Laboratory experiments on diffusion and sublimation of methane through ice dust layers to mimic cometary nucleus activity.

The desorption of volatiles from comets detains many pieces of information on cometary interiors, as well as the morphology of the ices hidden under the dust. The aim of this work is to study the sublimation and desorption of CH4 through amorphous solid water (ASW), and a layer of indene (C9H8, as a proxy for dust grains), during thermal processing, in order to simulate temperature changes occurring in cometary environments. Sublimation and diffusion experiments are performed for pure CH4, as well as CH4 layered or mixed with H2O. At about 30 K and with 1% of CH4 abundance ratio, the ices are deposited and the temperature is increased until (maximum) 200 K with a heating ramp of either 1 or 5 K/min. Mass and Infrared (IR) Spectroscopy are used to analyze the results through a Quadrupole Mass Spectrometer (QMS) and Fourier Transform Infrared (FTIR) spectrometry.

It has been noticed that depending on the heating ramp and type of deposition (layered or co-deposited ices), the desorption of methane varies, both in intensity and in desorption temperature. In the case of mixed ices, desorption is lower in intensity and occurs at higher temperatures compared to layered ices. Moreover, more methane desorbs due to the crystallization of water, rather than at its pure ice sublimation temperature. When the heating ramp is faster, instead, the desorption of methane occurs at higher temperatures, for both types of ice deposition.

In a second set of experiments, temperature cycles are applied, meaning that temperatures are increased up to 140 K, decreased down to 30 K, and then increased and decreased again. The desorption process of methane and water crystallization are slower for mixed ices than for layered ones, thus meaning that if comets present a mixed structure, they will undergo more cycles to lose the same quantity of volatiles. During the first temperature cycle performed, water is observed to perform the change of phase from amorphous to crystalline and then, during the second cycle, to a “better” crystalline cold. By reaching only 140 K and not 200 K still some methane is retained after one cycle, while water does not sublimate much.

Finally, a layer of indene is placed on top of a layered/mixed ice structure. The thicker the indene layer the higher the temperature at which methane desorbs since it is more difficult at colder temperatures to diffuse through the indene crust. However, the thicker the crust layer, the larger the quantity of methane that is observed to go out.

The desorption of methane that happens, even if slightly, during the entire process of warming up, is linked to the presence of the coma around the comet and its evolution. Moreover, the shift due to different ramps used experimentally allows extrapolating the desorption temperature shifts experienced by comets along their orbit. The temperature cycles performed give us high hopes for astrophysical results regarding comets since by noticing that the sub-layers retain material, it is understood the evolution of comets during their lives. The crust experiments can give insight into the behavior of cometary nuclei and their layer of crust on top. The more the dust surrounding the nucleus and thus the thicker its layer, the more difficult it will be for molecules trapped in the nucleus to desorb. Therefore, higher temperatures and thus multiple orbital cycles are needed in order for these lower layers to desorb.