WWW.ASTRONOMY.COM 33
heated metal is exposed to oxy-
gen. “That was what led us on to
the discovery,” Tomkins says.
“We thought, ‘Oh, wow, this hap-
pened as they went through the
upper atmosphere.’ ”
The finding was significant
because it coincided with the
period before when scientists
believe oxygen first appeared in
Earth’s atmosphere, during the
Great Oxidation Event some
2.4 billion years ago. The oxidized
metals in the micrometeorites
provided the first empirical evi-
dence that the upper atmosphere
may have been more oxygen-rich
than the lower atmosphere at that
time. A next step could be to find
and study micrometeorites from
both before and after the event,
to see how the upper atmosphere
responded to the biggest atmo-
spheric change in Earth’s history.
Fossil record
While Heck is delighted by the
discovery of fossil micrometeor-
ites, he’s not surprised. He and
his colleagues recognized a simi-
lar fossilization process at work
in large meteorites years ago.
“There, it is really obvious,” he
says. “It still looks like a meteor-
ite, but it is not anymore, because
as the fluids went through, they
replaced the minerals without
changing the shape of the object.”
But even though researchers
have been studying ancient
micrometeorites for some three
decades, they may not have rec-
ognized them as true fossils.
Heck says his team ignored the
more common melted spherules
and worked with the rarer
unmelted ones. This was because
they contain chromite — a min-
eral that retains trace elemental
characteristics indicative of
extraterrestrial material. Since
chromite is not one of the miner-
als affected by fossilization, “it’s
something we didn’t look at or
didn’t even see,” says Heck.
Schmitz also says he was aware
that most micrometeorites, espe-
cially the melted spherules, had
TOP: The external and internal textures of fossilized iron-silicide spherules, as seen through a
microscope, exhibit voids (F), protrusions (D), and cavities (B), with the latter being the result
of venting volatile gases while entering Earth’s atmosphere. MARTIN SUTTLEABOVE: Fossilized iron-oxide spherules, seen here, include dendrites on their surfaces (A-C),
as well as circular cavities (D and F) — the result of corrosion while on the seafloor. MARTIN SUTTLE40 μm40 μm10 μm50 μm20 μm40 μm40 μm20 μm10 μm10 μm5 μm5 μm10 μm10 μm30 μmA
A
D
D
G
B
B
E
E
H
C
C
F
F
I