A study reveals what the seeds of the solid bodies of the Solar System looked like: porous dust particles a few millimetres long

We know that all solid bodies in the universe, whether planets, comets or asteroids, are formed from small to large: tiny particles of dust add up to larger and larger objects. But how were the seeds with which the process began, and that billions of years later gave rise to the Moon, Venus, or the ground we step on? The answer can be found in comets, which constitute the least processed objects in the Solar System and, therefore, are witnesses of what the early solar nebula was like. Now a study shows that its seeds were very porous particles and the size of millimeters, a different scenario than the one contemplated until now.

“Dust particles are present in scenarios as diverse as the interstellar medium, planetary atmospheres, comet tails or discs around young stars -says Olga Muñoz, researcher at the Institute of Astrophysics of Andalusia (IAA-) CSIC) that coordinates the study-. Knowing the properties of these particles is essential not only to evaluate their effects, such as the increase or decrease of temperatures in the case of the Earth's atmosphere, but also to obtain information about the structure and evolution of the objects where they are found. And, even, dust in certain environments can reveal the history of the formation of rocky bodies”.

And, in the Solar System, the objects that constitute the key to unraveling that history are comets. Cometary nuclei are described as icy dust balls and, since their formation in the origins of the Solar System, they have remained far from the sun's radiation and at very low temperatures, so that the material that composes them has barely changed. In fact, this pristine character of comets was confirmed thanks to the Rosetta (ESA) mission, which accompanied comet 67P in its orbit around the Sun and was able to study it in situ.

 

"The data from 67P touched on a problem already known about the properties of dust particles in the universe. We had, on the one hand, the data from the 67P observations from the ground that pointed to micron-sized particles (one thousandth of a millimetre), and which coincided with those from the Giotto mission on Halley's comet. And, on the other hand, we had data from the instruments that analysed the 67P dust in situ and which indicated that the dominant particles measured, approximately, from a tenth of a millimetre to several millimetres, a conclusion that coincides in turn with the data from the dust observed in the planet-forming discs around young stars", says Fernando Moreno, a researcher at the Institute of Astrophysics of Andalusia (IAA-CSIC) who is participating in the work.

To solve the problem, Muñoz and collaborators used the Cosmic Dust Laboratory (CODULAB) at the Institute de Astrophysics of Andalusia, which works with cosmic dust analogues and whose previous results already showed that the very common practice of assuming that dust grains are spherical can lead to great errors in the interpretation of observations.

Until now, experimental studies on cosmic dust worked with tiny particles (from less than one micron to about one hundred microns) and, in order to contrast measurements, CODULAB was modified in 2017 to study particles of up to several millimeters. The team tested with dust particles of different sizes and characteristics and found the ideal ones, those that managed to reproduce both the signal from the ground-based observations of comet 67P and those from the instruments on board Rosetta: large, porous, oblate particles with inclusions of a few microns.

"The results have been spectacular, because they solve the problem and provide a new panorama. If at the time we abandoned the idea that the dust grains were spherical, now we have a new model that points out that the seeds of the rocky bodies can measure several millimetres and have porous structures joined by small organic particles: something similar to small dirty cotton balls", concludes Olga Muñoz (IAA-CSIC).