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The sticking properties of dust in early phases of planet formation depend on the thermal history and ambient atmosphere. Therefore, dust will change its ability to build larger aggregates in collisions, depending on its location in protoplanetary disks. We aim at quantifying the change in sticking properties as chondritic dust is heated under various atmospheric conditions. In laboratory experiments, we milled two different meteorites (Sayh al Uhaymir 001 and Allende) to dust and formed millimeter-size cylinders. These cylindrical aggregates were sequentially heated from 600 to 1400 K in vacuum and in a hydrogen atmosphere, with compositional changes being tracked via Mössbauer spectroscopy. Using a Brazilian splitting test, the splitting tensile strength was determined. At higher temperatures, iron in silicates is reduced to metallic Fe(Ni) within the hydrogen atmosphere. In any case, adhesive forces are strongly increased by orders of magnitude from 1000 to 1400 K with minimum variations, depending on the atmospheric conditions. The dust in protoplanetary disks becomes ever more sticky, approaching a sublimation line upon exposure to temperatures of about 1400 K.

Dust drifting inward in protoplanetary disks is subject to increasing temperatures. In laboratory experiments, we tempered basaltic dust between 873 K and 1273 K and find that the dust grains change in size and composition. These modifications influence the outcome of self-consistent low speed aggregation experiments showing a transition temperature of 1000 K. Dust tempered at lower temperatures grows to a maximum aggregate size of 2.02 ± 0.06 mm, which is 1.49 ± 0.08 times the value for dust tempered at higher temperatures. A similar size ratio of 1.75 ± 0.16 results for a different set of collision velocities. This transition temperature is in agreement with orbit temperatures deduced for observed extrasolar planets. Most terrestrial planets are observed at positions equivalent to less than 1000 K. Dust aggregation on the millimeter-scale at elevated temperatures might therefore be a key factor for terrestrial planet formation. [ABSTRACT FROM AUTHOR]

It is an open question how elevated temperatures in the inner parts of protoplanetary disks influence the formation of planetesimals. We approach this problem here by studying the tensile strength of granular beds with dust samples tempered at different temperatures. We find via laboratory experiments that tempering at increasing temperatures is correlated with an increase in cohesive forces. We studied dust samples of palagonite (JSC Mars-1a) which were tempered for up to 200 hr at temperatures between 600 and 1200 K, and measured the relative tensile strengths of highly porous dust layers once the samples cooled to room temperature. Tempering increases the tensile strength from 800 K upwards. This change is accompanied by mineral transformations, the formation of iron oxide crystallites as analyzed by Mössbauer spectroscopy, changes in the number size distribution, and the morphology of the surface visible as cracks in larger grains. These results suggest a difference in the collisional evolution toward larger bodies with increasing temperature as collisional growth is fundamentally based on cohesion. While high temperatures might also increase sticking (not studied here), compositional evolution will already enhance the cohesion and the possibility of growing larger aggregates on the way toward planetesimals. This might lead to a preferred in situ formation of inner planets and explain the observed presence of dense inner planetary systems. [ABSTRACT FROM AUTHOR]