A meteorite that fell on Earth more than a century ago may contain some of
the first concrete evidence for a cosmic mash-up in the early solar system.
Following the birth of our sun 4.5 billion years ago, it is thought that
Jupiter’s formation caused two reservoirs of asteroids to gather in the
solar system, one inside the giant planet’s orbit and one outside. The
former are known as non-carbonaceous asteroids, because they are
traditionally low in carbon, whereas the latter are carbonaceous asteroids,
richer in things such as water and carbon that were able to survive further
from the sun’s heat.
There had been some evidence that dust from these two reservoirs had mixed,
possibly as a result of Jupiter and the other outer planets migrating
slightly in their orbits towards and away from the sun as the solar system
settled.
Now, Fridolin Spitzer at the University of Münster in Germany and his
colleagues have found some of the best evidence yet for mixing. It comes
from a meteorite that appears to contain both carbonaceous and
non-carbonaceous material. “This implies it’s a mixture between these two
reservoirs,” says Spitzer.
This space rock, called the Nedagolla meteorite, fell in India in 1870. It
was already unusual in that its chemical composition didn’t match any other
meteorites analysed to date, suggesting it didn’t share a common origin with
any meteorites on Earth.
When Spitzer and his team studied the meteorite to learn more about its
origins, they were surprised to find pockets of the element molybdenum with
different concentrations, some corresponding to carbonaceous meteorites and
others to non‑carbonaceous ones.
Measurements of the radioactive decay of the meteorite suggest that it
formed at least 7 million years after the birth of our solar system. Two
asteroids from the inner and outer reservoirs appear to have smashed
together to form the meteorite, possibly as a result of Jupiter’s
gravitational pull as it moved towards or away from the sun.
“It’s nice to find evidence that this actually happened,” says Spitzer. If
the team is correct, it would be some of the first meteoritic evidence that
the mixing really took place.
“It’s exciting,” says Harold Connolly at Rowan University in New Jersey.
“It’s what we should be able to see if Jupiter was migrating and throwing
material in and out of the solar system.”
Other indirect evidence for this migration does exist, says Sara Russell at
the Natural History Museum in London, such as the small size of the planet
Mars – possibly the result of Jupiter being closer to Mars than it is today
and sweeping up material, preventing Mars from growing larger.
“Then there’s the massive diversity of material in the asteroid belt, which
is really a jumble of super-ice-rich stuff and stuff that’s at [relatively]
high temperature,” says Russell. “You need to have something stirring the
pot to mix things up.”
The Nedagolla meteorite would be the first physical evidence of a collision
between a pair of asteroids from the two reservoirs, though. Meteorites
already in collections on Earth may also contain similar evidence.
“There’s so many parts of the collections that are really unexplored,” says
Russell. “I’m sure there are loads and loads of secrets to find out.”
Reference:
Collisional mixing between inner and outer solar system planetesimals
inferred from the Nedagolla iron meteorite
by Fridolin Spitzer, Christoph Burkhardt, Jonas Pape, Thorsten Kleine
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