Discovered in 1801 by theastronomer Giuseppe Piazzi, Ceres is a celestial body almost 1,000 kilometers in diameter located in the main belt, between the orbits of March and Jupiter. Overflown by the probe dawn as of 2015, it is now classified as dwarf planet : although massive enough to have a spherical shape, it shares its orbit with a multitude of other smaller bodies — mainly asteroids, some reaching several hundred kilometers in diameter. But its location in our Solar system intrigues scientists: much more massive than its neighbors — Ceres alone represents almost a third of the mass total of the main belt –, it also has spectral characteristics that are very different from the objects with which it shares its orbit, leading astronomers to consider it as an intruder in the asteroid belt. According to them, it would have formed much further from the Sun than it is today…
A planetary body very different from its neighbors
According to scientists, the physical characteristics of Ceres differ far too much from those of its neighbors in the main belt for them to have a common origin: representing almost a third of the total mass of the asteroid belt, it is the only body massive enough to be in hydrostatic equilibrium — it therefore has a spheroid shape. Scientists even suspect the presence of a internal oceanpotentially preserved partly in the state liquid. But that’s not all: its composition seems to differ from that of the other corps of the main belt, also allowing it to be the only body to possess one. atmospherealthough very thin, formed by the sublimation water ice andammonia on its surface — the weak gravity of Ceres does not, however, allow it to retain its atmosphere in a sustainable manner.
This is a crucial element here: asteroids typically do not emit vapor and do not have an atmosphere, and the presence of volatile elements, such as ammonia, is generally typical of comets formed in the outer and colder regions of the Solar System, where they could have accreted these volatile elements; it is moreover the sublimation of these elements which form the cometary tail, when it approaches the Sun.
Based on these observations, scientists suspect that Ceres formed in cooler regions of the Solar System, where temperatures would allow the condensation such volatile elements, beyond the limit of freeze — a border delimiting the outer and cold regions of our Solar System, from the inner and warmer regions. Currently located around the orbit of Jupiter, its distance from the Sun has changed over time during the evolution of the Solar System — in its first phases of existence, the Sun emitting less heatthe frost limit was probably closer to it than it is today.
So how did Ceres end up there, surrounded by bodies that are so foreign to her? A team of scientists proposes in their study a new model to explain the migratory history of the dwarf planet.
A new model of planetary migration
To understand the evolution of Ceres in the Solar System, it is necessary to go back to the first phases of its history, when the planets we know today were still in the process of formation in the protoplanetary disc, more than 4.5 billion years ago. From this initial state, the team of scientists simulated the formation of Jupiter and Saturnof them gas giants massive enough to cause gravitational disturbances to surrounding bodies, as well asembryos planetary — considered here as the planets Uranus and Neptune becoming –, to then include the formation of a group of objects of compositions and sizes similar to those of Ceres, starting from the idea that the dwarf planet would constitute one of the first planetesimals (celestial objects in formation in a accretion diskin the process of becoming a planetary body) of the Solar System.
Their simulations show an extremely chaotic history: during the formation of the two gas giants, Jupiter and Saturn, many bodies formed in the outer regions of the Solar System would have been ejected from them, while others would have migrated to the inner regions, because of the gravitational disturbances to which they would have been subjected. Objects similar to Ceres, formed in the outer regions of the Solar System, then scattered, could thus very well have stabilized in the orbit of the asteroid belt.
Based on their simulations, the team of astronomers proposes a four-step model to explain the migration of Ceres from its formation zone, in the outer and cold regions of the Solar System beyond the orbit of Saturn, to the asteroid belt where it now evolves. After a phase of rapid chaotic mixing in the position of the planetesimals in the protoplanetary disc external, Ceres would have been captured in a resonance gravity with the giants Jupiter and Saturn. Then would come a third eventful phase during which interactions with other bodies (qualified as “invaders”) could have allowed Ceres to migrate towards the inner regions of our Solar System, to then stabilize there during the shutdown. of these gravitational disturbances.
Another interesting finding: according to their simulations, Ceres would be only one body among more than 3,600 formed in this zone of the Solar System. This result, in line with previous studies of objects beyond Saturn and in the Kuiper Belt, confirms the scientists in the validity of their model of the formation of the Solar System. Thus, Ceres would indeed be an intruder in the asteroid belt!
The dwarf planet Ceres would have been born beyond Neptune
A chance discovery reinforces the scientific appeal of Ceres. The presence of dust exogenous on the surface of the dwarf planet, otherwise the largest object in the main belt, indicates that it formed much further away from its current location. As Pierre Vernazza, the researcher behind this discovery, explains to us, Ceres is one of the asteroids on which it is appropriate to send a mission to its surface.
Article of Remy Decourtpublished on January 19, 2017
For nearly two years, the Dawn probe introduces us to the dwarf planet Ceres, an object linking small bodies, of which it is not a part, and larger ones like the satellites of Jupiter. The history of its formation is beginning to be better understood, starting with its place of birth, which is quite different from its present position. “It most likely formed in the outer Solar System, beyond the orbit of Jupiter.explained to us in July 2016 Pierre Vernazza, researcher at the CNRS and researcher at the Marseille Astrophysics Laboratory. The migration of giant planets would have ejected her inside, where she is today”.
This same scientist very likely provided a definitive answer to this question in the study published in The Astronomical Journal. At the head of an international team, composed mainly of French researchers from AML (CNRS, Aix-Marseille University), Pierre Vernazza revealed the presence of a “anhydrous component on the surface of Ceres [des particules fines de pyroxène, NDLR] ». A completely fortuitous discovery that we do not owe to the probe dawn but at theSOFIA observatory and his telescope infrared of 2.5 m installed in an aircraft.
It is the absence of spectrum infrared of Ceres which motivated Pierre Vernazza to use this telescope. Of course, he did not expect to discover exogenous dust. Further investigating its origin, the researcher and his team have determined that this contamination comes from “probably from a cloud of dust located in the outer main belt and which formed as a result of a recent collision, less than 10 million years ago ».
If the pyroxene observed on the surface of Ceres is exogenous in nature, then “nothing connects Ceres to other asteroids of its spectral class (so-called type C)”. The fact that the silicates hydrates observed on its surface by Dawn are rich in ammonia, this “opens up the possibility of a trans-Neptunian origin: Ceres and Orcus could be ‘twins'”. Thus Ceres, as well as the P and D type asteroids which are primordial residues of the formation of planets and for which a trans-Neptunian origin is evoked, could have been “formed at the edge of the solar system [au-delà de 10 UA, NDLR] and would have landed in the main belt as a result of the migration of the giant planets”. Added to this is that Ceres may not be the only asteroid in this situation. Others likely migrated from these outer regions, “including Iga, the fourth asteroid, in terms of mass, of the Main Belt”.
A major target for the future
If this discovery is confirmed, which is hardly in doubt, it makes Ceres a “major target for the future and increases the interest of a lander that would land on it”. With Ceres, astronomers have, a stone’s throw from Earth, “an object that can give information about the primordial external disk”. Certainly it does not allow “go directly back to the global primordial disk because the elementary bricks have been altered by liquid water” but certainly, significant data on the history of the formation of the planets are to be recovered.
As for future missions to Cérès, a few projects exist, including that of the JPLwho wants to land a rover. There is also the Nautilus by Pierre Vernazza. His idea is to “to land a Philae-type lander with a payload of around twenty kilograms and to use a minimalist orbiter with only two instruments and a payload for communications with Earth”. In view of the latest advances in knowledge of Ceres, Nautilus should carry instruments to “measure the isotopic composition of the elements, determine the nature of the silicates, as well as the presence or absence of organic matter”.
In the short term, the chances of one of the two projects seeing the light of day are very low. The selection by Nasa from Psyche and Lucy missions bound for two asteroids makes it unlikely to select another mission, again bound for one of these objects on the next selection. On the side of the European Space Agency, theESA, the next selection of a mission is expected in November 2018 with a budget that may not be sufficient for Nautilus. That said, “I remain optimistic”. With a horizon of ten years, it is inconceivable that we will not return there “with a more ambitious mission than Dawn and that we land on it”.
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