For planetary scientists, the Galilean moon Europa is amongst the most interesting
bodies in the solar system. Its surface composition is revealing the past and recent
geophysical processes both on and below the surface, and a dust mass analyzer
onboard a satellite orbiting the moon will allow us to acquire this invaluable knowledge.
In particular, hydrated forms of minerals such as sodium carbonates and magnesium
sulfates present in the ice surface probably represent deposits of materials from
below the ice crust.
The basic idea of compositional mapping is that moons without an atmosphere are
engulfed in clouds of dust particles released from their surfaces by meteoroid
bombardment. The ejecta cloud particles can be detected and their composition
analyzed from orbit or during a spacecraft flyby. The ejecta production process is
very efficient: a typical interplanetary 10-8 kg micrometeoroid impact on Europa
produces a large number of ejecta particles with a total mass on the order of a few
thousand times of that of the impactor. These ejecta particles move on ballistic
trajectories and most of them re-collide with the satellite due to the lower initial
speed. As a consequence, an almost isotropic dust exosphere is present around
the moon. Since these grains are direct samples from the Europa surface, unique
composition data will be obtained that will help to define and constrain the geological
activities on and below the moons’ surface. In 1999, the Galileo dust instrument
demonstrated that surface ejecta can be detected from orbit.
We developed a dust mass spectrometer to measure the composition of ballistic
dust particles populating the thin exosphere that was detected around Europa.
The SUrface Dust Mass Analyzer (SUDA) is a time-of- flight, reflectron-type impact
mass spectrometer, optimised for a high mass resolution which only weakly depends
on the impact location. The small size (268×250×171 mm^3), low mass (< 7 kg) and
large sensitive area (220 cm^2) makes the instrument well suited for the challenging
demands of a mission to Europa. A full-size prototype SUDA instrument was built in
order to demonstrate its performance through calibration experiments at the Heidelberg
dust accelerator with a variety of cosmo-chemically relevant dust analogues. The effective
mass resolution of m/Δm of 200 is achieved for mass range of interest m = 1-150.