ausblenden:
Schlagwörter:
minor planets; asteroids: general; meteorites; meteors; meteoroids;
planets and satellites: physical evolution; Astrophysics - Earth and
Planetary Astrophysics
Zusammenfassung:
Aims: Radiometric ages for chondritic meteorites and their components provide information on the accretion timescale of chondrite parent bodies, and on cooling paths within certain areas of these bodies. However, to use this age information for constraining the internal structure, and the accretion and cooling history of the chondrite parent bodies, the empirical cooling paths obtained by dating chondrites must be combined with theoretical models of the thermal evolution of planetesimals. Important parameters in such thermal models include the initial abundances of heat-producing short-lived radionuclides (26Al and 60Fe), which are determined by the accretion timescale and the terminal size, chemical composition and physical properties of the chondritic planetesimals. The major aim of this study is to assess the effects of sintering of initially porous material on the thermal evolution of planetesimals, and to constrain the values of basic parameters that determined the structure and evolution of the H chondrite parent body.
Methods: We present a new code for modelling the thermal evolution of ordinary chondrite parent bodies that initially are highly porous and undergo sintering by hot pressing as they are heated by decay of radioactive nuclei. The pressure and temperature stratification in the interior of the bodies was calculated by solving the equations of hydrostatic equilibrium and energy transport. The decrease of porosity of the granular material by hot pressing due to self-gravity was followed by solving a set of equations for the sintering of powder materials. For the heat-conductivity of granular material we combined recently measured data for highly porous powder materials, relevant for the surface layers of planetesimals, with data for heat-conductivity of chondrite material, relevant for the strongly sintered material in deeper layers.
Results: Our new model demonstrates that in initially porous planetesimals heating to central temperatures sufficient for melting can occur for bodies a few km in size, that is, a factor of ≈10 smaller than for compact bodies. Furthermore, for high initial 60Fe abundances small bodies may differentiate even when they had formed as late as 3-4 Ma after CAI formation. To demonstrate the capability of our new model, the thermal evolution of the H chondrite parent body was reconstructed. The model starts with a porous body that is later compacted first by "cold pressing" at low temperatures and then by "hot pressing" for temperatures above ≈700 K, i.e., the threshold temperature for sintering of silicates. The thermal model was fitted to the well-constrained cooling histories of the two H chondrites Kernouvé (H6) and Richardton (H5). The best fit was obtained for a parent body with a radius of 100 km that accreted at t = 2.3 Ma after CAI formation, and had an initial 60Fe/56Fe = 4.1 × 10-7. Burial depths of 8.3 km and 36 km for Richardton and Kernouvé were able to reproduce their empirically determined cooling history. These burial depths are shallower than those derived in previous models. This reflects the strong insulating effect of the residual powder surface layer, which is characterised by a low thermal conductivity.
