Cometary dust in Antarctica?

Discovered by researchers from the Center for Nuclear Spectrometry and Mass Spectrometry (CSNSM) (CNRS/Université Paris-Sud 11), attached to IN2P3 (Institut national de physique nucléaire et de physique des particules), the micrometeorites, which are remarkably well preserved, are made up of organic matter containing small assemblages of minerals from the coldest and most remote regions of the Solar System. This work is published in the journal Science.

The French-Italian scientific base Concordia is located at Dome C in the central region of the Antarctic continent. This is one of the most remote places in the world, where the amount of dust of terrestrial origin is extremely small.

Thanks to logistic support from the French Paul-Émile Victor Polar Institute (IPEV) and its Italian counterpart PNRA (Programma Nazionale Ricerche in Antartidea), CSNSM (Centre de spectrométrie nucléaire et spectrométrie de masse) researchers have discovered a new family of extraterrestrial particles in layers of ultra-clean snow located at a depth of four meters in the vicinity of the base. The CSNSM team identified ultracarbonaceous micrometeorites (particles less than 1 mm in size containing 50-80% carbonaceous material). The approximately 0.1 mm-sized particles have no equivalent in collections of extraterrestrial material available in laboratories until now, and have given rise to a series of additional analyses as part of a collaboration between CSNSM, the French Natural History Museum, the University of Lille 1, and the Ecole Normale Supérieure in Paris.

Analyses using transmission electron microscopy have shown that the micrometeorites are made up of only very slightly altered organic matter containing small assemblages of minerals. Analysis with an ion microprobe has revealed that its hydrogen isotopic composition shows exceptionally high deuterium/hydrogen (D/H) ratios (around 10 times the value found in terrestrial oceans). Taken all together, the results indicate that the particles very likely come from the most distant bodies in the Solar System, comets.

Comets are made up of a mixture of icy materials and dust. Occasionally, some of them enter the inner Solar System. When they pass near the Sun, the rise in temperature causes massive sublimation of the icy materials, leading to an ejection of a mixture of gases and cometary grains into interplanetary space. Some dust grains may cross Earth's orbit as they drift towards the Sun, where most of them finish their journey. It is probably some of these cometary grains that the CSNS scientists discovered in Antarctica.

Until now, only the US Stardust space mission had enabled international teams to carry out mineralogical and geochemical analyses of cometary grains (emitted by comet 81P/Wild2 as it passed near the Sun). The micrometeorites discovered at Concordia show numerous similarities to the samples from the Stardust mission. For the first time, they allow scientists to study extremely well preserved assemblages of minerals and organic material that were present beyond Jupiter's current orbit at the time when the Sun and the planets were being formed. Their chemical and isotopic composition should make it possible to comprehend the physical and chemical processes at work inside the disk of gas and dust that surrounded the early Sun 4.5 billion years ago.