The dust mass distribution of comet 81P/Wild 2

The Dust Flux Monitor Instrument ( DFMI) made direct measurements of the dust environment in the mass range 10(-14) < m < 10(-5) kg at comet 81P/Wild 2 during the Stardust flyby on 2 January 2004. We describe the techniques for derivation of the particle mass distribution, including updated calibration for the acoustic subsystem. The dust coma is characterized by "swarms'' and "bursts'' of particles with large variations of flux on small spatial scales, which may be explained by jets and fragmentation. The mass of the dust coma is dominated by larger particles, as was found for comets 1P/Halley and 26P/Grigg-Skjellerup. However, almost 80% of the particles were detected many minutes after closest approach at a distance of similar to4000 km, where small grains dominated the detected mass flux. The mass distribution varies on small spatial scales with location in the coma, consistent with the jets and fragmentation inferred from the highly heterogeneous dust spatial distribution. The cumulative mass distribution index alpha (where the number of particles of mass m or larger, N(m) proportional to m(-alpha)) in the coma ranges from 0.3 to 1.1. It is possible that jets and fragmentation occur in all comets but have not previously been well observed due to the limitations of detectors and flyby geometry. We estimate that 2800 +/- 500 particles of diameter 15 mum or larger impacted the aerogel collectors, the largest being similar to6 x 10(-7) kg (diameter similar to 1 mm), which dominates the total collected mass. Of these, only 500 +/- 200, representing just 3% of the collected mass, originated in the far postencounter region.