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When comet nuclei approach the Sun, the increasing energy flux through the surface layers leads to sublimation of the underlying ices and subsequent outgassing that promotes the observed emission of gas and dust. While the release of gas can be straightforwardly understood by solving the heat-transport equation and taking into account the finite permeability of the ice-free dust layer close to the surface of the comet nucleus, the ejection of dust additionally requires that the forces binding the dust particles to the comet nucleus must be overcome by the forces caused by the sublimation process. This relates to the question of how large the tensile strength of the overlying dust layer is. Homogeneous layers of micrometer-sized dust particles reach tensile strengths of typically 10 3 to 10 4 Pa . This exceeds by far the maximum sublimation pressure of water ice in comets. It is therefore unclear how cometary dust activity is driven. To solve this paradox, we used the model by Skorov and Blum (Skorov, Y.V., Blum, J. 2012. Icarus 221, 361–11), who assumed that cometesimals formed by gravitational instability of a cloud of dust and ice aggregates and calculated for the corresponding structure of comet nuclei tensile strength of the dust-aggregate layers on the order of 1 Pa. Here we present evidence that the emitted cometary dust particles are indeed aggregates with the right properties to fit the model by Skorov and Blum. Then we experimentally measure the tensile strengths of layers of laboratory dust aggregates and confirm the values derived by the model. To explain the comet activity driven by the evaporation of water ice, we derive a minimum size for the dust aggregates of ∼1 mm, in agreement with meteoroid observations and dust-agglomeration models in the solar nebula. Finally we conclude that cometesimals must have formed by gravitational instability, because all alternative formation models lead to higher tensile strengths of the surface layers.

The systems used to manage municipal solid waste (MSW) in Germany and the UK have been compared on the basis of carbon equivalent emissions. The legislative background to MSW management in Germany emphasises recycling and recovery, while the UK continues to rely on much higher levels of landfill, relatively low use (∼9%) of energy from waste (EfW), with currently about 34% recycling and composting. Analysis indicates that the carbon emissions associated with current MSW management in the UK is approximately five times higher than that for Germany. UK management of MSW is calculated to produce 175 kg CO2 equivalents/t, while MSW in Germany generates 34 kg CO2 equivalents/t. These results show the benefits of German initiatives in waste management introduced over the past 20 years. To put these figures into perspective, the difference in CO2 emissions would be approximately equivalent to removing 1.2 million cars from roads in England and Wales. The potential for the UK to achieve the Waste Management Strategy targets through increased recycling/separation and landfill legislation is discussed.

By modifying the usual Morse potential a more realistic potential function for describing the vibrational motion of a hydrogen-bonded complex XH.. Y is presented. It is shown that the broadening of the v (XH) absorption bands can essentially be influenced by the intramolecular interactions.