What Crystals Older Than the Sun Reveal About the Start of the Solar System
Arthur T Knackerbracket writes:
The standard story of the origin of our solar system has gone like this: 4.6 billion years ago, a giant cloud of dust hung frozen in space. Then the explosion of a nearby star caused part of that dust cloud to collapse. Pulled by gravity toward a central point, the dust coalesced into a radiating ball of hydrogen and helium about 1.4 million kilometers in diameter - what would become our sun. The remainder, which fell into orbit, collected into our solar system's planets, along with a mess of asteroids and other cosmic leftovers.
To test the validity of this story, researchers need to peer back in time to the solar system's first moments and beyond. And the cosmochemist Nan Liu has a way to do that: Locked in a safe on her desk at Boston University's Institute for Astrophysical Research is a shard of meteorite flecked with material older than the sun.
[...] Over the past decade or so, scientists have used meteorites like Liu's to challenge the story of how the solar system formed. Instead of a supernova, the solar system and everything in it might owe its existence to a more placid-sounding cosmic scenario: Maybe our solar system cobbled itself together from the winds blown off of a gargantuan star. New studies of presolar grains could offer a way to determine whether this new story is correct.
Scientists got their first clue about what could have triggered the formation of the solar system when a fireball appeared over Mexico in 1969. The now-famous Allende meteorite spread its debris over more than 500 square kilometers.
In 1976, researchers reported that samples from Allende contained a surprise: an unexpectedly large amount of a stable isotope called magnesium-26. They proposed that the meteorite formed with an abundance of aluminum-26, which is radioactive and leaves behind magnesium-26 when it decays.
Yet aluminum-26 was not known to be a normal component of the interstellar medium - the dusty space between stars that would have provided the materials for Allende. Ordinary stars don't make that particular isotope. "Most of these isotopes as we observe them in the early solar system, they were just the natural product of galactic chemical evolution," said Maria Lugaro, an astrophysicist at the Konkoly Thege Miklos Astronomical Institute in Hungary. "The most important exception is aluminum-26."
So where'd it come from? In 1977, two eminent astrophysicists proposed that the anomalous aluminum likely came from a nearby supernova explosion. Other phenomena can produce aluminum-26, but the supernova shock wave could also have caused the collapse of the cloud. With a single event, astronomers could explain how two rare occurrences - the injection of aluminum-26 and the formation of a new solar system - happened at virtually the same moment. "Everybody felt that we needed something to trigger the collapse," said Vikram Dwarkadas, an astronomer at the University of Chicago.
The supernova trigger remained the favored scenario for decades, supported by detailed astrophysical models, as well as further measurements of enriched magnesium-26 in pristine meteorites. But over the past decade or so, that view has run up against other measurements that don't seem to match. The problem: The solar system has an iron deficiency.
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