Successful Capture of Plume Ice Samples and Organic Molecules for Biomarker Analyses in a Valid Laboratory Model for the Enceladus Encounter

Abstract:

We carried out ice particle impact experiments to directly evaluate the feasibility of collecting Enceladus plume samples for high sensitivity organic molecule and biomarker analyses in a fly-by and/or orbiter mission profile (New et al. DOI: 10.1111/maps.13448; New et al. DOI:10.1111/maps.13554). Following the suggestion in Mathies et al. (DOI: 10.1089/ast.2017.1660), we evaluated soft metal foils (Au, Al, and In) as media for non-destructive organic capture. Our experiments were carried out using a light gas gun (Univ. of Kent, UK) to determine the efficacy of the Capture Surface (CS) under realistic plume impact conditions. Epifluorescence microscopy was used to quantitatively evaluate the capture efficiency and survival of organic molecules entrained in ice particles. An organic fluorophore (Pacific Blue) was dissolved in a buffer solution, frozen, and fired at the metal foil targets at velocities from 1–3 km/s. The salinity, organic mass and size of the ice particles were representative of Enceladus plume particles.

Our experiments are the most realistic and the only quantitative evaluation of the feasibility of organic capture and analysis from the Enceladus plume. We present our methods and quantitative results for organic capture as a function of crater size, CS and impact velocity. The results clearly demonstrate that an indium CS can provide successful capture of intact organic molecules entrained in high-velocity (1–3 km/s) ice particles and that capture efficiencies of ~10% can be expected when integrating over a large number of impacts, with capture as high as ~50% for individual impacts. The optimal velocity was between 1–2 km/s where high capture efficiencies (10-50%) were observed in over 25% of the craters formed by 5-15 µm diameter particles.

Our instrument, the Enceladus Organic Analyzer (EOA, http://eoa.ssl.berkeley.edu), utilizes an open capture chamber to collect organic content on a CS during plume transits. The capture chamber is then closed and the organic residue is dissolved in a low volume solution and transported to the analyzer. Unlike impact ionization TOF MS instruments, accumulative capture on the CS can be achieved via multiple plume transits, providing EOA with a large ice sample that provides in principle nanomolar biomarker sensitivity when deployed as an Enceladus orbiter.