An extraordinary Egyptian meteorite apparently formed in the wake of a rare Type Ia supernova, the type of explosion that revealed the existence of dark energy. If the claim is right, the rock is an unparalleled scientific treasure that could upend our theories of Solar System formation, and provide fresh insights into some of the universe's greatest explosions.
Like the Antarctic ice sheet, the Sahara desert is a good place to hunt for meteorites because there is no vegetation to hide them and dark rocks tend to stand out. In 1996, a 30-gram piece of rock found near the Egypt/Libya border was recognized as being so unusual it was named the Hypatia Stone after the great Egyptian mathematician (c 360-415 CE) murdered by religious zealots, arguably for being a woman.
The stone is extremely rich in microdiamonds, unlike anything we have seen in other meteorites. Initial speculation suggested the stone might be the remnant of a comet nucleus, but the scientists studying it have now presented a much more dramatic explanation, published in Icarus. Not only was the stone formed in the aftereffects of a supernova explosion, they claim, but we can even tell which type.
The stone's elemental make-up is so different from any other known meteorite Professor Jan Kramers of the University of Johannesburg and co-authors propose it must have formed from interstellar dust. Even the outer Solar System, where the stone was first thought to originate, just isn't different enough to the inner regions to account for this make-up, particularly the extraordinary lack of silicon. At least one meteorite older than the Solar System has been proposed before, but Hypatia's composition is stranger still.
The authors set about a slow process of comparing the stone's elemental abundance with the conditions in which elements are formed. The closest match is in material blown off during a Type Ia supernova, after a white dwarf star draws off so much material from a neighboring star that it explodes.
The authors propose the supernova occurred within a dust cloud left behind when the white dwarf formed from a collapsing red giant. They think gas thrown off by the supernova stuck to dust particles in the surrounding cloud and gradually condensed into a larger object. When this larger object hit the Earth's atmosphere the parts that did not burn up fragmented, and the Hypatia stone is the one piece we have found.
“If this hypothesis is correct, the Hypatia stone would be the first tangible evidence on Earth of a supernova type Ia explosion,” Kramers said in a statement. “In a sense we could say, we have ‘caught’ a supernova Ia explosion ‘in the act’, because the gas atoms from the explosion were caught in the surrounding dust cloud, which eventually formed Hypatia’s parent body.”
Evidence of Type Ia supernovas have been reported on Earth before, but these were in the form of trace elements scattered across the ocean floor.
We know many of the elements found on Earth and other planets are only formed in supernova explosions, neutron star collisions, and possibly Wolf-Rayet stars. However, it has been thought that all the elements from these sources seeded into the Solar System were well mixed through the original gas cloud. Only later, it has been believed, did atoms have a chance to stick together with others of their kind. If the Hypatia stone is what the authors think, that idea could only be sustained in the highly unlikely event the stone was part of an interstellar visitor like 'Oumuamua.
However, the evidence the paper presents is not yet watertight. The explanation offered fits the abundance of iron in Hypatia very well, something that alternative explanations, such as red giant neighborhoods and Type II supernova don't. It's also a good match for the abundance of seven other elements and the paucity of silicon.
On the other hand, the Hypatia stone includes 10-100 times more of six elements measured so far than a Type Ia supernova would produce. “Since a white dwarf star is formed from a dying red giant, Hypatia could have inherited these element proportions for the six elements from a red giant star,” Kramers said. Even the authors acknowledge this explanation is speculative, particularly when tweaked to explain the missing silicon.