Research press release





今回、Martin Bizzarroたちの研究グループは、火星の南半球の高地に由来すると考えられている隕石NWA 7034の試料から、7点のジルコン(耐熱性と耐腐食性が非常に高い鉱物)を抽出した。Bizzarroたちは、ウラン-鉛同位体年代測定法を用いて、これらのジルコンが約44億7600万年~44億2900万年前のものであることを明らかにした。そして、Bizzarroたちは、これらのデータとジルコンのハフニウム同位体組成を用いて、火星の原始地殻が45億4700万年前に存在しており、それから約1億年間存続し、その後、何らかの原因(天体の衝突かもしれない)で地殻の再形成が起こり、NWA 7034に含まれるジルコンが結晶化したマグマが形成したという考えを示している。

An analysis including the oldest directly dated material from Mars suggests that a primordial Martian crust formed no later than 4,547 million years ago. Published in this week’s Nature, the findings suggest that some of the early processes involved in the creation of Mars, including accretion, core formation and magma ocean crystallization, were completed less than 20 million years after the formation of the Solar System.

The meteorite record suggests that the accretion of Mars was largely complete within approximately five million years of the formation of the Solar System. However, the crystallization of the magma ocean, leading to the extraction of a primordial crust, was thought to have occurred about 30 to 100 million years later.

Martin Bizzarro and colleagues extracted seven zircons (zircon is a mineral very resistant to heat and corrosion) from a sample of the NWA 7034 meteorite, thought to have originated from the southern highlands of Mars. Using uranium-lead isotope dating, the authors found that the zircons were approximately 4,476 to 4,429 million years old. Using these data and the hafnium isotope compositions of the zircons, the authors suggest that a primordial crust existed on Mars by 4,547 million years ago and it survived for approximately 100 million years before it was reworked, possibly by impacts, to produce the magmas from which the NWA 7034 zircons crystallized.

doi: 10.1038/s41586-018-0222-z

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