New Insights on Martian Atmospheric Neon from Martian Meteorite, Dhofar

Introduction: Martian space probe, Viking landers could not measure Ne isotopic ratios, but only had the concentration of Ne as 2.5 ppm (error +3.5, -1.5) in Martian atmosphere [1]. After that, there have been many efforts to determine Ne isotopic composition of Martian atmosphere, by examining the Martian atmospheric gases trapped in impact glasses of the EET 79001 and other shergottites [2-6]. The suggested Ne/Ne ratios of Martian atmosphere are various from ∼7.0 to 10.6, assuming the trapped Ne/Ne ratio of 0.028 0.03. For example, Pepin [5] suggested the Martian atmospheric Ne/Ne ratio of 10.1±0.7, compromising two different values 10.6±0.6 [4] and 9.6±0.6 [3]. Afterward, Garrison and Bogard [6] inferred Ne composition of Martian atmosphere as a value of ∼7.0 for Ne/Ne ratio, with Ne/Ne ratio of 0.029 from the extrapolation of Y-793605, while other data for shergottites seemed to be scattered between 7∼10 in Ne/Ne ratios. Even though Martian atmospheric Ar, Kr, and Xe have been fairly constrained, the composition of atmospheric Ne is poorly determined, because of predominant cosmogenic and terrestrial Ne components in Martian meteorites. Accurate determination of Ne/Ne and Ne/Ne ratios of Martian atmosphere is essential to clarify the Martian atmospheric history of volatile element degassing and subsequent evolution, because the ratios will constrain the origin of Ne, degree of degassing from interior, degree of atmospheric loss to space, and contribution of spallogenic Ne to atmosphere. Here we present a new data on Ne/Ne ratio of Martian atmosphere inferred from the noble gas data obtained from Dhofar 378 shergottite (hereafter DHO 378). The full data set of noble gas concentrations and isotopic ratios of DHO 378 had been reported by Park [7]. Experimental Method: The DHO 378 shergottite is a single stone, which has no pairing meteorite. It has transparent plagioclase glass (47 vol. %) and blackcolored clinopyroxenes (49 vol. %) with minor accessory minerals [8, 9]. The plagioclase glassy part has been recrystallized, indicating that DHO 378 must have experienced very strong shock on Mars, followed by slow cooling to allow recrystallization of plagioclase [10]. Grains taken from the DHO 378 were washed in diluted-HNO3-solution with an ultrasonic bath to avoid terrestrial materials and weathering products adhering on the sample surfaces. After the treatment, microscopic observation shows clean glassy surface and numerous cavities inside the samples, suggesting volatile rich bubbles of Martian origin. Noble gases were analyzed using a mass spectrometer (modifiedVG5400/MS-II) at the Laboratory for Earthquake Chemistry, University of Tokyo. In the series of noble analyses of DHO 378, stepwise heating on pyroxenerich separates (0.1070g) showed high concentrations and low Ne/Ne ratios of trapped Ne, which seems to be closely related to the Martian atmospheric Ne. It is noteworthy that trapped Ne was highly enriched in DHO 378 compared to those of other shergottites. The trapped Ne is not enriched in the plagioclase glass phase but in pyroxene-rich phase of this meteorite. Results and Discussion: Here we present a new data on Ne/Ne ratio of Martian atmosphere inferred from the noble gas data obtained from DHO 378. Three data points of pyroxene-part in DHO 378 (Fig. 1) for the temperature fractions of 600, 800 and 1000°C, which have the Ne-enrichments, show a perfect linear correlation. This means a mixing of two Ne components of different origins; one is cosmic-ray produced Ne and another trapped (Martian atmospheric) Ne. Other three points especially for the lowest (400°C) and the highest (1750°C) temperature fractions are plotted above the linear array, which would be caused by additional component(s) probably from terrestrial Ne contamination. Extrapolation of the line fitted to the 600, 800 and 1000°C data points to Ne/Ne ratio of ∼0.03 (typical cosmogenic-free solar system Ne) gives the trapped Ne/Ne ratio of 7.3 0.1. Almost the same value of ∼7.0 has been proposed as the Martian atmospheric Ne based on the Ne data for Y 793605 and EET 79001 glasses by Garrison and Bogard [6]. The Ne isotopic ratios for Y 793605 glass by them are also plotted in Fig. 1, where the data point for 350°C is also plotted on the line within experimental errors. The good agreement supports the Ne/Ne ratio of 7.3 0.1 for the Martian atmosphere. The value, however, does not agree with the higher values around 10 by Swindle et al. [4] and Wiens et al. [3]. Origin of noble gases in Earth’s atmosphere has been discussed in Ozima & Podosek [11, 12] and Ozima et al. [13]. They showed a linear trend for the abundance ratios among Ne, Ar, Kr and Xe in