Roger R. Fu^a and Linda T. Elkins-Tanton^b

^aDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
^bDepartment of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC, USA

High abundances of short-lived radiogenic isotopes in the early solar system led to interior melting and differentiation on many of the first planetesimals. Petrologic, isotopic, and paleomagnetic evidence suggests that some differentiated planetesimals retained primitive chondritic material. The preservation of a cold chondritic lid depends on whether deep melts are able to ascend and breach the chondritic crust. We evaluate the likelihood of melt ascent on a range of chondritic parent bodies. We find that, due to the efficient ascent of free volatiles in the gas and supercritical fluid phases at temperatures still below the solidus for silicates and metals, mobile silicate melts on planetesimals were likely volatile-depleted. By calculating the densities of such melts, we show that silicate melts likely breached crusts of enstatite chondrite compositions but did not ascend in the CV and CM parent bodies. Ordinary chondrite melts represent an intermediate case. These predictions are consistent with paleomagnetic results from CV and CM chondrites as well as spectral observations of large E-type asteroids.

Reference
Fua RR and Elkins-Tanton LT (2014) The fate of magmas in planetesimals and the retention of primitive chondritic crusts. Earth and Planetary Science Letters 390:128–137.
[doi:10.1016/j.epsl.2013.12.047]
Copyright Elsevier

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A. Izidoro^1,2,3, N. Haghighipour^4,5, O. C. Winter¹ and M. Tsuchida⁶

¹UNESP, Univ. Estadual Paulista – Grupo de Dinâmica Orbital & Planetologia, Guaratinguetá, CEP 12.516-410, São Paulo, Brazil
²Capes Foundation, Ministry of Education of Brazil, Brasília/DF 70040-020, Brazil
³University of Nice-Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Laboratoire Lagrange, BP 4229, F-06304 Nice Cedex 4, France
⁴Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii-Manoa, Honolulu, HI 96822, USA
⁵Institute for Astronomy and Astrophysics, University of Tübingen, D-72076 Tübingen, Germany
⁶UNESP, Univ. Estadual Paulista, DCCE-IBILCE, São José do Rio Preto, CEP 15.054-000, São Paulo, Brazil

Models of terrestrial planet formation for our solar system have been successful in producing planets with masses and orbits similar to those of Venus and Earth. However, these models have generally failed to produce Mars-sized objects around 1.5 AU. The body that is usually formed around Mars’ semimajor axis is, in general, much more massive than Mars. Only when Jupiter and Saturn are assumed to have initially very eccentric orbits (e ~0.1), which seems fairly unlikely for the solar system, or alternately, if the protoplanetary disk is truncated at 1.0 AU, simulations have been able to produce Mars-like bodies in the correct location. In this paper, we examine an alternative scenario for the formation of Mars in which a local depletion in the density of the protosolar nebula results in a non-uniform formation of planetary embryos and ultimately the formation of Mars-sized planets around 1.5 AU. We have carried out extensive numerical simulations of the formation of terrestrial planets in such a disk for different scales of the local density depletion, and for different orbital configurations of the giant planets. Our simulations point to the possibility of the formation of Mars-sized bodies around 1.5 AU, specifically when the scale of the disk local mass-depletion is moderately high (50%-75%) and Jupiter and Saturn are initially in their current orbits. In these systems, Mars-analogs are formed from the protoplanetary materials that originate in the regions of disk interior or exterior to the local mass-depletion. Results also indicate that Earth-sized planets can form around 1 AU with a substantial amount of water accreted via primitive water-rich planetesimals and planetary embryos. We present the results of our study and discuss their implications for the formation of terrestrial planets in our solar system.

Reference
Izidoro A, Haghighipour N, Winter OC and Tsuchida M (2014) Terrestrial Planet Formation in a Protoplanetary Disk with a Local Mass Depletion: A Successful Scenario for the Formation of Mars. The Astrophysical Journal 782:31.
[doi:10.1088/0004-637X/782/1/31]

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