Low-Temperature Production of Cosmic Grains Analogs From Gas-Phase Molecular Precursors with the NASA Ames' COSmIC Facility
Complex carbon molecules and ions are ubiquitous in space and form the building blocks of carbonaceous cosmic dust grains, ultimately contributing to the formation of planets. Here, we report the first experimental investigations of the low temperature chemical pathways leading to the production of cosmic grain analogs from gas phase molecular precursors in order to better understand the evolution of cosmic carbon. The experiments were performed using the COsmic SImulation Chamber (COSmIC) to generate and characterize grains formed from gas phase precursors under controlled conditions representative of astrophysical environments [1]. Using COSmIC, it is possible to investigate the evolution of cosmic carbon from the formation of neutral and ionized gas phase hydrocarbons and PAHs [2], to carbon grains [3, 4]. This is achieved by using a discharge nozzle to (1) produce an adiabatic jet expansion and cool down Ar-hydrocarbon/PAH gas mixtures to astrophysically relevant temperature (150 K) before inducing chemistry by generating a plasma discharge in the stream of the expansion. This plasma-induced chemistry results in the formation of complex molecules and solid particles, analogs of cosmic grains. Solid grains are produced in-situ in the plasma expansion, carried by the accelerated gas in the expansion, and collected on substrates placed a few centimeters downstream of the electrodes. The results of a preliminary solid phase ex-situ analysis of cosmic grain analogs produced at low temperature in COSmIC from gas mixtures including molecular precursors of cosmic carbon grains, i.e., such as CH4 and C2H2 found in circumstellar ejecta of late C stars [3] as well as PAHs ubiquitously detected in IR emission bands seen in galactic and extragalactic environments [4] will be discussed. Scanning Electron Microscopy imaging was used to provide insight on the morphology and growth structure of the grains produced in COSmIC, and to investigate how the precursors used to produce the grains affect these parameters [3]. Laser desorption mass spectrometry was used to identify the molecules making up the main structures within the condensed grains [4]. NIR to FIR optical properties are characterized to provide critical information (functional groups, optical constants) to the scientific community, for use in radiative transfer models in particular, and to help decipher and enhance the return data from space observations.