Showing papers by "Eberhard Grün published in 1999"

Dust in the local interstellar wind

Abstract: The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, Rg/d = 94. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10-14 to 10-13 g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of CMa, give Rg/d = 427 for assumed solar reference abundances and Rg/d = 551 for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward λ Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H0 indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s-1. If solar reference abundances are correct, the low Rg/d value toward λ Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.

Dust in the Local Interstellar Wind

Abstract: The gas-to-dust mass ratios found for interstellar dust within the Solar System, versus values determined astronomically for the cloud around the Solar System, suggest that large and small interstellar grains have separate histories, and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, $R_{g/d} = 94^{+46}_{-38}$. Hubble observations of the cloud surrounding the solar system yield a gas-to-dust mass ratio of Rg/d=551+61-251 when B-star reference abundances are assumed. The exclusion of small dust grains from the heliosheath and heliosphere regions are modeled, increasing the discrepancy between interstellar and in situ observations. The shock destruction of interstellar grains is considered, and comparisons are made with interplanetary and presolar dust grains.

Detection of an impact-generated dust cloud around Ganymede

Abstract: Dust pervades the Solar System, and is concentrated in the ring systems surrounding the giant planets and along the plane of the planetary orbits (the Zodiacal cloud). Individual dust grains are thought to be generated when impacts loft material from larger bodies20,21,23,24,25,26, 27 such as satellites. Uncertainties in theoretical models of this ejection process are large, and there have hitherto been no direct measurements with which to constrain these models. Here we report in situ measurements of submicrometre dust within a few radii of Jupiter's satellite Ganymede. The directions, speeds and distribution of masses of the grains indicate that they come from Ganymede, and are consistent with an ejection process resulting from hypervelocity impacts of interplanetary dust onto Ganymede's surface. Dust appears also to be concentrated near Callisto and Europa, suggesting that these satellites too are significant sources of dusty debris.

Deflection of the local interstellar dust flow by solar radiation pressure.

Abstract: Interstellar dust grains intercepted by the dust detectors on the Ulysses and Galileo spacecrafts at heliocentric distances from 2 to 4 astronomical units show a deficit of grains with masses from 1 x 10(-17) to 3 x 10(-16) kilograms relative to grains intercepted outside 4 astronomical units. To divert grains out of the 2- to 4-astronomical unit region, the solar radiation pressure must be 1.4 to 1.8 times the force of solar gravity. These figures are consistent with the optical properties of spherical or elongated grains that consist of astronomical silicates or organic refractory material. Pure graphite grains with diameters of 0.2 to 0.4 micrometer experience a solar radiation pressure force as much as twice the force of solar gravity.

Dynamics of dust ejected from Enceladus: Application to the Cassini dust detector

Abstract: The satellite Enceladus is obviously the main source of Saturn's E ring. Up to now, different mechanisms of how particles are delivered from this satellite have been suggested. In this paper, we try to answer the question of whether these different launch processes can be distinguished by the cosmic dust analyzer (CDA) aboard the Cassini spacecraft. To this aim, the dynamics of dust particles just launched from the surface of Enceladus is studied numerically. We have integrated the equations of motion for a wide range of initial conditions including ejecta from interplanetary and E ring impactors onto Enceladus. According to our simulations, Cassini will encounter a significant dust stream about the time of closest approach to Enceladus. The duration and intensity of this expected enhanced impact rate onto the CDA depends on the way the particles are ejected from the satellite. The counting rate yields information about the distribution of ejecta sources on the surface of Enceladus. For instance, an anisotropy of the ejecta between the leading and the trailing hemispheres of Enceladus should be detectable, and impactors of different origin should be distinguishable. Furthermore, the vertical component of the ejecta velocities can explain the vertical extent of the E ring.

Observations Of Electromagnetically Coupled Dust In The Jovian Magnetosphere

Abstract: We report on dust measurements obtained during the seventh orbit of the Galileo spacecraft about Jupiter The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system The impact rate varied by more than an order of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileo’s position relative to the Jovian magnetic field This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field In addition to the 5 and 10 h periodicities, a longer period which is compatible with Io’s orbital period is evident in the dust impact rate This feature indicates that Io most likely is the source of the dust streams During a close (3,095 km altitude) flyby at Ganymede on 5 April 1997 an enhanced rate of dust impacts has been observed, which suggests that Ganymede is a source of ejecta particles Within a distance of about 25 RJ (Jupiter radius, RJ = 71,492 km) from Jupiter impacts of micrometer-sized particles have been recorded which could be particles on bound orbits about Jupiter

Techniques for Galactic Dust Measurements in the Heliosphere

Abstract: Galactic interstellar dust (ISD) is the major ingredient in planetary formation. However, information on this important material has been extremely limited. Recently the Ulysses dust detector has identified and measured interstellar dust outside 1.8~AU from the Sun at ecliptic latitudes above $50^{\circ}$. Inside this distance it could not reliably distinguish interstellar from interplanetary dust. Modeling the Ulysses data suggests that up to 30 % of dust flux with masses above $10^{-16}\rm kg$ at 1~AU is of interstellar origin. From the Hiten satellite in high eccentric orbit about the Earth there are indications that ISD indeed reaches the Earth's orbit. Two new missions carrying dust detectors, Cassini and Stardust, will greatly increase our observational knowledge. In this paper we briefly review instruments used on these missions and compare their capabilities. The Stardust mission [{\em Brownlee et al.}, 1996] will analyze the local interstellar dust population by an in-situ chemical analyzer and collect ISD between 2 and 3~AU from the Sun. The dust analyzer on the Cassini mission will determine the interstellar dust flux outside Venus' orbit and will provide also some compositional information. Techniques to identify the ISD flux levels at 1~AU are described that can quantify the interstellar dust flux in high-Earth orbit (outside the debris belts) and provide chemical composition information of galactic dust.

Aspects of the Mass Distribution of Interstellar Dust Grains in the Solar System from In-Situ Measurements

Abstract: The in-situ detection of interstellar dust grains in the Solar System by the dust instruments on-board the Ulysses and Galileo spacecraft as well as the recent measurements of hyperbolic radar meteors give information on the properties of the interstellar solid particle population in the solar vicinity. Especially the distribution of grain masses is indicative of growth and destruction mechanisms that govern the grain evolution in the interstellar medium. The mass of an impacting dust grain is derived from its impact velocity and the amount of plasma generated by the impact. Because the initial velocity and the dynamics of interstellar particles in the Solar System are well known, we use an approximated theoretical instead of the measured impact velocity to derive the mass of interstellar grains from the Ulysses and Galileo in-situ data. The revised mass distributions are steeper and thus contain less large grains than the ones that use measured impact velocities, but large grains still contribute significantly to the overall mass of the detected grains. The flux of interstellar grains with masses $> 10^{-14} {\rm kg}$ is determined to be $1\cdot 10^{-6} {\rm m}^{-2} {\rm s}^{-1}$. The comparison of radar data with the extrapolation of the Ulysses and Galileo mass distribution indicates that the very large ($m > 10^{-10} {\rm kg}$) hyperbolic meteoroids detected by the radar are not kinematically related to the interstellar dust population detected by the spacecraft.

Collisional Consequences of Big Interstellar Grains

Abstract: Identification by the Ulysses spacecraft of interstellar grains inside the planetary system provides a new window for the study of diffuse interstellar matter. Dust particles observed by Ulysses and confirmed by Galileo are more massive ($\geq 10^{-13} {\rm g}$) than the 'classical' interstellar grains. Even bigger grains ($\approx 10^{-7} {\rm g}$) were observed in form of interstellar meteors. We analyze the consequences of the plentiful existence of massive grains in the diffuse interstellar medium. Astronomically observed 'classical' interstellar grains can be described by a size distribution ranging from about 5 to 250 nm in radius (about $10^{-18}$ to $10^{-13} {\rm g}$). Lifetimes of these particles, due to mutual collisions in interstellar space, can be as short as $10^{5} f$ years, where f = 10 to 1000, is the fraction of total lifetime to the time when grains are exposed to supernova shocks. Shattering is a source of the smallest of these grains, but grains more massive than about $10^{-16} {\rm g}$ of the classical interstellar grain population are rapidly destroyed. When applying the same shattering mechanism to the more massive grains found recently, we suggest that collisions of particles bigger than about $10^{-15} {\rm g}$ provide a source for smaller grains. Because massive grains couple to the interstellar gas only over large (100 to 1000 pc) length scales, the cosmic abundance ratio of gas-to-dust needs only to be preserved averaged over corresponding volumes of space.

Dust Measurements in the Outer Solar System

Abstract: Dust measurements in the outer solar system are reviewed. Only the plasma wave instrument on board Voyagers 1 and 2 recorded impacts in the Edgeworth-Kuiper belt (EKB). Pioneers 10 and 11 measured a constant dust flux of 10-micron-sized particles out to 20 AU. Dust detectors on board Ulysses and Galileo uniquely identified micron-sized interstellar grains passing through the planetary system. Impacts of interstellar dust grains onto big EKB objects generate at least about a ton per second of micron-sized secondaries that are dispersed by Poynting-Robertson effect and Lorentz force. We conclude that impacts of interstellar particles are also responsible for the loss of dust grains at the inner edge of the EKB. While new dust measurements in the EKB are in an early planning stage, several missions (Cassini and STARDUST) are en route to analyze interstellar dust in much more detail.

New Results from In Situ Measurements of Cosmic Dust-Data from the GORID Experiment

Abstract: The impact detector GORID, on the Russian Express II satellite has now collected data on Cosmic Dust and Space Debris for more than one year from its geostationary location at 80° East. During this time a large number of events have been registered and these are now being analysed and categorised. Dust and Debris particles are in the first approximation separated by evaluating the velocity, assuming that particles in Earth orbit are related to Space Debris and that particles with velocities above the Earth escape velocity are Cosmic Dust particles. Now that a full year of data collected with constant instrument settings is available proper analysis is possible where the random process of the impacts best can be described by statistical methods on the whole set of data. Data sets from one or several full years are required to suppress the possible biases that can result from spatial, temporal and directional variations in the flux and to reduce the statistical errors. Interesting observations include clusters of particles and indications of highly charged particles.

Prediction of the In-Situ Dust Measurements of the Stardust Mission to Comet 81P/Wild 2

Abstract: We predict the amount of cometary, interplanetary, and interstellar cosmic dust that is to be measured by the Cometary and Interstellar Dust Analyzer (CIDA) and the aerogel collector on-board the Stardust spacecraft during its fly-by of comet P/Wild 2 and during the interplanetary cruise phase. We give the dust flux on the spacecraft during the encounter with the comet using both, a radially symmetric and an axially symmetric coma model. At closest approach, we predict a total dust flux of $10^{6.0} m^{-2} s^{-1}$ for the radially symmetric case and $10^{6.5} m^{-2} s^{-1}$ for the axially symmetric case. This prediction is based on an observation of the comet at a heliocentric distance of $1.7 {\rm AU}$. We reproduce the measurements of the Giotto and VEGA missions to comet P/Halley using the same model as for the Stardust predictions. The planned measurements of {\em interstellar} dust by Stardust have been triggered by the discovery of interstellar dust impacts in the data collected by the Ulysses and Galileo dust detector. Using the Ulysses and Galileo measurements we predict that 25 interstellar particles, mainly with masses of about $10^{-12} g$, will hit the target of the CIDA experiment. The interstellar side of the aerogel collector will contain 120 interstellar particles, 40 of which with sizes greater than $1 \mu m$. We furthermore investigate the ``contamination'' of the CIDA and collector measurements by interplanetary particles during the cruise phase.

Analysis of the sensor characteristics of the Galileo dust detector with collimated Jovian dust stream particles

Abstract: The Dust Detector System onboard Galileo records dust impacts in the Jupiter system. Impact events are classified into four quality classes. Class 3 -- our highest quality class -- has always been noise-free and, therefore, contains only true dust impacts. Depending on the noise environment, class 2 are dust impacts or noise. Within $20 R_J$ from Jupiter (Jupiter radius, $R_J = 71,492 km$) class 2 shows clear indications for contamination by noise. We analyse the dust data from Galileo's prime Jupiter mission (1996 and 1997), separate dust impacts from noise events and derive a complete denoised set of Galileo dust data (class 2 and class 3). Collimated streams of nanometer-sized dust particles which have been detected throughout the Jovian system (Gr\"un et al. 1998, JGR, 103, 20011-20022) are used to analyse the sensitive area and the field of view of the dust detector itself. The sensitive area for stream particles which trigger class 3 events is $110 \pm 37 cm^2$. This is almost a factor of ten smaller than the total sensitive area for class 2 impacts (1,000 cm^2). Correspondingly, the field of view of the detector for class 3 stream particles is reduced from $140^{\circ}$ to $96^{\circ}$. The magnetometer boom and other instruments on board Galileo cause a significant shadowing of the field of view of the dust sensor. Our analysis is supplementary to ground calibrations of the dust instrument because the low masses and high speeds of the stream particles could not be achieved in the laboratory. Our new results have important consequences for the analysis of dust in the Jupiter system.