The interplanetary dust cloud has been studied for many
years in order to understand its nature, origin, and relationship to
planetary systems (our own, as well as extrasolar systems).
The
interplanetary dust particles (IDPs) not only scatter solar light (called
the "zodiacal
light", which is confined to the
ecliptic plane),
the IDPs also produce
thermal emission, which is the most prominent feature of the night-sky light
in the 5-50 micrometer wavelength domain (Levasseur-Regourd, A.C.
1996).
The grains characterizing the infrared emission near the
earth's
orbit have
typical sizes of 10-100 micrometers (Backman, D.,
1997). The total
mass of the interplanetary dust cloud is about the mass of an
asteroid of
radius 15 km (with density of about 2.5 g/cm3).
The sources of IDPs include at least: asteroid collisions,
cometary activity
and collisions in the inner solar system,
Kuiper
Belt collisions, and interstellar medium (ISM) grains (Backman, D., 1997).
Indeed, one of the longest-standing controversies debated in the interplanetary
dust community revolves around the relative contributions to the interplanetary
dust cloud from asteroid collisions and cometary activity.
The main physical processes "affecting" (destruction or expulsion mechanisms)
IDPs are: expulsion by
radiation
pressure, inward
Poynting-Robertson (PR) radiation drag,
solar wind
pressure (with significant electromagnetic effects), sublimation, mutual
collisions, and the dynamical effects of planets (Backman, D., 1997).
The lifetimes of these dust particles are very short compared to the lifetime
of the
Solar System. If one finds grains around a star that is older than about
10^8 years, then the grains must have been from recently released fragments of
larger objects, i.e. they cannot be leftover grains from the
protoplanetary nebula (Backman, private communication). Therefore, the
grains would be "later-generation" dust. The zodiacal dust in the solar system
is 99.9% later-generation dust and 0.1% intruding ISM dust. All primordial
grains from the Solar System's formation have been removed long ago.
The interplanetary dust cloud has a complex structure (Reach, W., 1997).
Apart from a background density, this includes:
- At least 8
dust trails -- their source is thought to be short-period comets.
- A number of Dust bands, the sources of which are thought to be
asteroid families in the
main asteroid
belt. The three strongest bands arise from the
Themis family, the
Koronis family, and the
Eos family.
Other source families include the
Maria,
Eunomia, and possibly the
Vesta
and/or
Hygiea families (Reach et al 1996).
- at least 2 resonant dust rings are known (for example, the Earth-resonant
dust ring, although every planet in the solar system is thought to have a
resonant ring with a "wake") (Dermott, S.F. et al., 1994, 1997)
