Measurement of Organic Molecules and D-H Ratios in Laboratory Mass-Spectra of Hypervelocity Dust Impacts into Ice

Introduction: Water ice is prevalent in the solar system, and there have been numerous studies of the effects of radiation and charged particle bombardment of water ice in laboratory settings [1-6]. However, many questions yet remain concerning the effect of interplanetary dust particle (IDP) bombardment on icy surfaces and bodies, as IDP impacts into ice have not been heavily investigated. This is despite the fact that IDP impacts are expected to be at least as important as radiation and charged particle bombardment [7]. Since liquid water is regarded as a prerequisite for life, icy ocean worlds such as Europa and Enceladus are the focus of several planned NASA and ESA fly-by missions [8-9]. Water plumes erupting from Enceladus's surface have been observed [10], and analysis of the plume using impact ionization time of flight (TOF) mass spectrometry from the Cosmic Dust Analyzer (CDA) on the Cassini spacecraft strongly indicate that the plume originated from the subsurface ocean [11]. There have also been observations of what appear to be similar water plumes on Europa [12-14]. These observations indicate that the environment around these bodies is rich with dust from both the ice surface and the subsurface oceans, and that fly-by spacecraft with TOF spectrometers will be able to study surface and subsurface chemistry in situ without landing. Isotopic ratios have been used as metrics for solar system formation and evolution models, and the deuterium-hydrogen (D-H) ratio is of particular importance in studying the formation of planetary bodies. Temperature-dependent chemical processes result in deuterium-enrichment of water ice relative to hydrogen at low-temperatures [15]. Such enrichment processes enable measurements of D-H ratios in outer solar system bodies to be used to constrain the time and location that planetary bodies formed in the solar system as well as other geophysical phenomena [16-18]. While laboratory work has been performed to match CDA flight spectra, these studies have used laser ablation of flowing liquid sources rather than actual dust impact into actual ice surfaces [19]. However, the University of Colorado dust accelerator at the Institute for Modeling Plasma, Atmospheres, and Cosmic Dust (IMPACT, impact.colorado.edu), paired with a cryogenic target capable of creating H2O ice mixtures, allows for unique and tightly controlled experiments to study hypervelocity dust impact into ice. Such experiments will answer significant questions about the long term chemical evolution of icy bodies under dust bombardment as well as the survivability and detectability of certain types of chemistry in icy dust grains, be they isotopic ratios or complex organics, studied by impact ionization TOF instruments on flyby spacecraft. Experimental Setup: The IMPACT dust accelerator at the University of Colorado uses a 3 MV linear electrostatic potential to launch micron-sized dust particles at velocities up to 100km/s [20]. The accelerator features non-destructive inline beam detectors that record particle mass, velocity, charge, and radius. Active particle down-selection is provided by an FPGAcontrolled particle selection unit. This unit prevents impact of particles outside of a user-defined mass, velocity, radius, or radius parameter space. Selected particles are impacted onto the ice target, shown in Fig. 1.