Comparative study of degassing between Mars and Earth bade on noble gases in Martian meteorites

大气和地球内部的稀有气体同位素组成表明，早期从内部脱气形成大气。与此相反，从海盗号着陆器和火星陨石上观测到的火星惰性气体分布与地球上的完全不同。为了详细证实这一点，我们使用高灵敏度质谱仪和中子辐照样品测量了许多火星陨石。主要研究结果如下：1.在火星大气中，Dhofar378雪尔果石中的富Ne比值为7.3±0.1。这个数值表明火星大气严重流失。NWA2737榴辉石的^&lt；40；Ar/^&lt；36&gt；Ar和^&lt；129&gt；Xe/^&lt；132&gt；Xe明显高于Chassigny。这意味着火星内部的稀有气体同位素分布不均。火星内部可能没有发生均匀的脱气现象。MIL 03346纳赫利特有两个相，含有严重的元素分馏的火星大气。火星上形成的风化产物可能是分离出的稀有气体的载体。正如在查斯蒂观察到的那样，大多数雪戈提岩的端部成分之一被认为是火星内部的惰性气体。我们的橄榄石和沙漠沙风化产物的稀有气体数据有力地表明，该组分不是来自火星，而是分馏地球大气。在700-1400℃的温度范围内，SAU060型闪锌矿的Ar-Ar年龄为1-1.8Ga，在较高温度下为3-4Ga。较老的年龄可能代表火星地壳的年龄。

Noble gas isotopic compositions in atmosphere and interior of the Earth suggest early degassing from interior to form atmosphere. Contrary to this, noble gas distributions of Mars observed from the Viking lander and Martian meteorites are completely different from those in the Earth. To confirm this in detail, we measured many Martian meteorites using high sensitive mass spectrometers and neutron irradiated samples. Some of the results are followings :1. Enriched Ne in Dhofar 378 shergottite shows ^<20>Ne/^<22>Ne ratio of 7.3±0.1 for Martian atmosphere. The value suggests a severe loss of Martian atmosphere.2. NWA2737 chassignite shows much higher ^<40>Ar/^<36>Ar and ^<129>Xe/^<132>Xe than those for Chassigny. This means heterogeneous distribution of noble gas isotopes in the interior of Mars. Uniform degassing might not have occurred from the Martian interior.3. MIL 03346 nakhlite have two phases containing heavily elementally fractionated Martian atmosphere. Weathering products formed on Mars are probable carriers for the fractionated noble gases.4. One of the end components observed for most shergottites is thought to be a Martian interior noble gas as is observed in Chassigny. Our noble gas data for weathering products of olivine and desert sands strongly indicate that the component is not Martian origin but fractionated Earth atmosphere.5. Ar-Ar ages for SaU 060 shergottite are 1.0-1.8 Ga at the temperatures of 700-1400℃ and 3-4 Ga at the higher temperatures. The older age might represent the age of Martian crust.