Analyzing Enceladus' plume: First steps to experimentally simulating hypervelocity impacts

Abstract

The Saturnian moon Enceladus is thought to be one of the most ideal places to search for extraterrestrial, aq.-based life. Enceladus has a global, subsurface ocean that is sandwiched between an outer, icy shell and the moon's rocky core. Fractures in the ice shell at the southern pole of Enceladus give rise to the now-famous plume, which expresses the subsurface ocean into space. The plume was interrogated by two mass spectrometry-based instruments aboard the Cassini spacecraft. Measurements by the Ion and Neutral Mass Spectrometer and the Cosmic Dust Analyzer (CDA) confirmed the presence of H2, CH4, and silica nanometer-sized grains, which are indicative of hydrothermal processes. Large org. mols. and ammonia were also obsd. Enceladus' plume could enable ocean sampling with a flyby spacecraft. Flyby missions ease some of the constraints of orbital or lander missions; however, flyby measurements at hypervelocity (>1 km/s) present a challenge. In order to obtain the most useful and detailed information from a future flyby mission, it is imperative to understand hypervelocity sampling. Specifically, it is unknown at what velocities different org. mols. will be volatilized, ionized, or fragmented, and it is important to det. if mass spectral patterns are sensitive to impact velocity. To better understand hypervelocity sampling, a lab-based instrument is currently being characterized that generates hypervelocity species. The Hypervelocity Ice Grain System (HIGS) generates ions and neutrals ranging from bare mols. to small hydrated clusters to nanometer-sized ice grains by laser-induced desorption (LID) from a water jet. Charged LID products are then extd. into a time-of-flight mass spectrometer (TOF-MS) for anal. A similar system was used to reproduce mass spectra collected by CDA. The current focus of this research effort is to characterize LID products and to probe how the presence of salts and pH affect amino acid and polypeptide ion distributions. It has been demonstrated that charged LID products are accelerated to speeds >1 km/s before being extd. into the TOF-MS. Quantifying orgs. ensconced in water ice grains is also a focus of this work. Ultimately, vaporization, ionization, and fragmentation thresholds of ions of known size and velocity will be exptl. detd. by impacting LID species on a plate before extg. into the TOF-MS for mass anal.

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