Orbital Evolution of Interplanetary Dust
The two most important dynamical features of the zodiacal cloud are: (i) t he dust bands associated with t he major Hirayama asteroid families, and (ii) the circumsolar ring of dust particles in resonant lock with th e Eart h. Oth er important dynamical f
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University of Florida, Gainesville, Florida, USA NASA Goddard Space Flight Center, Greenbelt, Maryland, USA Southwest Research Institute, Boulder, Colorado, USA Vanguard Research Inc., Scotts Valley, California, USA University of Maryland, College Park, Maryland, USA Carnegie Institution of Washington, Washington, District of Columbia, USA Uk Astronomy Technology Centre, Royal Observatory, Edinburgh, UK
Abstract. The two most important dynamical features of the zodiacal cloud are: (i) the dust bands associated with the major Hirayama asteroid families, and (ii) the circumsolar ring of dust particles in resonant lock with the Earth. Other important dynamical features include the offset of the center of symmetry of the cloud from the Sun, the radial gradient of the ecliptic polar brightness at the Earth, and the warp of the cloud. The dust bands provide the strongest evidence that a substantial and possibly dominant fraction of the cloud originates from asteroids. However, the characteristic diameter of these asteroidal particles is probably several hundred microns and the migration of these large particles towards the inner Solar System due to Poynting-Robertson light drag and their slow passage through secular resonances at the inner edge of the asteroid belt results in large increases in their eccentricities and inclinations. Because of these orbital changes, the dividing line between asteroidal and cometary-type orbits in the inner Solar System is probably not sharp, and it may be difficult to distinguish clearly between asteroidal and cometary particles on dynamical grounds alone.
I. INTRODUCTION
Advances in infrared astronomy have revealed that the structure of the zodiacal cloud is complex and substantially different from the smooth, rotationally symmetric cloud assumed prior to the launch of the IRAS (Infrared Astronomical Satellite) and COBE (Cosmic Background Explorer) spacecraft (Giese et al. 1986). We now know that the Sun is not at the center of symmetry of the cloud (Kelsall et al. 1998; Dermott et al. 1999; Wyatt et al. 1999b), that the cloud contains dust bands originating from the disintegration of asteroids (Low et al. 1984; Dermott et al. 1984), dust trails derived from known comets (Sykes and Walker 1992), and clouds of dust associated with a circumsolar ring of dust particles trapped in resonant lock with the Earth (Dermott et al. 1994a; Reach et al. 1995). These features pose challenging dynamical problems. However, E. Grün et al. (eds.), Interplanetary Dust © Springer-Verlag Berlin Heidelberg 2001
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they are also our best source of information on the sources that supply the cloud. The plan of this review is as follows. In Section II, we discuss the LDEF (Long Duration Exposure Facility) results on the sizes of the particles accreted by the Earth. These results show very clearly that, in the vicinity of the Earth, the dominant particles in the cloud have characteristic diameters'" 102/-lm (Love and Brownlee 1993). A similar conclusion was reached by Griln et
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