Astronomy - USA (2020-08)

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And yet, the clues micromete-

orites provide about our solar sys-

tem make them worth the hunt.

“They are samples of distant

objects like asteroids or comets

that we don’t typically have direct

samples from,” says David

Nesvorny, a planetary scientist at

the Southwest Research Institute

in Boulder, Colorado. The rocks

and minerals that make up larger

bodies like Earth and the Moon

have been altered by geological

forces over time. But smaller bod-

ies like asteroids are largely

unchanged, so sampling them

gives scientists an idea of the raw

materials that existed when our

solar system was in its infancy.

When it comes to space relics,

bigger isn’t necessarily better.

“The question I get asked is,

‘They’re so small — how much

can you get out of them? Why

don’t you study large meteorites?’ ”

says Penny Wozniakiewicz,

a planetary scientist at the

University of Kent, who was not

involved in the research but stud-

ies micrometeorites herself. One

reason is abundance. “At the

moment, far more material is

arriving as micrometeorites than

meteorites,” she says. While “mac-

rometeorites” are rare and hard

to find, it’s estimated that up to

88 million pounds (40,000 metric

tons) of space dust sprinkles down

on Earth each year, making it a

plentiful source of extraterrestrial

material if you know how to look

for it. “It’s much more fruitful to

work with [micrometeorites],

because they’re more easy to find,”

agrees cosmochemist Philipp

Heck, curator of meteorites at the

Field Museum in Chicago.

Scientists have studied a rare

cache of large ancient meteorites

preserved in a limestone quarry in

Sweden, but such finds are few

and far between. “The chance of

finding one in a random outcrop-

ping is extremely small,” he says.

“You might have to look 10 or 20

years to find one meteorite — but

only if you look every day.”

Furthermore, while large mete-

orites tend to come from only cer-

tain parts of the asteroid belt,

particles from any body that pro-

duces dust are pulled inward

toward the Sun from all direc-

tions, giving researchers a much

larger sample size. “They have the

potential to come from anything

in the solar system,” Suttle says.

Many micrometeorites are now

collected in Antarctica, where sci-

entists melt large volumes of snow

or ice, filter the water, and then

pick through the leftover particles

under a microscope one by one.

Scientists have also collected

micrometeorites and cometary

dust by f lying research planes

high in the stratosphere, trapping

the particles on plates coated with

sticky silicon oil. Others have

been found in sediment dredged

ABOVE: Different types

of micrometeorites

have distinct mineral

structures and grain

patterns. Here we see:

A) fine-grained

unmelted;

B) coarse-grained

unmelted;

C) scoriaceous;

D) relict-grain-bearing;

E) porphyritic;

F) barred olivine;

G) cryptocrystalline;

H) glass;

I) CAT (calcium,

aluminum, titanium);

J) G-type;

K) I-type; and

L) single mineral.

S. TAYLOR/SHAW STREET/

WIKIMEDIA COMMONS

LEFT: By taking

microscopic cross

sections of

micrometeorites,

researchers can

determine what

specific structures

and minerals they

contain, revealing

more about their

past. This scoriaceous

micrometeorite is

characterized by a

relatively low melting

point, being very

vesicular, and

containing olivine

crystals. MARTIN SUTTLE

A

E

I

B

F

J

C

G

K

D

H

L

50 μm