of organic matter, as it may be due to disordered solid CO, and allows to determine the build-up temperature of CO ice.

Astrophy. J., 961, 1 (2021)