隕石「黑美人」加深了我們對於火星生命可能於何時出現的瞭解
僅在太陽系形成的2000萬年之後，早期火星表面的液態岩漿海就以極快的速度開始結晶。之後，這座紅色行星的原始地殼之上可能孕育了由水組成並含有生命的海洋。相較而言，地球的固體地殼要在1億3000萬年之後才出現。哥本哈根大學丹麥自然歷史博物館的恆星與行星形成研究中心最近發表的研究中，呈現了火星表面迅速結晶固化而形成地殼的新證據。透過分析一顆來自火星的罕見隕石「黑美人」，這項研究大幅加深了我們對於火星的生命可能存在於何時的瞭解。
來自火星的隕石「黑美人」。圖片來源：NASA
火星隕石「黑美人」對地球來說不過是新聞上的一則短訊。在類地形星的發育過程中，地殼形成是非常重要的一步。而使「黑美人」如此特別且珍貴的原因便是其含有火星的小塊地殼碎片。更精確地來說，黑美人含有一種稀有的礦物――鋯石，研究人員發現它們的鉿含量相當地高。
鋯石是一種相當堅固的礦物，使其相當適合提供其形成的絕對年代。教授Martin Bizzarro表示因為如此，我們可以利用鋯石建立出的時序來瞭解火星地殼形成的歷史。他繼而表示：
「鋯石就像是微小的時間膠囊一樣，可以保存它們形成當時所在地點的環境資訊。在此研究中，有一顆像這樣的時間膠囊擁有源自的鉿來自於火星最早的地殼，顯示在『黑美人』中最古老的鋯石形成的1億年前，火星最早的地殼已然形成。因此，相較於固體地殼在許久之後才形成的地球來說，火星可說是起步得相當早。
然而，要得到此結論需要一定程度的勇氣。
磨碎這顆隕石！
在2011年撒哈拉沙漠發現的「黑美人」隕石原本有319.8克重。很快地人們便發現這顆隕石的來頭並不平凡，而它的拍賣價格目前是每1公克值大約1萬美元。在一年前，Martin Bizzarro教授在許多機構的資助之下，加上以自家博物館的隕石收藏品來交換，而成功拿到44克的「黑美人」。
他們面臨到的艱鉅挑戰之一是黑美人中的鋯石顆粒極為微小。Martin Bizzarro說他們採取一種相當大膽的策略來解決：他們將這顆珍貴的隕石給磨碎。確切來說，他們磨碎了其中的5公克。Bizzarro 繼而表示：
「現在，我很高興我們選擇了這項方法。藉此我們得到了七顆鋯石，其中一顆是火星已知最古老的鋯石。我們從這顆鋯石以及其鉿含量得到的結論顯示，火星表面的結晶過程發生地極為快速――在太陽系形成的2千萬年之後，火星就已經擁有固體地殼，而其上方可能孕育了由水組成的海洋，甚至還有生命存於其中。
The meteorite 'Black Beauty' expands
the window for when life might have existed on Mars
The early surface of Mars consisting of a liquid
magma ocean crystallized extremely rapidly, just 20 million year after the
formation of the solar system. Thereafter, a solid crust emerged on the red
planet potentially housing oceans with water and life. This was about 130
million years before a corresponding solid crust appeared on Earth. New
evidence for this rapid crystallization and crust formation on Mars has just
been published in a study from the Centre for Star and Planet Formation at the
Natural History Museum of Denmark, University of Copenhagen. The study, based
on the analysis of the rare Mars meteorite Black Beauty, significantly expands
the window for when life might have existed on Mars.
The Mars meteorite Black Beauty has literally brought crisp news
to Earth. Crust formation is an important step in the development of
terrestrial planets, and what makes Black Beauty special and expensive is that
it contains small pieces of the crust from Mars. More precisely, Black Beauty
contains the rare mineral zircon, in which researchers have found a high
concentration of hafnium.
Zircon is a very robust mineral that is ideally suited to
provide absolute ages. In this context, the zircons can be used to establish a
temporal framework to understand the formation history of the Martian crust,
says Professor Martin Bizzarro, and continues:
Zircon also acts as a small time capsule as it preserves
information about the environment where and when it was created. In this case,
a time capsule with hafnium that originates from the earliest crust of Mars,
which was present approximately 100 million years before the oldest zircon of
Black Beauty was created. Thus, Mars got an early start compared to Earth,
whose solid crust wasn’t formed until much later.
However, it required a certain amount of courage to reach this
result.
We crushed
the meteorite
The original 319.8 grams heavy meteorite Black Beauty was found
in the Sahara Desert in 2011. It soon became apparent that the meteorite was
something special and it currently has a sales price of approx. $ 10,000 per
gram. A year ago, Professor Martin Bizzarro managed to acquire 44 grams of
Black Beauty with help from various funding agencies and exchange of meteorites
from the museum's collection.
One of the big challenges has been that the zircons in Black
Beauty are extremely small. This called for a courageous strategy: We crushed
our precious meteorite. Or to be precise: We crushed 5 grams, says Martin
Bizzarro and continues:
Today, I'm glad we chose that strategy. It released seven
zircons, one of which is the oldest known zircon from Mars. And from the
zircons and their content of hafnium, we can now conclude that the
crystallization of the surface of Mars went extremely fast: already 20 million
years after the formation of the solar system, Mars had a solid crust that
could potentially could house oceans and perhaps also life.
原始論文：Laura C.
Bouvier, Maria M. Costa, James N. Connelly, Ninna K. Jensen, Daniel Wielandt,
Michael Storey, Alexander A. Nemchin, Martin J. Whitehouse, Joshua F. Snape,
Jeremy J. Bellucci, Frédéric Moynier, Arnaud Agranier, Bleuenn Gueguen, Maria
Schönbächler, Martin Bizzarro. Evidence for extremely rapid magma ocean
crystallization and crust formation on Mars. Nature, 2018; 558
(7711): 586 DOI: 10.1038/s41586-018-0222-z
引用自：University of Copenhagen. "Meteorite
'Black Beauty' expands window for when life might have existed on Mars."
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