Science

Two main types of dust populate interplanetary space: interplanetary dust that originates from asteroids and comets and interstellar dust. Between these different types there exists a genetic relation: interstellar dust is the basic component from which stars and planets form today. Cometary dust is the most primitive material that still exists in the planetary system - it is suspected that it partially consists of unaltered interstellar dust. Dust from the asteroid belt was generated by collisions between bigger objects, the material of which has been modified during their formation.

There exist several links between meteorites (the best studied extraterrestrial material) and asteroids: (1) Photographically determined orbits of four ordinary chondrites resemble near-Earth-asteroid orbits, (2) mineralogical evidence indicates an origin of most meteorites from differentiated asteroids sized objects, and (3) optical properties of meteorite classes resemble the spectral reflectance data of asteroid classes. But a tiny (10"5) fraction of some primitive meteorites has a strange isotopic signature that deviates significantly from the 'average solar system abundance' of the bulk meteoritic material: these presolar grains have retained their interstellar identity. However, it must be noted that the collection of meteorites is a biased sample and does not represent the meteoroid population in near-Earth space. Some of the very friable material may not survive the passage through the Earth's atmosphere as is evidenced in the meteor records.

Besides the 'meteorite-window' there is another 'window' through which extraterrestrial material reaches the surface in more or less undisturbed state. Small interplanetary dust particles of a few to 10 |xm diameter are decelerated in the tenuous atmosphere above 100 km height. At this height the deceleration is so gentle that the grains will not reach the temperature of substantial evaporation. These dust particles subsequently sediment through the atmosphere and become accessible to collection and scientific examination. According to their elemental composition IDPs come in three major types: 1. chondritc 60% 2. iron-sulfur-nickel 30%, and 3. mafic silicates (iron-magnesium rich silicates, i.e. olivine and pyroxene) 10%. Most chondritic IDPs are porous aggregates but some smooth chondritic particles are found as well. Aggregates of 0.1-0.5 (xm-sized grains may contain varying amounts of carbonaceous material of unspecified composition. A significant enrichment in volatile (low condensation temperature) elements is found if compared to primitive chondrites. This observation is being used for the argument that these particles consist of some very primitive solar system material that had never seen temperatures above about 500°C.

A remarkable feature of IDPs is their large variability in isotopic composition. Extreme isotopic anomalies have been found in some IDPs (e.g. factors of 1000 off the solar hydrogen isotope ratio). Under typical solar system condition only fractions of a percent isotopic variations can occur. These huge isotopic variations indicate that some grains are not homogenized with other solar system material but have preserved much of their presolar character. These aggregate IDPs are believed to originate from comets, based on their porous structure, high carbon content, and relatively high atmospheric entry velocities. The match between the mineral identifications in cometary spectra and the silicates seen in the IDPs strengthens the link between comets and anhydrous chondritic aggregate IDPs.

In-situ measurements of the Halley missions and modern infra-red observations brought a wealth of information on cometary dust. It was found that the cometary particulates are intimate mixtures of two end-member components, one refractory organic component rich in the elements H, C, N, and O, and the other one rich in rock-forming elements as Si, Mg, Fe, respectively. The bulk abundance of the rock-forming elements in Halley's dust are indistinguishable from the solar and chondritic abundance within a factor of two. However, the presence of the wide range of isotopic compositions of carbon [17] excludes any equilibration processes affecting the carbon carrier during comet formation or later in its history. Infrared-spectrometry has identified the mineral assemblage in Comet Hale-Bopp [33] dust that resembles closely anhydrous chondritic aggregate IDPs: it consists of crystalline Mg-rich, Fe-poor enstatite and forsterite minerals that are formed at high temperatures plus lower-temperature glassy or amorphous grains. Mg-rich, Fe-poor particles in turn dominate Halley's dust.

The solid material that makes up the Earth and other planets resided in galactic interstellar dust (ISD) grains 5 109 years ago, before it was mixed and altered during the planetary formation process. However, information on galactic dust in the solar vicinity is extremely limited.

ISD research began around 1930, when the observed reddening of starlight was attributed to the presence of dust in interstellar space. With the advent of infrared astronomy it became possible to study the thermal emission of ISD and to detect it at lower column densities. Infrared spectroscopy allowed the analysis of molecular features and thus gave insight into the chemical composition of ISD. Prominent silicate features near 10 and 20 |am were detected. The infrared spectra also contained features at 3.3, 6.2, 7.7, and 11.3 which were tentatively attributed to polycyclic aromatic hydrocarbons (PAHs).

The depletion of some chemical elements in the gas phase of the interstellar medium compared to their cosmic abundance, was interpreted as a consequence of the condensation of these elements onto dust grains. A grain model by Mathis [24] suggests three grain types: small graphite grains; silicate grains; composite grains containing carbon (amorphous, hydrogenated, or graphitic), silicates, and oxides. On the basis of the same elementary composition, Li Greenberg [25] propose three different grain types: large grains consisting of a silicate core and an organic refractory mantle; small carbonaceous grains; very small grains (or large molecules) of PAHs.

What is known about the origin and evolution of ISD grains? Evolved stars continuously lose mass. About 90% of the stellar mass loss is provided by cool, high-luminosity stars, in particular by asymptotic giant branch (AGB) and post-AGB stars. As the ejected gas cools in expanding stellar winds, and also in supernova remnants, solid particles condense out of the gas phase. This so-called stardust provides the seeds for ISD grains that grow in cool interstellar clouds by accretion of atoms and molecules and by agglomeration. Interstellar shock waves provide an effective destruction mechanism for ISD grains as small grains are caught in the shock and collide with bigger grains that don't follow the shock's motion due to their larger inertia. In diffuse interstellar clouds, the grains lose their volatile constituents due to ultraviolet (UV) irradiation and thermal sputtering in interstellar shock fronts. Ultimately, an ISD grain can be incorporated (and destroyed) in a newly forming star, or else, it can become part of a planetary system. In this way, ISD grains are repeatedly recycled through the galactic evolution process.

The first goal of a dust telescope is to distinguish by their kinem atic properties dust particles from different sources: interstellar grains from the different types of interplanetary dust grains. Differences in the chemical and isotopic composition can then be used to determine their formation and history.

2.1 Distinction of interstellar from interplanetary dust

The kinematics of interstellar dust (ISD) is determined by the solar motion through the local interstellar cloud, which thus defines the encounter velocity of ISD in the heliocentric frame and its dynamics in the immediate solar vicinity. The Sun moves through the local interstellar cloud (LIC) which consists of warm partially ionized material. The diffuse interstellar gas in the Sun's vicinity is ablated from the parent molecular cloud complex when impacted by the combined stellar winds and supernova shock fronts from star formation and destruction in the Scorpius-Centaurus association [10]. The LIC has an extent of a few pc and is located close to the edge of the local bubble, a region of about 100 pc in extent which is believed to be excavated by supernova shocks.

From the in situ measurements of the Ulysses and Galileo dust instruments, the upstream direction of ISD particles was found to be close to ecliptic coordinates A.eci = 259°, Ped = 8° [5], This direction is approximately the upstream direction of interstellar gas as measured by the Ulysses GAS experiment [11], The initial velocity of ISD grains with respect to the Sun is compatible with 26 km/s, which is also the initial velocity of the gas. Close to the Sun, the motion of ISD grains is modified by their interaction with solar gravity, radiation pressure, and the solar electromagnetic field.

The zodiacal cloud of interplanetary dust has a flattened, lenticular shape which extends along the ecliptic plane about 7 times further from the Sun than perpendicular to the ecliptic plane. Zodiacal light observations found this cloud to extent out to the asteroid belt. Most interplanetary dust particles orbit the Sun in prograde elliptical orbits just like the planets do. Meteor and satellite observations provided the present dynamical state of the interplanetary dust cloud. Models developed by Divine [12] and Staubach [13] give orbital element distributions for various dust populations that describe the measurements. Cometary and asteroidal dust orbits can partially be distinguished by their orbital characteristics.

A dust telescope can use two different, complementary strategies to establish the interstellar origin of small (m < 10"11 g), and big (m > 10"u g) dust grains. For big ISD grains, the primary electrostatic charge is sufficiently high in order to determine the impact velocity and direction, by use of a large area charge and trajectory sensing instrument [14]. ISD grains can then be easily distinguished from interplanetary grains, because their velocity exceeds the local solar system escape velocity.

An ensemble of small ISD grains can be distinguished from the interplanetary dust population by statistical arguments, even if the exact trajectories of the individual particles are not known. This method has been demonstrated by the identification of an interstellar subset of the Ulysses and Galileo dust impacts [15] [16]. The Galileo and Ulysses instruments can only determine the impact direction of the dust particles with an accuracy of 70°. However, the statistical analysis of the distribution of impact directions of a large set of impacts, gives the interstellar upstream direction within 10° accuracy in ecliptic latitude and 20° accuracy in ecliptic longitude [5], With a similar argument, the mean velocity of dust impacts on the Ulysses sensor after Jupiter flyby has been found to exceed the solar system escape velocity at Jupiter [15]. Compared to the Ulysses and Galileo measurements, a dust telescope can provide an improved identification capability of small ISD grains. This is achieved by instruments with a narrow field of view of only 25° half-width.

2.2 Chemical and isotopic analysis

The dust telescope will use time-of-flight mass spectrometers to analyze the chemical and isotopic dust composition. The method of in situ mass spectrometry was demonstrated by the PUMA and PIA instruments that took data in the coma of comet Halley on board the spacecraft Vega 1, Vega 2, and Giotto, respectively. The data collected by PIA/PUMA demonstrate that each individual event detected by the mass spectrometer contains a wealth of scientific information.

The statistical analysis of spectra obtained from many individual dust particles gives information about the composition of their source. In the case of Halley, it was found that the abundances of elements more refractory than O, resemble solar composition and the composition of CI chondrites alike. H, C and N on the other hand, are less depleted than in CI chondrites, indicating the even more primitive (unequilibrated) character of Halley's dust compared to CI. For C and N, the composition of dust from comet Halley is nearly solar [17], The organic component of Halley dust has been inferred from coincidence analysis of molecular lines [18], The organic component consists mainly of highly unsaturated hydrocarbon polymers containing C-H and C-N-H compounds.

The dust telescope will use techniques for the analysis of dust which have previously successfully been applied to cometary dust. For the isotopic analysis of the detected grains, the mass spectrometer must have a mass resolution on the order of M/AM > 100. A lower mass resolution of M/AM 20 - 50 is sufficient to obtain an inventory of heavy elements in the detected grains.

2.3 Measurement of the interstellar dust flux near Earth

At large heliocentric distances, the Ulysses and Galileo dust instruments measured an average ISD flux for 10"15 g particles of 1.5 10"4 m'V1 [15]. The local ISD flux measured on a satellite moving with the Earth around the Sun exhibits an annual variation. Assuming a constant ISD flux of 1.5 10"4 m'V1 and a velocity of 26 km/s from a direction of Aeci = 259°, Peci = 8°, the maximum interstellar flux of 3 10"4 m'V1 occurs in February and remains more than 4 times larger than the interplanetary flux during 6 months around this time [14]. This flux gives a rough estimate for the expected impact rates onto dust instruments with sensitive areas of 0.01 m , 0.1 m2 and 1 m2. The corresponding rates of 10"15 g particles, are 10"6 s"1 (1 per 12 days) for 0.01 m2 sensitive area, and 10"5 s"1 (1 per day) for 0.1 m2 sensitive area, or 10"5 s"1 (1 per day) of 10"12 g particles for 1 m2 sensitive area, respectively.

The dynamics of ISD grains in the solar system is governed by solar gravity, radiation pressure, and the Lorentz force induced by the radially expanding solar wind magnetic field. These forces shape the trajectory of an individual grain and determine the ISD flux distribution at any given location in the solar system.

If electromagnetic forces are neglected, the distribution of ISD in the solar system can be calculated analytically [19]. In this case, and if gravity dominates radiation pressure (|3 < 1), ISD is concentrated in a wake downstream of the Sun. In the case (3 > 1, an interstellar dustfree cavity forms around the Sun [20]. The influence of the solar wind magnetic field on the dynamics of ISD grains is important for grains smaller than 0.4 in diameter. It was shown numerically [21] and confirmed by the in situ measurements of Ulysses [19] that the flux of small ISD grains varies with the 22-year polarity cycle of the solar magnetic field. As a result of the electromagnetic interaction, the ISD flux in the outer solar system dropped in mid-1996 from 1.5 10"4 m" s"1 to 5 10"5 m'V1. According to this model, the best time to measure a large number of ISD grains will be between 2004 and 2012, when the flux, and thus the impact rate, of 10"14 g grains at 1 AU will be enhanced by up to a factor of 10. The flux of large (m > 10"'1 g) grains is permanently increased at 1 AU by gravitational focusing.

2.4. Measurement of the interstellar dust size distributions

While the size distribution of interplanetary dust is well understood [22] [23] the size distribution of ISD grains is less well established. In order to match the observed wavelength dependence of interstellar extinction, ISD models introduce a grain size distribution [24] [25], This provides a basic test for astrophysical ISD models. The measurements of local ISD by Ulysses and Galileo have already shown that the local ISD population contains more large grains than expected by these models [5]. However, because of their limited target area of 0.1 m2, Ulysses and Galileo detected fewer than 10 ISD grains with masses above 10"11 g, which corresponds to a particle diameter of 2 |im.

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