On 2 June, 2016, at 10:56:26 UTC (03:56 local time), a very bright fireball fell over Arizona.
It was seen by hundreds of witnesses and caught on multiple cameras,
including some designed to look for bright meteors in the sky. In many
respects it was a typical meteor, if extremely luminous due to its size
(something less than a meter wide).
Except for one thing: It's likely this rock was the older brother of
the one that fell over Chelyabinsk, Russia in 2013, the largest impact
the Earth has seen in over a century. And that makes it very interesting
indeed.
The meteor came in from the north-northeast over central Arizona at
16.6 km/sec (60,000 kph), slammed into Earth's atmosphere, and
decelerated heavily. Cameras picked it up when it got hot enough to
start to glow at an altitude of 108 kilometers above Earth. The ram
pressure of it moving at hypersonic velocity squeezed it so hard that at
44 km up it split, ejecting a large fragment. The main body then began
to disintegrate, experiencing major breakup episodes at 34, 29, and 25
km up, ejecting a lot of mass as dust and flashing brilliantly as it
did. The total energy released was equivalent to about 30 tons of TNT
exploding. A decent sized event — people heard a boom and it was even detected by seismometers! It left behind a glowing trail of vaporized rock, called a persistent train, that lasted long enough to be illuminated by the rising Sun later.
The "persistent train," a trail of vaporized rock, left in the sky by the Dishchii'bikoh meteor. Credit: Mike Lerch/ via Jenniskens et al.
Doppler radar picked up several pieces falling to the ground on a
reservation of the White Mountain Apache tribe. A team was dispatched
and, with the permission of the tribe elders, searched for meteorites,
finding 15 of them. The descent track of the parent meteoroid took it
over the city of Cibecue, and so the meteorites have been named
Dishchii'bikoh (pronounced dish-ki-i-bi-go) meaning "valley of the red
mountains," the Apache word for the area.
Here's a local news report with video of the fall (note the size and speed hadn't been fully determined at this point):
Analysis of the meteorites was fruitful.
It's what's called an LL chondrite. The LL means it has low iron and
low metal content, and chondrites are meteorites with small grains of
minerals in them called chondrules. LLs are relatively rare, making up
about roughly 10% of all meteorites found on Earth. I'll get back to
that fact in a moment.
Meteorites like these contain very small amounts of uranium, and that
radioactively decays to lead at a known rate. Measuring that,
scientists found the rock had an age of 4.471 ±0.006 billion years.
A dozen of the Dishchii'bikoh meteorites collected in Arizona.
The cube on the left is for scale, and is one centimeter on a side (the
size of a normal six-sided die). Credit: Jenniskens et al.
However, when they're out in space, meteoroids* like this
are bombarded by subatomic particles zipping around the galaxy. These
change the chemistry of the elements inside at a known rate, and that
can be measured to get an age. The Dishchii'bikoh rock that fell to
Earth had an age of only 11 million years… which means that's how long
it was in space by itself. It must have come from a parent body, an
asteroid 4.471 billion years old, which suffered an impact 11 million
years ago that blasted material out into space.
That's where this gets cool. The asteroid that fell over Chelyabinsk Russia in 2013 was 19 meters across
— much larger than this one — but was also an LL chondrite. Radioactive
dating of Chelyabinsk meteorites gives an age of 4.452 ±0.021 billion
years. Within the uncertainty, that's identical to the age of
the Dishchii'bikoh meteorite, heavily implying they came from the same
parent body! Not only that, some of the elemental abundances are the
same between the two as well, again implying a common parent.
That's pretty cool. Only four LL chondrite meteors have ever been
caught on camera, including Chelyabinsk and Dishchii'bikoh, so finding
out these two may be from the same parent body is pretty interesting
scientifically! The cosmic ray exposure age of Chelyabinsk meteorites indicates
they were in space for only about 2 million years, which means the
parent body got slammed again 9 million years after the impact that made
the Dishchii'bikoh body; this time perhaps it was a bigger impact,
dislodging the much larger Chelyabinsk rock.
Video of Crash Course Astronomy: Meteors, Meteoroids, and Meteorites, Oh My!
Crash Course Astronomy: Meteors, Meteoroids, and Meteorites, Oh My!
Having these LL chondrite meteors on camera also means these objects
can have their orbits calculated by tracing their paths in the sky
backwards into space. It looks like they came from an asteroid in the
inner asteroid belt (that is, closer to Mars than Jupiter) with an orbit
roughly in the same plane as the major planets. But… Dishchii'bikoh
came in at a relatively high angle. That implies it's had an encounter
with Earth at least one time before which affected its orbit, slinging
it up into a higher angle. That's not too surprising; if it was on an
orbit that resulted in an actual impact, then it likely got close to
Earth several times in the past.
I collect meteorites; I have quite a few, including a small piece of
Chelyabinsk given to me by a friend. When I hold them in my hand I like
to think about how old they are, how long they were in space, what their
history was. And now, with cameras capable of triangulating their path
through our air, the ability to find their orbits, and the measurements
of their various ages, we can find out a lot more about them than merely
wondering.
Including that this 2016 Dishchii'bikoh impact likely came from the same parent asteroid as Chelyabinsk. They're siblings!
The next time you're out under a dark, moonless sky, you may see a
meteor or two flashing across the sky. For centuries humans wondered
what they were, where they came from. Now when you see one, remember
that it's only been in the past few decades that we can say we know.
Doppler radar picked up several pieces falling to the ground on a
reservation of the White Mountain Apache tribe. A team was dispatched
and, with the permission of the tribe elders, searched for meteorites,
finding 15 of them. The descent track of the parent meteoroid took it
over the city of Cibecue, and so the meteorites have been named
Dishchii'bikoh (pronounced dish-ki-i-bi-go) meaning "valley of the red
mountains," the Apache word for the area.
However, when they're out in space, meteoroids* like this
are bombarded by subatomic particles zipping around the galaxy. These
change the chemistry of the elements inside at a known rate, and that
can be measured to get an age. The Dishchii'bikoh rock that fell to
Earth had an age of only 11 million years… which means that's how long
it was in space by itself. It must have come from a parent body, an
asteroid 4.471 billion years old, which suffered an impact 11 million
years ago that blasted material out into space.
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