Modelling CDA mass spectra

Abstract

We present the initial results from a simulation of ion behaviour within Cassini's cosmic dust analyser (CDA) instrument, using an inhouse ion dynamics code. This work is to enable and enhance the detailed interpretation of dust impact ionisation mass spectra returned from the Saturnian system. Early work has already provided insights into the properties of the impact plasma in both low-and highvelocity impacts. We find that the isotropic emission of ions from the impact plasma successfully reproduces features seen in flight spectra and that the emitted ions have a higher range of energies (tens to hundreds of eV) than previously reported in some studies. Using these new ion characteristics, we have successfully modelled CDA flight mass spectra. r

Macromolecular organic compounds from the depths of Enceladus

“…The ions created by impact ionization in the impact speed regime considered here (4 – 18 km/s) are almost exclusively singly charged. Unfortunately, the TOF is also influenced by the broad distribution of initial ion velocities, slightly varying flight paths, and plasma shielding effects 32 . For that reason species with identical mass are distributed over a range of sampling points and the mass resolution drops below integer values usually around 25 - 30u.…”

Detection of phosphates originating from Enceladus’s ocean

“…The ions created by impact ionization in the impact speed regime considered here (roughly 10 km s –1 ) are almost exclusively singly charged. Unfortunately, ToF is also influenced by the broad distribution of initial ion velocities, slightly varying flight paths and plasma-shielding effects 40 . For that reason, species of identical mass are distributed over a range of sampling points and the mass resolution drops below integer values, usually at 20–30 u and above.…”

The composition of Saturn's E ring

“…The trajectory of a ‘just detected’ ion is indicated, with the inset diagram showing the ion behaviour within the acceleration region due to non‐zero initial ion energy. Taken from Hillier et al (2006).…”

“…Conversion of the QM spectra from time to mass space is, in principle, simple (arrival time being proportional to the square root of the ion mass). In reality this process is complicated by the ions' initial velocity distribution, instrument geometry, plasma shielding effects and unknown precise impact times, which can broaden and shift peaks making calibration of spectra non‐trivial (Hillier et al 2006). The dominant uncertainty in determining the exact impact time (and hence the ‘zero‐point’ of the mass scale) arises from the instrument triggering method.…”