Particles were collected at an altitude of 35 km by two flights of a volume sampling micrometeorite collector. The collection scheme is very sensitive and is capable of collecting a significant number of particles. Many of the particles collected have chemical compositions similar to solar or to iron meteorites. Morphology of collected particles indicates that both true micrometeorites and ablation products were collected

Brownlee, D. E.

Bucher, W.

Hodge, P. W.

English

NASA Technical Reports Server

30. The Physical Nature of Interplanetary Dust as Inferred
by Particles Collected at 35 km
DONALD E. BROWNLEE
PAUL W. HODGE
AND
WILLIAM BUCHER
University of Washington
Seattle, Washington
Particles were collected at an altitude of 35 km by two flights of a volume-sampling
mierometeorite collector. The collection scheme is 1:cry sensiti_,_e and is capable of collect-
ing a significant _lumber of particles. Many of the particles collected have chemical
compositions similar to solar or to iron meteorites. Morphology of collected particles
indicates that both true micrometeorites and ablation products were collected.
A BALLOON'-BORNE MICROMETEORITE COLLECTORhas been built which is sensitive enough to
collect significant numbers of true micrometeorites
even if the actual particle flux is an order of
magnitude less than that indicated by penetration
detectors mounted on spacecraft. Collections are
made at 35-km altitude, a level where under
normal conditions the probability of finding
terrestrial aerosols larger than 3u is exceedingly
small (Junge, 1961). The collector is a volume-
sampling dcvice and is orders of magnitude more
sensitive than other collection devices, primarily
because it takes advantage of the tremendous
spatial concentration of extraterrestrial dust
particles in the stratosphere.
COLLECTOR DESIGN
The collector consists basically of an air pump
which pulls air through an array of cylindrical col-
lection rods. Particles are collected on the oil-
coated rods by inertial deposition. The collection
rods are 0.25-in. diameter X 2.25-in. long and are
nearly 100 percent efficient for collecting particles
291
(larger than 2t_) from the 350-ft s -_ moving air
stream. The design of the collector is shown
schematically in figure 1. The pump is an air
ejector system similar in principle to the one de-
_ribed by Wood et al. (1966). The driving gas for
the pump is derived from the catalytic decomposi-
tion of hydrazene. The collector's fuel tanks carry
150 lb of hydrazene and the pump is capable of
sampling over 10 _ ft 3 of ambient air at 35 km dur-
ing a 5-hr period. The pump draws air through the
horn-shaped air inlet and the collection-rod as-
sembly. The 22 collection rods are mounted nor-
mal to the air flow in a slide-gate mechanism that
seals the collection rods from contamination before
and after the collection phase of the flight. A
cover over the end of the horn and a butterfly
valve just behind the collection-rod assembly pre-
vent contamination of the air channel upstream
and downstream of the collection rods before col-
lect.ion. The rod-holding assembly and the interior
portions of the butterfly valve and inlet horn are
critically cleaned in our class-!00 cleanroom in
Seattle, coated with silicone oil and sealed from
https://ntrs.nasa.gov/search.jsp?R=19740011353 2020-03-17T14:31:40+00:00Z
292 EVOLUTIONARY AND PHYSICAL PROPERTIES OF METEOROIDS
Collection Rod
emblyHoldIng
Flow Meter _s
_I _/] - I | L _I Air Inlet
i -  ,,el k\P:',',t,dl-Fll I
"°d°l_____LI\\_"+_'r"//III I1 ",1
Wind Stream
tteat ShteId _"_-
NOTE: Wind vane
tall excluded
_-- Suspension Points
]_ 8 places
I
_..L_____l_U.J . -- II .dkL._ I
-ij
FmVRE 1.--Collector schematic diagram.
contamination until the collector begins to func-
tion 35 km over Texas.
To prevent contamination from the balloon,
rigging, and exterior portions of the gondola, the
collector is suspended 3000 ft below the balloon
on a nylon line. The wind-velocity gradient in the
stratosphere typically produces a _ind against the
collector of 5 ft s-L This wind completely isolates
the collector from particulate matter emitted by
the balloon and is used to isolate the collector's air
inlet from dust generated by the gondola and
pump exhaust. A 30-ft boom (see fig. 2) is at-
tached to the exhaust end of the collector. At alti-
tude, 400 ft 3 of sail area is deployed at the end of
the tail which gives the collector the aerodynamic
characteristics of a wind vane. The bind acting on
the tail orients the gondola so that the air inlet
horn is at all times upwind of all other parts of the
system. The environment is exactly analogous to
a laminar-flow cleanroom. The orientation of the
collector is monitored both with a flux gate mag-
netometer and with a sequence camera.
Because of the orientation of the cylindrical col-
lection rods relative to the airstream, particles can
only bc collected on one half of each rod. For each
rod the upstream half is considered a collection
surface and the downstream half is considered a
FIGURE2.--Collector just prior to launch.
control. Both halves of each rod go through ex-
actly the same cleaning and handling procedures
so that the control technique is nearly ideal.
COLLECTION FLIGHTS
The collector, the Vacuum Monster (VS,[), has
now had two collection flights, one on April 21,
1970 and one on May 2, 1971. The ambient-air
volume actually sampled by the collection rods on
the first and second fights was 171 000 ft a and
200 000 ft + respectively. On the first flight the
microscopic rod scanning techniques were 100 per-
cent effective for only particles 10t_ and larger.
THE PHYSICAL NATURE OF INTERPLANETARY DUST AS INFERRED BY PARTICLES COLLECTED AT 35 KM 293
On this flight 12 particIes (>10_) were found
on the upwind sides of the collection rods and
seven on tile downwind sides. This is not a good
signal-to-noise ratio but morphological analysis
_qth a scanning electron microscope (SEM) and
chemical analysis with a microprobe indicates
that at least two of the particles are very probably
extratcrrestriM. One is a 12-u opaque spherule
(fig. 3) and the other is a 10_ X 30u cellular par-
ticle (fig. 4). Microprobe analysis indicates that
the major constituents of the sphere are Fc and
5Ig (Si could not be detected because of interfer-
ence from the substrate) and the minor constitu-
ents are Ni, Ti, Cr. The Fe content of the sphere
is ,-_15 percent and the Mg content is roughly
similar. Tile composition of the cellular particle is
Si, 5Ig, S, and Fe in order of abundance.
On tile second collection flight., techniques were
improved so that the scanning process was 100
percent efficient for particles >5_ and about 25
percent efficient for 3-u particles. The scanning
process detected 306 particles (>__3_) (fig. 5) on
the upwind sides of the rods and none on the down-
stream sides. This is an excellent signal-to-noise
ratio and indicates that all the particles on the rods
came in through the horn during the collection
phase of the flight. The largest particle collected
was a 12-u transparent spherulc with chondritic
composition. The number of large particles col-
lected on this flight is in agreement with the num-
ber on the first flight if one subtracts the back-
ground contamination. The size distribution of the
second collection is very steep and most of the
particles are 2_ to 3_ in size. Over 80 percent of
all the particles collected arc spheres. Ninety per-
cent of the spheres are transparent. Of the irregu-
lar particles many are highly irregular _qth a very
low density structure. Microprobe analysis of 50
of the particles indicates that a little over 10 per-
cent of the collection has meteoritic composition.
Most of the transparent spheres are Muminum
oxide. It seems unlikely that these could be extra-
terrestrial and at this point we are not sure
whether the transparent spheres came from the
pump exhaust or the atmosphere. Neither source
seems reasonable and at present the A],03 spheres
are a real mystery. All of the nontransparent
spheres analyzed to date have meteoritic composi-
tions. Some are Fe, Ni (Fe/Ni _ 20) and others
are Fe, Ni, S. Of the irregular particles analyzed,
few contained detectable elements. This is pos-
sibly due to the difficulty in microprobing small
low-density particles. One of the irregular particles
did, however, contain Si, Mg, S, Fe and Ni.
FIGURE 3.--A 12-_ sphcrule with chondritic composition. (A stereographic pair of photographs.)
294 EVOLUTIONARY AND PHYSICAL PROPERTIES OF METEOROIDS
FIGURE 4.--A 30 × I0_ cellular particIe.
CONCLUSION
Both of the VM flights sampled large quantities
of air and the time-area-products (TAP) of tile
collections is quite large. For the last flight the
TAP for 3-_ spheres (density = 3 gcm -_) was 7 ×
10 _ m2s. This is a very large TAP and it is pre-
dicted that the collection should contain approxi-
mately 30 true micrometeorites (>_3u) on the
basis of the flux model of Kerridge (1970). Our
analysis so far indicates that the micrometeorite
flux in the stratosphere is certainly not higher
than that predicted from the Kerridge model and
is probably in reasonable agreement with it.
On the basis of chemical and morphological
considerations we feel that we have collected ex-
traterrestrial particulate matter from the strato-
sphere both in the form of micrometeorites and
ablation products. The cellular particle from the
first flight probably could not be an ablation prod-
uct because of its complicated porous structure.
FtGURE 5.--Part, iclcs of 3 to 4 _ diameter from the second
collection flight.
THEPHYSICALNATUREOF INTERPLANETARY DUST AS INFERRED BY PARTICLES COLLECTED AT 35 KM 295
The low-density structure and primitive near-
solar composition of this particle is strongly sug-
gestive of a cometary origin. The spherule from
the first flight has a similar composition to the
cellular partic]e x_fith the exception of the sulfur
content. The sphere has an ordinary chondritic
composition and wc feel that it is most probably
an ablation product of a larger body. Of the par-
ticles analyzed from the second flight most of those
with meteoritic compositions are spheres and are
probably also ablation products. Excluding the
AlcOa spheres, the spheres scorn to fall into three
compositional groups: chondritic, nickel-iron and
nickel-iron-sulfur. The irregular particles that con-
tain detectable elements have compositions com-
patible with carbonaceous chondrites.
We plan to do extensive quantitative analysis on
the meteoritic particles from the balloon collec-
tions for accurate comparison with the compo_-
tions of the standard source of interplanetary
matter, meteorites. At this point in the analysis it
seems that the composition of most of the mete-
oritic particles is similar to the most primitive
meteorite types. This is consistent with the studies
of trace elements in the lunar soil by Ganapathy
et al. (1970) that indicate that the mean com-
position of micrometeoritcs is similar to that of the
Type I carbonaceous chondrite meteorites. The
structure of some of the irregular particles is very
low density which is consistent both with comet
models and meteor studies. The structure and
compositions of these particles will be studied with
the goal of determining the physical nature,
origin, and space density of interplanetary dust
grains.
ACKNOWLEDGMENT
We gra_fuIly acknowledge the partial support of the
National Aeronautics and Space Administration through
Grant NGR 48-002-033.
REFERENCES
GAN._.PATIIY, R., REID, R., KEAYS, J., LAUL, ft., AND ANDERS, E., 1970. Trace elements in Apollo
11 lunar rocks: Implications for meteorite influx and origin of moon, Prec. Apollo 11 Lunar
Scl. Conf., Geoch ira. Cosmochim. Acta, Suppl., 1, 1117-1142.
JUN_E, C. E., 1961. Stratospheric aerosol studies, J. Geophys. Res., 66, 2163-2182.
KERmDaE, J. F., 1970. Micrometcorite environment at the Earth's orbit, Nature, 288, 616--619.
WOOD, R._ ZELLER, H., OLSON, R., DEETER, It., MAI_'FITT, K., AND ANDERSON, ]:[., 1966. Studies
of extraterrestrial dust at 40 kin, Applied Sci. Div., Litton Systenm, Inc., Rcpt. No. 3063.



The physical nature of interplanetary dust as inferred by particles collected at 35 km

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