  The gas-driven dust activity of comets is still an unresolved question in
cometary science. In the past, it was believed that comets are dirty snowballs
and that the dust is ejected when the ice retreats. However, thanks to the
various space missions to comets, it has become evident that comets have a much
higher dust-to-ice ratio than previously thought and that most of the dust mass
is ejected in large particles. Here we report on new comet-simulation
experiments dedicated to the study of the ejection of dust aggregates caused by
the sublimation of solid water ice. We find that dust ejection exactly occurs
when the pressure of the water vapor above the ice surface exceeds the tensile
strength plus the gravitational load of the covering dust layer. Furthermore,
we observed the ejection of clusters of dust aggregates, whose sizes increase
with increasing thickness of the ice-covering dust-aggregate layer. In
addition, the trajectories of the ejected aggregates suggest that most of the
aggregates obtained a non-vanishing initial velocity from the ejection event.
