The asteroid that hit the Earth in the Gulf of Mexico region 66 million years ago left behind a large layer called the K-Pg boundary. Scientists studied the content of ruthenium isotopes in it and compared them with other samples. This helped determine the composition and origin of the cosmic body.

Mass extinctions of flora and fauna have occurred several times on Earth. The last one is 66 million years old. More than half of the species died out, including non-avian dinosaurs. This event coincided with the end of the Cretaceous period and the beginning of the Paleogene period.

There are many indications that a 10-kilometer asteroid hitting the Earth was responsible for the change of geological eras and the mass extinction. Its trace is clearly visible on the surface – the Chicxulub crater in the Gulf of Mexico and on the Yucatan Peninsula. The impact scattered debris across the planet, visible in the stratigraphy of rocks. A thin layer of that time, enriched in platinum-group metals, which are quite rare on Earth, is found on almost all continents. Geologists call this the K-Pg boundary or the Cretaceous-Paleogene boundary.

According to an alternative hypothesis, the K-Pg boundary was formed under the influence of strong volcanic activity, the most striking example of which is the Deccan Traps in India. However, the proportion of platinum group elements suggests a meteoric rather than volcanic origin. Extraterrestrial material at the K-Pg boundary is also indicated by the findings of chromium isotopes and meteorite fragments in samples from the Pacific Ocean.

An international group of scientists led by Mario Fischer-Gedde from the University of Cologne (Germany) decided to find out what kind of asteroid it could be. To do this, they analyzed ruthenium isotopes in samples of the K-Pg boundary. Their content varies in different types of meteorites and is different from that found on Earth. An extraterrestrial source can thus be identified. The results are published here: Science.

The most interesting thing is that the content of ruthenium isotopes in meteorites is related to the fact that the parent asteroid was formed far from the Sun. According to their composition, meteorites are divided into two large classes: carbonaceous chondrites and non-carbonaceous (including iron). Among the latter, the most common on Earth are rocky meteorites. Their parent bodies are S-class asteroids, which formed in the inner Solar System together with rocky planets. The composition of ruthenium isotopes in them does not differ much from that of the Earth. Angular chondrites stand out more strongly, but there is a large diversity within this class. Their source is considered to be C-class asteroids, which were born at a greater distance from the Sun, beyond the orbit of Jupiter.

For comparison, the authors studied the isotopic composition of samples taken from impact (i.e. impact-origin) rocks of Phanerozoic age (not older than 540 million years), Archean (3.5-3.2 billion years), as well as fragments of bicarbonate chondrites. It turned out that the composition of ruthenium isotopes at the K-Pg boundary is similar to those found in uniform and curly chondrites. This means that their source is a C-class rocky asteroid born in the outer Solar System and collided with the Earth at the end of the Cretaceous period. This result refuted the hypothesis of cometary origin of the Chickshulub crater.

The isotopic composition of the other five Phanerozoic samples showed a greater similarity to S-class asteroids originating from the outer Solar System orbiting Jupiter, but the archaeal samples contained extraterrestrial material from C-class asteroids that bombarded the planet during the accretion phase.