19 Μαΐου 2022 (Ειδήσεις Nanowerk) In an article published in the journal Ικάρος (“Dynamical origin of the Dwarf Planet Ceres”), researchers at São Paulo State University (UNESP) and collaborators report the findings of a study reconstituting the formation of the dwarf planet Ceres.
The research was conducted by Rafael Ribeiro de Sousa, a professor in the program of graduate studies in physics on the Guaratinguetá campus. The co-authors of the article are Ernesto Vieira Neto, who was Ribeiro de Sousa’s PhD thesis advisor, and researchers affiliated with Côte d’Azur University in France, Rice University in the United States, and the National Observatory in Rio de Janeiro.
Ceres is the largest object in the Asteroid Belt, a collection of celestial bodies located between the orbits of Mars and Jupiter. It is roughly spherical and comprises a third of the Asteroid Belt’s total mass, with a diameter of almost 1,000 km, less than a third of the Moon’s.
Its orbit around the Sun is almost perfectly circular, with 0.09 eccentricity, and an inclination of 9.73° to the invariable plane of the Solar System, much greater than Earth’s, which is 1.57°.
Ceres has too little mass to retain an atmosphere by gravitational attraction, but sunlight evaporates the ammonia and water ice below its surface, forming a mist that disperses into outer space. Ice deposits shine brightly at the bottom of its craters. The possibility of primitive life forms has not been ruled out. The craters were mapped by NASA’s 2007-18 Dawn Mission, which orbited Vesta, the second-largest body in the Asteroid Belt, as well as Ceres. A very interesting video of Occator crater made using data from the Dawn spacecraft can be watched on the mission website.
The dwarf planet’s core is probably made up of heavy matter – iron and silicates – but what differentiates it from nearby objects is its mantle of ammonia and water ice. Most bodies in the Asteroid Belt do not have ammonia, so the hypothesis is that Ceres was formed outside it, in the colder region beyond Jupiter’s orbit, and then thrust into the middle of the Asteroid Belt by the huge gravitational instability caused by the formation of gas giants Jupiter and Saturn.
“The presence of ammonia ice is strong observational evidence that Ceres may have been formed in the coldest region of the Solar System beyond the Frost Line, in temperatures low enough to cause condensation and fusion of water and such volatile substances as carbon monoxide [CO], carbon dioxide [CO2] and ammonia [NH3],” Ribeiro de Sousa said.
The Frost Line is now located very near Jupiter’s orbit, but when the Solar System was being formed 4.5 billion years ago, the position of this zone varied according to the evolution of the protoplanetary gas disk and the formation of the giant planets. “The intense gravitational disturbance produced by the growth of these planets may have changed the density, pressure and temperature of the protoplanetary disk, displacing the Frost Line. This disturbance in the protoplanetary gas disk may have led the expanding planets to migrate to orbits closer to the Sun as they acquired gas and solids,” Vieira Neto said.
“In our article, we propose a scenario to explain why Ceres is so different from neighboring asteroids. In this scenario, Ceres began forming in an orbit well beyond Saturn where ammonia was abundant. During the giant planet growth stage, it was pulled into the asteroid Belt as a migrant from the outer Solar System, and survived for 4.5 billion years until now,” Ribeiro de Sousa said.
To test the hypothesis, Ribeiro de Sousa and collaborators ran a large number of computer simulations of giant planet formation inside the protoplanetary gas disk that surrounded the Sun. In their model, the disk contained Jupiter, Saturn, embryonic planets (precursors of Uranus and Neptune), and a collection of objects similar to Ceres in size and chemical composition. The assumption was that Ceres was a planetesimal, one of a class of bodies thought to have been building blocks of planets, asteroids and comets.
“Our simulations showed that the giant planet formation stage was highly turbulent, with huge collisions between the precursors of Uranus and Neptune, ejection of planets out of the Solar System, and even invasion of the inner region by planets with masses greater than three times Earth’s mass. In addition, the strong gravitational disturbance scattered objects similar to Ceres everywhere. Some may well have reached the region of the Asteroid Belt and acquired stable orbits capable of surviving other events,” Ribeiro de Sousa said.
Three main mechanisms acted to keep these objects in the region, he added: the action of gas, which smoothed their orbital eccentricities and inclinations; mean motion resonances with Jupiter, protecting them against ejections and collisions caused by that giant planet; and close encounters with invader planets, scattering planetesimals to more stable inner regions of the Asteroid Belt.
“Our main finding was that in the past there were at least 3,600 Ceres-like objects beyond Saturn’s orbit. With this number of objects, our model showed that one of them could have been transported and captured in the Asteroid Belt, in an orbit very similar to Ceres’s current orbit,” he said.
Other research groups had already estimated this number of Ceres-like objects, based on observation of craters and on the sizes of other populations of celestial bodies beyond Saturn, such as those of the Kuiper Belt, where Pluto and other small planets orbit. “Our scenario enabled us to confirm the number and explain Ceres’s orbital and chemical properties.