Solar-system dust

Tiny solid cosmic particles - often referred to as dust - are the ultimate source of solids from which rocky planets, planetesimals, moons, and everything on them form. The study of the dust particles genesis and their evolution from interstellar space through protoplanetary disks into forming planetesimals provides us with a bottom-up picture on planet formation. These studies are essential to understand what determines the bulk composition of rocky planets and, ultimately, to decipher the formation history of the Solar System. Dust in many astrophysical settings is readily observable and recent ground- and space-based observations have transformed our understanding on the physics and chemistry of these tiny particles. 

In this chapter we will present information obtained by laboratory analysis of Solar System dust grains as well as observational constraints on the composition of dust around other stars. Each method has both advantages and disadvantages. The laboratory studies provide us with a very detailed view on the composition of the... 

Below we review the characteristics of fine-grained Solar System dust as preserved in primitive chondrite matrices and draw inferences from these observations about the mechanisms of coagulation of nebular dust as well as the timing and location of coagulation. 

In order to consider the processes of dust coagulation in the early Solar System, we first review the characteristics of this material. Of considerable importance is the fact that these samples - represented principally by chondritic meteorites, but also by IDPs and by samples from Comet Wild 2 collected by the Stardust mission - all come from parent bodies of different kinds. As a result, even the most primitive of these materials has been processed, both physically and chemically, to different degrees. The processes that affected Solar System dust may have occurred in different environments such as the solar nebula (e.g. evaporation/condensation, annealing) and asteroidal parent bodies (aqueous alteration and/or thermal processing, mild compaction to extensive lithihcation). A major challenge is to understand the effects of this secondary processing. 

K. Muinonen, Coherent backscattering by solar system dust particles, in Asteroids, Comets and Meteors, ed. by A. Milani, M. Di Martino, and A. Cellino (Kluwer, Dordrecht, 1974) 271-296. 

While orrly a few in-situ observatiorts of dust are available, nttmerous remote observations of the above-mentioned bodies have been performed. Observation of emitted light, mostly in the infrared for solar system dust, provides information about temperatrrre and compositiorr, with the silicate emission feature near 11 pm. Observation of solar light scattered by dust, mostly in the optical and near infrared domains, provides clues to dust physical properties. 

According to present-day concepts, our solar system was formed from a huge gas-dust cloud several light years across in a side arm of the Milky Way. The particle density of this interstellar material was very low, perhaps 108-1010 particles or molecules per cubic metre, i.e., it formed a vacuum so extreme that it can still not be achieved in the laboratory. The material consisted mainly of hydrogen and helium with traces of other elements. The temperature of the system has been estimated as 15 K. 

Fig. 2.2 The state of the incipient solar system during the T Tauri phase of the young sun. The central region around the sun was blown free from the primeval dust cloud. Behind the shock front is the disc with the remaining solar nebula, which contained the matter formed by the influence of the solar wind on the primeval solar nebula. From Gaffey (1997)... 

Fig. 1. Left panel. Post-explosive yields versus mass of the Ge isotopes for a 25 M of solar composition by [10]. Arrows represent a production factor of 200 over the initial mass fraction of each isotope. Right panel. Logaritmic abundances relative to O and to solar ratio observed in the DLA-B/FJ0812+32 System (dust corrected) [5]. The observed [Zn/O] value is represented by a full square.

At 2000 K there is sufficient energy to make the H2 molecules dissociate, breaking the chemical bond the core density is of order 1026 m-3 and the total diameter of the star is of order 200 AU or about the size of the entire solar system. The temperature rise increases the molecular dissociation, promoting electrons within the hydrogen atoms until ionisation occurs. Finally, at 106 K the bare protons are colliding with sufficient energy to induce nuclear fusion processes and the protostar develops a solar wind. The solar wind constitutes outbursts of material that shake off the dust jacket and the star begins to shine. 

Flynn G. J., Keller F. P., Feser M., Wirick S. and Jacobsen C. (2003). The origin of organic matter in the solar system evidence from the interplanetary dust particles. Geochimica et Cosmochimica Acta 67 4791. 

The lithium resonance doublet line X 6707 is fairly easy to observe in cool stars of spectral types F and later, and it has also been detected in diffuse interstellar clouds. There is thus an abundance of data, although in the ISM the estimation of an abundance is complicated by ionization and depletion on to dust grains. The youngest stars (e.g. T Tauri stars that are still in the gravitational contraction phase before reaching the main sequence) have a Li/H ratio that is about the same as the Solar System ratio derived from meteorites, Li/H = 2 x 10-9, which is thus taken as the Population I standard. 

Each known type of grain is made from a particularly refractory form of material. Themselves born in extreme heat conditions, these grains survived the formation of the Solar System without the least difficulty. They have been able to carry down the isotopic composition of their source quite intact, throughout the whole prehistory of the Sun. But their message has not yet been perfectly decoded. The story of this star dust will therefore be continued, especially as it is radioactive and can be identified by its gamma emissions. 

Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or IDPs, returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula - the disk-shaped cocoon of gas and dust that enveloped the early Sun some 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). 

When the elements are ejected from the stars where they were produced, they are in the gas phase. Subsequently, they combine in various chemical compounds and most condense as solids. The nature of those compounds and their behavior in the various environments encountered on their way to becoming part of the solar system can, in principle, be determined from the basic chemical properties of the elements. Evaporation and condensation are also important in the solar system and have played a defining role in determining the properties of planets, moons, asteroids, and the meteorites derived from them, comets, dust... 

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