Using the Mesospheric Lithium Layer to Monitor the Atmospheric Impacts of Space Debris
Using the Mesospheric Lithium Layer to Monitor the Atmospheric Impacts of Space Debris
The impacts of space debris (defunct satellites, rocket motors etc.) re-entering the atmosphere is of growing societal concern: the satellite launch rate is increasing rapidly and, in order to avoid a buildup of space debris in the near-Earth environment above 400 km, satellite operators are now being required to place spacecraft in relatively low orbits so that re-entry into the atmosphere occurs after 2 – 5 years. The rate of space debris mass entering the atmosphere is thus projected to increase markedly over the next decade. At the present time, this rate is around 3% of the ~30 tonnes of cosmic dust being deposited in the atmosphere each day (Schulz and Glassmeier, 2021) . However, the rate of deposition of particular metals such as Al, Li, Mn and Cu, which are used in the construction of satellites, already exceeds the natural cosmic dust input (Murphy et al., 2023). Li is increasing particularly rapidly because of its use in Li-ion batteries and Li-Al alloys. Space debris tends to ablate (i.e. vaporize) in the atmosphere around 60 km, and so concern is focussing on the impacts of the resulting metal-rich particles on the stratospheric ozone layer and on climate. The objective of this project is to determine whether Li can be used to monitor the rate of ablation of re-entering space debris.
A natural layer of Li atoms occurs globally in the upper mesosphere (around 90 km altitude), produced by the ablation of cosmic dust particles entering the atmosphere. The layer was first observed in 1958, using the technique of twilight emission photometry at 670.8 nm. Following extensive atomic bomb tests during 1961 (in particular a 50 Mt detonation by the Soviet Union at a height of 4 km in October 1961), these early observations were shown to have been contaminated by the artificial injection of Li from bomb tests (Sullivan and Hunten, 1964) . Subsequently, a lidar study in 1980 showed that the natural wintertime Li column density at Observatoire d’Haute Provence (44°N) varied within the range (1.5 – 3) x 106 atom cm‑2. This decreased during summer by a factor of about 5, indicating that Li has a larger seasonal variation than Na (Jegou et al., 1980) . As in the case of Na, the wintertime enhancement was shown to be greatest in the polar mesosphere, where a column density of 1.9 x 107 cm-2 has been observed at Svalbard (78°N) (Henricksen et al., 1980). The winter profiles of atomic Li are located around 92 km, roughly 4 km higher than the wintertime Na peak. The observations also show that the Li layer exhibits extreme variability: fluctuations in column density exceeding a factor of 4 have been measured over a single day, whereas the Na and K layers tend to vary by less than a factor of 1.5. These unexpected differences between Li and the other alkali (Group I) metals have not been explained.
At the University of Leeds we have developed detailed chemistry models of the major metals – Fe, Mg, Na, K and Ca – that ablate from cosmic dust (Plane et al., 2015). This chemistry has then been input into the Whole Atmosphere Community Climate Model (WACCM), in order to explore the chemistry of these metals in the mesosphere (Marsh et al., 2013; Feng et al., 2013; Plane et al., 2021). A similar approach will be used here.
The purpose of this project is to: 1) develop a scheme of the atmospheric chemistry of Li in the mesosphere, using available measured reaction rate coefficients together with electronic structure theory and rate theory calculations; 2) put this chemistry into a global chemistry-climate model together with the Li injection rate from the ablation of cosmic dust; 3) compare with the available observations of the natural Li layer at different locations, and understand why Li behaves so differently from Na and K; 4) model the transport of Li compounds, produced from the ablation of space debris around 60 km, to above 80 km where Li atoms should be released (as evidenced from the bomb tests in the 1960s); and 5) collaborate with a lidar observatory in Germany to make present-day observations of the Li layer, searching for evidence that the Li abundance is now increasing as a result of the ablation of space debris re-entering the atmosphere.
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