Il Misterioso Caso Degli Asteroidi Mancanti

" La possibilit`a che la migrazione dei pianeti abbia perturbato gli asteroidi, potrebbe essere la causa di un pesante bombardamento nel Sistema Solare interno."Nel febbraio del 2009, `e stato pubblicato uno studio dell'Universit`a dell'Arizona molto interessante che vale la pena di rileggere nell'adattamento in italiano. (26 febbraio 2009)I planetologi David A. Minton e Renu Malhotra, analizzando la Fascia Principale, degli asteroidi compresa tra Marte e Giove, hanno calcolato che mancherebbero all'appello un numero assai elevato di oggetti.Sostengono che questa potrebbe essere la prova, che ai primordi della formazione planetaria del Sistema Solare, sia avvenuto un intenso bombardamento asteroidale verso l'interno.Il loro studio `e partito con l'osservazione della Fascia Principale, analizzando delle aree prive di asteroidi, chiamate "Kirkwood gaps".Questi vuoti appaioni in distinte regioni della cintura, dove la forte gravit`a di Giove e Saturno perturba fortemente ed espelle gli asteroidi.Quello che gli studiosi vogliono scoprire `e qualcosa in pi`u sui vuoti come la kirkwood gap.Minton e Malhotra hanno osservato la distribuzione degli asteroidi con diametro superiore a 50 km.Tutti gli asteroidi di questa taglia sono rimasti al loro posto nella cintura, da quando essa si `e formata 4 miliardi di anni fa.Hanno effettuato un intenso numero di simulazioni al computer che hanno rivelato la presenza di un numero maggiore di asteroidi rispetto a quelli presenti attualmente.La situazione, si `e verosimilmente avvicinata a quella attuale soltanto dopo aver simulato anche la migrazione dei pianeti nella loro attuale posizione.La loro interpretazione di questi risultati `e che la migrazione di Giove e Saturno, con la loro risonanza orbitale, hanno espulso molti asteroidi dalla loro posizione nella cintura.Questi corpi sarebbero stati deviati verso il Sole e moltissimi di essi hanno pesantemente bombardato i pianeti interni.Minton ha detto inoltre che il loro studio non pu`o considerarsi definitvo, ma `e sicuramente la prova certa che qualcosa ha destabilizzato la cintura degli asteroidi in un periodo relativamente breve di tempo.Tutti gli asteroidi, ha detto infine, sono stati lanciati come proiettili verso Marte, la Terra, la Luna, Venere e Mercurio. E' evidente che un pesante bombardamente deve essere avvenuto sui pianeti terrestri miliardi di anni fa. Marte, la Luna e Mercurio sono fortemente craterizzati. Lo sarebbero anche la Terra e Venere, se la loro atmosfera non avesse in parte protetto la superfice e quest'ultima non fosse stata fortemente rimodellata da una intensa tettonica.Di sicuro, il passato del nostro Sistema Solare, non ci `e del tutto chiaro ed `e assai probabile che siano avvenuti fenomeni del tutto anomali.ENGLISH:SCIENTISTS UNCOVER A CURIOUS CASE OF MISSING ASTEROIDS"The possibility that planet migration perturbed asteroids may have contributed to a heavy bombardment of the inner solar system."Provided by University of Arizona, TucsonThis artist's concept depicts a distant hypothetical solar system, similar to the one recently discovered with the Spitzer Space Telescope. In this artist's rendering, a narrow asteroid belt filled with rocks and dusty debris, orbits a star similar to our own Sun when it was approximately 30 million years old (about the time Earth formed). Within the belt a hypothetical planet also circles the star. NASA/JPL-Caltech/T. Pyle (SSC) [View Larger Image]February 26, 2009The main asteroid belt is a zone containing millions of rocky objects between the orbits of Mars and Jupiter. But University of Arizona scientists are now finding that there should be more asteroids in this area of the solar system than what researchers observe. The missing asteroids may be evidence of an event that took place about 4 billion years ago, when the solar system's giant planets migrated to their present locations.David A. Minton, University of Arizona planetary sciences graduate student, and Renu Malhotra, planetary sciences professor, say missing asteroids are an important piece of evidence to support an idea that the early solar system underwent a violent episode of giant planet migration. This migration could be responsible for a heavy asteroidal bombardment of the inner planets.Minton and Malhotra began by looking at the distribution of asteroids in the main asteroid belt. Astronomers first discovered a series of gaps in the asteroid belt, now called the Kirkwood gaps, back in the 1860s when only a handful of asteroids were known. The gaps occur at distinct regions of the asteroid belt where Jupiter and Saturn's gravity strongly perturbs and ejects asteroids. The present-day orbits of Jupiter and Saturn explain why these unstable regions are devoid of asteroids."What we wanted to know was how much of the structure of the asteroid belt could be explained simply by the gravitational effects of the giant planets, as are the Kirkwood gaps," Minton said.Minton and Malhotra looked at the distribution of all asteroids with diameters greater than 30 miles (50 kilometers). All asteroids of this size have been found, giving the UA researchers an observationally complete set for their study. Also, almost all asteroids this large have remained intact since the asteroid belt formed more than 4 billion years ago."We ran massive sets of simulations with computer planets where we filled up the asteroid belt region with a uniform distribution of computer asteroids," Minton said. The scientists then had the computers simulate the billions of years of solar system history.Their simulations ultimately ended with far more asteroids remaining than are actually observed in the asteroid belt. When comparing the simulated asteroid belt with the actual asteroid belt, they discovered a peculiar pattern in the differences. The simulated asteroid belt matched the real asteroid belt quite well on the sunward-facing sides of the Kirkwood gaps, but the real asteroid belt seemed to be depleted in asteroids on the Jupiter-facing sides."Then we simulated the migration of the giant planets," Minton said. "The perturbing effects of the migrating planets sculpted our simulated asteroid belt. After the migration was over, our simulated asteroid belt looked much more like the observed asteroid belt.""Our interpretation is that as Jupiter and Saturn migrated, their orbital resonances swept through the asteroid belt, ejecting many more asteroids than is possible with the planets in their current orbits," Malhotra said. "And the particular pattern of missing asteroids is characteristic of the pattern of Jupiter's and Saturn's migration.""Our work explains why there are fewer asteroids on the Jupiter-facing side of the Kirkwood gaps compared to the Sun-facing side," Minton said. "The patterns of depletion are like the footprints of wandering giant planets preserved in the asteroid belt."Their results corroborate other lines of evidence indicating that the giant planets - Jupiter, Saturn, Uranus, and Neptune - formed in a more tightly compacted configuration, and Jupiter moved slightly closer to the Sun while the other giant planets moved farther apart from each other and farther away from the Sun.Minton and Malhotra say that their result has implications for how far and how fast the planets migrated early in solar system history, and the possibility that planet migration perturbed asteroids may have contributed to a heavy bombardment of the inner solar system."Our result doesn't directly answer the question of whether the timing of this can be tied to inner solar system heavy bombardment - that's open for debate," Minton said. "But what it does say is that there was an event that destabilized asteroids over a relatively short period of time."All the asteroids being kicked out of the asteroid belt had to go somewhere," he added. "The implication of this is that, when all those asteroids were getting kicked out of the main belt, they could have become projectiles impacting Earth, the Moon, Mars, Venus and Mercury."LINK: HTTP://WWW.ASTRONOMY.COM/ASY/DEFAULT.ASPX?C=A">traduzione a cura di Arthur McPaul