Asteroid data from the Hayabusa2 mission support collisional evolution of a pristine body

The Japanese spacecraft Hayabusa began its >2-billion-mile journey (in Dec 2014) to rendezvous and land on a rocky body with a diameter of no more than a kilometer; it arrived on Asteroid Ryugu in June 2018, and various studies are now being published concerning geology, morphology, and composition of the asteroid [see attached article and editorial]. Although these GEITP pages see little relevance to gene-environment interactions 😉 this mission represents an amazing, mind-boggling engineering feat that will perhaps clarify our understanding of the formation of the solar system 4.5 billion years ago (the figure in the editorial is very informative).


Viewed on a larger cosmic scale Earth’s planetary system is of course not the only one in our galaxy. [Extra-solar planets orbiting other stars are detected regularly, these days.]  Recently, a planet barely emerging from its birthplace was imaged, sculpturing the remains of the flat disk of gas and dust in which it formed. Proto-planetary disks of gas and dust can develop rings and gaps, and early shaping of planetary systems often leaves behind belts of smaller bodies.


Earth’s solar system harbors two such belts. [a] At the outer perimeter is the Edgeworth-Kuiper belt. Situated beyond Neptune, it is

considered a reservoir of cold, icy, short-period comets (those orbiting the Sun in <200 years); the European Rosetta mission was the first spacecraft to rendezvous and land on a comet (Churyumov-Gerasimenko) from this belt (however, the collection of samples and returning to Earth for analysis was unsuccessful). Planet formation in these cold regions — beyond what is known as the snowline — strongly depends on existence of water-ice; condensation of water to ice makes it easier for giant planets (e.g. Jupiter) to form, because there are more solids available to initiate the growth. [b] The other belt in our Solar System — the asteroid belt — is a ring of small bodies situated just inside Jupiter’s orbit; inhabitants of this belt, between Mars and Jupiter, feel the gravitational presence of Jupiter (as we also do on Earth, which influences our climate by affecting Earth’s precession, eccentricity and axial tilt)  This asteroid belt region is too warm for water-ice to persist, and low ambient pressure makes water only stable in the gas phase.


However, as temperatures varied with the evolution of the proto-planetary disk early on, the outer edge of the asteroid belt could have reached up to, or slightly beyond, the snowline; if so, some water might have “made it into” the minerals of these asteroids. Therefore, the asteroid belt links different aspects and regions of planet formation, and as part of the inner Solar System might provide details about the formation of Earth 4.54 billion years ago.


Authors [see attached article] present Hayabusa observations of Asteroid Ryugus shape, mass, and geomorphology. Ryugu has an oblate spinning topshape, with a prominent circular equatorial ridge. Its bulk density indicates a high-porosity interior; large surface boulders suggest a “rubble-pile structure“. Surface slope analysis shows Ryugus shape may have been produced from having once “spun” at twice the current rate. Coupled with the observed global material homogeneity, this suggests that Ryugu was reshaped by centrifugally induced deformation during a period of rapid rotation. From these remote-sensing investigations, authors have chosen a suitable sample collection site on the equatorial ridge. Analysis of what has been collected will be published later.     🙂






Science   19 Apr 2o19; 364: 268-272 & editorial, pp 230-221

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