Evaluation of Science Measurement Capabilities for Enceladus Plume Fly-By Flight Profiles Based on Experimental HyperVelocity Particle Impact Studies and Enceladus Organic Analyzer (EOA) Sample Analysis Capabilities

Enceladus is an appealing location to look for biosignatures, but the small amount of plume sample that can be gathered and the rapid transit time require careful examination of the sampling parameters and instrument capabilities to ensure high science value. We consider organic analyses for two mission formats: A Saturn orbiter with Enceladus plume pass heights from 30-50 km and velocities from 3-5 km/s, and an Enceladus orbiter with a 50 km orbit height at 180 m/s.

The Enceladus Organic Analyzer (EOA, http://eoa.ssl.berkeley.edu) consists of a Plume Capture System with an area of 120 cm² and a microfluidic Organic Analyzer. After collection, the capture chamber is closed to dissolve and transport the organics in the ice sample for analysis. Unlike time-of-flight instruments, capture can be performed once or multiple times, providing a larger ice sample for analysis. Our concentration limits of detection depend on plume capture efficiency, accumulation, dissolution and transport to the analyzer. The capability of the EOA analyzer is determined by its 100 pM concentration sensitivity at the detector, which translates to a 200 pM target concentration in the solution delivered from the collector. The capture efficiency of organic molecules in ice particles depends on particle size, impact velocity and impact surface. We have demonstrated significant organic capture at velocities up to 3 km/s. For these simulations with a 50% efficiency each for capture and dissolution, we achieve a detection limit of 1 pg of analyte (100 MW) in the captured ice.

To evaluate various flight and detection scenarios, we benchmark against the goal of 1 pmol analyte/gram ice (1 nM or 0.1 ppb at 100 MW) recommended in the Europa Lander SDT for icy moon studies of biosignatures. Considering one 5 km/s pass at 50 km, ~20 μg of plume ice results in a sensitivity of <50 ppb; for eight 5 km/s integrated passes at 30 km we reach 3 ppb; and for lower velocity orbits with a 3 km/s pass velocity at 50 km we predict 1 ppb. Considering Enceladus orbit formats: for 10 passes at 180 m/s and 50 km height we predict <5 ppb and for 100 passes accumulating 2000 μg ice (practical because of the 2.8 hr orbit time) we achieve <0.5 ppb. We conclude that there are several mission formats that provide a valuable science measurement capability for detecting chemical biosignatures at Enceladus.