Vesta is a primitive body, a protoplanet, orbiting in the main asteroid belt between Mars and Jupiter. It is the second largest body, with an average radius of around 270 km. A recent study published in the journal Nature, one of whose authors is a member of the LTE, reveals an unexpected internal structure that opens up new scenarios for the formation of this body.
From 16 July 2011 to 5 September 2012, NASA’s Dawn space mission orbited this asteroid to carry out geodesy measurements using the radioscience experiment and the onboard camera.
The radioscience experiment involves measuring the Doppler shift (variation in frequency) and the time taken by the radio telecommunications signal between a probe orbiting another body and an Earth antenna. From these measurements, it is possible to extract the speed of the probe along the line of sight and its distance from the Earth. This information is then used to determine the probe’s trajectory around the body. The perturbed trajectory of the probe depends on Vesta’s gravitational field, which is deduced in this way.
The images from the camera not only show a cratered surface, but also enable the overall shape of the asteroid to be reconstructed. This corresponds to a flattened ellipsoid in the shape of an orange. By combining the topography, gravitational field and rotational motion data, the authors of the article were able to study Vesta’s properties in order to learn more about its interior.
The measurement of the rotational motion, extracted from successive images of the surface, has proved particularly valuable, as it has made it possible, in combination with the gravitational field data, to determine Vesta’s moment of inertia.
What is a star’s moment of inertia and why is it important to know it ?
The moment of inertia is a physical quantity that represents a body’s resistance to torques of force and depends on the distribution of mass within a body. This quantity therefore influences rotational movement. For example, when an ice skater turns on himself with his arms stretched outwards and brings his arms alongside his body, he reduces his moment of inertia and at the same time increases his speed of rotation (by conservation of angular momentum). By analogy, by measuring the speed of rotation we can determine Vesta’s moment of inertia and thus understand the distribution of mass within it. If its inertia is low, there will be a greater concentration of mass towards its centre. If it is high, the mass will be more evenly distributed throughout the body.
Based on the mass distribution inside the body and using data from the gravitational field and topography, the authors of the study were able to deduce a realistic density profile. They showed that the interior of Vesta would be less differentiated than expected. Vesta’s core would be about half the size predicted in previous studies and its mantle is therefore thicker and denser.
Two hypotheses about the formation of Vesta are put forward to explain these unexpected results.
The first hypothesis puts forward the idea that Vesta formed later, leaving no time for the interior of Vesta to differentiate completely.
The second hypothesis proposes that Vesta is the remnant of a larger body, the result of a catastrophic collision. This would be an interesting hypothesis, as it would suggest that larger protoplanets may have existed and that the collisional effect may have been significant during primordial formation.
Vesta is not a dwarf planet, because according to the definitions of the International Astronomical Union (IAU), the shape of a dwarf planet corresponds to hydrostatic equilibrium, but this is not the case with Vesta, whose northern and southern hemispheres are not symmetrical. The southern hemisphere is cratered by two major collisions. Vesta would therefore be the remnant of a dwarf planet whose formation process has been interrupted.
The method used can be applied to all asteroids and small bodies, so there’s no doubt we’ll have plenty more surprises in store !
Read NASA press release.
Read article : « A small core in Vesta inferred from Dawn’s observations »
Listen to the programme broadcast on France Culture on 12 May « Comment l’astéroïde Vesta a failli être une planète ? ».
Image credits : NASA/JPL/MPS/DLR/IDA/B.Jónsson
