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Title: On The Effect of Giant Planets on the Scattering of Parent Bodies of Iron Meteorite from the Terrestrial Planet Region into the Asteroid Belt: A Concept Study
Authors: Nader Haghighipour, Edward R. D. Scott

In their model for the origin of the parent bodies of iron meteorites, Bottke et al proposed differentiated planetesimals that were formed in the region of 1-2 AU during the first 1.5 Myr, as the parent bodies, and suggested that these objects and their fragments were scattered into the asteroid belt as a result of interactions with planetary embryos. Although viable, this model does not include the effect of a giant planet that might have existed or been growing in the outer regions. We present the results of a concept study where we have examined the effect of a planetary body in the orbit of Jupiter on the early scattering of planetesimals from terrestrial region into the asteroid belt. We integrated the orbits of a large battery of planetesimals in a disk of planetary embryos, and studied their evolutions for different values of the mass of the planet. Results indicate that when the mass of the planet is smaller than 10 Earth-masses, its effects on the interactions among planetesimals and planetary embryos is negligible. However, when the planet mass is between 10 and 50 Earth-masses, simulations point to a transitional regime with ~50 Earth-mass being the value for which the perturbing effect of the planet can no longer be ignored. Simulations also show that further increase of the mass of the planet strongly reduces the efficiency of the scattering of planetesimals from the terrestrial planet region into the asteroid belt. We present the results of our simulations and discuss their possible implications for the time of giant planet formation.

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Iron meteorite evidence
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Title: Iron meteorite evidence for early formation and catastrophic disruption of protoplanets
Authors: Jijin Yang, Joseph I. Goldstein & Edward R. D. Scott

In our Solar System, the planets formed by collisional growth from smaller bodies. Planetesimals collided to form Moon-to-Mars-sized protoplanets in the inner Solar System in 0.11 Myr, and these collided more energetically to form planets. Insights into the timing and nature of collisions during planetary accretion can be gained from meteorite studies. In particular, iron meteorites offer the best constraints on early stages of planetary accretion because most are remnants of the oldest bodies, which accreted and melted in <1.5 Myr, forming silicate mantles and iron-nickel metallic cores. Cooling rates for various groups of iron meteorites suggest that if the irons cooled isothermally in the cores of differentiated bodies, as conventionally assumed, these bodies were 5200 km in diameter. This picture is incompatible, however, with the diverse cooling rates observed within certain groups, most notably the IVA group, but the large uncertainties associated with the measurements do not preclude it. Here we report cooling rates for group IVA iron meteorites that range from 100 to 6,000 K Myr^-1, increasing with decreasing bulk Ni. Improvements in the cooling rate model, smaller error bars, and new data from an independent cooling rate indicator show that the conventional interpretation is no longer viable. Our results require that the IVA meteorites cooled in a 300-km-diameter metallic body that lacked an insulating mantle. This body probably formed ~4,500 Myr ago in a 'hit-and-run' collision between Moon-to-Mars-sized protoplanets. This demonstrates that protoplanets of ~10 km size accreted within the first 1.5 Myr, as proposed by theory, and that fragments of these bodies survived as asteroids.

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Scientists analysed certain patterns in meteorites, such as those in this Carlton Meteorite, to obtain the cooling rates of asteroids.

Credit: J Goldstein UMass Amherst, and H. Yakowitz, NIST

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A fender-bender between two celestial bodies that left a 200 mile-wide metallic chunk to cool in space was the likely source of a group of meteorites known as the IVA iron meteorites, suggests new research by University of Massachusetts Amherst scientists. Their findings, published in the April 19 issue of the journal Nature, help explain conflicting meteorite data that has long puzzled scientists, and sheds new light on how and when asteroids form.
Jijin Yang and Joseph Goldstein of the UMass Amherst department of mechanical and industrial engineering, and Edward Scott of the Hawaii Institute of Geophysics and Planetology at the University of Hawaii at Manoa collaborated on the research.
The standard model of asteroid formation says asteroidal bodies are just leftover debris from the collisions and subsequent melting that happens when planets form. Scientists find that these leftover chunks typically have a dense iron core containing nickel, surrounded by an insulating layer of silicate. Evidence has suggested that the iron-nickel core cools relatively evenly, thanks to the insulating silicate mantle.

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