R.H. Giese

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.

Three-dimensional models of the zodiacal dust cloud: A comparative study

Abstract Several analytical presentations of the three-dimensional distribution of interplanetary dust have been derived in the literature from measurements of the zodiacal light such as fan, ellipsoid, sombrero, and multilobe models. To provide a basis for comparisons with infrared measurements these classical and some new optical approaches are reviewed and compared with observations of the zodiacal light all over the sky and in selected viewing directions. Strengths and weaknesses of the models are discussed and qualitatively explained. It is shown that multilobe models can be refuted. The remaining models predict in surprising agreement that the interplanetary spatial dust density decreases “above” the Earths orbit by a factor of 2 within 0.2 to 0.3 AU. Beyond about 3 AU in the ecliptic plane and about 1.5 AU off the ecliptic no reliable density values can be obtained from the zodiacal light.

Three-dimensional models of the zodiacal dust cloud: II. Compatibility of proposed infrared models

Abstract Simple 3D distributions of the interplanetary dust cloud derived from observations in the visible and distributions suggested by IR investigators are compared. Predictions of visual brightness based on IR models are fairly compatible with observations for large ( e > 70°) elongations. They would however fail completely if extrapolated closer to the Sun. The reasons for both agreement and disagreement are explained by the shape of the isodensity surfaces of dust and by the relative contributions of scattered vs emitted radiation along the line of sight.

Icy particles from comets

The survival probability of the particles leaving a comet is examined based on their orbital changes at the moment of ejection. Considering the repulsive radiation pressure on them plus their emission velocities relative to the cometary nucleus, the critical radii s1 and sc are defined in a grain radius s ≧ 10−5 cm, where the ejected grains with s1 ≦ s ≦ sc cannot stay in the Solar System. We have found empirical formulae of s1 and sc; e.g., for long-period comets with semimajor axes a ≧ 103 AU and perihelion distances q ≧ 3 AU, sc (cm) = 2.3 × 10−6 a (AU) and s1 (cm) < 10−5 in the case of zero emission velocity. Water-ice particles with radii s ≧ 100 μm contaminated by “dirty” inclusions, which are injected into bound orbits with q ≧ 7 AU, are controlled by catastrophic collisions with interplanetary dust, and those with s ≦ 100 μ are controlled by the Poynting-Robertson drag forces. Contrary to that, almost all dirty water-ice grains on orbits with q ≦ 7 AU lose their ice parts due mainly to sublimation.

Dirty ice grains in comets

Abstract Based on the computed equilibrium temperature of evaporating dirty water-ice grains, dirty water-ice halo is examined, taking into account of a size dependence of terminal velocity of dust at P/Halley. It is found that due to an enhanced grains temperature caused by dirtiness, icy halo cannot extend over 100 km from the nucleus when comet approaches inside a solar distance r of 1 AU. Therefore, it is unlikely that the ice bands in the near infrared wavelengths could be detected in the cometary coma at r

Cometary dust observations by optical in-situ methods

Abstract Remote optical observations of comets provide information only along the whole line of sight and require some assumptions to be interpreted. Due to the advent of cometary space missions, a two-step strategy has been defined to derive without any assumption spatial distribution and physical properties of dust by in-situ optical observations. First, an Optical Probe Experiment , suitable for a fast fly-by, should provide passive in-situ measurements in the direction of the approaching (or receding) comet near encounter; by suitably differencing such observations, the brightness and polarization per unit volume can be recovered along the trajectory of the spacecraft. Secondly, a Light Scattering Dust Analyzer , suitable for a rendez-vous mission, should permit the determination of the scattering properties of individual particles . Both experiments also provide a connecting link between non-optical in-situ measurements (from mass spectrometers or impact detectors) and remote optical observations.

The three-dimensional (3D) distribution of zodiacal dust derived from infrared and visual measurements and their compatibility including dust dynamics

Abstract Several different models describe the three-dimensional global behaviour of the zodiacal dust cloud. They have been derived from measurements of the thermal emission of the dust particles (infrared) and from measurements of the scattered sunlight (optical). A sufficient agreement of the infrared models with the optical models is achieved for comparison with observations at larger elongations. Close to the Sun the infrared models become, however, flattened at the poles without showing any bulge. Dynamical considerations seem to favour these flattened models because they need a lower amount of retrograde orbits. This small component of retrograde orbits is confirmed by an analysis of the Doppler-shift in the Fraunhofer-lines from sunlight scattered at zodiacal dust. The comparison of optical and infrared models shows there are more observations necessary especially in the sunward direction.

Mass Input into and Output from the Meteoritic Complex

Direct observations have established the size distribution of interplanetary meteoroids at 1 AU as well as the dependence of the spatial density with respect to the distance from the sun. After evaluating the consequences of mutual collisions and the effect of radiation pressure the following conclusions can be drawn: 1. Catastrophic collisions dominate the lifetimes of meteoroids with masses m ≳ 10 -5g. About 10 tons per second are lost within 1 AU (mostly in form of 10 -4g to 10-1g particles). Under steady state conditions these meteor sized particles have to be replenished by other sources, e. g. comets. 2. After being crushed by collisions 70 to 85% of this mass will be in form of particles with masses 10-10g ≲ m ≲ 10-5g. Part of these “zodiacal light” particles (about 0.3 tons per second) are transported by the Poynting Robertson effect towards the sun where they will evaporate. However, since the collisional production of these intermediate sized particles exceeds their losses this population is presently not in equilibrium. 3. 15 to 30% of the collisional fragments have masses m ≲ 10-10g. Most of these small particles will be injected into hyperbolic orbits by radiation pressure (s-meteoroids)

A spectral photopolarimeter for Giotto - Halley Optical Probe Experiment

Abstract The Halley Optical Probe Experiment (HOPE) is designed to provide in situ photopolarimetric data on both the dust cloud and the gaseous atmosphere in Halleys coma. The optical probe concept is presented here, together with a description of the instrumentation and with the possibilities for cross-checks between HOPE results and those of other space and ground-based experiments.

The impact of IRAS results on the three-dimensional models (3D) of the global distribution of interplanetary dust

Abstract The three-dimensional (3D) distribution of micrometeoroids, representing the interplanetary dust cloud, has been investigated by light scattering analysis of the zodiacal light. Classical models imply, however, that there are homogeneous physical properties of the grains throughout the visible zodiacal cloud. IRAS results have questioned this assumption and suggested, e.g., a possible spatial decrease of albedo with solar distance, which would result in a more moderate decrease of number densities. The discussion of the conventional and corresponding modified models shows that the IRAS measurements propose a flatter interplanetary dust cloud than the visual models. This tendency is also supported by other IR-observations. A dynamical analysis reveals that this flattening of the dust cloud on account of the IRAS-data is achieved by a change in the major axis distribution of the particle orbits only, whereas in the other models the inclination distribution is also changed.