Department of Earth and Planetary Sciences, McDonnell Center for the Space Sciences, Washington University in St. Louis, St. Louis, MO 63130;
Section of Isotope Geochemistry and Geochronology, Czech Geological Survey, CZ-118 21 Prague, Czech Republic.
Proc Natl Acad Sci U S A. 2021 Sep 28;118(39). doi: 10.1073/pnas.2101155118.
The abundances of water and highly to moderately volatile elements in planets are considered critical to mantle convection, surface evolution processes, and habitability. From the first flyby space probes to the more recent "Perseverance" and "Tianwen-1" missions, "follow the water," and, more broadly, "volatiles," has been one of the key themes of martian exploration. Ratios of volatiles relative to refractory elements (e.g., K/Th, Rb/Sr) are consistent with a higher volatile content for Mars than for Earth, despite the contrasting present-day surface conditions of those bodies. This study presents K isotope data from a spectrum of martian lithologies as an isotopic tracer for comparing the inventories of highly and moderately volatile elements and compounds of planetary bodies. Here, we show that meteorites from Mars have systematically heavier K isotopic compositions than the bulk silicate Earth, implying a greater loss of K from Mars than from Earth. The average "bulk silicate" δK values of Earth, Moon, Mars, and the asteroid 4-Vesta correlate with surface gravity, the Mn/Na "volatility" ratio, and most notably, bulk planet HO abundance. These relationships indicate that planetary volatile abundances result from variable volatile loss during accretionary growth in which larger mass bodies preferentially retain volatile elements over lower mass objects. There is likely a threshold on the size requirements of rocky (exo)planets to retain enough HO to enable habitability and plate tectonics, with mass exceeding that of Mars.
行星中水和高挥发性元素的丰度被认为对地幔对流、表面演化过程和宜居性至关重要。从第一次飞越空间探测器到最近的“毅力号”和“天问一号”任务，“寻找水”，更广泛地说，“挥发物”一直是火星探索的关键主题之一。挥发性元素与难熔元素（例如 K/Th、Rb/Sr）的比值表明，火星的挥发性元素含量高于地球，尽管这两个天体的现今表面条件截然不同。本研究通过一系列火星岩石的光谱提供了 K 同位素数据，作为比较行星体高挥发性元素和化合物含量的示踪剂。在这里，我们表明火星陨石的 K 同位素组成比地球的整体硅酸盐更重，这意味着火星的 K 损失比地球更大。地球、月球、火星和小行星 4-Vesta 的平均“整体硅酸盐”δK 值与表面重力、Mn/Na“挥发性”比以及最显著的行星 HO 丰度相关。这些关系表明，行星挥发性丰度是由于在吸积生长过程中挥发性元素的可变损失所致，其中较大质量的物体优先保留挥发性元素，而较低质量的物体则优先保留。可能存在一个对 rocky（exo）行星的大小要求的阈值，以保留足够的 HO 来实现宜居性和板块构造，其质量超过火星。
