common interplanetary dust particles 
are black aggregates of tiny grains 
of submicron size, smaller than the 
wavelength of light. A single ten-mi- 
cron particle is actually a collection 
of nearly a million grains ranging in 
size from several microns down to 100 
angstroms or less. (Angstroms are used 
to measure the wavelengths of light 
and intermolecular distances. One 
angstrom equals one ten-thousandth 
of a micron.) The individual grains 
are usually iron-magnesium silicates 
or iron sulfides. Some of the submi- 
cron grains themselves are aggregates 
of carbonaceous materials and tiny 
crystalline silicates. The bulk elemen- 
tal compositions of the aggregates are 
similar to carbon-rich meteorites, yet 
the structure and to some extent the 
mineralogy of the particles are dif- 
ferent from those of established me- 
teorite types. Many of the particles 
are fragile, and some are porous, form- 
ing weakly bonded structures that re- 
semble bunches of grapes. The ma- 
terial that makes up the most common 
type of interplanetary dust has not 
been found in the bigger meteorites, 
presumably because it cannot survive 
entry into the earth’s atmosphere in 
large pieces. 
In addition to the aggregates of sub- 
micron particles, a variety of other 
types of micrometeorites from the 
stratosphere have been studied. Some 
of these particles are single crystals 
of silicate minerals or iron sulfides 
that are coated with a thin micro- 
powder of material identical to the 
fine-grained material that composes 
the aggregates. These particles were 
probably embedded in the aggregate 
material at one time but were broken 
out either by processes that occurred 
in space or during atmospheric entry. 
Other types of particles have more 
mysterious origins. These types in- 
clude crystals of metallic nickel-iron, 
particles composed of carbonates or 
phosphates, particles composed of 
stacks of iron-sulfide platelets, and 
particles composed of glass spheres 
coated with tiny mounds of metallic 
nickel-iron. 
Laboratory studies of micromete- 
orites and cosmic deep-sea spheres in- 
dicate that most interplanetary dust 
particles are primitive, complex ma- 
terials that are often more fragile and 
porous than meteorites. While the ori- 
gin of these extraterrestrial particles 
is not certain, it is probable that they 
are samples of comet dust. The most 
porous particles are so fragile that 
their mere existence may be incon- 
sistent with their ever having been 
inside a pure rocky body such as an 
This crater in a lunar rock was 
created by a high-velocity cosmic 
dust particle striking the moon. The 
crater was discovered when the rock 
sample was viewed with a scanning 
electron microscope. 
Donald E Brownlee 
asteroid. If the particles are comet 
dust, they may be samples of the build- 
ing materials of the giant outer planets 
preserved at low temperatures ever 
since the formation of the solar sys- 
tem. Comets are unique bodies com- 
posed of dust and ice that accumu- 
lated in the cold outer regions of the 
primordial solar nebula. The dust con- 
tained in comets is expected to be 
well preserved because comets are 
cold objects that have never been 
warm and have spent most of their 
lifetime at great distances from the 
sun. The typical solar system comet 
has a surface temperature only a few 
tens of degrees above absolute zero 
and exists so far from the sun that 
the sun viewed from a comet would 
be just a small star in the sky, only 
a little brighter than Sirius, the 
brightest star visible from the earth. 
The truly exciting aspect of comet 
dust is the possibility that it might 
be older than the solar system itself. 
The solar system was formed from 
interstellar gas and dust; in the cold 
regions where the comets formed, 
some ancient presolar system grains 
may have survived with minimal al- 
teration. The aggregate interplanetary 
dust particles may be collections of 
these grains. In that case, the study 
of interplanetary dust particles may 
not only give us information about 
comets and how solid bodies formed 
in the solar nebula but it may also 
give us direct, detailed information 
about environments elsewhere in our 
Galaxy. The comet dust that falls on 
you each week may contain a little 
stardust, perhaps as old as the Galaxy 
itself. □ 
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