The Origin of Mercury

Mercury’s unusually high mean density has always been attributed to special circumstances that occurred during the formation of the planet or shortly thereafter, and due to the planet’s close proximity to the Sun. The nature of these special circumstances is still being debated and several scenarios, all proposed more than 20 years ago, have been suggested. In all scenarios, the high mean density is the result of severe fractionation occurring between silicates and iron. It is the origin of this fractionation that is at the centre of the debate: is it due to differences in condensation temperature and/or in material characteristics (e.g. density, strength)? Is it because of mantle evaporation due to the close proximity to the Sun? Or is it due to the blasting off of the mantle during a giant impact? In this paper we investigate, in some detail, the fractionation induced by a giant impact on a proto-Mercury having roughly chondritic elemental abundances. We have extended the previous work on this hypothesis in two significant directions. First, we have considerably increased the resolution of the simulation of the collision itself. Second, we have addressed the fate of the ejecta following the impact by computing the expected reaccretion timescale and comparing it to the removal timescale from gravitational interactions with other planets (essentially Venus) and the Poynting–Robertson effect. To compute the latter, we have determined the expected size distribution of the condensates formed during the cooling of the expanding vapor cloud generated by the impact. We find that, even though some ejected material will be reaccreted, the removal of the mantle of proto-Mercury following a giant impact can indeed lead to the required long-term fractionation between silicates and iron and therefore account for the anomalously high mean density of the planet. Detailed coupled dynamical–chemical modeling of this formation mechanism should be carried out in such a way as to allow explicit testing of the giant impact hypothesis by forthcoming space missions (e.g. MESSENGER and BepiColombo).     

AMETHYST: automatic alarm assessment becoming a reality

The aim of the AMETHYST (AutoMatic Event auTHentication SYSTems) project is to encourage the development of a high performance perimeter detection system which combines Video Motion Detection (VMD) technology with another type of Perimeter Intrusion Detection System (PIDS). AMETHYST will automatically assess the cause of PIDS alarms and pass to an operator those alarms likely to be caused by an intruder. It will filter out alarms not likely to have a human cause. A previous paper explaining and exploring the AMETHYST concept was presented at the 1995 Carnahan Conference. Since then PSDB has produced a single channel AMETHYST demonstrator and placed a contract for the development of an eight channel prototype AMETHYST system. This updated paper gives details of the hardware and software used with these two systems. Also described is PSDB's approach to the development of AMETHYST's automatic assessment algorithms. These will combine current expertise from Video Motion Detection (VMD) and Intelligent Scene Monitoring (ISM) systems with the unique AMETHYST approach. AMETHYST analyses picture sequences from before and after an alarm instead of continuously analysing live video. Sequences are provided by a Loop Framestore, either connected to or part of the AMETHYST system. The algorithms will be assessed and developed using PSDB's growing collection of over 150 alarm sequences