In this scanning electronmicrograph, 
the surface of a stony deep-sea 
sphere, etched by ocean water, 
reveals crystals of different minerals. 
that collection of a significant number 
of them from the atmosphere is be- 
yond our present ability. The way to 
collect these larger dust particles is 
not to try to capture them before they 
hit the ground, but to find some region 
on the earth where they accumulate 
over a fairly long period with a mini- 
mum of terrestrial contamination. The 
best such place is the sea floor. 
Because of the relative ease of re- 
covery and the durability of large par- 
ticles in sea-floor sediments, most of 
the cosmic particles from that source 
are in the range of fifty microns to 
a few millimeters. The majority of 
these particles melted during entry 
into the atmosphere, forming small 
round bodies commonly called cosmic 
spheres. True unmelted micrometeor- 
ites must exist in ocean sediments but 
they are probably not very magnetic, 
and because of their contact with mud 
and water, they would be in much 
less pristine condition than those col- 
lected in the atmosphere. The sea floor 
is the best collection site for large 
melted cosmic spheres, but the smaller 
true micrometeorites are best col- 
lected in the stratosphere despite the 
difficulties involved. 
Cosmic deep-sea spheres are highly 
degraded samples of interplanetary 
dust that were melted in the atmos- 
phere and kiter at least partially modi- 
fied chemically in the ocean. They 
are valuable because they are rela- 
tively large. A one-millimeter sphere 
contains a million times more mass 
than a ten-micron particle. The larger 
size is a great advantage when at- 
tempting to measure trace elements 
in the material or when trying to de- 
termine the isotopic composition of 
stable elements such as oxygen or of 
radioactive isotopes such as manga- 
nese 53, produced by cosmic-ray bom- 
bardment in space. Many of the valu- 
able measurements require spheres a 
millimeter in diameter but spheres of 
this size are so unusual that only a 
few can be found in a ton of sediment. 
The first cosmic spheres were found 
more than a century ago when sci- 
entists on the British Challenger ex- 
pedition in the 1870s ran a hand mag- 
net though mud samples recovered 
from the sea floor. They found curious 
spheres, some of which contained me- 
tallic iron. Metallic iron is very rare 
in nature, and the researchers cor- 
rectly concluded that the particles 
were extraterrestrial. 
To collect large particles without 
bringing tons of mud into the lab- 
oratory the sensible approach is to do 
the magnetic separation in situ on the 
ocean floor. This was first done in 
the 1950s by Danish oceanographers 
using magnets mounted in a wooden 
plank that was dragged along the sea 
floor at several locations around the 
world. This technique was recently im- 
proved on by University of Washing- 
ton researchers who used a 300-kilo- 
gram magnetic collection device, 
something like a sled, called the Cos- 
mic Muck Rake. The collection sled 
was towed in the Pacific Ocean at 
a depth of 5,300 meters by the Uni- 
versity of Hawaii research vessel Kana 
Keoki , and tens of thousands of cosmic 
spheres, ranging in size up to three 
millimeters, were collected. 
Studies of the deep-sea cosmic 
spheres made by an international 
group of investigators have concen- 
trated on elemental and isotopic com- 
position because most of the other 
properties of these particles were de- 
stroyed by melting during atmospheric 
entry. The compositions of the spheres 
fall into two basic groups: type I (for 
iron), composed of metallic iron and 
magnetite; and type S (for stony), 
composed of silicates and magnetite. 
Since the origin of the type I spheres 
remains unknown, it is not clear 
whether the parent materials are pure 
iron meteoroids or stony bodies that 
contain metallic iron. The elemental 
compositions of the type S spheres 
indicate that approximately 80 
percent of them are solidified droplets 
of a material similar in composition 
to the carbonaceous chondrites but 
different from the ordinary stony me- 
teorites. This is noteworthy because 
only 2.5 percent of freshly fallen me- 
teorites that are larger than dust size 
are of the primitive carbon-rich va- 
riety. Apparently, carbonaceous chon- 
dritelike objects are more common in 
space than their scarcity in meteorite 
collections would suggest. The cosmic 
spheres are probably a more repre- 
sentative sample of the millimeter-size 
objects in space than normal mete- 
orites are of the bigger objects. All 
materials presumably melt and form 
spheres, but only fairly strong mate- 
rials become traditional meteorites. 
Studies of micrometeorites col- 
lected with U2 aircraft reveal detailed 
information on interplanetary dust be- 
cause these samples are the least al- 
tered by atmospheric entry. Extensive 
investigation with optical and electron 
microscopes has shown that the most 
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