Quantitative evaluation of the feasibility of sampling the ice plumes at Enceladus for biomarkers of extraterrestrial life

Abstract

Enceladus, an icy moon of Saturn, is a compelling destination for a probe seeking biosignatures of extraterrestrial life because its subsurface ocean exhibits significant organic chemistry that is directly accessible by sampling cryovolcanic plumes. State-of-the-art organic chemical analysis instruments can perform valuable science measurements at Enceladus provided they receive sufficient plume material in a fly-by or orbiter plume transit. To explore the feasibility of plume sampling, we performed light gas gun experiments impacting micrometer-sized ice particles containing a fluorescent dye biosignature simulant into a variety of soft metal capture surfaces at velocities from 800 m ⋅ s−1 up to 3 km ⋅ s−1. Quantitative fluorescence microscopy of the capture surfaces demonstrates organic capture efficiencies of up to 80 to 90% for isolated impact craters and of at least 17% on average on indium and aluminum capture surfaces at velocities up to 2.2 km ⋅ s−1. Our results reveal the relationships between impact velocity, particle size, capture surface, and capture efficiency for a variety of possible plume transit scenarios. Combined with sensitive microfluidic chemical analysis instruments, we predict that our capture system can be used to detect organic molecules in Enceladus plume ice at the 1 nM level—a sensitivity thought to be meaningful and informative for probing habitability and biosignatures.

Mass Spectrometric Fingerprints of Organic Compounds in NaCl-Rich Ice Grains from Europa and Enceladus

“…Spaceborne impact ionization mass spectrometers analyze projectile ions from ice grains impacting onto metal targets. It is challenging to recreate this impact process in laboratories because micrometer-sized ice grains cannot currently be easily accelerated at relevant speeds. , Therefore, the Laser Induced Liquid Beam Ion Desorption (LILBID) technique has been developed to simulate the impact ionization mass spectra of ice grains recorded in space at impact speeds from 3 to >20 km/s − without the need of a dust accelerator. This technique has been used to reproduce compositional differences of ice grains detected from Enceladus with the CDA instrument ,, and to investigate the mass spectral characteristics and fragmentation patterns of organics between different ionization methods .…”

A multi-lander New Frontiers mission concept study for Enceladus: SILENUS

“…After the EOI, at least ten passes through the plumes are necessary in order to fulfill the science requirements (Table 1). Ideally, the relative velocity between the spacecraft and plume particles should be less than 2 km/s (New et al, 2021). The velocity of the plume particles may be up to 1.2 km/s near where they exit the Tiger Stripes (Hansen et al, 2020), so the spacecraft must move slowly during these passes to avoid increasing its relative velocity.…”

Unraveling the Fate of Impacted Ice Particles and the Consequences for Plume Fly‐Through Missions

“…Alternatively, experiments accelerating projectiles into ice (New, Kazemi, et al., 2020) to create a spray of ice grains that can then interact with the surface have also been successfully performed. This technique generates a high instantaneous flux of water ice particles with large size distributions, and has been used to show the survival of organics on the impacted surface (Mathies et al., 2021; New, Kazemi, et al., 2020; New, Mathies, et al., 2020; New et al., 2021), but has limited control of temperature and does not allow for a detailed analysis of the impacts themselves. These experiments create short impulses of high particle fluxes and have demonstrated an important piece of the puzzle (i.e., organic survival), but are challenging to correlate to the particle profiles expected in the plume.…”