H. Fechtig

Collisional balance of the meteoritic complex

Taking into account meteoroid measurements by in situ experiments, zodiacal light observations, and oblique angle hypervelocity impact studies, it is found that the observed size distributions of lunar microcraters usually do not represent the interplanetary meteoroid flux for particles with masses ⪅10−10g. From the steepest observed lunar crater size distribution a “lunar flux” is derived which is up to 2 orders of magnitude higher than the interplanetary flux at the smallest particle masses. New models of the “lunar” and “interplanetary” meteoroid fluxes are presented. The spatial mass density of interplanetary meteoritic material at 1 AU is ∼10−16g/m3. A large fraction of this mass is in particles of 10−6 to 10−4 g. A detailed analysis of the effects of mutual collisions (i.e., destruction of meteoroids and production of fragment particles) and of radiation pressure has been performed which yielded a new picture of the balance of the meteoritic complex. It has been found that the collisional lifetime at 1 AU is shortest (∼104years) for meteoroids of 10−4 to 1 g mass. For particles with masses m > 10−5g, Poynting-Robertson lifetimes are considerably larger than collisional lifetimes. The collisional destruction rate of meteoroids with masses m ⪆ 10−3g is about 10 times larger than the rate of collisional production of fragment particles in the same mass range. About 9 tons/sec of these “meteor-sized” (m > 10−5g) particles are lost inside 1 AU due to collisions and have to be replenished by other sources, e.g., comets. Under steady-state conditions, most of these large particles are “young”; i.e., they have not been fragmented by collisions and their initial orbits are not altered much by radiation pressure drag. Many more micrometeoroids of masses m ⪅ 10−5g are generated by collisions from more massive particles than are destroyed by collisions. The net collisional production rate of intermediate-sized particles 10−10g ⪅ m ⪅ 10−5g is found to be about 16 times larger at 1 AU than the Poynting-Robertson loss rate. The total Poynting-Robertson loss rate inside 1 AU is only about 0.26 tons/sec. The smallest fragment particles (m ⪅ 10−10g) will be largely injected into hyperbolic trajectories under the influence of radiation pressure (β meteoroids). These particles provide the most effecient loss mechanism from the meteoritic complex. When it is assumed that meteoroids fragment similarly to experimental impact studies with basalt, then it is found that interplanetary meteoroids in the mass range 10−10g ⪅ m ⪅ 10−5g cannot be in temporal balance under collisions and Poynting-Robertson drag but their spatial density is presently increasing with time.

The Galileo Dust Detector

The Galileo Dust Detector is intended to provide direct observations of dust grains with masses between 10−19 and 10−9 kg in interplanetary space and in the Jovian system, to investigate their physical and dynamical properties as functions of the distances to the Sun, to Jupiter and to its satellites, to study its interaction with the Galilean satellites and the Jovian magnetosphere. Surface phenomena of the satellites (like albedo variations), which might be effects of meteoroid impacts will be compared with the dust environment. Electric charges of particulate matter in the magnetosphere and its consequences will be studied; e.g., the effects of the magnetic field on the trajectories of dust particles and fragmentation of particles due to electrostatic disruption. The investigation is performed with an instrument that measures the mass, speed, flight direction and electric charge of individual dust particles. It is a multicoincidence detector with a mass sensitivity 106 times higher than that of previous in-situ experiments which measured dust in the outer solar system. The instrument weighs 4.2 kg, consumes 2.4 W, and has a normal data transmission rate of 24 bits s−1 in nominal spacecraft tracking mode. On December 29, 1989 the instrument was switched-on. After the instrument had been configured to flight conditions cruise science data collection started immediately. In the period to May 18, 1990 at least 168 dust impacts have been recorded. For 81 of these dust grains masses and impact speeds have been determined. First flux values are given.

Some consequences of meteoroid impacts on Saturn's rings

Abstract High-velocity impacts of interplanetary meteoroids on Saturns rings are discussed. It is shown that the neutral gas emitted by impact vaporization may be responsible, to a large part, for the observed neutral ring atmosphere. Both the predicted neutral gas injection rate and the gas temperature (or kinetic energy) are compatible with the measurements (see Broadfoot, A. L., B. R. Sandel, D. E. Shemansky, J. B. Holberg, G. R. Smith, D. F. Strobel, J. C. McConnell, S. Kumar, D. M. Hunten, S. K. Atreya, T. M. Dohnahne, H. W. Moos, J. L. Bertaux, J. E. Blamont, R. B. Pomphrey, and S. Linik, Science 212, 206–211, 1981 ). Heavy ejecta particles produce a particulate ring “halo”. The physical properties of this halo are calculated, and it appears to be identical with the tenous particle population discussed by Baum and Kreidl (1982) . Erosion of Saturns ring particles, the resulting mass balance, and regolith formation are estimated. This provides some constraints on surface properties and optical albedo.

The comet Halley dust and gas environment

Quantitative descriptions of environments near the nucleus of comet P/Halley have been developed to support spacecraft and mission design for the flyby encounters in March, 1986. To summarize these models as they exist just before the encounters, we review the relevant data from prior Halley apparitions and from recent cometary research. Orbital elements, visual magnitudes, and parameter values and analysis for the nucleus, gas and dust are combined to predict Halleys position, production rates, gas and dust distributions, and electromagnetic radiation field for the current perihelion passage. The predicted numerical results have been useful for estimating likely spacecraft effects, such as impact damage and attitude perturbation. Sample applications are cited, including design of a dust shield for spacecraft structure, and threshold and dynamic range selection for flight experiments. We expect that the comets activity may be more irregular than these smoothly varying models predict, and that comparison with the flyby data will be instructive.

Micrometeoroids within ten Earth radii

Abstract The micrometeoroid population within 10 Earth radii (60,000 km) has been observed by the HEOS 2 dust experiment between 7 February 1972 and 2 August 1974. A total of 431 particles has been observed. Of those 90 particles are classified as random, the rest as particle bursts. The random particles only show a slight increase (factor 3) in flux within 10 Earth radii, compared to the deep space flux at 1 AU and this is interpreted as being due to the gravitational field of the Earth. The bursts are divided into groups and swarms according to their time profiles. The 19 recorded groups are observed both within 10 Earth radii and above, again with a slight increase below 60,000 km. The 15 recorded swarms are exclusively observed within 10 Earth radii. The total micrometeoroid flux in this region is enhanced by 2–3 orders of magnitude. The interpretation is that larger bodies in the 10–10 6 g mass range of the type III fireballs are disintegrating while travelling through the Earths auroral zones. The fragmentation process proposed is that of electrostatic disruption. This leads to one (or several) swarm(s) of small individual particles, which originally made up the flurry type (cometary) parent body.

Lunar microcraters: Implications for the micrometeoroid complex

Abstract The contributions of lunar microcrater studies to understand the overall micrometeoroid environment are summarized and compared to satellite data. In comparison with small-scale laboratory studies, most lunar crater morphologies are compatible only with impact velocities > 3·5 km/sec and projectile densities between 1–8 g/cm 3 ; a mean value is most likely 2–4 g/cm 3 . The particles arenon-porous and fairly equi-dimensional; needles, platelets, rods, whiskers and other highly asymmetric particle shapes can be excluded. Data on projectile chemistry is sparse and non-diagnostic at present. The crater diameters are converted into projectile masses via small scale laboratory impact experiments. Accordingly, the observed span of crater pit diameters (0·1 μm–1 cm) corresponds to a particle mass range of ≈ 10 −15 –10 −3 g. This large, dynamic detection range is a unique feature of the lunar rock detector. Absolute crater densities on different rocks vary from “production” to “equilibrium” conditions. After normalization of such densities, relative microcrater size frequencies are obtained to deduce a mass frequency distribution for particles 10 −15 –10 −3 g. There is evidence that this distribution is bimodal. A radiation pressure cutoff at 10 −12 g particle mass does not exist. The micrometeoroid flux obtained from lunar rocks is compatible with satellite data. There is indication that the micrometeoroid flux may have been lower in the past. Some speculative astronomical consequences concerning the origin of micrometeoroids are discussed.

Orbital and physical characteristics of micrometeoroids in the inner solar system as observed by Helios 1

Abstract The Helios 1 spacecraft was launched in December 1974 into a heliocentric orbit of 0.3 AU perihelion distance. Helios 2 followed one year later on a similar orbit. Both spaceprobes carry on board micrometeoroid experiments each of which contains two sensors with a total sensitive area of 121 cm 2 . To date, only preliminary data are available from Helios 2 . Therefore the results presented here mainly apply to data from Helios 1 . The ecliptic sensor of Helios 1 measures dust particles which have trajectories with elevations from −45° to + 55° with respect to the ecliptic plane. The south sensor detects dust particles with trajectory elevations from −90° (ecliptic south-pole) to −4°. The ecliptic sensor is covered by a thin film (3000 A parylene coated with 750 A aluminium) as protection against solar radiation. The other sensor is shielded by the spacecraft rim from direct sunlight and has an open aperture. Micrometeoroids are detected by the electric charge produced upon impact. During the first 6 orbits of Helios 1 around the sun the experiment registered a total of 168 meteoroids, 52 particles were detected by the ecliptic sensor and 116 particles by the south sensor. This excess of impacts on the south sensor with regard to the impacts on the ecliptic sensor is due predominantly to small impacts which are characterized by small pulse heights of the charge signals. But also large impacts were statistically significantly more abundant on the south sensor than on the ecliptic sensor. Most impacts on the ecliptic sensor were observed when it was pointing in the direction of motion of Helios (apex direction). In contrast to that the south sensor detected most impacts when it was facing in between the solar and antapex direction. Orbit analysis showed that the “apex” particles which are predominantly detected by the ecliptic sensor have eccentricities e a ⩽ 0.5 AU . From a comparison with corresponding data from the south sensor it is concluded that the average inclination f of “apex” particles is - i e > 0.4 and semimajor axes a > 0.5 AU . β-meteoroids leaving the solar system on hyperbolic orbits are directly identified by the observed imbalance of outgoing (away from the sun) and ingoing particles. It is shown that “eccentric” particles, due to their orbital characteristics, should be observable also by the ecliptic sensor. Since they have not been detected by this sensor it is concluded that the only instrumental difference between both sensors, i.e. the entrance film in front of the ecliptic sensor, prevented them from entering it. A comparison with penetration studies proved that particles which do not penetrate the entrance film must have bulk densities ρ ( g / cm 3 ) below an upper density limit ρ max . It is shown that approximately 30% of the “eccentric” particles have densities below ρ max = 1 g / cm 3 .

Galileo observes electromagnetically coupled dust in the Jovian magnetosphere

Measurements of dust coupled to the Jovian magnetosphere have been obtained with the dust detector on board the Galileo spacecraft. We report on data obtained during the first four orbits about Jupiter that had flybys of the Galilean satellites: Ganymede (orbits 1 and 2), Callisto (orbit 3), and Europa (orbit 4). The most prominent features observed are highly time variable dust streams recorded throughout the Jovian system. The impact rate varied by up to 2 orders of magnitude with a 5 and 10 hour periodicity, which shows a correlation with Galileos position relative to the Jovian magnetic field. Around 20 RJ (Jupiter radius, RJ=71, 492 km) in bound a dip in the impact rate has been found consistently. At the same times, reversals by 180° in impact direction occurred. This behavior can be qualitatively explained by strong coupling of nanometer-sized dust to the Jovian magnetic field. At times of satellite flybys, enhanced rates of dust impacts have been observed, which suggests that all Galilean satellites are sources of ejecta particles. Inside about 20 RJ impacts of micrometer-sized particles have been recorded that could be particles on bound orbits about Jupiter. (Less)

Three years of Galileo dust data

Abstract From its launch in October 1989 until the end of 1992, the Galileo spacecraft traversed interplanetary space from Venus to the asteroid belt and successfully executed close flybys of Venus, the Earth, and the asteroid Gaspra. The dust instrument has been operating most of the time since it was switched on in December 1989. Except for short time intervals near Earth, data from the instrument were received via occasional (once per week to once per month) memory read outs containing 282–818 bytes of data. All events (impacts or noise events) were classified by an onboard program into 24 categories. Over the three-year time span, the dust detector recorded 469 “big” dust impacts. These were counted in 21 of the 24 event categories. The three remaining categories of very low amplitude events contain mostly noise events. The impact rate varied from 0.2 to 2 impacts per day depending on heliocentric distance and direction of spacecraft motion with respect to the interplanetary dust cloud. Because the average data transmission rate was very low, some data were not received on the ground. Complete data sets for 358 “big” impacts were received, but the other 111 “big” impacts were only counted. The observed impact rates are compared with a model of the meteoroid complex.

Temporal fluctuations and anisotropy of the micrometeoroid flux in the Earth-Moon system measured by HEOS 2

Abstract The HEOS detector measures the mass and speed of micrometeoroids in the Earth-Moon system. They are detected by the plasma produced by particle impacts on the sensor. During 2 yr of data collection 384 particles have been registered. As shown earlier (COSPAR 1973), they can be divided into 3 categories according to their temporal distribution: particles that are (1) randomly distributed or (2) appear in “groups” or (3) appear in “swarms” In this paper the origin of the groups and swarms is discussed. For this purpose the article orbits with respect to the Earth and the Moon were traced back. The results imply a lunar origin of the groups, whereas the swarms are correlated with the vicinity of the Earth. In addition, the dependence of the cumulative flux upon the detectors viewing direction indicates clearly an anisotropic particle flux.