It was more than 20 years ago. In December 1996, specialists discovered a small yet very special pebble in the east of the Sahara desert. Since then, analysis has been done, revealing that the object is a lot more modest than its appearance would suggest and it has brought into question theories on how the solar system was created.
Named Hypatia in reference to Hypatia of Alexandria, the first female mathematician and astronomer in the Western World, this interstellar rock has certain properties that have never been seen on Earth before, or in our surroundings.
A geological “fruit cake”
In 2013, a team in Johannesburg, South Africa suggested that the rock came from a nucleus of a comet that crashed into Earth. This hypothesis would be concurrent with the fact that Hypatia was found in a part of the desert that was littered with lybic glass. These fragments of glass of a pale yellowy colour that were scattered around for more than 6,000 square kilometres in the Sahara could have possibly been formed when the comet hit the ground, which could have resulted in the sand surrounding it to melt and fuse together.
Then in 2015, the same team studied the noble gases and nitrogen found in Hypatia, concluding that it is extraterrestrial. This information was published in an article in the Geochimica et Cosmochimica Acta journal which is today analysing what it is made of in more detail, revealing the presence of unexpected elements. To better understand these incongruities, the professor Jan Kramers, co-author of the study, has a cake related metaphor to help us.
“The majority of Hypatia can be compared to the mixture of a badly mixed fruit cake that we would refer to in geology as a two mixture compound. The candied fruit would represent the mineral grains that we’ve found embedded in Hypatia,” he explains. “Finally, a layer of icing sugar would come in to play the role of the secondary materials from outer space that have been discovered in Hypatia’s fractures.”
An unusual compound
The fragment, which was barely a few millimetres in size, originally came from an object that was much bigger, probably many meters wide. However, for the team, there is no doubt that Hypatia is not a classic meteorite. “If we could grind up the Earth in a giant mortar, we would get a powder whose composition would be very similar to that of chondritic meteorites,” Kramers continues.
“In chondrites, we usually find a small amount of carbon and a good dose of silicates. But the mold of Hypatia contains a huge amount of carbon and a surprisingly low proportion of silicate,” he adds. Furthermore, it contains compounds of a specific carbon, known as polycyclic aromatic hydrocarbons or PAHs.
These PAHs are a major element in cosmic dust that came before our solar system. We can find this dust among other things in comets and meteorites which haven’t been heated up for a long part of their existence. The majority of these carbon compounds in Hypatia have, for their part, been changed into microscopic diamonds, probably after entering our atmosphere or on impact, preserving the elements until they were discovered.
During analysis of the grains found embedded in Hypatia (the “dried fruits” of the cake), many surprising results have been obtained. “We have found aluminium in its purest form. […] It is sometimes possible to find gold nuggets, but normally this never happens for aluminium. It is an extremely rare case on Earth and in the rest of the solar system as far as we know,” states George Belyanin, co-author.
“We have also found a part that contains silver; iodine and phosphor, as well as grains of moissanite (silicon carbon), which was also completely unexpected. […] There are also grains consisting of a mix of nickel and phosphor and very little iron: a mineral compound never before found on Earth or in meteorites.” There are so many elements that just raise new questions about the origins of the solar system.
Unique elements in our solar system
Together, the PAHs as well as the phosphor, metallic aluminium and the moissanite suggest that Hypatia is formed by pre-solar and unchanged materials that have bonded together, meaning that these materials formed before our solar system appeared. The bonding of nickel, phosphor and iron is particularly interesting because it belongs to a category of heavier elements that form terrestrial planets.
The ratios of these three elements are however completely different to those that have been calculated on Earth or measured in some meteorites. Researchers think that the nickel-phosphor-iron grains are pre-solar because they are found at the heart of the compound and it would have been difficult for the impact to affect this, but above all because this bonding of elements doesn’t resemble anything that is currently known about.
Hypatia’s compound reinforces this hypothesis. Generally, science admits that the solar system was created from a relatively smooth cloud of cosmic dust (the solar nebula). The lack of silicates in the compound – however very present in our system - suggests that if Hypatia was formed after the solar system began the circumstances of the latter have to be revised.
“We know that Hypatia was formed in a cold environment, probably below the temperature of liquid nitrogen on Earth (-196°C). In our solar system, this would suggest an area that is located further away than the asteroid belt between Mars and Jupiter, where the majority of meteorites come from,” explains Kramers.
“The comets mainly come from the Kuiper belt, located beyond Neptune’s orbit […]; some come from the Oort cloud, others from further still. We still don’t know a lot about the chemical compounds of objects from space from these regions. As a consequence, we have to study the case of Hypatia further,” he concludes.