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The gas-driven dust activity of comets is still an unresolved question incometary science. In the past, it was believed that comets are dirty snowballsand that the dust is ejected when the ice retreats. However, thanks to thevarious space missions to comets, it has become evident that comets have a muchhigher dust-to-ice ratio than previously thought and that most of the dust massis ejected in large particles. Here we report on new comet-simulationexperiments dedicated to the study of the ejection of dust aggregates caused bythe sublimation of solid water ice. We find that dust ejection exactly occurswhen the pressure of the water vapor above the ice surface exceeds the tensilestrength plus the gravitational load of the covering dust layer. Furthermore,we observed the ejection of clusters of dust aggregates, whose sizes increasewith increasing thickness of the ice-covering dust-aggregate layer. Inaddition, the trajectories of the ejected aggregates suggest that most of theaggregates obtained a non-vanishing initial velocity from the ejection event.

Experiments on cometary activity: ejection of dust aggregates from a sublimating water-ice surface