The DORN instrument—for Detection of Outgassing RadoN and also for the physicist Friedrich Dorn, the discoverer of radon—will for the first time ever measure the concentration of radon on the surface of the Moon. From lunar orbit, the U.S. Apollo 15 and Apollo 16 (1971-1972) and Lunar Prospector (1998-1999) missions and the Japanese Kaguya probe (2007-2009) revealed spatial and temporal variations in this radioactive gas generated by the decay of uranium. But no in-situ data have been acquired before. DORN will be capable of detecting alpha particles emitted by the decay of radon within a radius of a several tens of metres around the lander with ten times more sensitivity than its orbital predecessors.
A tracer for lunar outgassing
DORN will estimate outgassing from the lunar crust and its contribution to the Moon’s exosphere, which is extremely tenuous and short-lived due to its interaction with ultraviolet radiation and the solar wind, as a result of which it has to be permanently replenished. One source for this process is outgassing from the crust (of helium-4, argon-40 and radon), the others being the solar wind (helium-4 and argon-36), interactions between the solar wind and the lunar surface (sodium, potassium, methane, nitrogen, carbon dioxide, etc.) and meteorite impacts (mainly H2O).
Radon is the ideal tracer of lunar outgassing.
“It can only come from the lunar regolith. What’s more, the short life of its isotopes—5½ days for radon-222 and just 1 minute for radon-220—means that emissions can be more easily located than for argon-40, which remains for about 25 days in the exosphere,” explains Pierre-Yves Meslin, a research scientist at the IRAP astrophysics and planetology research institute and DORN principal investigator (PI).
Provisional logo for the DORN instrument.
Artist’s concept of the Chang’e 5 lander with its lunar sample ascent module. The Chang’e 6 lander will be a carbon copy of this rover. Credits: CNSA/CLEP.
Pierre-Yves Meslin is an expert in extraterrestrial radon. Funded by CNES and IRSN, the French nuclear radioprotection and safety institute, his thesis in 2008 was on radon on Mars. He has also studied radon on Mercury.
Solving the mystery of water ice in the Moon’s craters
The different half-lives of the two isotopes of radon will allow the dynamics of its transport to be constrained and make it possible to distinguish radon outgassed locally and that coming from a wider area around the lander. Lunar samples returned by the Chang’e 6 mission will also enable in-situ and laboratory measurements—for example of uranium content, radon emanation, radon daughter surface deposits and gas transport properties—to be compared.
Understanding radon transport on the Moon will also provide new insights into the transport of water molecules and the presence of water ice at its poles. “The large amounts of water ice detected in permanently shadowed craters suggest that water migrates to the poles, where it’s trapped like in a freezer. But the transport of gases in the lunar exosphere is still poorly understood. By measuring radon, we’ll learn more about how one of them migrates and then be able to transpose what we find to transport models to study how water molecules migrate,” says Pierre-Yves Meslin.
Is dust in suspension on the Moon?
DORN is an alpha spectrometer that will detect the decay of radon daughters. Scientists are especially interested in studying the decay of polonium-210, which on average decays 32 years after radon-222. The presence of polonium-210 on the lunar surface would indicate that dust from its regolith is not in suspension as a result of radiation, solar particles or micro-meteorites.
As of end 2019, Chang’e 6’s landing site had not yet been decided. If the Chang’e 5 mission is a success, Chang’e 6 will target the South Pole. After landing, DORN will operate for 48 hours before the ascent module departs. These two days are not expected to be sufficient to reveal any temporal variations in radon and notably any bursts possibly related to lunar seismic activity. “DORN is designed to demonstrate the potential value of doing these kinds of measurements on the lunar surface,” points out Francis Rocard, in charge of solar system exploration programmes at CNES.
DORN will be built at the IRAP astrophysics and planetology research institute (Toulouse III University, CNRS, CNES) with oversight from CNES, in partnership with the French atomic energy and alternative energies commission CEA (including the LNHB laboratory overseen by the LNE national metrology and testing laboratory), the Subatech subatomic physics and technologies laboratory, the Arronax public research consortium, and outside France with support from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS), the China University of Geoscience in Beijing, the Christian-Albrechts University of Kiel and the Planetary Science Institute (PSI) in the United States.
IRAP research scientist, DORN principal investigator
E-mail: pierre-yves.meslin at rap.omp.eu
Head of solar system exploration programmes, CNES
E-mail: francis.rocard at cnes.fr
Tel.: +33 (0)1 44 76 75 98