Lunar Xenon from Ancient Earth-Wind

Lunar Xenon from Ancient Earth-Wind

 The apparent secular variability of the Xe/Kr abundance ratio in the solar wind implanted in grains of the lunar regolith is a long-standing problem (Wieler, 2016): Recently irradiated soils (<100 Ma ago) show Xe/Kr ratios comparable to the ratio found in solar wind targets of the Genesis mission (Meshik et al. 2014, Vogel et al. 2011), while lunar samples exposed billions of years ago to the solar wind exhibit a Xe/Kr ratio about twice as high. 

It has been argued that this observation is the consequence of the variability of the solar wind composition. More recently it has also been suggested that, over time, cometary impacts have contributed significantly to the inventory of noble gases and other volatiles of the lunar regolith.

From our understanding of the development of typical G-stars, such as the Sun, we consider it unlikely that such a strong variation could have occurred several 100 My after the lunar regolith had started to build up. Today, variations of this ratio even in very different solar wind regimes are marginally distinguishable (Vogel et al. 2019). On the other hand, it seems also unlikely that early cometary impacts could have implanted sufficient amounts of Xe to noticeably modify the Xe/Kr inventory in the regolith with the correct isotopic compositions.

While we are currently unable to clearly outrule any of the above hypotheses, we here propose an alternative explanation: The ancient lunar regolith has been exposed to a xenon-rich Earth-Wind. An ancient Earth-Wind has been invoked previously (e.g., Geiss and Bochsler, 1991, Ozima et al. 2005) in order to explain the secular variability of the isotopic composition of nitrogen in the lunar regolith.

The apparent secular depletion of light isotopes of atmospheric xenon combined with the presumed large deficit of Xe in the atmosphere (Avice et al. 2018) recently led Zahnle et al. (2019) to postulate a loss of Xe ions over the first 3 Gy from the upper atmosphere, concomitant with the hydrogen escape and the oxygenation of the atmosphere. The loss mechanism devised by Zahnle and co-authors selectively involves Xe without affecting the other noble gases. It operates through resonant charge exchange of H⁺ with Xe, leading to a low-lying excited state of Xe⁺.

We believe that escaping terrestrial Xe ions will undoubtedly be incorporated into the flow of the magnetotail of the Earth and impact the lunar surface, whenever the Moon crosses the tail directed away from the Sun. From the present cross section of the magnetotail near the orbit of the Moon and the amount of xenon lost from the terrestrial atmosphere over the first few Gy, we conclude that the Xe-fluence of the Earth-Wind could be sufficient to account for the apparent secular variation of the lunar Xe/Kr ratio. Since we expect Earth-Wind-xenon to be strongly fractionated in favour of light isotopes, we expect its isotopic composition to deviate significantly from the present-day terrestrial atmosphere, approaching the composition of the solar wind. Unfortunately, given the experimental uncertainties of the isotopic composition of xenon in ancient lunar soil, it is difficult to obtain conclusive evidence in favour or against the Earth-Wind hypothesis from isotopic abundances.

                                                              

 

In a simple box model as outlined in Figure 1, we investigate the potential contribution of the Earth-Wind to the lunar regolith using the compilation of data on the isotopic composition of Xe in the ancient atmosphere of Avice et al. (2018) and the abundance of Xe in the mantle to determine free parameters. Our first results indicate that the Earth-Wind is a viable alternative to explain the apparent secular decrease of the Xe/Kr ratio in the lunar regolith, even if the solar wind has decreased in intensity over the life-time of the Sun.

The Earth-Wind hypothesis could be tested by investigation of ancient lunar regolith samples with present-day state-of-the-art mass spectrometry and by analysis of lunar samples at different lunar longitudes, particularly from the lunar backside, which at least at present, is mostly shielded from the ion-flow in the geotail.     

References:

Avice G. et al. (2018) Geochimica et Cosmochimica Acta  232, 82-100.

Geiss J., and Bochsler P. (1991) In: The Sun in Time, The University of Arizona Press, 98-117.

Meshik A. et al. (2014) Geochimica et Cosmochimica Acta 127, 326-347.

Ozima M. et al. (2005) Nature 436, 655-659.

Vogel N. et al. (2011) Geochimica et Cosmochimica Acta 75, 3057-3071.

Vogel N. et al. (2019) Geochimica et Cosmochimica Acta 263, 182-194.

Wieler R. (2016) Chemie der Erde - Geochemistry 76, 463-480.

Zahnle K.J., Gacesa M., and Catling D.C. (2019), Geochimica et Cosmochimica Acta 244, 56-85.