NELIOTA: A Terrain-Dependent Dichotomy of Lunar Impact Flashes

IntroductionHypervelocity impacts are among the most violent processes in the Solar System. On the Moon, impacts of meteoroids produce luminous transient events in the visible and near-infrared, known as lunar impact flashes (LIFs). During the initial stages of the cratering process, the LIF is generated by the partial conversion of the impactor’s kinetic energy into thermal radiation, a process governed by the critical parameter known as luminous efficiency [1]. Current models generally assume this is solely dependent on impactor velocity[2, 3], overlooking the target’s composition. While lunar surface lithology is known to shape late-stage crater morphology, its influence on the initial stages of impact and the resulting lunar impact flashes (LIFs) has remained elusive. A LIF lightcurve dichotomyThe extensive catalogue of LIFs, resulting from nine years of systematic monitoring by the ESA-funded NELIOTA programme [4, 5, 6, 7], provides a unique opportunity to investigate flash decay across different terrain types. Similar to the study in [8] on lunar craters, LIFs can be classified according to their location into three lunar terrain types: mare, highland, and ‘border’. The latter refers to regions near the interface between mare and highland, where both types of target material may be involved.By analysing a sample of 124 light curves of multi-frame LIFs, we identify a clear dichotomy between events occurring on lunar mare and highland terrains (Fig. 1). Specifically, highland LIFs exhibit a shallower and longer-lasting decay than those on the maria, which show a faster and steeper decay. ‘Border’ LIFs display decay profiles that are closer to those of mare LIFs. This extended emission in the highlands results in approximately 30% higher total luminous energy. Assuming that the initial peak brightness is directly linked to impactor’s kinetic energy, these results indicate that luminous efficiency is influenced by lunar lithology. Our findings provide a predictive framework for estimating luminous energy based on impact location and establishing a critical foundation for the forthcoming ESA LUMIO mission [9, 10]. NELIOTA databaseNELIOTA-III aims to broaden participation and foster collaborations by expanding its database to allow the wider astronomy community to contribute their own observations. Through this system, professional and citizen astronomers can register specific details of their telescope setup and location and share their planned observation sessions. Along with observation reports, the observational data of detected potential LIFs can be uploaded and undergo expert review. The accepted candidate LIFs will be integrated into the NELIOTA database and possibly linked with other reports.AcknowledgmentsThis project is funded by the Horizon Europe Programme of the European Union and implemented by ESA. Views and opinion expressed are however those of the authors only and the European Commission cannot be held responsible for any use which may be made of the information contained therein. References[1] Bouley, S., et al. "Power and duration of impact flashes on the Moon: Implication for the cause of radiation." Icarus 218.1 (2012): 115-124.[2] Swift, W. R., et al. "An exponential luminous efficiency model for hypervelocity impact into regolith." Meteroids 2010: An International Conference on Minor Bodies in the Solar System. No. M10-0209. 2010.[3] Fuse, R., et al. "An experimental study of the impact flash: the relationship between luminous efficiency and vacuum level." Planetary and Space Science 187 (2020): 104921.[4] Bonanos, A. Z., et al. "NELIOTA: First temperature measurement of lunar impact flashes." Astronomy & Astrophysics 612 (2018): A76.[5] Xilouris, E. M., et al. "NELIOTA: The wide-field, high-cadence, lunar monitoring system at the prime focus of the Kryoneri telescope." Astronomy & Astrophysics 619 (2018): A141.[6] Liakos, A., et al. "NELIOTA: methods, statistics, and results for meteoroids impacting the Moon." Astronomy & Astrophysics 633 (2020): A112.[7] Liakos, A., et al. "NELIOTA: New results and updated statistics after 6.5 years of lunar impact flashes monitoring." Astronomy & Astrophysics 687 (2024): A14.[8] Osinski, G. R., et al. "Transitional impact craters on the Moon: Insight into the effect of target lithology on the impact cratering process." Meteoritics & Planetary Science 54.3 (2019): 573-591.[9] Cervone, A., et al. "LUMIO: A CubeSat for observing and characterizing micro-meteoroid impacts on the lunar far side." Acta Astronautica 195 (2022): 309-317.[10] Ferrari, F., et al. ESA's LUMIO Mission: detecting meteoroid impacts on the lunar farside. No. EPSC-DPS2025-1514. Copernicus Meetings, 2025.