M. J. Cole

Hypervelocity impact studies using the 2 MV Van de Graaff accelerator and two-stage light gas gun of the University of Kent at Canterbury

The hypervelocity impact facilities of the University of Kent are described. They comprise a 2 MV Van de Graaff accelerator for the electrostatic acceleration of dust particles (mass and velocities ) and a two-stage light gas gun firing millimetre-sized particles at . Results for impact ionization studies using iron dust accelerated in the Van de Graaff and hitting a variety of metal targets (gold, silver, indium, iron, rhodium and molybdenum) are presented. Over the range , the ionization yields are found to be similar to within a factor of 20 at low velocity and converge to within a factor of five at high velocity. The light gas gun is used to investigate the volumes of craters in metal targets for impacts of 1 mm diameter stainless steel spheres on aluminium at velocities in the range . For normal incidence the crater volume scales with the square of the impact velocity. For oblique impacts at a fixed velocity it is found that the crater volume scales with the cosine of the impact angle.

Time of flight mass spectra of ions in plasmas produced by hypervelocity impacts of organic and mineralogical microparticles on a cosmic dust analyser

The ionic plasma produced by a hypervelocity particle impact can be analysed to determine compositional informa- tion for the original particle by using a time-of-flight mass spectrometer. Such methods have been adopted on interplanetary dust detectors to perform in-situ analyses of encountered grains, for example, the Cassini Cosmic Dust Analyser (CDA). In order to more fully understand the data returned by such instruments, it is necessary to study their response to impacts in the laboratory. Accordingly, data are shown here for the mass spectra of ionic plasmas, produced through the acceleration of microparticles via a 2 MV van de Graa accelerator and their impact on a dimensionally correct CDA model with a rhodium target. The microparticle dusts examined have three dierent chemical compositions: metal (iron), organic (polypyrrole and polystyrene latex) and mineral (aluminosilicate clay). These microparticles have mean diameters in the range 0.1 to 1.6 m and their velocities range from 1-50 km s 1 . They thus cover a wide range of compositions, sizes and speeds expected for dust particles encountered by spacecraft in the Solar System. The advent of new low-density, microparticles with highly controllable attributes (composition, size) has enabled a number of new investigations in this area. The key is the use of a conducting poly- mer, either as the particle itself or as a thin overlayer on organic (or inorganic) core particles. This conductive coating permits ecient electrostatic charging and acceleration. Here, we examine how the projectiles chemical composition influences the ionic plasma produced after the hypervelocity impact. This study thus extends our understanding of impact plasma formation and detection. The ionization yield normalized to particle mass was found to depend on impact speed to the power (3.4 0.1) for iron and (2.9 0.1) for polypyrrole coated polystyrene and aluminosilicate clay. The ioization signal rise time was found to fall for all projectile materials from a few microseconds at low impact speeds (3 km s 1 ) to a few tenths of a microsecond at higher speeds (approximately 16 km s 1 for aluminosilicate particles and approximately 28 km s 1 for iron and polystyrene particles). At speeds greater than these the rise time was a constant few tenths of a microsecond independent of impact speed. The mass resolution of the time of flight spectrometer was found to be non-linear at high masses above 100 amu. It wasm=m= 5f orm= 1 amu and 40 for m= 200 amu. However, although at high masses most mass peaks had the resolution quoted, there were also occasional much narrower mass peaks observed, suggesting that at 250 to 280 amum=m= 80 to 100. The lower resolutions may be due to closely spaced mass peak signals eectively merging into one observed peak due to the (greater but still finite) resolution found for the isolated mass peaks. Complex mass spectra have been reproducibly obtained from a number of dierent projectiles that display many charged molecular fragments with masses up to 250 amu and with periodicities of 12-14 amu. These new studies reveal an extremely strong dependence of the time-of-flight mass spectra on the impact speed, particularly at low velocities (1-20 km s 1 ). In some impact velocity regimes it is possible to distinguish time-of-flight spectra originating from organic microparticles from those obtained from iron microparticles. However, such discrimination was not possible at high impact speeds, nor was it possible to distinguish between the time-of-flight spectra obtained for aluminosilicate particles from those obtained for iron projectiles.

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

The known encounter velocity (6.1 kms(-1)) and particle incidence angle (perpendicular) between the Starchist spacecraft and the dust emanating from the nucleus of comet Wild-2 fall within a range that allows simulation in laboratory light-gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre-existing major- and trace-element composition of the foil, geometrical issues for energy dispersive X-ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.

Acceleration of conducting polymer-coated latex particles as projectiles in hypervelocity impact experiments

A series of sterically-stabilized polystyrene latex particles in the size range 0.1-5.0 µm have been coated with ultrathin ( 1 km s-1) using a Van de Graaff accelerator. These coated latexes have two main advantages compared to the sterically-stabilized polypyrrole particles of 0.1-0.3 µm diameter reported previously. First, a wider particle size range can be accessed. Second, the particle size distributions of the coated latexes are much narrower than those of the pure polypyrrole particles reported earlier. Preliminary studies confirm that, after charging and acceleration, these conducting polymer-coated latex particles have very similar mass-velocity profiles to those reported for colloidal iron particles in the hypervelocity literature. The hypervelocity impact generated ionization has been measured for latex spheres impacting copper targets. This is compared to previous work for impact ionization by iron particles, thus demonstrating the ability to study the dependence of impact ionization on widely different projectile materials.

Laboratory calibration of the Cassini Cosmic Dust Analyser (CDA) using new, low density projectiles

The Cassini Cosmic Dust Analyser (CDA), developed from the Galileo and Ulysses dust instruments with the addition of a Chemical Analyser, is currently travelling outward from the Earth (collecting data from March 1999 onward) to the Saturnian system (arrival 2004) via Jupiter. The Chemical Analyser will provide information on the elemental composition of impacting micrometeoroids through impact ionisation, time-of-flight mass spectrometry. A rigorous calibration programme primarily focussed upon the Chemical Analyser is in progress at the University of Kent at Canterbury. A 2-MV Van de Graaff electrostatic accelerator and CDA laboratory model are used to simulate impacts. Acceleration of revolutionary low density, polymer dust particles has enabled an insight into the response of CDA to molecularly bonded material with increasing event velocity. These conducting polymer coated polystyrene latex particles represent significantly better analogues for carbonaceous cosmic grains than more traditionally accelerated projectiles (e.g. iron) and have enabled complex organic spectra to be produced in the laboratory. The current status of an ongoing programme is reported. Three samples are presented, two polypyrrole coated latexes of differing size and one PEDOT-coated latex sample.

Velocity thresholds for impact plasma production

Abstract Experiments have been performed on the dust accelerator facilities at the University of Kent at Canterbury (UK) and the Max-Planck-Institut fur Kernphysik (Germany) in which the production of plasma from impacts of micron and sub-micron particles at velocities from 1 to 90 km s −1 has been measured. Various projectile and target materials have been investigated. Time-of-flight mass spectrometry of the positive ions in the plasma allows their atomic species to be identified. By accumulating large amounts of data over a range of impact velocities it has been possible to identify the threshold velocities required to produce ions of different species, whether present in the system as the nominal projectile and target materials or as contaminants. The results obtained have been compared with theoretical predictions based on the principles of molecular dynamics and with the results of hydrocode simulations.

ROLE OF PARTICLE CHARGE IN IMPACT IONIZATION BY CHARGED MICROPARTICLES

Abstract The role of particle charge in generating ionization of projectile and target in impacts of iron microparticles on copper has been investigated at impact velocities above 1 km s −1 . Charged microparticles are accelerated in an electrostatic accelerator and their surface charge subsequently reduced in flight by passage through a thin conducting film. It is found that a reduction in projectile charge of 80% at 5 km s −1 reduces noticeably the degree of impact ionization. This supports the recent assertion that heating due to current flow between the particle and target immediately prior to impact may play a significant role in impact ionization at such velocities.

Survival of Organic Materials in Hypervelocity Impacts of Ice on Sand, Ice, and Water in the Laboratory

The survival of organic molecules in shock impact events has been investigated in the laboratory. A frozen mixture of anthracene and stearic acid, solvated in dimethylsulfoxide (DMSO), was fired in a two-stage light gas gun at speeds of ~2 and ~4 km s(-1) at targets that included water ice, water, and sand. This involved shock pressures in the range of 2-12 GPa. It was found that the projectile materials were present in elevated quantities in the targets after impact and in some cases in the crater ejecta as well. For DMSO impacting water at 1.9 km s(-1) and 45° incidence, we quantify the surviving fraction after impact as 0.44±0.05. This demonstrates successful transfer of organic compounds from projectile to target in high-speed impacts. The range of impact speeds used covers that involved in impacts of terrestrial meteorites on the Moon, as well as impacts in the outer Solar System on icy bodies such as Pluto. The results provide laboratory evidence that suggests that exogenous delivery of complex organic molecules from icy impactors is a viable source of such material on target bodies.

Limits on methane release and generation via hypervelocity impact of Martian analogue materials

The researchers at Kent acknowledge the STFC, UK for funding this work. Nisha Ramkissoon thanks the UK Space Agency for her support via an Aurora studentship. KM’s work is funded by the UnivEarthS LabEx project of the University of Sorbonne Paris Cite.

Characterization of space dust using acoustic impact detection

This paper describes studies leading to the development of an acoustic instrument for measuring properties of micrometeoroids and other dust particles in space. The instrument uses a pair of easily penetrated membranes separated by a known distance. Sensors located on these films detect the transient acoustic signals produced by particle impacts. The arrival times of these signals at the sensor locations are used in a simple multilateration calculation to measure the impact coordinates on each film. Particle direction and speed are found using these impact coordinates and the known membrane separations. This ability to determine particle speed, direction, and time of impact provides the information needed to assign the particles orbit and identify its likely origin. In many cases additional particle properties can be estimated from the signal amplitudes, including approximate diameter and (for small particles) some indication of composition/morphology. Two versions of this instrument were evaluated in this study. Fiber optic displacement sensors are found advantageous when very thin membranes can be maintained in tension (solar sails, lunar surface). Piezoelectric strain sensors are preferred for thicker films without tension (long duration free flyers). The latter was selected for an upcoming installation on the International Space Station.