Department of Earth Sciences

Impact ionization of ice grains is used to explore the subsurface oceans of Enceladus with the Cosmic Dust Analyzer onboard Cassini, and Europa with the SUrface Dust Analyzer (SUDA) on Europa Clipper. Impact ionization of ice grains occurs by hypervelocity (velocities > 1km/s) impacts of ice grain into a detector, during which a fraction of the ice particle is converted into elemental and molecular ions that can then be analyzed by mass spectrometry. With increasing impact speeds, the greater energy available results in the generation of more ions, the breakup of molecular ions into elemental ions, and a reduction in the amount of cluster formation, but these effects are complex and compositionally dependent. Simulating hypervelocity ice grain impacts onto space detectors is crucial for interpreting the results of ongoing and future space missions. Laboratory analogue experiments are an essential element of the quantitative interpretation of impact ionization mass spectra recorded in space. For mineral, metallic and/or organic dust particles, this can be achieved by electrostatic acceleration of micrometer and sub-micrometer sized grains, by firing them upon duplicates of the space detector. Our research group participates in such experiments at accelerator facilities in Germany (at Stuttgart University) and the USA (at the University of Colorado at Boulder).

However, such experiments are technically much harder to perform with water-rich ice grains and acceleration of icy particles to the desired speed is currently not possible. Therefore, a laser-based analogue technique has been implemented to simulate the impacts of water-rich ice grains onto impact ionization mass spectrometers.This technique uses the principle of Laser Induced Liquid Beam Ion Desorption (LILBID) coupled to a reflection time-of-flight mass spectrometer. In the LILBID process, a pulsed infrared laser intersects a micrometer-scale water beam (or similarly sized water droplets) of known composition, to simulate an ice grain impact. The laser acts onto the liquid in the same way as a mechanical hit. Energy from the laser creates a cloud of cations, anions, electrons and neutral molecules which is similar to the impact cloud created by a hypervelocity ice grain impact in space. The cations or anions are then directed into a Time-of-Flight mass spectrometer which, after a predefined delay time, generates and records the analogue mass spectra. By setting a delay time, we select ions with specific ranges of velocities. By combining different delay times with variable laser energies, we can mimic spectra generated by different speed impacts in space. The laboratory laser apparatus is therefore key for the compositional analysis of ice grains detected in space, and has proved itself invaluable for interpreting Cassini Cosmic Dust Analyzer spectra from the Saturnian system. It will be crucial for the interpretation of SUDA impact mass spectra at Europa, and future missions to Enceladus and other icy ocean moons. The recorded spectra are stored in a comprehensive spectral reference library from a wide variety of organic and inorganic analogue materials, in matrix solutions which simulate a range of icy grain bulk compositions.