30 ASTRONOMY • AUGUST 2020
chemical process that converts
sediment into sedimentary rocks.
Researchers have found
ancient micrometeorites pre-
served in sedimentary rock like
chert and limestone before, and
some think they were diageneti-
cally altered. But Suttle and
Genge are the first to show they
have truly found micrometeoritefossils, also demonstrating exactly
how the fossilization process
played out.
In their paper, published in
2017 in Earth and Planetary
Science Letters, Suttle and Genge
outlined five new criteria for posi-
tively identifying cosmic spher-
ules that have been fossilized.
These include their spherical
shape, treelike “dendritic” tex-
tures typical of molten metals
that have been rapidly cooled, and
large central cavities, where the
beads of metal typically found at
the center of iron-rich microme-
teorites have weathered away.
“They’ve done this very careful
study to show that you can iden-
tify [fossilized micrometeorites]
even without the typical chemical
signatures,” says Larry Nittler, a
cosmochemist and planetary sci-
entist at the Carnegie Institution
for Science. “[It’s] just like a dino-
saur bone is replaced by new min-
erals, but you can still identify it
as a bone.”This methodology gives
researchers a tool to identify
ancient micrometeorites that
might have been previously over-
looked because they didn’t have
the right chemical composition.
“Without this method, studies
can be skewed toward specimens
that survive better than others,”
Nittler says.Ancient grains
Micrometeorites are particles of
dust and rock that fall to Earth
from space. A few enter the
planet’s atmosphere at a low angle,
causing them to drift slowly to
the ground and remain intact.
But most arrive at high speed,
largely melting in the heat of their
descent. They then quickly harden
into spherules as they cool, shap-
ing into microscopic droplets.
Their tiny size also makes them
hard to find — the largest might
be as big as a grain of sand, while
the smallest can’t be seen without
a microscope.TOP: Martin Suttle takes
a break from collecting
samples of late
Cretaceous rocks found
in the white chalk hills
of North Downs, Kent,
England. MARTIN SUTTLE
ABOVE: A block
extracted from the white
chalk hills, a site hiding
many micrometeorites.
MARTIN SUTTLE