Cosmic Dust 
Interplanetary dust particles from comets and asteroids fall by the ton 
on the earth, where they can be collected and studied 
by Donald E. Brownlee 
Meteorites can often be found on 
Antarctic blue-ice fields and near cer- 
tain large meteorite impact craters 
after only a few hours of searching, 
but finding them elsewhere on Earth 
is an extraordinarily rare experience. 
Only about a dozen newly fallen me- 
teorites are found each year by the 
entire global population of four billion 
people. This apparent rarity, however, 
applies only to extraterrestrial mate- 
rials large enough to be noticed with 
the naked eye. Small cosmic particles 
as fine as dust are actually very com- 
mon, and although invisible to the un- 
aided eye they are present everywhere 
in our daily environment. Each year 
more than 10,000 tons of extraterres- 
trial particles smaller than one mil- 
limeter in diameter enter the atmos- 
phere and fall to the surface of the 
earth. For particles of ten-micron size 
(a micron is 0.001 millimeter, or about 
one-sixth the diameter of a fine human 
hair), the flux hitting the earth’s sur- 
face is one per square meter per day. 
If you spend an appreciable amount 
of time outdoors, you will be “hit” 
by several cosmic particles a week. 
However, with fall speeds of a centi- 
meter per second and masses of a bil- 
Most of the cosmic spheres that 
accumulate on the sea floor are 
magnetic, a fortuitous circumstance 
that facilitates their collection. The 
spheres in this enlargement were 
gathered in 1979 in the Pacific 
Ocean southeast of Hawaii. 
lionth of a gram, impacts are less than 
noticeable. These particles are in the 
air we breathe, the food we eat, and 
the water we drink. They are ancient, 
primitive materials that formed during 
the earliest history of the planetary 
system and were stored inside comets 
and asteroids for most of the age of 
the solar system — about 4.5 billion 
years. 
Some of the dust particles are frag- 
ments of larger objects that broke up 
while entering the earth’s atmosphere, 
but the majority are believed to have 
been small particles of interplanetary 
dust out in space. Interplanetary dust 
is material that orbits the sun, forming 
a giant, flattened cloud centered on 
the ecliptic — the orbital plane of the 
planets. This cloud can be seen in 
the night sky as a faint glow extending 
along the ecliptic and fading with in- 
creasing distance from the sun. The 
glow is caused by light from the sun 
scattered off the zodiacal cloud: the 
vast cloud of dust particles one-micron 
to one-millimeter in diameter that ex- 
ists in the solar system. 
The interplanetary dust particles 
that are currently falling into the 
earth’s atmosphere had only a brief 
existence as small particles in space. 
Prior to that they were stored inside 
larger “parent bodies.” In almost all 
cases the parent bodies are comets 
and asteroids, dusty objects without 
permanent atmospheres and with low 
surface gravities. The dust must be 
generated on a semicontinuous basis 
because the zodiacal cloud is long- 
lived in spite of the fact that individual 
particles are disintegrated by high-ve- 
locity collisions with one another on 
time scales of only 10,000 to 100,000 
years. Comets are believed to be the 
major source of the dust since they 
are the dominant place of origin of 
the millimeter-sized particles that pro- 
duce visible meteors, or shooting stars, 
in the night sky. From the luminous 
path of a meteor it is possible to de- 
termine the particle’s orbital path 
around the sun, and in most cases 
the orbits are similar to those of com- 
ets. For some annual meteor showers, 
the meteors can be shown to have 
originated from specific comets. 
All of the cosmic particles on the 
earth entered the atmosphere at ex- 
tremely high velocity (greater than 
twelve kilometers per second) and 
were subjected to elevated tempera- 
tures and, sometimes, extreme me- 
chanical stress as they decelerated. 
The character of a particle’s high-ve- 
locity entry into the atmosphere is de- 
pendent on its size. Large meteoroids, 
say larger than ten centimeters, pen- 
etrate deeply into the atmosphere at 
high velocities until they reach alti- 
tudes of less than thirty kilometers. 
There the air density is significant and 
generates large pressures on the lead- 
ing edge of the meteoroid, causing 
its surface to melt and vaporize. Al- 
though the surfaces of the big me- 
teoroids melt, their interiors remain 
cool, much like a baked Alaska. Their 
interiors may escape strong heating, 
but the pressures acting on big me- 
teoroids during entry are so great that 
fragile materials disintegrate and only 
fairly strong rocks survive intact. This 
selective process may explain why the 
majority of recovered meteorites are 
thought to be asteroid chunks even 
Donald E Brownlee 
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