68 ASTRONOMY • JUNE 2020
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
HOW DID STONY METEORITES
FORM FROM THE DUST CLOUD
OF THE EARLY SOLAR SYSTEM TO
BECOME DENSE ROCKS? I THOUGHT
THEY WOULDN’T HAVE HAD ENOUGH
GRAVITY TO BECOME ROCKS. AND
WHERE DID THE IRON FOUND IN SOME
METEORITES COME FROM?
Michale Olsen
Leaf River, IllinoisAI
The three major types of meteorite (stony,
stony-iron, and iron) cover a wide range of den-
sities, from the least dense stony meteorites (roughly 2
to 3 grams per cubic centimeter), which have densities
similar to some rocks from Earth, to the very dense iron
meteorites (7 to 8 g/cm^3 ). Although they only account
for about 5 percent of meteorite falls, iron meteorites
are more easily recognized than other types of meteorite
because they are so heavy.
The processes by which small grains stick together
and accrete into small bodies are not well understood,
and it is an area of active research and debate. Gravity
isn’t the only force that attracts bodies to one another.
Electrostatic forces, which pull together particles with
opposite magnetic charges, may explain how small
grains stuck together to form the first solids in the early
solar system.
Iron is present in different amounts in all meteorites,
obviously being most abundant in iron meteorites. All
iron in planetary materials (including Earth) was inher-
ited from the molecular cloud that collapsed to form ourHow
meteorites
form
solar system and would have originally formed during
nuclear fusion in massive stars.
All asteroids probably accreted from a mix of stony
material, but the largest were able to undergo melting and
differentiation to form a nickel-iron core and stony outer
mantle, similar to the structure of Earth. Indeed, iron
meteorites are thought to represent the nickel-iron metal
cores of large asteroids that were disrupted and broken
apart by impacts. Stony-irons (nearly equal mixtures of
stony material and nickel-iron metal) may come from
the boundary between the molten metal core and the
outer silicate mantle. Stony meteorites come from a vari-
ety of different places in the solar system, including the
Moon, Mars, and the asteroid belt. While some of these
originated from bodies that differentiated (like the
Moon, Mars, and large asteroids such as Vesta), many are
remnants of asteroids that never grew large enough to
undergo differentiation and remain largely unaltered
since they formed over 4.5 billion years ago.
Jemma Davidson
Assistant Research Scientist, Center for Meteorite Studies,
Arizona State University at TempeQI
WHEN YOU WRITE ABOUT “SUPER-
EARTH” EXOPLANETS, ARE YOU
COMPARING THE MASS OF EARTH AS IT IS
NOW, OR EARTH BEFORE THE MOON WAS
FORMED? I HAVE READ SEVERAL ARTICLES
ON WHAT EARTH WOULD BE LIKE WITHOUT
THE MOON. WOULD THIS AFFECT THE
DEFINITION OF SUPER-EARTH PLANETS?
Doreen McLaughlin
Monument, ColoradoAI
Super-Earths are rocky exoplanets with masses
greater than Earth’s. But, in fact, there is no
firm, agreed-upon definition for the mass range of
these planets. Most studies consider planets with masses
between about one and 10 times that of Earth a super-
Earth. (However, as noted above, this number can vary.
Some studies consider slightly more mass — two to threeThere are three major
types of meteorites,
shown here from left
to right: iron, stony-
iron, and stony. Each
is characterized by
the amount of iron
present. H. RAAB/WIKIMEDIA
COMMONS; JAMES ST. JOHN/
FLICKR; H. RAAB/WIKIMEDIA
COMMONS