Collecting Micrometeorites from the South Pole Water Well
SUSAN TAYLOR, JAMES H. LEVER, RALPH P. HARVEY, AND JOHN GOVONI
they had chondritic ratios of magnesium, silicon,
INTRODUCTION
and iron (Blanchard et al. 1978), but it required
Micrometeorites are the dominant mass contri-
trace chemical analyses to show their extraterres-
bution to the present-day Earth and add one hun-
trial origin (Ganapathy et al. 1978).
dred tons of material to the Earth each day
Cosmic spherules are distinctive-looking par-
(Brownlee 1981, Love and Brownlee 1993). Al-
ticles and have been found in many terrestrial en-
though ubiquitous in terrestrial environments,
vironments. They have been found in the swamps
relatively little of this material has been collected
of Siberia (Krinov 1959), in desert sands
because micrometeorites occur in low concentra-
(Fredriksson and Gowdy 1963), in beach sand
tions and generally weather rapidly. To collect mi-
(Marvin and Einaudi 1967), in deep sea sediments
crometeorites one needs to find deposits where
(Brownlee 1979), in lithified abyssal sediments
they are concentrated. Like meteorites, micro-
exposed on land (Czajkowski et al. 1983, Jehanno
meteorites range from unaltered primordial ma-
et al. 1988, Taylor and Brownlee 1991), in Green-
terials to those that have seen extensive differen-
land's cryoconite and melt-water drainage basins
tiation and alteration deep within a parent body.
(Maurette et al. 1986, 1987; Robin et al. 1990), and
They represent collision-produced fragments of
in Antarctic morainal sediments (Hagen 1988,
asteroids as well as materials of cometary origin
Koeberl and Hagen 1989), aeolian debris (Harvey
(Brownlee et al. 1993). Some of these small par-
and Maurette 1991, Hagen et al. 1992), and ice cores
ticles are thought to contain exotic materials, in-
(Yiou and Raisbeck 1990, Hagen et al. 1992).
cluding interstellar grains and fragments of solar
Unmelted micrometeorites have also been
system materials that have no counterparts among
sought and found. Interplanetary dust particles
meteorite collections (Bradley et al. 1989, Brownlee
(IDPs) were collected in the stratosphere
et al. 1993).
(Brownlee et al. 1977) and are routinely collected
In this paper we use the word micrometeorite
by NASA using U-2 high-flying aircraft. Larger
as a generic term referring to all types of terrestri-
unmelted micrometeorites were found in
ally collected extraterrestrial debris with sizes <1
Greenland ice (Maurette et al. 1987), and many
micrometeorites, primarily <100 m in size, were
cm. Cosmic spherules refer to spherical and/or
rounded particles that have been totally or par-
found upon melting and sieving 100 tons of Ant-
tially melted by atmospheric entry. The term in-
arctic blue ice (Maurette et al. 1991).
terplanetary dust particle (IDP) refers to the small
The South Pole Water Well (SPWW) is a 24-m-
(<50 m) extraterrestrial particles collected in the
diameter by 15-m-deep melt pool 100 m below the
stratosphere.
snow surface; it supplies potable water to the
More than a century ago a few magnetic cos-
AmundsenScott South Pole Station. Because of
mic spherules were found in deep-sea sediments
the large volume of ice melted, the well is the larg-
collected by members of the H.M.S. Challenger ex-
est source of micrometeorites yet discovered. The
pedition (Murray and Renard 1876). Both iron and
compressed-snow polar ice preserves a record of
silicate spherules were found, and it was thought,
melted and unmelted micrometeorites in an envi-
especially of the iron spherules, which contained
ronment low in terrestrial debris, and the well pro-
nickel in chondritic proportions, that they were of
vides the concentrating mechanism necessary for
meteoritic origin. The presence of wustite in these
collecting these particles. The large pool volume
spherules, a high-temperature iron oxide found in
and low water-injection rate produce low circula-
meteorite fusion crusts, also suggested an extra-
tion velocities that cannot entrain micrometeorites
terrestrial origin for iron spherules (Marvin and
melted out of the ice. These particles should, there-
Einaudi 1967). The extraterrestrial origin of the
fore, form a lag deposit on the well bottom. Based
silicate spherules, which are composed of olivine,
on this assumption, we built a collector to retrieve
micrometeorites larger than 50 m from the bot-
interstitial glass, and magnetite, was more diffi-
cult to prove. Major element analysis showed that
tom of the well. We deployed the collector in De-
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