Smithsonian

Contributions to Astrophysics

VOLUME 5, NUMBER 10

SAMPLING DUST FROM THE STRATOSPHERE

by PAUL W. HODGE

SMITHSONIAN INSTITUTION

Washington, D.C.

1961
Publications of the Astrophysical Observatory

This series, Smithsonian Contributions to Astrophysics, was inaugurated

in 1956 to provide a proper communication for the results of research conducted at the Astrophysical Observatory of the Smithsonian Institution.

Its purpose is the "increase and diffusion of knowledge" in the field of

astrophysics, with particular emphasis on problems of the sun, the earth,

and the solar system. Its pages are open to a limited number of papers by

other investigators with whom we have common interests.

Another series is Annals of the Astrophysical Observatory. It was started

in 1900 by the Observatory's first director, Samuel P. Langley, and has been

published about every 10 years since that date. These quarto volumes, some

of which are still available, record the history of the Observatory's researches

and activities.

Many technical papers and volumes emanating from the Astrophysical

Observatory have appeared in the Smithsonian Miscellaneous Collections.

Among these are Smithsonian Physical Tables, Smithsonian Meteorological

Tables, and World Weather Records.

Additional information concerning these publications may be secured

from the Editorial and Publications Division, Smithsonian Institution,

Washington, D.C.

FRED L. WHIPPLE, Director,

Astrophysical Observatory,

Smithsonian Institution.

Cambridge, Mass.

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington 25, D.C. - Price 20 cents
Sampling Dust from the Stratosphere1

By PAUL W. HODGE

Of the numerous meteoric objects that enter

the earth's atmosphere from interplanetaryspace, the smallest and most numerous are

dust particles. Some of these particles result

from ablation of larger bodies (Krinov, 1955),

while others enter the atmosphere relatively

unchanged (Whipple, 1950). They lose their

cosmic velocities at heights between 100 km

and 50 km above the earth's surface, slowly

fall to the surface, and gradually mix with atmospheric dust of terrestrial origin.

For nearly 100 years scientists have been attempting to identify and study this extraterrestrial dust (Hoffleit, 1952). Investigations

have been hampered, however, by the fact that

the terrestrial dust in the atmosphere is so

abundant and so varied that any meteoritic

component is difficult to isolate. Many investigators have identified particles as being extraterrestrial from their appearance or from their

magnetic or chemical properties. However, it

has become increasingly apparent that many

terrestrial particles have properties that resemble to a distressing degree those of meteoritic

particles (Handy and Davidson, 1953). For

this reason, we need to collect meteoritic dust

from places where the possibility of terrestrial

contamination is very slight. Artificial earth

satellites can detect particles in the purest environment that we can explore, but they are

not yet able to bring back samples for analysis.

The best method at present is to use vehicles

that reach somewhat lower altitudes, where the

meteoritic dust has been slowed down and concentrated by the upper atmosphere.

We have concentrated on collecting dust by

use of high-flying jet aircraft because (1) they

1

 The project reported in this paper has been supported in part by two

contracts from the Geophysics Rearch Directorate, AFCBC.

1

 Smithsonian Astrophysical Observatory, Cambridge, Mass.

are able to reach altitudes high enough above

the troposphere to ensure a sample relatively

free of terrestrial dust; (2) their speeds enable

them to intersect a large volume of air in a relatively short time; (3) their flights are still below

the altitudes where extraterrestrial dust has

cosmic velocities; and (4) their flights are more

frequent and more reliable than those of other

upper-atmosphere vehicles, such as rockets or

balloons.

The problems involved in obtaining and identifying high-altitude dust are numerous. The

worst problem is contamination of the sample.

The collection device—in our case usually a

filter—may be contaminated when it is manufactured or packaged, when it is installed, when

it is flown, or when it is dismounted; also, the

sample may be contaminated during laboratory

preparation or analysis. We have tried to

minimize the amount of contamination by

using only very clean filters; by constructing

the collector mechanism out of aluminum,

pieces of which are easily distinguished from the

sample; by placing the collector on the aircraft

in a position forward of possible sources of dust;

and by using a dust-free laboratory in which to

prepare and mount the samples.

Other problems, such as the breakage of

filters by vibrations in the aircraft and the

wearing of parts of the collector mechanism

by extreme weather conditions, have been

solved by numerous alterations to the design

of the collector and by changes of the location

of the collector on the vehicle.

Collections

Between January 1958 and December 1959,

45 filters were flown in collectors mounted on

high-altitude jet aircraft. Of these filters, 26

were flown on a U-2 out of Edwards Air Force

145
146 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

Base, Edwards, Calif., and 19 were flown on a

B-52 out of Seattle, Wash. Because of necessary experiments conducted to discover the

proper location of the collector on the aircraft,

the proper type of filter, and the possible

sources of contamination, many of the filters

flown could not be used for analysis. The

totals of usable filters were 21 from Edwards

and 8 from Seattle.

Collecting devices

We made our first collecting device as simple

as possible. A solenoid-operated shutter covers an air intake hole 6 mm in diameter at the

front of the collector; the shutter can be opened

by the pilot at a preselected altitude. Air

passes through the opening into a chamber 2

cm in diameter. At the back of the chamber

is a millipore filter in a removable housing.

The filter is supported by a wire screen. This

first collector (sketched in fig. 1) was used only

on the U-2 out of Edwards Air Force Base.

We later built a more streamlined collector.

While it has the same interior design, it has

a more compact shutter mechanism and better

characteristics of exterior air-flow. The filter

housing, which is readily removable, allows

fresh filters to be installed with a minimum of

time, movement, and contamination. Each

unit, consisting of a filter, a retaining ring, and

a screen, is packaged in a sealed box ready for

immediate installation. This collector (shown

in pis. 1, 2) was used on the B-52 out of Seattle.

Examination of samples

All filters were first examined microscopically.

We mounted the filter in glass, and examined

an exposed area of 16 mm2 under low power

(200X). We counted the number of particles

of all types on the filter, and recorded the size

and description of each particle. We next

examined the entire exposed area of the filter

under 400X magnification. We noted the

position of every opaque particle with a mean

dimension greater than 3jx, and recorded the

shape, dimensions, and surface characteristics

of each of these particles. The particles could

then be relocated immediately for further study

or for removal from the filter for chemical

analysis.

The results of these examinations are listed

in tables 1 and 2. In each table the first column gives the number of the filter; the second

column the number of particles of all types

found in the first survey of 16 mm2 of the filter

area, normalized to an area of 160 mm2; and

the third column gives the number of opaque

TABLE 1.—Particles on filters from first collector,

on U-2 flights

Filter series

and number

A-l

A-2

A-3

A-4

A-5

A-7

A-8

B-l

B-2

B-3

B-5

C-l

C-2

C-3

C-4

C-5

C-6

C-8

C-9

Number of particles

All types, diameter >6M

820

400

2000

1120

660

900

840

730

330

250

870

620

1120

490

260

310

360

170

420

Opaque, diameter >3jt

74

60

99

91

40

33

—

89

—

35

30

67

66

63

19

49

42

21

36

TABLE 2.—Particles on filters from second collector, on

B-52 flights

Filter series

and number

E-2

E-3

E-5

E-16

E-23

E-25

E-26

E-28

E-28(2)

E-30

Number of particles

All types, diameter >6/i

680

1100

1330

120

1090

190

160

50

100

1010

Opaque, diameter >3M

17

83

93

17

128

29

29

32

25

40
SAMPLING DUST FROM THE STRATOSPHERE 147

FILTER

I INCH

SCREEN

REMOVABLE FILTER

HOLDER

FIGURE 1.—Sketch of the first collector, which was carried by jet aircraft (U-2) out of

Edwards Air Force Base, Calif.

particles found on the entire exposed area of

the filter (160 mm2). In all cases, particles

that were obviously chips of aluminum derived

from the collector itself were disregarded.

The particles found on the filters are of a

wide variety of types and sizes. The nature

and relative amounts of these various types are

given in table 3, which is based on the data for

all of the filters from the first collector. The

filters from the second collector, on the B-52,

gave similar results.

Table 3 also shows the results of the optical

surveys with the microscope at 400 X ; included

are all particles larger than the effective limit

of recognition, i.e., 3/x in the mean dimension.

TABLE 3.—Classification of parliculate matter on the first

collector

Description

Clear transparent

Semitransparent

Aluminum

Opaque

Shiny, metalliclike

Dull, black irregular

Black spherules

Origin

Filter and terrestrial

Terrestrial

Walls of collector

(?)

(?)

(?)

Percent

of

sample

80

11

1

2

5

1

The clear, transparent, blob-like particles were

found to be as abundant on unexposed filters

as on those with the longest exposures at high

altitude; they were, in fact, found on millipore

filters taken directly from stock. These particles, which are easily identified, were disregarded in the final scanning.

The semitransparent particles have the wide

variety of shapes and colors characteristic of

the types of terrestrial particles that are so

common lower in the atmosphere. On the

average, their abundance on the filter increased

with collecting time, indicating that most were

gathered during the flight of the vehicle.

The aluminum particles, which are readily

identified by their appearance under the microscope, show no such relation; they are undoubtedly from the aluminum collecting apparatus. This conclusion has been verified

chemically (Fireman and Kistner, in press).

The number of opaque particles generally

increased with the length of the exposure of

the filter at high altitude. Most of them are

shiny, black chunks; some are smooth-surfaced

fragments with rounded edges, while others

are in the shape of cindery frameworks full

of holes. They are often quite large, up to

50jx or more across, and commonly have a

distinctly metallic appearance. A small percent are shiny, black spherules similar to those

found in ground collections and which numerous

investigators think to be meteoritic.
148

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

Space density of particles

The filters exposed by the first collector

mounted on the U-2 aircraft provide a value

of approximately 500 for the average number

of particles greater than Qn in diameter that

were collected per hour. The general problem

of the space density of particles cannot readily

be studied in more detail from the data derived

from these collections because we lack precise information concerning velocity altitude,

weather, handling, and other conditions that

characterized the flights. However, if we assume a velocity of 600 km/hr, we can obtain a

rough value for the density of the particles at

the assumed altitude of 60,000 feet. The area

of the opening of the collector is 3X10~5 square

meters; therefore, at a velocity of 600 km/hr

the collector would sweep out 18 cubic meters

of air per hour. From these data we derive a

tentative value of 28 particles per cubic meter

of air as the average space density. This

value is uncertain, of course, because the relation between the number of particles collected

and the exposure time cannot be exactly determined, and because the velocity had to be

assumed.

Since we know the details of each B-52 flight,

we can draw much more detailed conclusions

from the particles on the second collector.

For the five B—52 collections that can be strictly

compared, table 4 lists the necessary data for

determining space densities of the particles.

The indicated air speed was converted into true

air speed (column 3) by assuming standard

atmospheric conditions and an average altitude

of 45,000 feet. Column 4 gives the velocity in

kilometers per hour, and column 6 gives the

volume of air swept out during the exposure

by the 6-mm diameter opening of the collector.

In order to derive from the data of tables 2

and 4 a value for the space densities of particles,

we must first subtract from the number of

particles on the filter the probable number of

contamination particles. Although numerous

transparent particles are found on a filter taken

directly from the manufacturer's box, the

number of opaque particles is negligible. We

can expect, however, that some contamination

occurs while the filter is being prepared and

while it is being mounted on the aircraft. The

most reliable values for the number of particles

picked up at these times are therefore obtained

from an examination of filters that have been

flown but have not been exposed.

Two B-52 filters, E-25 and E-28, answer this

description. The average number of contaminating particles derived from the values in

table 2 for these two filters is 120 ±70 particles

of all types, and 30 ± 1 opaque particles.

After we have subtracted these values from the

counts for the other filters, we can obtain the

probable true space densities (table 5).

The values for the probable space densities

differ remarkably for the various filters. The

wide variety of values for all particles can be

partly explained by the great dispersion in the

amount of contaminating particles, especially

the transparent particles that are on the filters

when they are received from the manufacturer.

However, the number of opaque particles varies

in the same way and in approximately the same

amount as the transparent ones; it seems

probable, therefore, that the amount of dust in

the air at the sampled altitudes varies considerably. The mean space densities of the B-52

flights (table 6) agree remarkably well with

the value determined from the U-2 filters.

However, before we can draw further conclusions and in order to be confident that the

TABLE 4.—Characteristics of B-52 flights, which carried the second collector

Filter series

and number

E-16

E-23

E-26

E-28 (2)

E-30

Indicated air

speed (knots)

240

240

230

240

230

True air

speed (knots)

456

456

446

456

446

Velocity

(km/hr)

844

844

826

844

826

Exposure

(nrs)

0.25

0.33

0.33

0.28

1.0

Volume

(m»)

6

8

8

7

25
Plates 1 and 2
HODGE PLATE 1

Unassembled parts of the second collector, which was carried by jet aircraft (B-52) out of Seattle, Wash.
HODGE PLATE 2

The second collector as assembled. The strut is 14 inches long.

SAMPLING DUST FROM THE STRATOSPHERE

TABLE 5.—Space densities of the B-52 flights, which carried the second collector.

149

Filter series

and number

E-16

E-23

E-26

E-28(2)

E-30

Altitude

(in feet)

40, 000

42, 000

48, 000

44, 000

40, 000

Weather

Clear

Clear

Clear

Clear

Haze to 40, 000 ft.

No. of particles per cubic

meter

All particles

diameter > 6M

0

121

5

0

36

Opaque particles diame0

12

0

0

0.4

efficiency of our collector is what we have

assumed, we need many more samples and we

need independent measures.

The particle size distribution

For all types, the number of particles

collected increases markedly as their sizes

decrease. Table 7 gives the percentages in

various size groups of (1) all particles found

on the exposed filters of table 1; (2) only the

opaque particles found on these filters, and (3)

the opaque particles found on the exposed

filters listed in table 2. The size distributions

are in good agreement with each other, but they

differ somewhat from the size distribution of

all particles in general (fig. 2). The distribution of all particles is, of course, dominated by

particles of terrestrial origin. The greater

steepness of the distribution of the opaque

particles is what we expect when we consider

that their densities are probably greater.

Probable meteoritic particles

We can reasonably expect that among the particles collected at high altitudes there will be a

larger percentage of dust of extraterrestrial

origin than there will be among particles collected at ground level. We do not expect,

TABLE 6.—Mean space densities of the B-52 flights, which

carried the second collector

Particles

All types, diameter>6M

Opaque, diameter>3/x

Number per cubic

meter

32 ±46 (rmse)

2.5 ±4

however, that this difference will be very large,

even at 40,000 or 50,000 feet. The evidence in

the preceding sections suggests that probably

most of the dust at these altitudes is made up

of particles carried up from terrestrial sources

by convection. However, the space density of

particles at these heights is very much less

than that at sea level, and therefore our

chances of segregating the meteoritic component of the sample are greatly increased.

Except for particles collected from the vicinity of meteorite craters (Rinehart, 1958;

Krinov, 1955), there are no known samples of

meteoritic dust to guide one who wishes to

search for such objects. Even chemical analysis will not suffice as a criterion. We expect

part of the extraterrestrial component of atmospheric dust to be nickel-iron, like iron

meteorites, but the composition or appearance

of the rest is a matter of conjecture. What

TABLE 7.—Size distribution of particles collected

Diameter

to

3-6

6-9

9-12

12-15

15-18

18-21

21-24

24-27

27-30

First collector

(on U-2)

Percent of

all particles

54.0

18.9

9.7

5.4

4. 1

3.2

2.3

1.6

0.0

Percent of

opaque

particles

67.6

17.4

6.7

5.2

1.5

1.0

0.5

0. 1

0.0

Second

collector

(on B-52)

Percent of

opaque

particles

69.0

16.9

8.4

4.0

1.6

0. 1

0.0

0.0

0.0
150 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

70

60

50

4 0

30

20

10

• ALL PARTICLES, FIRST COLLECTIONS

X OPAQUE PARTICLES. B 52 COLLECTIONS

0 OPAQUE PARTICLES,FIRST COLLECTOR

15 20

DIAMETER (MICRONS)

FIGURE 2.—Stratospheric particles distributed according to size.

25 30
SAMPLING DUST FROM THE STRATOSPHERE 151

would be the appearance of dust derived from

large stony meteoritic bodies? Or from the

disintegration of cometary meteors? Or from

the zodiacal light particles? When we have

obtained many data from a large number of

collections at very high altitudes in the atmosphere and have analyzed chemically a large

number of individual particles we will be

better able to answer these important questions. At present, we have only fragmentary

data on the chemical composition of stratospheric dust (Fireman and Kistner, in preparation; Junge, Chagnon, and Manson, 1960).

We can tentatively select those samples that

appear likely to be extraterrestrial particles.

Among these are a number of shiny spherules

that are of great interest. They are identical

in appearance to those particles found near

recent meteorite falls—especially the SikhoteAlin fall—that have been identified as droplets

formed from the melting skin of the meteorite

(Krinov, 1955).

Such spherules have been identified in both

groups of samples; however, because we lack

flight data for the flights of the first collector,

we will discuss only spherules on the filters on

the B-52. Of the 10 filters listed in table 2,

five were found to contain a total of 10 spherules. These are described in table 8, where

the sizes of individual particles are given in

microns. That the spherules were actually

collected during flight and were not contamination particles is supported by the fact that

filters E-2 E-23, E-25, and E-28, which were

flown but not exposed, contained no such particles. The space density for these spherules

is calculated to be 0.2 per cubic meter at an

average height of 45,000 feet. Stokes' law suggests that in a quiescent atmosphere these

particles probably fall at a rate of approximately 1 cm per second from this altitude.

Therefore, the computed rate of fall on the

surface of the earth is 2 X 10~2 spherules/cm2/

day. This is l/50th the value obtained by

Hodge and Wildt (1958), who collected such

spherules from Arctic regions and mountain

tops. If our preliminary figures are correct,

we must conclude that the ground samples are

heavily contaminated by terrestrial spherules,

TABLE 8.—Spherules on filters carried by B-52

Filter

E-11

E-23

E-26

E-28 (2)

E-30

Number of

spherules

2

4

1

1

2

Diameters (ft)

3, 3, 6, 7

3

4

7,8

and that even the high-altitude spherules are

possibly of terrestrial origin. This latter statement is supported by the fact that we have

evidence that for the space densities for all

types of particles the ratio of the value for sea

level to that for 45,000 feet is approximately

100 to 1, which is, considering the uncertainties involved, equivalent to the ratio for the

spherules.

Conclusions

From a study of stratospheric dust collections

we find that at 45,000 feet there are per cubic

meter approximately 30 particles larger than

6M in diameter. There is evidence that these

particles are unevenly distributed in space at

this altitude. Most of the particles are transparent or semitransparent and are probably of

terrestrial origin. The space density of opaque

particles at 45,000 feet is roughly two particles

larger than 3M in diameter per cubic meter.

Some of these particles may be meteoritic, but

there is no convincing evidence that any large

percentage is not merely terrestrial dust carried to high altitudes by convective winds.

We can conclude from this comparison of the

number of spherules found at 45,000 feet with

the number at sea level that many opaque,

probably high-density particles are carried up

into the stratosphere. The actual number of

extraterrestrial particles collected must be very

small.

Acknowledgments

Dr. F. L. Wbipple, Dr. J. S. Rinehart, Dr. F. W.

Wright, and the writer have carried out the

work reported here. Dr. E. L. Fireman and

Dr. G. Kistner have developed methods of

chemical analysis for small particles.
152 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

We are greatly indebted to the Boeing Airplane Company and to Project Jetstream,

Geophysics Research Directorate, Air Force

Cambridge Research Center, for making the

collections.

Through cooperation with the Flight Research Center, National Aeronautics and Space

Administration, we are continuing to make

collections by means of aircraft flying to

altitudes of 85,000 feet and higher.

References

FIREMAN, E. L., AND KISTNEK, G.

In press. The nature of the dust collected at high

altitude. Geochim. et Cosmochim.

Acta.

HANDY, R. L., AND DAVIDSON, D. T.

1953. On the curious resemblance between fly

ash and meteoritic dust. Proc. Iowa

Acad. Sci., vol. 60, pp. 373-379.

HOFFLEIT, D.

1952. Bibliography on meteoritic dust with brief

abstracts. Harvard Coll. Obs. Reprints,

ser. 2, no. 43, 45 pp.

HODGE, P. W., AND WILDT, R.

1958. A search for airborne particles of meteoritic

origin. Geochim. et Cosmochim. Acta,

vol. 14, pp. 126-133.

JUNGE, C. E.; CHAGNON, C. W.; AND MANSON, J. E.

1960. Stratospheric aerosols. Report from Geophysics Research Directorate, Bedford,

Mass.

KBINOV, E. L.

1955. Osnovi meteoritiki. State Publishing House

of Technical Literature, Moscow. English

translation: Principles of meteoritics, H.

Brown, ed., Pergamon Press, New York,

1960, 535 pp.

RlNEHART, J. S.

1958. Distribution of meteoritic debris about the

Arizona meteorite crater. Smithsonian

Contr. Astrophys., vol. 2, pp. 145-160.

WHIPPLE, F. L.

1950. The theory of micro-meteorites. Part I.

In an isothermal atmosphere. Proc. Nat.

Acad. Sci., vol. 36, pp. 678-695.

Abstract

Airborne particles have been obtained from stratospheric air by collectors mounted on high-flying jet aircraft.

The estimated density of particles in the 10M size range is found to vary widely, with an average value of about

30 particles larger than 6M in diameter per cubic meter at 45,000 feet. Only a very small number of these particles

are possibly meteoritic in origin.