Alexander Hubbard^a and Denton S. Ebel^b

^aDepartment of Astrophysics, American Museum of Natural History, New York, NY 10024-5192, USA
^bDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024-5192, USA

The Earth is known to be depleted in volatile lithophile elements in a fashion that defies easy explanation. We resolve this anomaly with a model that combines the porosity of collisionally grown dust grains in protoplanetary disks with heating from FU Orionis events that dramatically raise protoplanetary disk temperatures. The heating from an FU Orionis event alters the aerodynamical properties of the dust while evaporating the volatiles. This causes the dust to settle, abandoning those volatiles. The success of this model in explaining the elemental composition of the Earth is a strong argument in favor of highly porous collisionally grown dust grains in protoplanetary disks outside our Solar System. Further, it demonstrates how thermal (or condensation based) alterations of dust porosity, and hence aerodynamics, can be a strong factor in planet formation, leading to the onset of rapid gravitational instabilities in the dust disk and the subsequent collapse that forms planetesimals.

Reference
Hubbard A and Ebel DS (in press) Protoplanetary dust porosity and FU Orionis Outbursts: Solving the mystery of Earth’s missing volatiles. Icarus
[doi:10.1016/j.icarus.2014.04.015]
Copyright Elsevier

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T. Kohout^a et al. (>10)*
*Find the extensive, full author and affiliation list on the publishers website.

^aInstitute of Geology, Academy of Sciences of the Czech Republic, Prague, Czech Republic

Airless planetary bodies are directly exposed to space weathering. The main spectral effects of space weathering are darkening, reduction in intensity of silicate mineral absorption bands, and an increase in the spectral slope towards longer wavelengths (reddening). Production of nanophase metallic iron (npFe⁰) during space weathering plays major role in these spectral changes. A laboratory procedure for the controlled production of npFe⁰ in silicate mineral powders has been developed. The method is based on a two-step thermal treatment of low-iron olivine, first in ambient air and then in hydrogen atmosphere. Through this process, a series of olivine powder samples was prepared with varying amounts of npFe⁰ in the 7-20 nm size range. A logarithmic trend is observed between amount of npFe⁰ and darkening, reduction of 1 μm olivine absorption band, reddening, and 1 μm band width. Olivine with a population of physically larger npFe⁰particles follows spectral trends similar to other samples, except for the reddening trend. This is interpreted as the larger, ∼40-50 nm sized, npFe⁰ particles do not contribute to the spectral slope change as efficiently as the smaller npFe⁰ fraction. A linear trend is observed between the amount of npFe⁰ and 1 μm band center position, most likely caused by Fe²⁺ disassociation from olivine structure into npFe⁰ particles.

Reference
Kohouta T et al. (in press) Space weathering simulations through controlled growth of iron nanoparticles on olivine. Icarus
[doi:10.1016/j.icarus.2014.04.004]
Copyright Elsevier

Link to Article