Laboratory Studies Required to Understand Cometary Emissions: O(1S) and O(1D) Yields from CO2 Photodissociation in the Extreme Ultraviolet

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This award's main goal is to produce laboratory measurements that are needed to reliably interpret and quantify carbon dioxide emissions from comets. Comets are relatively small bodies made of ice and dirt that orbit the Sun; however, they spend most of their time very far from the Sun at very cold temperatures. Thus, they are considered to be among the best preserved and most primordial objects in the solar system and their chemical composition is key to constraining solar system formation models. Carbon dioxide is of particular interest because the molecule may be present in comet nuclei in high enough quantities that sublimation (going directly from ice to gas upon heating without ever becoming liquid water) may drive a significant portion of the activity as comets orbit close to the Sun. The PI and his research team will conduct experiments in order to better understand how carbon dioxide degrades in sunlight after sublimating from comet nuclei that are close to the Sun. The project focuses on two atomic oxygen emission lines commonly observed in comet spectra, which are possibly largely formed during the breakup of carbon dioxide. Since carbon dioxide is unobservable from ground-based telescopes, the atomic oxygen emission lines serve as a valuable proxy for carbon dioxide. This work will also be useful for modeling atmospheres of Mars, Venus and some exoplanets. The PI will train and mentor a postdoctoral fellow and summer undergraduate students in this research. The planned experiments will be carried out at the Advanced Light Source of the Lawrence Berkeley National Laboratory. The team will use synchrotron radiation to determine O(1S) and O(1D) yields from carbon dioxide photodissociation at 90-115 nm. In order to amplify detection of these two atomic states, which normally have long radiative lifetimes, they will introduce xenon gas to the samples, which enhances the radiative rate by a factor of 75. Knowledge of these excited atomic oxygen lines from carbon dioxide photodissociation will enable the use of the atomic oxygen red and green line emissions (at 630/636.4 nm and 557.7 nm, respectively) as a probe of cometary composition.