COMETARY SCIENCE. 67P/Churyumov-Gerasimenko surface properties as derived from CIVA panoramic images.

The structure and composition of cometary constituents, down to their microscopic scale, are critical witnesses of the processes and ingredients that drove the formation and evolution of planetary bodies toward their present diversity. On board Rosetta's lander Philae, the Comet Infrared and Visible Analyser (CIVA) experiment took a series of images to characterize the surface materials surrounding the lander on comet 67P/Churyumov-Gerasimenko. Images were collected twice: just after touchdown, and after Philae finally came to rest, where it acquired a full panorama. These images reveal a fractured surface with complex structure and a variety of grain scales and albedos, possibly constituting pristine cometary material.

Towards time-dependent current-density-functional theory in the non-linear regime.

Time-Dependent Density-Functional Theory (TDDFT) is a well-established theoretical approach to describe and understand irradiation processes in clusters and molecules. However, within the so-called adiabatic local density approximation (ALDA) to the exchange-correlation (xc) potential, TDDFT can show insufficiencies, particularly in violently dynamical processes. This is because within ALDA the xc potential is instantaneous and is a local functional of the density, which means that this approximation neglects memory effects and long-range effects. A way to go beyond ALDA is to use Time-Dependent Current-Density-Functional Theory (TDCDFT), in which the basic quantity is the current density rather than the density as in TDDFT. This has been shown to offer an adequate account of dissipation in the linear domain when the Vignale-Kohn (VK) functional is used. Here, we go beyond the linear regime and we explore this formulation in the time domain. In this case, the equations become very involved making the computation out of reach; we hence propose an approximation to the VK functional which allows us to calculate the dynamics in real time and at the same time to keep most of the physics described by the VK functional. We apply this formulation to the calculation of the time-dependent dipole moment of Ca, Mg and Na2. Our results show trends similar to what was previously observed in model systems or within linear response. In the non-linear domain, our results show that relaxation times do not decrease with increasing deposited excitation energy, which sets some limitations to the practical use of TDCDFT in such a domain of excitations.

Two-dimensional spin-exchange solid-state NMR studies of 13 C-enriched wood.

Two-dimensional high-resolution solid-state NMR has been used to study 13C-enriched wood. Wood is a complex material containing three major polymers: cellulose, hemicelluloses and lignin. The use of an enriched 13C-compound allows the observation of intra-molecular spin-diffusion driven by dipolar couplings. Correlations between spins at progressively longer distances have been obtained as the mixing time is increased, corresponding, for each of the separate polymer chains, to intra-unit and then to inter-unit interactions, and in the case of cellulose to inter-chain interactions. A straightforward qualitative analysis of the spin diffusion spectra is shown to yield the assignment of the carbon-13 spectrum. The cellulose resonances can all be sequentially assigned using the spin diffusion experiment. Using the experiments it is shown that, at least on a distance scale of several nanometres explored by the spin diffusion process, the three main components of wood occur in separate phases. Also, a question concerning the structure of the hemicellulose units is resolved by locating the O-acetyl group at the 2-position of the xylan chains.