Influence of impact ionisation detection methods on determination of dust particle flux in space

Results are presented for a series of experiments investigating effects which can influence the interpretation of data from ionisation-based dust detectors carried on spacecraft. First, the variation of the impact ionisation yield with angle of impact was studied for impacts of iron microparticles onto gold at speeds of 1-. The angle of incidence was from 0° (normal incidence) to 80° (glancing incidence). Little or no variation was observed at angles up to 60°, but at 80° the total impact ionisation signal was around an order of magnitude lower than at smaller angles. In addition, the fast rising component of the ionisation signal rise time showed no variation with impact angle, but the total signal rise time showed a steady decrease. The effect of secondary impact ionisation resulting from particle impacts on detector side walls was also studied. Iron microparticles were fired on to an aluminium target at various angles, and the impact ionisation signal on a nearby gold target was measured. It was found that ionisation signals were observed on the gold target, and that these were very similar in appearance to those observed in direct impacts.The effect of reduction in particle charge on an impact ionisation signal was investigated. Iron microparticles were fired on to a gold target after passing through a thin film which reduced the charge which was used to accelerate them. It was found that there was a measurable drop in ionisation signal in the reduced-charge case. The empirical relation I_IONISATION=1.67×10⁻⁹Q_PARTICLE^0.35 (units of C) was found. This implies there is a component in the observed ionisation signal that is not related to the impact. To test this, charged tungsten carbide particles were dropped at very low velocity onto a replica of a dust detector used in space whilst placed in a vacuum chamber. Ionisation signals were frequently recorded by the detector. It was concluded that this signal originated from the incident particle charge.In the final section of work, as an example, the influence of oblique incidence, side-wall impacts and particle charge effects on data collected by the Gorid dust detector in Earth orbit were investigated. Corrections were applied to the mass and velocity distributions derived from Gorid data. In extremis, oblique incidence effects were found to shift the mass distribution down by an order of magnitude, and the velocity distribution up by a factor of two to three. If all the data had come from unrecognised side-wall impacts, the mass distribution would be shifted downwards to lower masses by three orders of magnitude, and the velocity distribution upwards by a factor of five. Possible particle charge effects were found to shift the mass distribution down by 30%, and did not alter the velocity distribution.Overall we have investigated a variety of impact-related phenomena and conclude that these can affect the interpretation of data from instruments deployed in space.     

Analysis of the hypervelocity impact process from impact flash measurements

Hypervelocity microparticle impact experiments were performed with a 2 MV Van De Graaff dust accelerator. From measurements of the light intensity I and the total light energy E, the relations I=c₁mv^4.1 and E=c₂mv^3.2 were obtained, where m is the projectile mass, ν the projectile velocity and c₁,c₂ are constants, depending on projectile and target material. Using the measured values of the spectral distribution of the light emitted during impact, the temperature of the radiating material was estimated to be between 2500 and 5000 K depending on the projectile velocity. From an analysis of these measurements the angular distribution of secondary particle velocities as well as the relative mass distribution of these particles was determined. Approximately 90% of the detected ejecta mass (ν?1 km/sec) is found between 50° and 70° ejection angle. For ejection angles smaller than 20°, ejecta velocities of up to 30 km/sec were detected when the primary particle velocity was 4.8 km/sec. Using the dependence of the light intensity on pressure in the target chamber, an estimate of the total amount of material vaporized during impact could be derived. It was concluded that at 7.4 km/sec particle impact velocity at least 1.6% of the displaced projectile and crater material was vaporized.     

A non-linear study of the possible effects of electron-ion collisions on the diffusion of a cylindrical column of ionisation in the Earth's ionosphere

The possible effects of electron—ion collisions on the diffusion of a cylindrical plasma irregularity orientated at right angles to the geomagnetic field lines, at altitude 300 km, are discussed. The results obtained indicate that the diffusion may proceed at a rate rather greater than for a case for which the effects of collisions of electrons with ions are assumed to be negligible. Furthermore, it is suggested that if the plasma density is great enough, the production of a 'fin' of ionisation along the magnetic field direction may be inhibited by the action of electron—ion collisions.     

GEOS-2 measurements of cold ions in the magnetosheath

The Suprathermal Plasma Analysers on GEOS-2 are able to make differential energy measurements of plasma particles down to sub-eV energies because the entire sensor package can be biased relative to the spacecraft. When the package is biased negatively with respect to space potential, low energy positive ions are sucked in and are more easily detected against the background. Large fluxes of ions with temperatures of the order of 1 eV or less were consistently detected at space potential when the spacecraft was in the magnetosheath though not when it was in the nearby magnetosphere. This apparent geophysical correlation, suggesting that the ions were part of the magnetosheath ion population, was contradicted by the fact that the ions showed no signs of the large drift velocity associated with the electric field in the magnetosheath. We conclude, after further investigation, that the observed ions were probably sputtered as neutrals from the spacecraft surface by the impact of solar wind ions and subsequently ionized by sunlight or electron impact. The effect of sputtering by solar wind ions has not been previously observed, although it could have consequences for the long-term stability of spacecraft surfaces.     

Measurements of the light flash produced by high velocity particle impact

The impact light flash produced by electrostatically accelerated iron particles with diameters meters ranging from 5 to 0.05 μm and velocities lying between 1 km/sec and 30 km/sec has been investigated by means of photomultipliers. As target materials mainly gold and tungsten were used. The pulse of the multiplier was registered directly and after electronic integration. The pulse height of the multiplier signal, the amplitude of the integrated signal as well as its rise time were found to be unique functions of the mass and velocity of the impacting particle. For the pulse height of the differential signal the relation I = c₁ × m^1.25 × v⁵ was obtained, and for the integrated signal the relation I = c₂ × m^1.25 × v^3.8, with only c₁ and C₂ depending on the target material. The rise time of the integrated signal follows the relation T = 2.2 × 10² × v^?0.4 using gold as target, and in the case of tungsten material follows the relation T = 9.8 × 10² × v^?1.2, where v is expressed in km/sec and T in μsec. Using the spectral distribution of the light intensity, measured by means of calibrated photomultipliers, the total amount of light energy emitted in the visible range could be calculated. As a result we obtained that for v = 4 km/sec and m = 10^?11 g about 3 × 10^?4 of the kinetic energy of the particle was converted into light energy. The variation of the impact flash intensity with the target material and the measured spectral distribution allowed the temperature of the crater after the impact to be estimated as between 2000 and 3000 K.     

Horizontal electric fields in the middle latitude

Three dimensional electric fields were measured at the altitude of about 27 km in the stratosphere over the Pacific Ocean about 200–400 km away from the Sanriku coast of Honsyu Island (L = 1·4) on 16–17 October 1973, which was magnetically disturbed. The average horizontal electric field thus measured is about 10 mV/m, and the electric field vectors made clockwise semidiurnal rotations rather than diurnal. Daily variation of this electric field was compared with data at L = 2·7–3·5 published by Mozer (1973) and was found to be very similar. This suggests that these electric fields are of common origin in the plasmasphere. From their mean daily variation it is estimated that the plasmaspheric convection is decreased in the night side and is increased in the day side by 200–300 m/sec, and there is an outward flow in the first half of the afternoon and an inward flow in the plasma bulge region of about 500 m/sec.     

Sputtering of sulfur by kiloelectronvolt ions: Application to the magnetospheric plasma interaction with Io

Measurements of total yields, temperature dependences, mass spectra, and energy spectra of molecules sputtered from condensed sulfur (S₈) at low temperatures by keV ions are reported and results are given for Jovian plasma ion bombardment of Io. A change in the reflectance of the sulfur, which can be removed by annealing, is produced by the most penetrating ions and may be connected with the darker, colder polar regions on Io. The measured sputtering yields are much lower than those estimated earlier for room temperature sulfur films but are comparable to previous measurements of keV ion sputtering of SO₂ at low temperatures. The corrected mass spectrum indicates that ≈66% of the total yield corresponds to S₂ ejection while only 5 and 16% correspond to S and S₃, respectively. Therefore, if ions reach the surface of Io its atmosphere will have a non-negligible sulfur component of primarily S₂. The ejection of S and S₂ is temperature independent for temperatures characteristic of most of the surface of Io. The energy spectrum for S has an approximate 1/E² dependence at high ejection energies, whereas S₂ and S₃ fall off more rapidly. Assuming 50% coverage of both sulfur and SO₂ and a thin atmosphere (e.g., nightside and polar region) the direct sputter injection of sulfur atoms and molecules into the Jovian plasma torus and the indirect injection due to coronal processes are estimated. These injection rates for sulfur are compared to those for SO₂ showing that injection from sulfur deposits contributes 13% to the total mass injection rate of ∼2–3 × 10²⁹ amu/sec.     

Steady-state observations of geomagnetically trapped energetic heavy ions and their implications for theory

Measurements of energetic heavy ions using the Explorer 45 and ATS-6 satellites are reviewed and the resulting implications for theory are evaluated. The measured ions are basically protons and helium ions in the energy range from 0.1 to 1 MeV/nucleon. The equatorial energetic ion distributions inside L = 4.5 are found to be very stable for extended periods of time. These ions are very closely confined to the equatorial plane and are sharply peaked as a function of L around a value designated as L_max. Beyond L = 5.0 the fluxes of these ions are more variable with order of magnitude variations being observed at L = 6.6 on the time scales of minutes, hours, or days. The region inside L = 4.5 appears to be well described by radial diffusive transport driven by fluctuations in the geomagnetic field coupled with losses due to charge exchange and Coulomb interactions with ambient hydrogen geocorona and terrestrial plasma environment. From an analysis relating the position in L-value of the maximum intensity, L_max, observed for a given ion species and energy, it is argued that the influence of fluctuations in the convection electric field as discussed by Cornwall (1972) are not effective in radially diffusing in L ions with energies greater than a few hundred kiloelectron volts per nucleon. The source of these ions remains basically undetermined and its determination must await further measurements.     

Inversion of Thick-Target Bremsstrahlung Spectra from Nonuniformly Ionised Plasmas

The effects of non-uniform plasma target ionisation on the spectrum of thick-target HXR bremsstrahlung from a non-thermal electron beam are analysed. In particular the effect of the target ionisation structure on beam collisional energy losses, and hence on inversion of an observed photon spectrum to yield the electron injection spectrum, is considered and results compared with those obtained under the usual assumption of a fully ionised target.The problem is formulated and solved in principle for a general target ionisation structure, then discussed in detail for the case of a step function distribution of ionisation with column depth as an approximation to the sharp coronal–chromospheric step structure in solar flare plasmas. It is found that such ionisation structure has very dramatic effects on derivation of the thick-target electron injection spectrum F₀(E₀) as compared with the result F*₀(E ₀) obtained under the usual assumption of a fully ionised target: (a) Inferred F*₀ contain more electrons than F ₀ and in some cases include electrons at energies where none are actually present. Although the total (energy-integrated) beam fluxes in the two cases do not differ by a factor of more than A_ee/A_eH, the spectral shapes can differ greatly over finite energy intervals resulting in the danger of misleading results for total fluxes obtained by extrapolation. (b) The unconstrained mathematical solution for F₀ for any photon spectrum is never unique, while that for F*₀ is unique. When the physical constraint F₀ 0 is added, for some photon spectra solutions for F₀ may not exist or may not be unique. (This is not an effect of noise but of real analytic ambiguity.) (c) For data corresponding to F*₀ with a low-energy cut-off, or a cut-off or rapid enough exponential decline at high energies, a unique solution F₀ does exist and we obtain a recursive summation for its evaluation.Consequently, in future work on the inversion of HXR bremsstrahlung spectra it will be vital for algorithms to include the effects of target ionisation if spurious results on thick-target electron spectra are not to be inferred. Finally it is pointed out that the depth of the transition zone, and its evaporative evolution during flares may be derivable from its effect on the HXR spectrum.     

On the impact of multiply charged heavy solar wind ions on the surface of Mercury, the Moon and Ceres

We have studied the impact of multiply charged solar wind O⁷⁺ and Fe⁹⁺ ions on the surfaces of Mercury, the Moon and on a Ceres-size asteroid using a quasi-neutral hybrid model.The simulations showed that heavy O⁷⁺ and Fe⁹⁺ ions impact on the surface of Mercury non-homogenously, the highest flux being near the magnetic cusps—much as in the case of impacting solar wind protons. However, in contrast to protons, the analyzed heavy ions do not create high ion impact flux regions near the open-closed magnetic field line boundary. Dawn-dusk asymmetry and the total ion impact flux were each found to increase with respect to the increasing mass per charge ratio for ions, suggesting that the Hermean magnetic field acts as a mass spectrometer for solar wind ions. The Moon, in contrast, does not have a global intrinsic magnetic field and, therefore, solar wind ions can freely impact on its surface when this body is in the solar wind. The same is true for a, non-magnetized, Ceres-size asteroid.The impact of multiply charged ions on a solid surface results in a large variety of physical processes, of often intimately inter-related atomic reactions, e.g. electron exchange between solid and approaching projectile, inelastic scattering of projectile, electronic excitation in the projectile and/or the solid, ejection of electrons, photons, neutral and iodized surface particles, and eventual slowing down and stopping of the projectile in the solid. The electron transfer process between impacting heavy ions and surface constituents can result in soft X-ray (E<1 keV) and extreme ultraviolet (EUV) photon emissions. These processes will eventually damage the target surface. Analysis of the hybrid Mercury model (HYB-Mercury) suggests that, at this planet the damaging processes result in non-homogenous ageing of the surface that is controlled by the intrinsic magnetic field of the planet and by the direction of the interplanetary magnetic field. In the corresponding Lunar model (HYB-Moon) and in the non-magnetized asteroid model (HYB-Ceres), surface ageing is demonstrated to take place on that side of the body that faces toward the flow of the solar wind.     

On the structure of the Io torus

It is now recognized that a number of neutral-plasma interaction processes are of great importance in the formation of the Io torus. One effect not yet considered in detail is the charge exchange between fast torus ions and the atmospheric neutrals producing fast neutrals energetic enough to escape from Io. Since near Io the plasma flow is reduced, the neutrals of charge exchange origin are not energetic enough to leave the Jovian system; these neutrals are therefore distributed over an extensive region as indicated by the sodium cloud. It is estimated here that the total neutral injection rate can reach 10²⁷ s^?1 if not more. New ions subsequently created in the distributed neutral atomic cloud as a result of charge exchange or electron impact ionization are picked up by the corotating magnetic field. The pick-up ions are hot with initial gyration speed near the corotation speed. The radial current driven by the pickup process cannot close in the torus but must be connected to the planetary ionosphere by field-aligned currents. These field-aligned currents will flow away from the equator at the outer edge of the neutral cloud and towards it at the inner edge. We find that the Jovian ionospheric photoelectrons alone cannot supply the current flowing away from the equator, and torus ions accelerated by a parallel electric field could be involved. The parallel potential drop is estimated to be several kV which is large enough to push the torus ions into the Jovian atmosphere. This loss could explain the sharp discontinuous change of flux tube content and ion temperature at L = 5.6 as well as the generation of auroral type hiss there. Finally we show that the inner torus should be denser at system III longitudes near 240° as a result of the enhanced secondary electron flux in this region. This effect may be related to the longitudinal brightness variation observed in the SII optical emissions.     

Behavior of the Line Profiles of the Sodium Doublet and the Calcium Triplet in the Spectrum of R CrB in the Shallow Minima of 1998-1999

The behavior of profiles of the Na I D line and of the infrared Ca II triplet for the star R Coronae Borealis (R CrB) during shallow light minima of 1998-1999 is traced using high-resolution spectra. During a light maximum, the sodium lines had an absorption profile with a shift of —(2-4) km/sec. During a light minimum, a narrow emission feature, which has an almost constant absolute intensity and a shift of —(8-10) km/sec, and an intense circumstellar absorption feature, which has a variable profile and a variable relative shift corresponding to an increase to 220 km/sec in the velocity of mass ejection, appeared in the cores of absorption lines. For several days before the onset of a light minimum, all three calcium lines exhibited a narrow emission feature in the line core with a shift of —(1-5) km/sec. All the subsequent changes in a line involved mainly the shape of the absorption line profile. The narrow emission feature's absolute intensity and relative position were maintained during all our observations. The behavior of the Na I D line profiles can be described qualitatively within the framework of the model of a spherical dust shell.     

A study of F2 region daytime vertical ionization fluxes at Millstone Hill during 1969

A study has been undertaken of the vertical fluxes of ionization in the F2 region over Millstone Hill (L = 3.2) utilizing incoherent scatter measurements of electron density, electron and ion temperatures, ion composition and vertical velocity, made over 24-hr periods twice per month during 1969. The paper presents the results for all these parameters on five representative days, and discusses the distribution of the vertical flux observed during the daytime at other times during the year.Near noon the downward flux reached a peak near 300 km with an average value of ～3 × 10⁹ el/cm²/sec in winter and ～1.6 × 10⁹ el/cm²/sec in summer. The difference is thought to be real and be caused by the higher loss rates prevailing in summer. Above 550 km there is usually a transition to upward flux, which appears to be fully established by 700 km and has an average value of the order of 5 × 10⁷ l/cm²/sec. From ion composition measurements, it appears that this flux is carried almost entirely by O⁺ ions to at least ～900 km, as the H⁺ ion concentration is small (<2% at ～775 km altitude) in this region by day. While the value of the escape flux appears in fair agreement with theoretical estimates of the limiting flux for this portion of the sunspot cycle, the extremely low H⁺ concentrations do not appear to be in accord with existing models.The diurnal variation of the upward flux through 650 km exhibits an abrupt onset close to the time of sunrise at the 200 km level (χ = 103°). A reversal to downward flux usually begins before sunset, often in the early afternoon.     

Atmospheric expansion from Joule heating

An expression for the vertical velocity of the neutral atmosphere in the F-region is derived for Joule heating by the electric field that drives the auroral electrojet. When only vertical expansion is allowed, it is found that the vertical wind must always increase monotonically with altitude. The heating rate is proportional to the F-region ion density, so that appreciable heating, even during high electric fields, requires some production mechanism of ionization such as auroral secondary ionization or solar photoionization, in the lower F-region. Once started at night, when an ionizing source is present in the lower F-region, the expansion of the atmosphere transports ionization upward, thereby increasing the heating rate, and hence the expansion rate, i.e. positive feedback. Electric field strengths and F-region ion densities of 50 mV/m and 2 × 10¹¹e/m³, respectively, will produce vertal neutral wind speeds of several tens of m/sec in the 300–500 km altitude range. During periods of high magnetic activity, i.e. high electric field, Joule heating can produce large increases in the relative N₂ concentration in the upper F-region; computations made with a simple model suggest that tenfold increases can occur at 400 km altitude $\text{1}\text{2?1}\text{hr}$ after the onset of magnetic activity, a result in agreement with satellite observations. When the Joule heating theory is applied to incoherent scatter data taken during one period of high heating, the horizontal electric field in the F-region is found to decrease markedly, possibly approaching zero as the field penetrates a weak, discrete auroral arc; the decrease began 10–20 km from the arc.     

On the dynamics of the jovian ionosphere and thermosphere.: IV. Ion-neutral coupling

We use the fully coupled, three-dimensional, global circulation Jovian Ionospheric Model (JIM) to calculate the coupling between ions in the jovian auroral ovals and the co-existing neutral atmosphere. The model shows that ions subject to drift motion around the auroral oval, as a result of the E×B coupling between a meridional, equatorward electric field and the jovian magnetic field, generate neutral winds in the planetary frame of reference. Unconstrained by the magnetic field, these neutral winds have a greater latitudinal extent than the corresponding ion drifts. Values of the coupling coefficient, k(h), are presented as a function of altitude and cross-auroral electric field strength, for different incoming electron fluxes and energies. The results show that, with ion velocities of several hundred metres per second to over 1 km s⁻¹, k(h) can attain values greater than 0.5 at the ion production peak. This parameter is key to calculating the effective conductivities required to model magnetosphere-ionosphere coupling correctly. The extent to which angular momentum (and therefore energy) is transported vertically in JIM is much more limited than earlier, one-dimensional, studies have predicted.     

Impact experiments in low-temperature ice

New results of low-velocity impact experiments in cubic and cylindrical (20 cm) water-ice targets initially at 257 and 81 °K are reported. Impact velocities and impact energies vary between 0.1 and 0.64 km/sec and 10⁹ and 10¹⁰ ergs, respectively. Observed crater diameters range from 7 to 15 cm and are two to three times larger than values found for equal-energy impacts in basaltic targets. Crater dimensions in ice targets increase slightly with increasing target temperatures. Crater volumes of strength-controlled ice craters are about 10 to 100 times larger than those observed for craters in crystalline rocks. Based on similarity analysis, general scaling laws for strength-controlled crater formation are derived and are applied to crater formation on the icy Galilean and Saturnian satellites. This analysis indicates that surface ages, based on impact-crater statistics on an icy crust, will appear greater than those for a silicate crust which experienced the same impact history. The greater ejecta volume for cratering in ice versus cratering in silicate targets leads to accelerated regolith production on an icy planet.     

Transterminator ion flow in the Martian ionosphere

The upper ionospheres of Mars and Venus are permeated by the magnetic fields induced by the solar wind. It is a long-standing question whether these fields can put the dense ionospheric plasma into motion. If so, the transterminator flow of the upper ionosphere could explain a significant part of the ion escape from the planets atmospheres. But it has been technically very challenging to measure the ion flow at energies below 20 eV. The only such measurements have been made by the ORPA instrument of the Pioneer Venus Orbiter reporting speeds of 1-5 km/s for O⁺ ions at Venus above 300 km altitude at the terminator ( [Knudsen et al., 1980] and [Knudsen et al., 1982]). At Venus the transterminator flow is sufficient to sustain a permanent nightside ionosphere, at Mars a nightside ionosphere is observed only sporadically. We here report on new measurements of the transterminator ion flow at Mars by the ASPERA-3 experiment on board Mars Express with support from the MARSIS radar experiment for some orbits with fortunate observation geometry. We observe a transterminator flow of O⁺ and O₂⁺ ions with a super-sonic velocity of around 5 km/s and fluxes of 0.8×10⁹/cm² s. If we assume a symmetric flux around the terminator this corresponds to an ion flow of 3.1±0.5×10²⁵/s half of which is expected to escape from the planet. This escape flux is significantly higher than previously observed on the tailside of Mars. A possible mechanism to generate this flux can be the ionospheric pressure gradient between dayside and nightside or momentum transfer from the solar wind via the induced magnetic field since the flow velocity is in the Alfvénic regime. We discuss the implication of these new observations for ion escape and possible extensions of the analysis to dayside observations which may allow us to infer the flow structure imposed by the induced magnetic field.     

The role of neutral winds and ionospheric electric field in forming stable sporadic E-layers

Using the combined measurements from a rocket flight through a stable intense sporadic E-layer, we examine the shortcomings of conventional wind shear theory of ion layer formation, principally in underestimating the role of the ambient ionospheric electric field. Our results imply that the ionospheric electric field may control the stability and precise location of such ionisation layers within a region of convergent ion flow.     

Asteroidal agglutinate formation and implications for asteroidal surfaces

A large number of shock recovery experiments that address the ease of impact melt formation as a function of peak shock pressure lead to the conclusion that impacts at 5 km/sec into fragmental, porous surfaces will produce agglutinate-type glasses; no shock melts are produced at these velocities in dense silicate target rocks. While agglutinitic glasses dominate lunar surface soils, they are virtually absent in gas-rich, brecciated meteorites. This apparent paucity—if not complete lack—of agglutinate-type glasses is also inferred from remote IR-reflectance spectroscopy. The need to identify mechanisms that inhibit agglutinate formation on asteroidal sufaces was recognized previously and was predominantly attributed to lower projectile velocities and different gravitational environments.We will argue in this paper that additional mechanisms may be required. Specifically we propose that spall processes at a target's free surface play a major role in asteroidal surface evolution. At 5 km/sec collision velocity, a target (R_T) to projectile (R_P radius ratio of $\text{R}{}_{\mathrm{<mtext>T</mtext>}}\text{R}{}_{\mathrm{<mtext>P</mtext>}}\text{≈ 100}$ delineates the boundary between an “infinite half-space” and a “finite”-sized target. In the first case, collisional energy is expended in a pure cratering regime; in the latter, additional displacement of target material in the form of spallation products occurs. The spall volume may exceed the crater volume by an order of magnitude. Therefore fragmental impact deposits on small planetary bodies may be entirely controlled by spall products, rather than crater ejecta. Because tensile failure occurs at <0.2 GPa stress, spall velocities are measured in meters per second (contrary to crater ejecta) and therefore spallation products are efficiently retained even in low gravitational environments. Spall products are also more coarse grained than crater ejecta; they are also highly biased toward petrographically “unshocked” (<0.2 GPa) rocks.Thus asteroidal surface deposits should be more coarse grained and less shocked than lunar ones—consistent with meteorite evidence and remote-sensing observations. Because spall volume exceeds crater ejecta volume, the total growth rate of asteroidal surface deposits is accelerated, leading to relatively short surface residence times of individual meteorite components, another significant difference between lunar and asteroidal surface materials.     

Discriminating contamination from particle components in spectra of Cassini's dust detector CDA

The Chemical Analyser subsystem of the Cosmic Dust Analyser (CDA) aboard the Cassini spacecraft performs in situ measurements of the chemical composition of dust in space. The instrument records time-of-flight mass spectra of cations, extracted from the impact cloud that is created by high-velocity particle impacts onto the detector target. Thus, the spectra not only show signals of particle components but also of ions from the target material and target contamination. The aim of this work is to determine which non-particle ions are to be expected in the spectra obtained in space operation at Saturn.We present an analysis of the contamination state of the instrument's impact target. Beside investigations of the purity of the rhodium target surface, spectra from CDA calibration experiments at the dust accelerator facility are evaluated with regard to contamination signatures. Furthermore, contamination mass lines in spectra obtained by impacts of Jovian and Saturnian dust stream particles are analysed. Due to their small size and high speed, stream particle impacts predominantly produce ions from the target material and therefore the spectra are excellent probes of the contamination state of the target operating in space. With the exception of adsorbed hydrogen and carbon, the level of contamination is very low.Implications for CDA spectra of Saturnian E ring particle impacts are derived. The findings confirm the published interpretations. The low level of alkali metal contamination implies a significant sodium contribution in the composition of E ring ice particles. Additionally, ionisation thresholds for the occurrence of contamination mass lines can be utilised to set limits for the impact velocity.