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When comet nuclei approach the Sun, the increasing energy flux through thesurface layers leads to sublimation of the underlying ices and subsequentoutgassing that promotes the observed emission of gas and dust. The ejection ofdust requires that the forces binding the dust particles to the comet nucleusmust be overcome by the forces caused by the sublimation process. This relatesto the question of how large the tensile strength of the overlying dust layeris. Homogeneous layers of micrometer-sized dust particles reach tensilestrengths of typically $10^3$ to $10^4$ Pa. This exceeds by far the maximumsublimation pressure of water ice in comets. It is therefore unclear howcometary dust activity is driven. To solve this paradox, we used the model bySkorov and Blum (Icarus 221, 1-11, 2012), who assumed that cometesimals formedby gravitational instability of a cloud of dust and ice aggregates andcalculated for the corresponding structure of comet nuclei tensile strength ofthe dust-aggregate layers on the order of 1 Pa. Here we present evidence thatthe emitted cometary dust particles are indeed aggregates with the rightproperties to fit the model by Skorov and Blum. Then we experimentally measurethe tensile strengths of layers of laboratory dust aggregates and confirm thevalues derived by the model. To explain the comet activity driven by theevaporation of water ice, we derive a minimum size for the dust aggregates of$\sim 1$ mm, in agreement with meteoroid observations and dust-agglomerationmodels in the solar nebula. Finally we conclude that cometesimals must haveformed by gravitational instability, because all alternative formation modelslead to higher tensile strengths of the surface layers.

icarus

Comets formed in solar-nebula instabilities! -- An experimental and  modeling attempt to relate the activity of comets to their formation process  (corrigendum included)