Showing papers by "Mark C. Price published in 2009"

Comet 81p/wild 2: the updated stardust coma dust fluence measurement for smaller

Abstract: Introduction: Micrometre and smaller scale dust within cometary comae can be observed by telescopic remote sensing spectroscopy [1] and the particle size and abundance can be measured by in situ spacecraft impact detectors [2]. Initial interpretation of the samples returned from comet 81P/Wild 2 by the Stardust spacecraft [3] appears to show that very fine dust contributes not only a small fraction of the solid mass, but is also relatively sparse [4], with a low negative power function describing grain size distribution, contrasting with an apparent abundance indicated by the on-board Dust Flux Monitor Instrument (DFMI) [5] operational during the encounter. For particles above 10 μm diameter there is good correspondence between results from the DFMI and the particle size inferred from experimental calibration [6] of measured aerogel track and aluminium foil crater dimensions (as seen in Figure 4 of [4]). However, divergence between data-sets becomes apparent at smaller sizes, especially submicrometre, where the returned sample data are based upon location and measurement of tiny craters found by electron microscopy of Al foils. Here effects of detection efficiency ‘tail-off’ at each search magnification can be seen in the down-scale ‘flattening’ of each scale component, but are reliably compensated by sensible extrapolation between segments. There is also no evidence of malfunction in the operation of DFMI during passage through the coma (S. Green, personal comm.), so can the two data sets be reconciled? Recent work [7] suggests that the efficiency of Al foil crater excavation by very small particles may be lower than for larger grains, implying that a new calibration is required for this part of the size distribution. The calibrations of [4] were based upon light gas gun (LGG) shots of sodalime glass particles between ~10 and 100 μm, using projectiles with monodispersive size distributions. The data gave a good statistical fit to a line with a constant gradient across the measured sizes, which, within the errors, extrapolated close to the origin, albeit with relatively large error bars. At that time this uncertainty could not be resolved due to a lack of suitable projectiles of <10 μm diameter. This was particularly unfortunate as subsequent analysis of the returned Stardust Al foils indicated that the majority of impact craters were made by particles smaller than 10 μm. The availability of large numbers of monodisperse micrometre and smaller scale projectiles of known density, and their successful acceleration in LGG shots have proven elusive goals, only recently resolved. In this paper we describe new calibration experiments, their preliminary results and the implications for interpretation of particle sizes responsible for the smallest Stardust craters. Experimental methodology: Shots were performed using the two-stage LGG at the University of Kent [8]. Projectile materials were monodispersive silica spheres commercially available from Whitehouse Scientific (UK) and Micromod (Germany). SEM/EDX imaging of the foils was carried out at the Natural History Museum. Craters were measured following the method of [6]; crater diameters were defined as the distance from top of the crater lip to top of the diametrically opposed crater lip. Two measurements were made for each crater to minimise error. Results: Table 1: Measured projectile and crater diameters, mean crater diameter and impact speeds from LGG shots.

Comet 81p/Wild 2: The Updated Stardust Coma Dust Fluence Measurement for Smaller (Sub 10-Micrometre) Particles

Abstract: Micrometre and smaller scale dust within cometary comae can be observed by telescopic remote sensing spectroscopy [1] and the particle size and abundance can be measured by in situ spacecraft impact detectors [2]. Initial interpretation of the samples returned from comet 81P/Wild 2 by the Stardust spacecraft [3] appears to show that very fine dust contributes not only a small fraction of the solid mass, but is also relatively sparse [4], with a low negative power function describing grain size distribution, contrasting with an apparent abundance indicated by the on-board Dust Flux Monitor Instrument (DFMI) [5] operational during the encounter. For particles above 10 m diameter there is good correspondence between results from the DFMI and the particle size inferred from experimental calibration [6] of measured aerogel track and aluminium foil crater dimensions (as seen in Figure 4 of [4]). However, divergence between data-sets becomes apparent at smaller sizes, especially submicrometre, where the returned sample data are based upon location and measurement of tiny craters found by electron microscopy of Al foils. Here effects of detection efficiency tail-off at each search magnification can be seen in the down-scale flattening of each scale component, but are reliably compensated by sensible extrapolation between segments. There is also no evidence of malfunction in the operation of DFMI during passage through the coma (S. Green, personal comm.), so can the two data sets be reconciled?

HYPERVELOCITY SUB 10‐μM IMPACTS INTO ALUMINIUM FOIL: NEW EXPERIMENTAL DATA AND IMPLICATIONS FOR COMET WILD‐2’S DUST FLUENCE

Abstract: We report on the results of an experimental shot programme using a light gas gun (LGG) the principal purpose of which was to extend the existing calibration of projectile vs. crater diameter to aid in the interpretation of very small (<10 μm) impact craters observed on Stardust aluminium (Al) foils. Stardust was a NASA mission which flew past a comet at 6.1 km s−1 in 2004 and collected freshly emitted cometary dust via impact onto its exposed surface. The results show an unexpected change in the profile of the calibration curve resulting in a need to readdress the fluence measurement for Comet 81P/Wild‐2 and also gives an insight into the strain rate behaviour of Al‐1100 at the very high (∼109 s−1) rates experienced by the Al during impact of a micron sized projectile at 6.1 km s−1.