During an expedition in 1996 in the Libyan desert which extends over 700,000 km2 between western Egypt, eastern Libya and northwestern Sudan, an Egyptian geologist, Aly Barakat, discovered a black and shiny stone, regarding 3 centimeters by 3 and weighing regarding 30 grams. As he quickly realizes with the help of X-rays, the stone contains tiny diamonds. At the time, for lack of equipment to deepen the study, it stops. About ten years ago, a geochemist from the University of Johannesburg (South Africa), Jan Kramers, managed to recover a one-gram sample of the stone. With his colleagues from different specialties (cosmochemists, petrologists, etc.) and from different backgrounds (France, Switzerland, Germany, Italy, Hungary, Poland), he embarked on a new series of analyses.
The successive works on the stone, now called “Hypatia” in honor of the mathematician, astronomer and ancient philosopher Hypatia of Alexandria (between 350 and 370 – 415), thus show that it was not formed on Earth. – (Kramers & al. 2013), that its composition also does not match that of hitherto known meteorites or comets (Avice & al. 2015), and that it contains mineral species older than the formation of our Solar System (appeared regarding 4.5 billion years ago) (Belyanin & al. 2018). In a new study to be published in August in the journal Icarus (already available on line), Jan Kramers and his colleagues now put forward the idea that Hypatia would in fact be formed from interstellar dust but whose composition bears the traces of a rare and powerful phenomenon: the explosion of a white dwarf in the supernova of type Ia.
For this new work, they used proton-induced X-ray emission spectroscopy (PIXE), an extremely sensitive method of analysis, which involves using a beam of protons to pierce the stone and measure, at the using a microscope, the nature and quantity of the chemical elements contained in the sample. Fifteen elements were identified and their concentrations analyzed. According to the researchers, the chemical signature of this meteorite is unusual. So much so that they had to eliminate several stone formation scenarios before finding the one that might correspond to this signature. Is the parent body from a red giant (a star at the end of its life, very large, luminous and at low temperature, no longer having enough hydrogen in the core to fuel nuclear fusion)? No, because the meteorite contains too much iron and too little silicon for that. Could it then be a type II supernova (a phenomenon that occurs when a massive star runs out of fuel, collapses and explodes)? Again, Jan Kramers and his colleagues had to rule out this possibility, because the proportion of iron, as well as the presence of nickel phosphide in the stone, do not correspond to this type of supernova.
The team then looked at a type of explosion that would explain this strange composition: type IA supernovae. These supernovae occur in a binary system, that is to say containing two types of interacting stars including at least one white dwarf. In this case, Jan Kramers and his team imagine that the other star must be a red giant, because this type of explosion is the source of a large part of the presence of iron in the universe, which corresponds to the unusual chemical composition of Hypatia Stone.
During the transfer of matter from the red giant to the white dwarf, the mass of the latter increases, and when it reaches a certain critical mass, it explodes. After the explosion, a kind of cloud of atoms and subatomic particles is formeddescribes Georgy Belyanin, of the University of Johannesburg and co-author of the study. Then, following a very long cooling process, these atoms stick to the particles of interstellar dust and form larger particles. They are still not minerals, but primitive matter,” who would have eventually given rise to the parent body of Hypatia. “Somewhere nearby, the formation of the nebula at the origin of our solar system has begun. Hypatia’s parent body would have been caught in this nebula, and would have continued to cool and grow, incorporating more and more material, without this affecting the original signature of the supernova explosion in the body. parent of stone, once more explains Georgy Belyanin. It would have finally landed on Earth some 28 million years ago. How ? This remains a real mystery. »
If this hypothesis is the most plausible, the correspondence is far from perfect. For 6 of the 15 elements studied, the proportions are in fact between 10 and 100 times higher than the predictions of the models. Which makes Guillaume Avice, cosmochemist and CNRS researcher at the Institut de physique du globe de Paris, who worked on Hypatia a few years ago with Jan Kramers say: “The problem with Hypatia is that there might be observational bias. By dint of looking at an object more and more closely, on a very small scale and doing a lot of measurements on it, much more than on other objects, we may end up finding something special that we cannot not to fully explain.“. The mystery around the origin of Hypatia will continue to fuel scientific research for (at least) a few years…
by Odyssey Piettre
Image : Hypatia, Credit: Georgy Belyanin
