Why does the Moon emit so little radon ? New constraints from the DORN experiment aboard the Chang’E 6 spacecraft

The Moon has a tenuous atmosphere. This exosphere is short-lived due to its interactions with UV radiations and the solar wind, but it is also constantly regenerated. Several sources have been identified (solar wind, bombardment by meteorites). Some gases may also originate from the outgassing of the Moon, as evidenced by the detection of radon by the Apollo missions. Radon-222 is a radioactive gas produced in the soil by the decay of Ra-226. On Earth, a fraction of radon atoms is released from grains and migrates through soils into the atmosphere. This produces a series of decay products (Pb-210, Po-210, etc.) in the surface environment. Similarly, radon can be released from the lunar soil and diffuse or be advected to the surface. Due to their different mobilities and half-lives, radon and its progeny provide powerful tools for tracing the transport of gases, fluids, and aerosols in the lithosphere, hydrosphere and atmosphere. Since the early stages of the lunar exploration, they have thus been considered as key tracers of the lunar venting and potentially seismic activity. Measurements performed from the orbit (by Apollo 15-16, Lunar Prospector and Kaguya) and on returned samples and equipments have revealed temporal variations and significant differences in their spatial distribution. These variations have been attributed to the presence of degassing spots with variable outgassing intensities.However, the DORN instrument, which was embarked on the Chang’E 6 spacecraft that landed in the Apollo Crater on the farside of the Moon in June 2024, and which performed the first in situ measurements of these radionuclides on the lunar surface, could not detect any radon, and only traces of polonium, at levels much lower than the “hot spots” detected by Apollo 15-16 and by Lunar Prospector (with activities > 100 Bq.m-2), and much lower than the average values of 15 Bq.m-2 for Po-210 and ~10 Bq.m-2 for Rn-222 measured by the Apollo orbiters. Furthermore, models of radon transport predict average diffusive fluxes that are much larger than those measured by DORN. Finally, the DORN experiment confirms that the Moon — with its significantly lower radon exhalation rate — stands out in comparison to Earth, Mars, and even Mercury, despite the latter's presumed resemblance to the Moon. These results impose new, severe constraints on the diffusive component of radon flux and, consequently, on the physical properties of the regolith. At the conference, we will discuss several hypotheses to explain these discrepancies.