Introduction

Onboard the Cassini-Huygens mission destined for the Saturn system, the Cassini Cosmic Dust Analyser (CDA) instrument will provide extended in-situ observations of the dust environment of particle mass 10"15 - 10"9g. The seven year cruise enables an extended period for measurement of the interplanetary environment and of the Saturnian system on arrival. Additionally, during the Jupiter fly-by in December 2000 there arises the unique opportunity to take simultaneously measurements with the CDA instrument and its forefather the Galileo dust flux instrument [1], CDA is the next generation descendant of Galileo, with the inclusion of a chemical analyser capable of providing compositional information. The principle of a time-of-flight mass spectrometer relying on plasma production by hypervelocity impact is first reported in [2], Impact ionization is produced when a hypervelocity impact occurs; the velocity of an impact is greater than the speed of the compression wave produced in both the projectile and target. This results in immense shock waves within the projectile and target material causing intense heating and vaporisation hence producing an ionic plasma.

On CDA the curved chemical analyzer target has a potential of 1 kV with an earthed grid 3 mm above. This accelerates the positively charged constituent of the ionic plasma towards a centrally located detector beyond a short drift region, resulting in a time-of-flight spectrum detailing the anionic components of the impacting projectile and the target material [3].

In addition to CDA, the Cometary and Interstellar Dust Analyzer (CIDA) aboard the Stardust mission has collected data on several occasions since February 1999. Some CDA and most of the spectra measured by CIDA have shown features quite different from the laboratory calibrations, characterised by broad features with masses up to several hundred amu. The higher mass resolution of CIDA has led to interpretation of these features as due to complex organics containing nitrogen and/or oxygen [4], Interpretation of the CDA spectra to glean impactor compositional information is nontrivial with many perturbing influences. Such interpretation must rely upon extensive laboratory calibration to characterise the instrument and constrain the processes involved.

The University of Kent at Canterbury 2 MV Van de Graaff electrostatic accelerator [5] accelerates suitable projectiles to simulate impacts on a CDA representative model. Traditional projectiles (e.g. iron) are used to characterise instrument responses, although the resulting time-of-flight spectra contain no molecularly bonded high-mass components. Revolutionary low-density projectiles are enabling the investigation of complex organic spectral responses within a laboratory environment, aiding the interpretation of flight data.

Was this article helpful?

0 0

Post a comment