Planetary Science by Space Missions
Simulation of hypervelocity impacts of ice particles with laser dispersion
A crucial factor for on-going and future research projects are experiments that simulate hypervelocity impacts (v > 1km/s) onto space detectors. The basis for time-of-flight mass spectrometry (TOF-MS) of micron-sized dust particles in space is impact ionisation. Upon impact a substantial fraction of the dust particle is converted into elemental and molecular ions that can then be analysed by mass spectrometry.
For quantitative interpretation of impact ionisation spectra, analogue experiments in the lab are mandatory. For mineral and organic dust particles this is achieved by electrostatic acceleration of micron and sub-micron sized dust grains of known composition that are fired upon duplicates of the space detectors. Our group in Heidelberg frequently participates in respective experiments carried out at accelerator facilities in Germany and the US.
Acceleration of icy particles to the desired speed is currently not possible. However to simulate icy particle impact one can use a laser-driven analogue setup. Here a highly focussed pulsed laser intersects a micron-sized water beam (or similar sized water droplets) of controlled composition to simulate an ice particle impact. The ions are then directed into a TOF-MS to record analogue spectra (Figs. 4 and 5). In our lab in Heidelberg we run such a (worldwide unique) setup. It is currently in heavy use for the Cassini mission and will be needed for many years to come for the Europa Clipper and other future missions exploring subsurface oceans.
A 12μm wide liquid jet of desired composition consists of an intact ≈3 mm long beam, which disintegrates into droplets further down (Fig. 5). The tuneable infrared laser beam is generated by pumping an optical parametric oscillator (OPO) with a 20Hz Nd:YAG-laser. The laser acts onto the liquid like a mechanical hit (Fig. 5), producing a cloud of charged and neutral molecules very similar to the impact cloud from hypervelocity ice grain impacts. In the past years we learned to adjust the intensity of the laser pulse and the delay time to mimic different impact speeds of ice grains. A reflectron time-of-flight mass spectrometer is used to sample an adjustable number of spectra at varying delay times (3–10 μs). The spectra are digitized by an analogue to digital converter (ADC).
Our analogue experiment is used exclusively for a systematic simulation of icy impact ionisation spectra with varying impact energy and composition. For example from analogue mass spectra taken with this setup the high sodium salt content of some Enceladus' plume particles could be inferred. In addition to the chemical 'reconstruction' of ice grains detected by Cassini in the Saturnian system, the preparation for compositional analysis of ejecta from other icy worlds (i.e., Enceladus, Europa and other Galilean moons, or comets) are the main objectives. In October 2016 the setup was upgraded with a new bi-polar high-performance spectrometer. In the framework of our running ERC proposal another substantial upgrade is planned by employing a UV laser for secondary ionisation of neutral molecules, that abundantly form when the IR laser hits the water beam. This setup will then also be able to record cation and anion spectra simultaneously. In parallel another advanced experiment is currently developed in collaboration with the “Leipniz Institut für Oberflächenforschung” in Leipzig (Prof. Bernd Abel). This new setup will be used as a generator of high-speed ice particles, accelerating sub-micron ice grains to speeds of 5 km/s or higher, which then are available for impact experiments.