Strongly Enhanced Sticking of Dry (Hot) Grains and a Sweet Spot for Planet Formation

Sticking properties rule the early phases of pre-planetary growth. The role of water, being omnipresent under ambient conditions, is not well known in this. Therefore, we studied the effect of varying surface water content on contacts between grains, i.e. we measured the surface energies. To resemble dust in protoplanetary disks, we milled a chondrite to micrometer size. Water was reduced and finally removed by tempering a sample under vacuum at increasing temperatures up to 1400 K. The splitting tensile strength of millimeter-sized, cylindrical dust aggregates for each tempered sample was measured by means of a Brazilian test. Mössbauer spectroscopy was used in addition to measure the (iron-containing) composition of the samples and also link the evolution of surface energy to compositional changes. Under ambient conditions, surfaces of silicates hold many layers of water (wet samples). At the low pressure of protoplanetary disks and at moderate temperatures, grains can be considered as dry and they presumably hold a monolayer of surface water only. Even this monolayer may evaporate completely as the dust drifts further inwards towards higher temperature regions and grains are super dry. For wet samples, we measured an effective surface energy which monotonously decreases by a factor of 5 from room temperature to about 1300 K due to compositional changes [1]. This compositional reduction of surface energy still holds for dry samples but with an increase in the sticking force by a factor of 10 over the wet samples. Starting at about 900 K, super dry samples deviate strongly from this. Above this temperature, the surface energy is boosted and increases exponentially up to another factor of 100 with a peak at about 1200 K [2]. At still higher temperatures in the range of 1300 - 1400 K, grain sizes microscopically change (increase) for all samples, leading to an instability of aggregates and therefore making growth challenging. Beyond 1400 K no classical collisional growth is possible. As a consequence, there is a spatial region in protoplanetary disks with temperatures around 1200 K which favors aggregation and, therefore, will likely be a sweet spot for planetesimal or planet formation.