在隕石裡面找到地球上年代最久遠的物質：70億年前的星塵
古老的星塵揭露了恆星形成過程中的一段「嬰兒潮」
恆星的生命是道周而復始的循環。它們誕生於宇宙間漂浮的塵埃和氣體找到彼此、接著往內塌縮在一團、然後溫度越來越高的過程。在燃燒了數萬至數億年之後，它們最後會邁向死亡。恆星死亡的時候會把在恆星風中形成的物質往外拋到宇宙，這些微小的星塵最終會形成新的恆星，同時產生新的行星、衛星和隕石。科學家最近在五十年前墜落至澳洲的一顆隕石裡，發現了五十億年至七十億年前形成的星塵，這是目前為止在地球上找到的最古老固態物質。
描述這項發現的研究發表在《美國國家科學院院刊》(Proceedings
of the National Academy of Sciences)。「這是我進行過的研究中讓我感到最興奮的其中之一。」主要作者，菲爾德博物館的研究員暨芝加哥大學的副教授Philipp
Heck表示。「這是人類迄今發現最古老的固態物質，它們訴說了銀河系裡的恆星是如何形成。」
Heck和同事研究的物質形成於太陽誕生以前，稱為太陽前礦物顆粒(presolar
grains-mineral)。Heck說：「這是實際的恆星樣本，真正的『星塵』。」這些微量的星塵數十萬年來都被包覆在隕石當中而沒有產生變化，這讓它們成為時光膠囊，保存了太陽系形成以前的時間。
但是太陽前顆粒十分難以取得。它們相當稀有，墜落到地球上的隕石中大概只有5%能夠發現它們；此外它們也相當微小，即使是最大的顆粒也要湊齊100顆才能跟英文的句號差不多大。不過，地球上有個太陽前顆粒的寶庫：1969年墜落在澳洲的默奇森隕石。澳洲維多利亞州默奇森村的人將此隕石捐出作為科研用途，而菲爾德博物館收藏了的最大一部份。在這項研究中，科學家分析了大約30年前芝加哥大學從默奇森隕石中分離出來的太陽前顆粒。
「首先要把隕石碎片給磨成粉末。」研究共同作者，菲爾德博物館暨芝加哥大學的研究生Jennika
Greer解釋實驗流程。「當所有顆粒都分離開來之後就會變得像是一團糨糊，而且還會散發出一種刺鼻的氣味，就像是壞掉的花生醬。」
他們接著把這團「壞掉的花生醬隕石糨糊」用酸來溶解，直到剩下太陽前顆粒為止。「這就像是把整座海洋抽乾，來撈出裡面的針一樣。」
當太陽前顆粒分離出來之後，研究人員就能調查它們來自於什麼類型的恆星，以及它們有多老。「我們運用的數據稱為『露光年齡』(exposure age)，基本上是測量物質暴露在宇宙射線之下有多久。所謂的宇宙射線是穿梭在銀河系中，可以穿過固態物質的高能粒子。」Heck解釋。「某些宇宙射線會和物質產生交互作用而形成新的元素。因此物質暴露在宇宙射線的時間越長，這類元素就會形成越多。」
他補充說：「我把這比喻成外面下大雨時放個水桶一樣。假設雨勢保持固定，那麼水桶裡累積的水量就能告訴你它被放在外面多久。」透過測量太陽前顆粒含有多少這類宇宙射線產生的新元素，我們就能得出它在宇宙射線之下暴露了多久，進而算出它的年齡。
研究人員根據太陽前顆粒吸收了多少宇宙射線，得知他們的某些樣品是人類迄今發現過的最古老太陽前顆粒。大部分顆粒的年齡是46億年至49億年，其中某些甚至有55億年以上。作為比較，太陽的年齡是46億年，地球則是45億年。
但是得出太陽前顆粒的年齡還不是這項發現的最終成果。由於太陽前顆粒是恆星死亡過程的產物，因此它們可以告訴我們恆星的歷史。結果顯示在70億年前似乎有大批恆星形成――就像是宇宙中的嬰兒潮一般。
「我們找到的年輕顆粒比預期中的還多。」Heck表示。「我們提出的假說認為這些年代為46億年至49億年的顆粒，大部分形成於一段恆星產量提高的時期。因此在太陽系誕生以前，有某段時期形成了比平常還多的恆星。」
科學家對於新的恆星是以穩定的速率形成，或是隨著時間經過新的恆星數量會有所增減還尚無定論。這項發現為此增添了新的證據。「有些人認為星系裡的恆星形成速率是固定的。」Heck表示。「但是透過研究這些顆粒，我們從隕石樣品中得到的直接證據顯示七十億年前，銀河系裡曾有一段時期形成了更多恆星。這是我們研究的重要發現之一。」
Heck指出這不是他們團隊唯一的意外發現。研究人員探討了礦物顆粒跟宇宙射線之間有什麼交互作用――這幾乎只能算是主要研究問題旁的一道附註――結果他們得出太陽前顆粒在宇宙漂流時常常會聚集成一大團。「它們就跟穀麥片一樣黏在一起，」Heck表示，「沒人想過這會在如此小的尺度下發生。」
Heck和同事期望所有這類發現能讓我們更加了解銀河系。他說：「我們透過這項研究直接測出了星塵的壽命。希望研究結果之後可以被人們更加深入的探討，進而成為模擬整個星系的生命循環時輸入的參數之一。」
Heck強調太陽前顆粒和早期太陽系是值得投入終生去探討的課題。他說：「我希望有更多人從事這方面的研究，使我們可以更加瞭解我們居住的星系，也就是銀河系。」
「一但你開始學習這方面的知識，怎麼還會想去研究別的東西？」Greer表示。「這塊領域真的相當迷人，它是世界上最有趣的事物。」
「一直以來我都想用可以握在手中的地質樣品來進行天文學研究。」Heck表示。「可以親眼看見銀河系的歷史真的相當令人興奮。星塵是落到地球的物質中年代最古老的，我們可以運用它們來認識我們的太陽、我們體內的碳來自何方、以及我們呼吸的氧氣起源為何。透過星塵，我們可以追溯這些物質的起源，直到太陽誕生以前的時光。」
Greer表示：「想要得到比這個更好的樣品，就只能直接從一顆恆星上面取回來了。」
Meteorite contains the oldest material
on Earth: 7-billion-year-old stardust
The ancient stardust reveals a 'baby
boom' in star formation
Stars have life cycles. They're born when
bits of dust and gas floating through space find each other and collapse in on
each other and heat up. They burn for millions to billions of years, and then
they die. When they die, they pitch the particles that formed in their winds
out into space, and those bits of stardust eventually form new stars, along
with new planets and moons and meteorites. And in a meteorite that fell fifty
years ago in Australia, scientists have now discovered stardust that formed 5
to 7 billion years ago -- the oldest solid material ever found on Earth.
"This is one of the most exciting studies I've
worked on," says Philipp Heck, a curator at the Field Museum, associate
professor at the University of Chicago, and lead author of a paper describing
the findings in the Proceedings of the
National Academy of Sciences. "These are the oldest solid materials
ever found, and they tell us about how stars formed in our galaxy."
The materials Heck and his colleagues examined are
called presolar grains-minerals formed before the Sun was born. "They're
solid samples of stars, real stardust," says Heck. These bits of stardust
became trapped in meteorites where they remained unchanged for billions of
years, making them time capsules of the time before the solar system..
But presolar grains are hard to come by. They're
rare, found only in about five percent of meteorites that have fallen to Earth,
and they're tiny-a hundred of the biggest ones would fit on the period at the
end of this sentence. But the Field Museum has the largest portion of the
Murchison meteorite, a treasure trove of presolar grains that fell in Australia
in 1969 and that the people of Murchison, Victoria, made available to science.
Presolar grains for this study were isolated from the Murchison meteorite for
this study about 30 years ago at the University of Chicago.
"It starts with crushing fragments of the
meteorite down into a powder ," explains Jennika Greer, a graduate student
at the Field Museum and the University of Chicago and co-author of the study.
"Once all the pieces are segregated, it's a kind of paste, and it has a
pungent characteristic-it smells like rotten peanut butter."
This "rotten-peanut-butter-meteorite paste"
was then dissolved with acid, until only the presolar grains remained.
"It's like burning down the haystack to find the needle," says Heck.
Once the presolar grains were isolated, the
researchers figured out from what types of stars they came and how old they
were. "We used exposure age data, which basically measures their exposure
to cosmic rays, which are high-energy particles that fly through our galaxy and
penetrate solid matter," explains Heck. "Some of these cosmic rays
interact with the matter and form new elements. And the longer they get
exposed, the more those elements form.
"I compare this with putting out a bucket in a
rainstorm. Assuming the rainfall is constant, the amount of water that
accumulates in the bucket tells you how long it was exposed," he adds. By
measuring how many of these new cosmic-ray produced elements are present in a
presolar grain, we can tell how long it was exposed to cosmic rays, which tells
us how old it is.
The researchers learned that some of the presolar
grains in their sample were the oldest ever discovered-based on how many cosmic
rays they'd soaked up, most of the grains had to be 4.6 to 4.9 billion years
old, and some grains were even older than 5.5 billion years. For context, our
Sun is 4.6 billion years old, and Earth is 4.5 billion.
But the age of the presolar grains wasn't the end of
the discovery. Since presolar grains are formed when a star dies, they can tell
us about the history of stars. And 7 billion years ago, there was apparently a
bumper crop of new stars forming-a sort of astral baby boom.
"We have more young grains that we
expected," says Heck. "Our hypothesis is that the majority of those
grains, which are 4.9 to 4.6 billion years old, formed in an episode of
enhanced star formation. There was a time before the start of the Solar System
when more stars formed than normal."
This finding is ammo in a debate between scientists
about whether or not new stars form at a steady rate, or if there are highs and
lows in the number of new stars over time. "Some people think that the
star formation rate of the galaxy is constant," says Heck. "But
thanks to these grains, we now have direct evidence for a period of enhanced
star formation in our galaxy seven billion years ago with samples from
meteorites. This is one of the key findings of our study."
Heck notes that this isn't the only unexpected thing
his team found. As almost a side note to the main research questions, in
examining the way that the minerals in the grains interacted with cosmic rays,
the researchers also learned that presolar grains often float through space
stuck together in large clusters, "like granola," says Heck. "No
one thought this was possible at that scale."
Heck and his colleagues look forward to all of these
discoveries furthering our knowledge of our galaxy. "With this study, we
have directly determined the lifetimes of stardust. We hope this will be picked
up and studied so that people can use this as input for models of the whole
galactic life cycle," he says.
Heck notes that there are lifetimes' worth of
questions left to answer about presolar grains and the early Solar System.
"I wish we had more people working on it to learn more about our home
galaxy, the Milky Way," he says.
"Once learning about this, how do you want to
study anything else?" says Greer. "It's awesome, it's the most
interesting thing in the world."
"I always wanted to do astronomy with geological
samples I can hold in my hand," says Heck. "It's so exciting to look
at the history of our galaxy. Stardust is the oldest material to reach Earth,
and from it, we can learn about our parent stars, the origin of the carbon in
our bodies, the origin of the oxygen we breathe. With stardust, we can trace
that material back to the time before the Sun."
"It's the next best thing to being able to take
a sample directly from a star," says Greer.
原始論文：Philipp
R. Heck, Jennika Greer, Levke Kööp, Reto Trappitsch, Frank Gyngard, Henner
Busemann, Colin Maden, Janaína N. Ávila, Andrew M. Davis, Rainer Wieler.
Lifetimes of interstellar dust from cosmic ray exposure ages of presolar
silicon carbide. Proceedings of the National Academy of Sciences, Jan. 13,
2020; DOI: 10.1073/pnas.1904573117
引用自：Field
Museum. "Meteorite contains the oldest material on Earth:
7-billion-year-old stardust”
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