A scanning electron micrograph of a 
stratospheric dust particle shows 
that it is composed of large 
numbers of submicron grains. 
Particles such as this, collected 
with a U2 aircraft, are thought to 
be the most common type of 
interplanetary dust. 
tides of similar size. In an optical 
microscope, the cosmic particles are 
indistinguishable from the terrestrial 
particles. In an electron microscope, 
the chemical compositions of individ- 
ual particles can be determined, mak- 
ing it possible in principle to identify 
the single extraterrestrial particle 
among ten billion contaminants, but 
in practice that would take centuries 
of work. 
Fortunately, there are better places 
than cities in which to collect extra- 
terrestrial dust. Two regions where 
cosmic dust particles can be success- 
fully collected are the stratosphere 
and the deep-ocean floor. In the strato- 
sphere, at altitudes above twenty ki- 
lometers, “large” terrestrial particles 
are quite rare and most particles big- 
ger than ten microns are not particles 
that rose from the earth’s surface but 
ones that fell in from space. Large 
terrestrial particles are scarce in the 
stratosphere because they are only 
lifted to high altitudes by such rare 
events as huge thunderstorms or vol- 
canic explosions, and because of their 
relatively high fall speeds (a kilometer 
a day). 
The other good collecting locality 
for extraterrestrial dust is the deep- 
ocean floor in regions far from con- 
tinents and islands. The ocean floor 
may be one of the dirtiest environ- 
ments on the earth, but it is a place 
uniquely suited for the recovery of 
extraterrestrial material. Although 
cosmic particles that fall onto the sea 
floor are greatly diluted with terres- 
trial materials, they are less diluted 
than those recovered anywhere else 
on Earth. In the center of the Pacific 
Ocean, the accumulation of sediments 
from the continents, for example, is 
only a few meters in a million years. 
A cubic meter of sea-floor mud, then, 
contains all of the extraterrestrial par- 
ticles that fall onto a square meter 
of surface area in a period of nearly 
500,000 years. The long accumulation 
time of deep-sea sediments makes it 
possible to obtain samples of much 
rarer and therefore much larger par- 
ticles than can be collected in the 
stratosphere. 
The “trick” in collecting cosmic ma- 
terial from deep-sea sediments is that 
most of the extraterrestrial particles 
in the 0.1 millimeter to 1 millimeter 
size range are magnetic, a property 
that distinguishes them from nearly 
all other particles of similar size on 
the sea floor. The magnetic mineral 
magnetite forms in most meteoritic 
materials that are melted or heated 
to high temperatures in the atmos- 
phere. For example, the fusion crusts 
of stony meteorites are magnetic even 
though their unmelted interiors may 
be nonmagnetic. Because of their mag- 
netic property, the larger dust par- 
ticles can be easily extracted from 
deep-sea sediments by means of simple 
magnets. 
The first positively identified ex- 
traterrestrial particles collected in the 
atmosphere were obtained in 1970 
with a University of Washington high- 
altitude balloon experiment called the 
Vacuum Monster. At the present time, 
particles are routinely collected in the 
stratosphere with NASA U2 aircraft 
flown from Ames Research Center in 
California. The U2 carries a small py- 
lon underneath its wing that exposes 
a sticky flyswatter-sized collection 
plate to the airstream. After exposure 
on ten or fifteen consecutive flights 
the collection plate is removed to a 
clean room, where individual particles 
are picked off with tiny glass needles 
and mounted for study in analytical 
electron microscopes and other instru- 
ments. 
The extraterrestrial particles are a 
fairly large fraction of all the collected 
particles larger than a few microns 
and identification is not usually a prob- 
lem. Most of the cosmic particles have 
elemental compositions similar to 
primitive meteorites and are distinctly 
different from nearly all terrestrial 
contaminants. The most common con- 
taminants are pure aluminum oxide 
(sapphire) spheres, which are the com- 
bustion products of solid fuel rockets. 
The proof that the particles identified 
as cosmic are indeed extraterrestrial 
came from the detection of helium 
and other rare gases implanted in them 
by the solar wind. The rare-gas content 
is high, comparable to that in lunar 
soil, and is undoubtedly the result of 
implantation of the ions composing the 
solar wind while the particles were 
in space. 
Most of the 500 extraterrestrial par- 
ticles that have been collected in the 
stratosphere are true micrometeorites 
— dust particles that have survived en- 
try into the atmosphere without melt- 
ing. They are small, less than fifty 
microns in diameter, exceedingly rare, 
and difficult to collect. The collection 
of a single ten-micron particle requires 
sampling more than a thousand cubic 
meters of stratospheric air. Particles 
larger than 100 microns are so rare 
75 
