If the surface of carbonaceous asteroids such as Bennu and Ryugu – destinations respectively of the Osiris-Rex and Hayabusa 2 space missions – is not covered by large quantities of material known as fine regolith, similar to the sand of our beaches, the cause is to be found in the high porosity of their rocks. These are the results obtained by an international team, which sees an important participation of researchers from the National Institute of Astrophysics
When the NASA mission Osiris-Rex reached the asteroid Bennu in 2018, scientists expected to find a surface covered with fine regolith : material formed by small grains or pebbles at most a few centimeters large, similar to the sand found on beaches of our planet. There were also similar expectations for the Hayabusa 2 mission of the Japanese space agency Jaxa, which reached the asteroid Ryugu in the same year . Once in the vicinity of these two small bodies, however, both probes revealed rocky terrain, surprisingly covered with boulders and with very little fine regolith.
A new study, led by the Italian researcher Saverio Cambioni , with the participation of Giovanni Poggiali and John R. Brucato of the National Institute of Astrophysics, has finally solved the mystery: it would be the very porous nature of the rocks of these asteroids to explain the absence of fine regolith on their surface.
“When the first images from Bennu arrived, we noticed some regions where the resolution wasn’t enough to distinguish between small rocks and fine regolith, so we started using our new AI method to recognize fine regolith from rocks. using the infrared emission recorded by the probe “, explains Saverio Cambioni, first author of the article published today in Nature , who conducted this research during his doctorate at the University of Arizona, in the United States, and is now a postdoctoral researcher -doc at the Massachusetts Institute of Technology .
Osiris-Rex collected high-resolution images – up to 3 millimeters per pixel – across Bennu’s surface to map it out and study it in detail: characterizing the asteroid is one of the mission’s main objectives. The absence of fine regolith on the surface posed not only a scientific question, but also a technical challenge: the probe was designed to collect a sample of the asteroid from a surface rich in fine regolith, not among boulders and rocks. Thus, the sampling area on Bennu had to be drastically reduced from a region of about 160 meters, the size of a parking lot for 100 cars, down to 8 meters, comparable to a parking lot for only 5 cars. The operation was successful on 20 October.
“To distinguish the contribution of fine regolith from that of rock and to make a global study of the surface, it was necessary to put together a large database of regions observed both in the day and in the night,” says Giovanni Poggiali, co-author of the study and researcher. INAF in Florence.
The thermal emission released by the fine regolith, which depends on the size of its grains, is different from the emission produced by larger rocks, which instead depends on their porosity. For this the team simulated the emission produced by different mixtures of fine regolith and rocks of varying porosity, teaching an algorithm how to recognize the different soils and compare them with observations of the Bennu surface.
The analysis of the data showed that the fine regolith is not randomly distributed on Bennu but that, where the rocks are more porous – that is, on most of the asteroid – it is systematically found less. This suggests that the highly porous rocks of the celestial body produce less fine regolith because they are compacted – and not fragmented, as is the case with less porous rocks – as a result of meteoric impacts: the cavities inside the rocks would help to cushion the impact of the meteorites. resulting in less fragment production. Furthermore, the porous rocks would break down more slowly due to the diurnal cycle of heating and cooling of the asteroid, further inhibiting the formation of the fine regolith.
“With artificial intelligence, used for the first time in this type of research, we were able to go much deeper into the analysis of spectroscopic data by highlighting unique properties of the materials that make up primitive asteroids such as Bennu and Ryugu,” comments John Brucato, co-author of the study and INAF researcher in Florence.
The results of Bennu’s analysis are in agreement with findings from independent laboratory experiments. Also for the asteroid Ryugu, Cambioni and colleagues propose that the porosity of its rocks would explain the lack of fine regolith observed by Hayabusa 2. Similarly, the abundance of this material recorded in 2005 by the previous Jaxa mission, Hayabusa, on a another asteroid of a different type, Itokawa, would be due to a lower porosity of its rocks, determined by the researchers using observations from Earth. According to the team, fine regolith would be rare on carbonaceous asteroids such as Bennu and Ryugu, which are the most common type of asteroid and are thought to be formed from very porous rocks. The fine regolith would instead be abundant on S-type asteroids such as Itokawa,
“Asteroids are fossils of the formation of the Solar System, but recent studies – including ours – are showing how in reality some asteroids are very evolved,” Cambioni concludes. «Understanding the evolution processes of asteroids is important for understanding the evolution of the Solar System and our planet. To shed some light on this, we will need to visit more asteroids in the future to collect samples to be reported and analyzed on Earth. Our study will allow us to understand in advance the nature of the asteroid surfaces, and therefore to plan the missions accordingly ».
Featured image: The asteroid Bennu in a mosaic of images taken by the Nasa Osiris-Rex probe. Credits: NASA / Goddard / University of Arizona
To know more:
- Read in Nature the article ” Fine-regolith production on asteroids controlled by rock porosity “, by S. Cambioni, M. Delbo, G. Poggiali, C. Avdellidou, AJ Ryan, JDP Deshapriya, E. Asphaug, R.-L . Ballouz, MA Barucci, CA Bennett, WF Bottke, JR Brucato, KN Burke, E. Cloutis, DN DellaGiustina, JP Emery, B. Rozitis, KJ Walsh and DS Lauretta
Provided by INAF