32 ASTRONOMY • AUGUST 2020
up from the seaf loor, where they
are separated from the sludge
with magnets — although this
process only picks up particles
containing ferrous metals.
But while these methods have
resulted in a harvest of modern
micrometeorites, there’s growing
interest in studying ancient space
dust. Scientists have found cosmic
dust in sedimentary rock since the
1980s. Heck says his colleague,
geologist Birger Schmitz of Lund
University in Sweden, studied
these larger meteorites, which were
recovered from a limestone quarry
in Sweden. “They’re pretty rare, so
they only found about two or three
[large meteorites] a year,” Heck
says. “But the thinking was that
micrometeorites are far morefrequent and abundant than bigger
ones, so they might be hidden in
the sediment, still preserved.” Sure
enough, Schmitz’s team docu-
mented their discovery of extrater-
restrial particles in a 2003 paper.
Since then, researchers from
different disciplines have been
taking another look at these pre-
served particles, studying them
for clues about the solar system’s
distant past. “We’ve known
they’re there for a while,” says
Andy Tomkins, an earth and
planetary scientist at Monash
University in Melbourne,
Australia, who was not involved
in the study. “Now it’s [a question
of] what can they tell us.”Measuring cosmic
dust flux
Scientists are interested in learn-
ing the rate at which space dust
falls to Earth over time. Although
micrometeorites usually arrive in a
steady sprinkle, studies show peri-
ods in the planet’s geological past
when this “f lux” has brief ly spiked
for a few million years or so.
Researchers believe these sudden
showers coincided with a major
collision in the asteroid belt or
some other large dust-generating
event farther out in our solar sys-
tem. “This is the only empirical
evidence we can have of geologi-
cal activity in the asteroid belt,”
Suttle says. Although researchers
have hypothesized many of these
historical cosmic events by model-
ing the movements of asteroids
and planets, “it can’t be verified
through any other way than by
looking at the cosmic dust f lux
over time,” he says.
The chemical composition of
ancient micromete-
orites can also
offer clues about
the evolution of
the solar system.
Today, most meteor-
ites and micrometeorites
hitting Earth are ordi-
nary chondrites, con-
taining minerals that
come from S-type, orsiliceous, asteroids that are
abundant in the inner part of the
asteroid belt. But when research-
ers examine micrometeorites
found in sedimentary rocks that
are 466 million years old, they
find something different.
“Ordinary chondrites were not
ordinary at that time,” says Heck.
Other types of micrometeorites,
including achondrites (which
have no chondrules, or rounded
silicate droplets found in the
most primitive types of space
rocks), were more abundant.
These mineral types are linked
to other asteroid families. “From
the geological record, we will
reconstruct when different aster-
oid families formed, when there
were perhaps orbital perturba-
tions in the solar system — times
when the solar system may not
have been as stable as many
researchers believe today,” says
Schmitz, who works with Heck
on micrometeorite research.
“We say that we do astronomy
by looking down, instead of by
looking out in space.”
Scientists have also used
micrometeorites to study the
environment of early Earth.
Tomkins and his team discovered
micrometeorites in 2.7-billion-
year-old sedimentary rock in
Australia, the oldest fossilized
space dust ever recorded. When
they analyzed their find, they saw
that the iron and nickel in the
particles had been oxidized — a
chemical process also seen in
modern micrometeorites,
which occurs when theTOP: The branching
(dendritic) crystals of
this 17-micrometer (μm)
mote of fossilized dust
exhibit a treelike
structure. MARTIN SUTTLE
ABOVE: Zooming in
on a 75-μm wide
micrometeorite
reveals a roughly 4.5-
by-7 μm fossilized
coccolithophore — a
unicellular plankton
with microscopic
limestone plating.
MARTIN SUTTLE
RIGHT: A smattering of
melted micrometeorites
highlight that these
cosmic spherules
drastically vary in size
and appearance. The
largest particle in this
image is about 300 μm
wide. SHAW STREET/WIKIMEDIA
COMMONS