Solar System Dust 635
TABLE 4 Comparison of Various Forces Acting on Dust Particles of SizesUnder Typical Interplanetary Conditions
at 1 AU Distance from the Suna
s(μm) FG(N) FR(N) FL(N) FPR(N) FID(N)0.01 9 × 10 −^23 1.4× 10 −^21 1.5× 10 −^20 1.4× 10 −^254 × 10 −^26
0.1 9 × 10 −^20 1.4× 10 −^19 1.5× 10 −^19 1.4× 10 −^234 × 10 −^24
1 9 × 10 −^17 1.4× 10 −^17 1.5× 10 −^18 1.4× 10 −^214 × 10 −^22
10 9 × 10 −^14 1.4× 10 −^15 1.5× 10 −^17 1.4× 10 −^194 × 10 −^20
100 9 × 10 −^11 1.4× 10 −^13 1.5× 10 −^16 1.4× 10 −^174 × 10 −^18aNotes:Dominating forces are in bold. Subscripts refer to gravity, radiation pressure, Lorentz force, Poynting-Robertson drag, and ion drag.range of heliocentric distances but preferentially close to
the Sun, asteroid debris is mostly generated in the Asteroid
Belt, between 2 and 4 AU from the Sun. Collisions dom-
inate the fate of big particles and are a constant source of
smaller fragments. Meteoroids in the range of 1 to 100μm
FIGURE 14 Mass flow of meteoric matter through the solar
system. Most of the interplanetary dust is produced by collisions
of large meteoroids, which represent a reservoir continually
being replenished by disintegration of comets or asteroids. Most
of it is blown out of the solar system as submicrometer-sized
grains. The remainder is lost by evaporation after being driven
close to the Sun by the Poynting–Robertson effect. In addition to
the flow of interplanetary matter shown, there is a flow of
interstellar grains through the planetary system.
are dragged by the Poynting–Robertson drag to the Sun.
Smaller fragments are driven out of the solar system by
radiation pressure and Lorentz force.
Estimates of the mass loss from the zodiacal cloud in-
side 1 AU give the following numbers. About 10 tons per
second are lost by collisions from the big (meteor-sized)
particle population. A similar amount (on the average) has
to be replenished by cometary and asteroidal debris. Nine
tons per second of the collisional fragments are lost as small
particles to interstellar space, and the remainder of 1 ton per
second is carried by the Poynting–Robertson effect toward
the Sun, evaporates, and eventually becomes part of the
solar wind. Interstellar dust transiting the solar system be-
comes increasingly important farther away from the Sun. At
3 AU from the Sun, the interstellar dust flux seems to already
dominate the flux of submicrometer- and micrometer-sized
interplanetary meteoroids.4. Future Studies
New techniques will generate new insights. These tech-
niques will include innovative observational methods, new
space missions to unexplored territory, and new experimen-
tal and theoretical methods to study the processes affecting
solar system dust. Questions to address are: the composition
(elemental, molecular, and isotopic) and spatial distribution
of interplanetary dust; the quantitative understanding of ef-
fects or processes affecting dust in interplanetary space;
and the quantitative determination of the contributions
from different sources (asteroids, comets, planetary envi-
rons, and interstellar dust).
Analyses of brightness measurements at infrared wave-
lengths up to 200μm by theCOBEsatellite result in re-
fined models of the distribution of dust mostly outside 1 AU.
Spectrally resolved observations of asteroids, comets, and
zodiacal dust by the infrared space observatories (ISOand
Spitzer) show the genetic relation between these larger
bodies and interplanetary dust. Improved observations of
the inner zodiacal light and the edge of the dust-free zone
around the Sun will provide some clues to the composi-
tion of zodiacal dust. Optical and infrared observations of