India’s Chandrayaan-2 lunar orbiter has made the first-ever observation of the effects of the Sun’s Coronal Mass Ejection (CME) on the Moon, with one of its scientific instruments Chandra’s Atmospheric Composition Explorer-2 (CHACE-2) onboard. Observations from CHACE-2 showed an increase in the total pressure of the dayside lunar exosphere (very thin atmosphere) when the CME impacted the Moon. The total number density (number of neutral atoms or molecules present in an environment per unit volume) derived from these observations showed an increase by more than an order of magnitude. This increase is consistent with earlier theoretical models, which predicted such an effect, but CHACE-2 onboard Chandrayaan-2 has observed such an effect for the first time.

The Earth’s Moon has a very thin atmosphere, which falls under the category of ‘exosphere’, implying that the gas atoms and molecules in the lunar environment rarely interact despite their coexistence. The boundary of the exosphere is the surface of the Moon and hence the Moon’s exosphere falls under the category of ‘surface boundary exosphere’. Theexosphere on the Moon is produced by a number of processes, which involves the interaction of solar radiation,solar wind (ions of Hydrogen, Helium and a small quantity of heavier ions emanated from the Sun) and the impact of the meteorites with the surface of the Moon. These processes liberate atoms/molecules fromthe surface of the Moon, which become a part of the exosphere. In general, the exosphere of the Moon is highly sensitive to even small variations of the factors that are responsible for its creation, and such a factor is the emission of the coronal mass of the Sun, known as CME (short form of Coronal Mass Ejection). CMEs are the events when the Sun ejects significant quantities of its building material, comprising mostly Helium and Hydrogen ions. These effects are significant on the Moon, as Moon is an airless body, that too deprived of any global magnetic field, the presence of which would have shielded (even partially) the solar effects on its surface.

This opportunity to directly observe the effects of the CMEimpacting on the Moon came in a rare occurrence, on May 10, 2024, when a series of coronal mass ejections (CMEs) were hurled by the Sun. This increased quantity of the solar coronal mass that impacted on the Moon enhanced the process of knocking off the atoms from the lunar surface, thereby liberating them to the lunar exosphere, which manifested as the enhancement of the total pressure in the sunlit lunar exosphere.

This observation would provide scientific insight in the understanding of the lunar exosphere and space weather effects on the Moon. Apart from pushing the edge of our scientific understanding about the Moon and the lunar space weather (effect of the Sun’s emissions on the Moon), this observation also indicates the challenges of building scientific bases on the Moon. Lunar base architects need to account for such extreme events, which would temporarily alter the lunar environment, before the effects subside.

Reference: “Impact of a Coronal Mass Ejection on the Lunar Exosphere as Observed by CHACE-2 on the Chandrayaan-2 Orbiter”, Dhanya, M B,ChemukulaMathin Yadav, Smitha V Thampi, Tirtha Pratim Das, Satheesh Thampi, R, Anil Bhardwaj, Geophysical Research Letters, Volume 52, Issue 15, AR e2025GL115737, 16 AUG 2025 (DOI: 10.1029/2025GL115737)