The bodies of the asteroid belt of our solar system have violence in the past. Some, like the planet Dwarf Ceres, have seen their interiors restructure under the force of gravity, while their surfaces are destroyed by collisions. The smaller asteroids experienced collisions that completely destroyed them, leaving the weak gravity of their debris to pull the pieces together in a pile of debris. Now, new images of the fourth largest body in the asteroid belt, Hygiea, suggest there is a story somewhere between the two. After suffering a shocking collision in the distant past, the remains of the body had enough gravitational pull to turn it into a dwarf planet.
History vs. Observations
We know that Hygia has experienced a collision in the past because we can see his remains. An entire family of small asteroids have been discovered in orbits that are thought to have originated from an asteroid about two billion years ago.
In other cases where we have seen families of asteroids such as this, the body on which they have arisen is either littered with craters and / or has undergone at least one extremely large collision. So planetary scientists probably had similar expectations when pointing the very large telescope at the European Southern Observatory of Highea in 201
Instead, the body they depicted is remarkably smooth and almost spherical – its average radius is 217 km and probably ranges from only 225 km to 212 km. It also spun at about twice the speed we thought it was.
None of this is exactly what was expected: "None of our images and related contours revealed the large impact pool expected by the large size of the Hygiene [asteroid] family. "
To understand how the asteroid family could be formed, the researchers created a model that allowed them to test different collisions by testing different impact sizes and impact angles. After using these collision models to generate debris , researchers let gravity absorb and understand what transformed Hygiene and other debris would look like (which will continue to form the asteroid family).
Instead of Forming a large crater, collisions that worked instead, completely shattered the asteroid, which can be done either in a frontal collision with a 75 km object or in a 150 km center collision, but collisions outside the center tend not to leave large debris in debris (other than Hygiene itself), as some members of the resulting asteroid family are larger, researchers conclude that a head-on collision is likely.
In addition to the release of a debris field, the collision turns the remnants of Hygia into liquid, i.e. who experienced massive oscillations about four hours after the event. This allowed the residues to blend and balance evenly, creating the actual spherical shape of the body. This was probably possible due to the relatively low density of the material; Researchers estimate that Hygiene is less than twice as dense as water.
This is similar to the density of Ceres, the largest body in the asteroid belt. The authors suggest that the two formed further away from the "snow line" of water, the point at which the material was far enough from the Sun to remain frozen water.
The authors also claim that the collision may have accidentally turned Hygia into a dwarf planet. As they note, everything in the asteroid belt orbits the sun and does not orbit, fulfilling two of the four requirements for the state of the dwarf planet. These bodies must have also cleared their orbit; this is somewhat dependent on the orbits of other material from the same asteroid family – probably if they were close enough they would already be included back into Hygiene.
This leaves one last obstacle: Is Hygia mass enough to form a spherical shape under the force of its own gravity? It is not clear what Hygia looked like before the collision, as some other large asteroids like Eros have rather irregular shapes. But after its destruction, Higia's gravity was clearly enough to shape it into a sphere. As a result, researchers propose that it be designated the dwarf planet, the smallest so far.
Nature Astronomy, 2019. DOI: 10.1038 / s41550-019-0915-8 (About DOIs).