ALBERT

Beschreibung: [1]  We present the first direct measurement of neutral oxygen in the lunar exosphere, detected by the Chandrayaan-1 Energetic Neutral Analyzer (CENA). With the lunar surface consisting of about 60% of oxygen in number, the neutral oxygen detected in CENA's energy range (11 eV – 3.3 keV) is attributed to have originated from the lunar surface, where it is released through solar wind ion sputtering. We verify this proposition by comparing the measured oxygen content in two different mass spectra groups with ion sputtering theory. One group contains mass spectra that were recorded when the solar wind consisted of almost pure hydrogen and the other group contains mass spectra that were recorded when the helium content in the solar wind was very high (〉3.5%). Since helium is a much more effective sputtering agent than hydrogen (5% of alpha particles present in the solar wind typically contribute 30% of the total sputter yield), these two groups should show clear differences in the oxygen sputter yield. Fitting of CENA's mass spectra with calibration spectra from ground and in-flight data resulted in the detection of a robust oxygen signal, with a flux of 0.2 to 0.4 times the flux of backscattered hydrogen, depending, as expected, on the solar wind helium content and particle velocity. These measurements present the first in-situ detection of oxygen in the lunar exosphere. For the two solar wind types observed, we derive sub-solar surface oxygen atom densities of N 0  = (1.1 ± 0.3) ⋅ 10 7 m − 3 and (1.4 ± 0.4) ⋅ 10 7 m − 3 , respectively, which agree well with earlier model predictions and measured upper limits. From these surface densities we derive, by modeling, column densities of N C  = (1.5 ± 0.5) ⋅ 10 13 m − 2 and (1.6 ± 0.5) ⋅ 10 13 m − 2 . [2]  In addition, in the CENA mass spectra, we identified for the first time a helium component. This helium is attributed to backscattering of solar wind helium (alpha particles) from the lunar surface as neutral energetic helium atoms, which has been observed for the first time. This identification is supported by the characteristic energy of the measured helium atoms, which is roughly four times the energy of reflected solar wind hydrogen, and the correlation with solar wind helium content.