A meteorite crashed into Earth in 1969, ending an epic journey that had lasted billions of years, and maybe even much longer.
Inside this cosmic rock – the Allende meteorite that fell half a century ago in the Mexican desert – scientists have discovered interstellar material older than our solar system.
The discovery of such incredibly ancient matter – traces of stardust from interstellar space, called pre-solar grains – is rare, but not accidental.
Just a few weeks ago, a team of scientists announced that presolar grains found in another meteorite (which also fell to Earth in 1969, but in Australia) are the oldest known material on the planet, ranging from 5 to 7 billion years old.
In comparison, our own solar system is only about 4.6 billion years old, so we are wandering around some primitive territory here, at least in terms of our own neighborhood with space.
Now, in a new study by the University of Washington in St. Louis, scientists have uncovered evidence of the presence of pre-solar grains inside a portion of the Allende meteorite – and where they were found inside the space rock is contrary to our knowledge.
In this case, the identified presolar grains consisted of silicon carbide (SiC) and were found in an inclusion within the meteorite.
“What's surprising is that there are pre-solar grains,” says Olga Pravdivtseva, physicist and cosmochemist.
“Following our current understanding of the formation of the solar system, presolar grains cannot remain in the environment where these inclusions are formed.”
In this case, it is notable that silicon carbide can exist in a lump that is otherwise mostly calcium-aluminum-rich inclusion (CAI): a mineral mixture that is considered one of the oldest solids formed in the solar system.
CAI is believed to have formed from an overheated solar nebula – the scorching concentration of gas and dust that gave birth to the Sun and Solar System, and which should have been too hot for interstellar stardust.
“It is generally accepted that CAIs formed close to the Sun at temperatures above 1226 degrees Celsius, where the pre-solar grains could not remain in their previous form, and then were transferred to other regions of the nebula, where planetesimals were accreting,” the authors write in their article.
In experiments in which the researchers heated a tiny sample of a meteorite, they identified noble gas signatures that revealed SiC inside CAI – an unexpected combination of chemicals. This tells us that we will have to reconsider the understanding of what was possible inside the solar nebula.
“It's an elegant piece of work experimentally,” says Pravdivtseva.
'Then we had to solve the riddle of the isotopic signatures of noble gases. All noble gases pointed to the same source of anomalies – silicon carbide. '
Researchers don't know how silicon carbide from another star got into such primordial solids, but the fact that it did means we need to rethink some things about chemistry in the early solar system.
“Although CAI, the oldest dated particulate matter in the solar system, has been extensively studied, questions still remain regarding the nature and origin of the isotopic anomalies they carry, their distribution among primitive meteorite classes, and their relationship with other meteorite components,” the researchers write.
The findings are reported to Nature Astronomy.
Sources: Photo: The Planetary Society
