Dermott et al.: Asteroidal Dust 423 Asteroidal Dust Stanley F. Dermott University of Florida Daniel D. Durda Southwest Research Institute Keith Grogan NASA Goddard Space Flight Center Thomas J. J. Kehoe University of Florida There is good evidence that the high-speed, porous, anhydrous chondritic interplanetary dust particles (IDPs) collected in Earth’s stratosphere originated from short-period comets. How- ever, by considering the structure of the solar-system dust bands discovered by IRAS, we are able to show that asteroidal collisions are probably the dominant source of particles in the zodiacal cloud. It follows that a significant and probably the dominant fraction of the IDPs collected in Earth’s stratosphere also originated from asteroids. IDPs are the most primitive particles in the inner solar system and represent a class of material quite different from that in our meteorite collections. The structure, mineralogy, and high C content of IDPs dictate that they cannot have originated from the grinding down of known meteorite types. We argue that the asteroidal IDPs were probably formed as a result of prolonged mechanical mixing in the deep regoliths of asteroidal rubble piles in the outer main belt. 1. INTRODUCTION information on the nature of the particles in the preplanetary solar nebula (Bradley, 1999). Observations of microcraters In our collections on Earth, we have an abundance of on the Long Duration Exposure Facility (LDEF) confirmed meteorite samples from three major sources of extraterres- that each year Earth accretes 3 × 107 kg of dust particles, a trial material: the asteroid belt, the Moon, and Mars. Some mass influx ~100× greater than the influx associated with of the source bodies of these meteorites have experienced the much larger meteorites that have masses between 100 g major physical, chemical, and mineralogical changes since and 1000 kg. The action of sunlight on these small inter- the time of their formation; hence the resulting meteorites, planetary particles causes their orbits to decay into the Sun while providing vital information on the origin and evolu- on timescales of less than 10 m.y. Thus, the dust is not orig- tion of the parent bodies, give limited information on the inal but must be continuously replenished. The emphasis nature of the primordial particles out of which these bod- of this chapter is on the origin of these IDPs and MMs and ies accreted. However, this broad statement does not apply whether they are asteroidal or cometary. There is now good to unequilibriated ordinary chondrites. These primitive me- evidence that interstellar dust particles also penetrate the teorites are composed almost entirely of particles that were inner solar system (Grün et al., 1993) and may provide a freely orbiting in the solar nebula and although they show significant contribution to the dust flux at 1 AU (Grün et al., some signs of thermal metamorphism (McSween et al., 1988) 1997). In addition, small dust particles that originate in the and aqueous alteration (Zolensky and McSween, 1988), they Kuiper Belt may be transported to the inner solar system do provide information on initial conditions. (Flynn, 1996; Liou et al., 1996). However, the larger Kuiper A fourth class of extraterrestrial material consists of par- Belt particles, which have long orbital-decay periods, are ticles recently accreted by Earth from the zodiacal cloud. likely to be destroyed by collisions or removed by the giant These are the small, mostly 5–25-µm diameter, interplane- planets before reaching Earth. tary dust particles (IDPs) collected, gently and mostly unal- In section 2, we give a brief description of our current tered, in Earth’s stratosphere and the larger micrometeorites understanding of the nature of IDPs, emphasizing those fea- (MMs) and cosmic spherules collected from polar ices and tures that may separate the putative cometary particles from deep-sea floors (Brownlee, 1985; Jessberger et al., 2001). those that are asteroidal. Despite the enormous advances Some of the IDPs appear to be the most primitive material in that have been made in recent years in our understanding of the solar system and, at present, provide our best source of these highly interesting particles, the question of their prov- 423 424 Asteroids III enance, whether they are predominantly asteroidal, pre- et al., 1984). In addition, the nonsolar isotopic abundances dominantly cometary, or a useful mixture of both, remains of D, H, and N (Messenger and Walker, 1996) of some of controversial. In section 3, we attempt to answer this key these particles prove not only that they are extraterrestrial question by looking beyond the particles themselves to the but also that some of these particles existed in the presolar zodiacal cloud as a whole. nebula out of which the planets formed. Roughly 15% of Observations by the Infrared Astronomical Satellite the particles are essentially single-mineral grains or a few- (IRAS) in 1983 revealed that the zodiacal cloud is not de- mineral grains. However, the most common IDPs (75%) are void of large-scale structure (Low et al., 1984). We now unequilibriated, fine-grained mixtures of thousands to mil- know that the Sun is not at the center of symmetry of the lions of mineral grains and amorphous components. Their cloud (Kelsall et al., 1998; Dermott et al., 1999), that the composition is chondritic and similar, within a factor of 2, plane of symmetry is warped (Dermott et al., 2001), and to the most primitive, C-rich, CI carbonaceous chondrites that clouds of dust trail Earth in its orbit that are associ- (Schramm et al., 1989; Thomas et al., 1996). Jessberger et ated with a circumsolar ring of dust particles in resonant al. (2001) have noted that the fact that the composition of lock with the planet (Dermott et al., 1994a; Reach et al., these tiny particles is chondritic is remarkable “because 1995). Ultimately, after more analysis, these features should most chondritic meteorites do not typically have chondritic contribute significant information on the origin and evolu- elemental composition at the 10-micron size scale. Most tion of the cloud (Dermott et al., 2001). However, for the meteorites are coarser grained and common 15-micron vol- purposes of this chapter, it is clear that the most direct and umes are single mineral grains.” It follows that most IDPs important source of information on the origin of the par- cannot be formed from the simple grinding down of known ticles in the zodiacal cloud is provided by the solar-system meteorites, but must result “from the mechanical mixing of dust bands. Remarkably, these dust bands have now been large numbers of randomly selected tiny grains.” The com- observed from the ground, without the aid of a telescope, positions of MMs are similar to CM- and CR-type carbon- simply by using a wide-angle lens attached to a cooled CCD aceous chondrites. However, there are some very important camera (Ishiguro et al., 1999). In section 3, we show that differences between carbonaceous chondrites and both MMs these dust bands must originate from the disintegration of (Maurette et al., 1996) and IDPs. In particular, both MMs asteroids (Low et al., 1984; Dermott et al., 1984; Sykes and and IDPs are markedly C-rich. In fact, some IDPs have C Greenberg, 1986; Grogan et al., 2001), and we use the IRAS contents about 2–3× higher than the most primitive, C-rich, observations to estimate, with comparative confidence, the carbonaceous chondrites (Thomas et al., 1994). Moderately total asteroidal contribution to the zodiacal cloud. volatile elements such as Zn and Ga also show systematic In section 4, we discuss the collisional evolution of the enrichments in IDPs by a factor of 2–4 above CI chondrites asteroid belt. Earlier discussions on the sources of the par- (Flynn et al., 1996). ticles in the cloud were based on estimates of the dust pro- On the basis of mineralogy, as revealed by transmission duction rates from comets (Whipple, 1967; Delsemme, 1976) electron microscopy (TEM), infrared (IR) spectroscopy, and and asteroids (Dohnanyi, 1976). In the case of comets, these X-ray diffraction analyses, IDPs can be divided into two calculations may have been strengthened by the discovery major groups: chondritic and nonchondritic (Klöck and of cometary trails (Sykes and Walker, 1992), and the analysis Stadermann, 1994). The nonchondritic group is poorly stud- of these trails may lead to more accurate estimates of the ied, with only a few very refractory IDPs having received average cometary input to the cloud (Lisse et al., 1998; thorough mineralogic and isotopic characterization (Zolen- Lisse, 2001). However, we consider that the uncertainties sky, 1987; McKeegan, 1987). The chondritic group is well in these types of calculation, both for the asteroidal and the studied and has been further subdivided into hydrous and cometary sources, are still too large to provide definitive anhydrous subgroups. The hydrous chondritic IDPs have results. We support this statement in section 5 by a discus- been extensively altered by liquid water inside a parent sion of the current uncertainties in the asteroidal dust pro- body (Zolensky and McSween, 1988; Zolensky and Barrett, duction rate. 1994), and this fact suggests that they are asteroidal. The anhydrous chondritic IDPs have, in general, higher porosi- 2. INTERPLANETARY DUST PARTICLES ties (Corrigan et al., 1997) and consist mainly of olivine and/or pyroxene. In particular, the pyroxene-rich particles Important reviews of the nature of IDPs and MMs have are physically, chemically, and isotopically more primitive been given by Brownlee (1985), Bradley et al. (1988), and than any other materials available for laboratory study. Most Rietmeijer (1988) and are to be found in the conference pro- of these particles are complex admixtures of 0.1–5-µm-di- ceedings edited by Zolensky et al.