Speaker
Description
SMASI is a table-top simulation chamber developed at IAA-CSIC from a forerunner instrument at the University of Leeds known as MASI (Meteor Ablation Simulator). The new instrument is designed to perform thermal ablation experiments with space debris analogs, with the objective of studying the evaporation of their elemental constituents upon atmospheric re-entry and the subsequent formation of molecules in the presence of atmospheric gases. SMASI has improved capabilities of measuring simultaneously two chemical species ablated from the samples by laser induced fluorescence and recording videos of the evolution of the samples while controlling the ablation temperature. In addition, SMASI has the ability to detect polyatomic molecules downstream of the heated sample by mass spectrometry, which provides a glimpse of the chemistry that occurs under atmospheric conditions. We have performed so far experiments using different aluminium alloys (e.g. AA1421, AA6061, AA7075) with the aim of understanding the oxidative ablation of aluminium under atmospheric re-entry conditions as well as the evaporation of mayor components of these alloys such as lithium and copper. It is worth pointing out that although extensive literature on the ignition and combustion of nano- and micron-sized aluminium partiles and aluminium foils exist, experiments looking at evaporation under relevant atmospheric heating rates, oxygen concentrations and particle sizes have not been reported yet. Similarly, the fate of aluminium atoms beyond its fast bimolecular reaction with oxygen had not been investigated. Here we show that oxygen starts playing a role in hindering aluminium evaporation at concentrations higher than those typical at 60 km altitude, but this is dependent on the heating rate. For high heating rates (50 ºC/s) oxygen barely plays a role in preventing evaporation, while at lower rates (4 ºC/s) the effect of the oxide layer is clearly distinguished as a succession of aluminium bursts caused by oxide shell cracking. The onset of aluminium evaporation occurs around 1300 ºC and is determined by the vapour pressure of aluminium. The main product of atmospheric chemistry is aluminium hydroxide (not alumina, as commonly assumed). For lithium, we observe that evaporation starts around 800 ºC and lithium depletion is almost complete by the time aluminium starts evaporating. These observations carry important consequences for the ablation of Li-Al alloys. The relatively high threshold for aluminium evaporation most likely determines the fraction of this metal that partitions to the gas phase, considering the relatively low apparent temperatures observed during real re-entries. Aluminium hydroxide is potentially more effective in activating chorine when taken up by stratospheric aerosol. The low-threshold, fast evaporation of lithium from alloys implies that anthropogenic lithium from re-entries overlaps with the natural lithium layer, which suggest that Li lidar could be use to track the antropogenic mass input globally. The potential effect of lithium and copper content on aluminium evaporation is currently under investigation. The datasets generated in these simulations can be used to fine tune re-entry break-up simulation codes.
