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Description
The Moon possesses neither global magnetic field no atmosphere and the solar wind directly impinges upon the Moon regolith. To now it was assumed that the solar wind consisting of mostly protons is fully absorbed by the lunar surface. Recent observations from the Japanese Kaguya, NASA IBEX, and, in particular, Indian Chandrayaan-1 mission showed that the lunar surface neutralizes and backscatters up to 20% of the solar wind flux as hydrogen atoms of an energy of few hundreds eV. The Moon thus emits around 300 tons of hydrogen a year. Beside hydrogen atoms, up to 1% of the impinging protons are reflected without changing the charge state. The protons injected into the solar wind result in a number of plasma effects such as pick-up acceleration and generation of ion beam - instabilities and waves.
The backscattered hydrogen atoms are not affected by electromagnetic forces and can be used for remote diagnostics of the processes taking place at the lunar surface. The surface maps of the hydrogen flux measured from Chandrayaan-1 showed that the solar wind cannot reach some areas, even if they are illuminated by the Sun. Lunar crustal magnetizations with a field up to 100 nT at the surface can form mini-magnetospheres partially shielding the magnetization areas.
Recently the studies of the solar wind – moon interaction became unexpectedly relevant to the fundamental question for the lunar research, namely, the presence of water on the Moon. Since the first sample return missions of the 1960s, lunar scientists firmly believed that the Moon is entirely dry. The data recently obtained by Chandrayaan-1 and the re-analyzed observations from Cassini and Deep Impact fly-by’s challenged that notion: infrared spectroscopic measurements have unambiguously detected absorptions near 3 μm on the lunar surface that are almost certainly due to hydroxyl and/or water. The absorption line strength increases toward the lunar poles and the spectral feature varies in strength with time, suggesting that the water is in a process of rapid and dynamic migration across the lunar surface. Two most important continuous sources of water include (1) reduction of lunar divalent iron in minerals to metallic iron by solar wind protons being implanted into the surface, producing water, and (2) liberation of water from the impact of interplanetary dust and small meteoroids.
All these recent findings and discoveries clear indicate that our knowledge of the only Earth’s moon is surprisingly limited.
Host: Robert Cumming
