34 ASTRONOMY • AUGUST 2020
almost certainly been altered
through diagenesis. “That’s what
we expected,” he says, “and that’s
why we have never worked with
that type of micrometeorite,
because you cannot really get a lot
of information. It’s the same with
the fossil meteorites that we find.
You cannot get any information
from them, because they’re so
altered — just like a fossil.” But
Schmitz questions whether all the
spherules identified in Suttle’s
paper were truly extraterrestrial,
saying it is difficult to prove with-
out specific chemical signatures.
After all, lots of spherical things
that resemble micrometeorites
appear in sediments, he says.
Suttle agrees that fossilized
micrometeorites may have limited
value in traditional meteoric
research because it’s no longerpossible to analyze the chemical
traces of their parent asteroids.
But there’s still much that scien-
tists can learn from them, he says.
In addition to measuring the total
cosmic dust flux over time and
investigating the oxygen content
of the early atmosphere, they can
also be used to study diagenesis in
sedimentary rocks. “Just as with
fossils of biological remains,
many important investigations
can still be conducted, even when
the animal’s tissues are no longer
preserved,” Suttle says.
Investigating and proving the
true nature of the tiny grains cer-
tainly wasn’t easy. Suttle first
scanned the outside of his micro-
meteorites with an electron
microscope to capture images
of their external shapes and tex-
tures, especially characteristicsthat would f lag them as extrater-
restrial. Looking inside was
harder. To cut these microscopic
grains in half, Suttle first cast
them in tiny cylinders of resin.
He then polished them down
until the interiors were exposed.
“When I looked at the external
textures, they looked exactly as
I expected,” Suttle says. “It was
only when I cut them open and
started analyzing their minerol-
ogy that I realized they weren’t
what they should be.”
The discovery was even more
surprising because searching for
cosmic dust in ancient sediments
started off as a side project that
was outside the scope of Suttle’s
original Ph.D. plan. Suttle’s main
project was supposed to focus on
investigating the sources of con-
temporary micrometeorites, butABOVE LEFT: The
dips and divots in this
micrometeorite show
where internal water
has pushed its way
to the surface and
vaporized. This piece
of cosmic dust is about
75 μm wide by 130 μm
long. IMPERIAL COLLEGE LONDON
ABOVE RIGHT: Cross
sections of detrital
pyrite grains from
the chalk hills reveal
pentagonal structures
commonly found in
cubic crystals like
pyrite. MARTIN SUTTLE
BELOW: The white
chalk hills of North
Downs in Kent, England,
may be reminiscent of
Tolkien lore, but the
area also contains relics
from space — namely,
fossilized grains of
microscopic dust.
MARTIN SUTTLE
AB
50 μm 50 μm