- 4164 - OLDEST METEORITE - how do we know? A 4.6 billion-year-old meteorite could reveal how Earth formed different layers. The meteorite “Erg Chech 002” found in the Sahara desert in 2020 is one of the oldest known space rocks.
-------------- 4164 - OLDEST METEORITE - how do we know?
- The rock analysis
could reveal secrets about the solar system in its infancy during the birth of
the planets and also help scientists better determine the ages of the oldest
meteorites that fall to Earth.
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- The meteorite is
encrusted with green crystals.
Meteorites like this are believed to have formed from material in a disk
of gas and dust around the infant sun. Cold, dense patches of this "solar
nebula" collapsed to birth the planets, but leftover material formed
comets and asteroids from which meteors break away, often finding their way to
the surface of Earth in the form of meteorites.
-
- Erg Chech 002
contained the radioactive isotope Aluminum-26 when it formed, which is
significant because this unstable form of Aluminum is believed to have been
important in a later stage of Earth's evolution, so-called "planetary
melting”.
-
- “Planetary melting”
is believed to be the process by which rocky planets like ours
"differentiated" or formed different compositions at different
layers. This is because the melting allows denser material to sink to the core
of planets. So, for Earth, an example of this differentiation would be the
formation of a dense metal core and, above it, a less dense rocky mantle.
-
- Understanding how
Aluminum-26 was distributed as the planets were forming around 4.6 billion
years ago is thus important to building a picture of how the rocky inner
planets of the solar system evolved.
Because Aluminum-26 decays to Magnesium-26, a stable form of Magnesium,
it can be used as a dating system for space rocks.
-
- To determine the
age they measured the amounts of lead isotopes within it. Aluminum-26 decays over time within the first
four or five million years of the solar system's life. The half-life of
Aluminum-26 is around 717,000 years, meaning it is too short-lived to be
directly found in large quantities in the 4.6-million-year-old space rock. But,
when it decays, this radioactive isotope of Aluminum leaves behind Magnesium-26,
a stable non-radioactive isotope of Magnesium.
-
- That means
Magnesium-26 can be used to determine the starting amount of Aluminum-26 in a
space rock like Erg Chech 002, and this could be used as a dating system (also
known as a “chronometer”) for space rocks.
-
- The Aluminum-26 –
Magnesium-26 decay system also serves as a high-resolution relative
chronometer. Developing a generalized
approach for isotopic dating with Aluminum-26 – Magnesium-26 and other extinct
isotope chronometers that accounts for heterogeneous distribution of the parent
radionuclide would allow us to produce more accurate and reliable age data for
meteorites and asteroidal and planetary materials to advance a better
understanding for the formation of our solar system.
-
- It’s sometimes
hard to remember that meteorites have been hitting our planets for millions of
years. And some of them are made of valuable materials such as titanium or
iron. Our bronze and iron age ancestors
could have utilized these ready-made metallic rocks without having to dig
underground to access them, like they would with regular tin or iron veins.
-
- A new study of an
arrowhead made out of a meteorite points out just how valuable iron age society
thought these meteorites were and hints at a trade network that reached farther
than archeologists initially thought.
-
- They only had two
sites in Poland that turned up objects made by meteorites. Now, there is a third. An arrowhead found in
a dwelling near Lake Mörigen in Switzerland in the late 19th century was
confirmed to be made from a meteorite. It was dated back to the Bronze Age,
somewhere between 900-800 BCE. But several key features about it make it
particularly interesting to archeologists.
-
- They were looking
for objects of possible meteoritic origin in that part of Switzerland because
there had been a known meteorite strike known as the “Twannberg iron meteorite”
that fell nearby. When the Twannberg
meteorite fell, it broke into pieces. So far, 2,000 individual pieces have been
found with a total weight of over 150 kg.
-
- That’s a lot of
easily recoverable metal sitting only a few kilometers from the site at Lake
Mörigen, where the arrowhead was found. But strangely, the study found that the
arrowhead was conclusively not made of the meteor that fell near Twannberg.
-
- Instead, they
believe it was created using pieces from a different meteorite that fell in
“Estonia” in 1500 BCE. Known as the
“Kaalijarv meteorite”, it is the best fitting of the three other meteorites
with the same chemical signature as the arrowhead. However, its landing site
was over 1,600 km away. That is quite the distance for an arrowhead to travel
in the Bronze Age.
-
- Some metal
meteorites have a tiny magnetic field. But how?
One of the striking things about iron meteorites is that they are often
magnetic. The magnetism isn’t strong, but it holds information about their
origin. This is why astronomers discourage meteorite hunters from using magnets
to distinguish meteorites from the surrounding rock, since hand magnets can
erase the magnetic history of a meteorite, which is an important scientific
record.
-
- Magnetic meteorites
occur because they form in the presence of a magnetic field. The iron grains
within the meteorite are aligned along the external magnetic field, which gives
the meteorite its own magnetism. For example, the Martian meteorite known as
Black Beauty gained its magnetism from the strong magnetic field of young Mars.
-
- Some meteorites are
magnetic but shouldn’t have formed in a strong magnetic field. Iron meteorites
are typically categorized by chemical composition, such as their ratio of
nickel to iron. One type, known as “IVAz', is known to be fragments of smaller
asteroids. Small asteroids don’t have strong magnetic fields, so IVA meteorites
shouldn’t be magnetic, but many of them are. There’s a new study showing how
that’s possible.
-
- Small asteroids
form through what is known as the rubble pile method. Small chunks of iron-rich
rock aggregate over time, building up to become an asteroid. For a body to
generate a strong magnetic field, there needs to be liquid iron to create a
dynamo effect, and since small asteroids don’t experience this, they can’t have
magnetic fields. Or can they?
-
- Asteroids are also
subject to collisions over time. It’s these collisions which break off
fragments that become the meteorites we find on Earth. Impacts can create a magnetic dynamo within
an asteroid. If a colliding body is not big enough to shatter the asteroid, but
large enough to melt a layer of material near the surface, then a chain of
events can occur.
-
- When a cold rubble
core is surrounded by a molten layer, the core is heated up. Lighter elements
evaporate out of the core and migrate toward the surface, which churns the
layers to generate convection. The convection of iron generates a magnetic
field, which imprints itself on parts of the asteroid. Later collision then
creates magnetic fragments, some of which reach Earth.
-
- So the magnetism of
IVA meteorites comes not from the original formation of their parent asteroid,
but rather from later collisions that stirred up their core. Knowing this,
researchers can gain a better understanding of the history of our solar system,
and how things such as planetary drift might have triggered more frequent
asteroid collisions.
-
- Yet another reason
not to look for meteorites with hand magnets. The very act of finding a
meteorite could also erase the history of its collisions.
-
-
September 18, 2023 OLDEST METEORITE - how
do we know? 4164
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