巨型小行星撞毀產生的塵埃造成許久之前的一段冰期
距今4.66億年前左右，遠在恐龍出現許久之前，地球變的相當寒冷。這段期間兩極的海洋開始結凍，同時全球氣溫進入新的範疇，提供新物種大量演化出來的良好條件。這段冰期的成因一直是個謎團，但最近發表在《科學前緣》(Science Advances)的新研究主張，一顆巨型小行星在外太空撞毀之後產生的塵埃進入地球大氣層，造成全球冷化而引發冰期。
無時無刻都有許多外太空來的塵埃落到地球，偶爾還有小行星或是彗星，但這類塵埃相較於其他大氣中的塵埃，像是火山灰、沙漠沙塵、海鹽等，正常來說只佔了一小部分。然而4.66億年前，火星和木星之間一顆93英里(約150公里)寬的小行星四分五裂之後，產生了比平常多出許多的塵埃。「地球每年接受到的地外物質通常是40000噸左右。」本篇研究的作者之一，菲爾德博物館的研究員暨芝加哥大學的副教授Philipp
Heck表示。「以實物來比喻的話，平常每年落到地球的星際塵埃大概相當於一千輛聯結車。而小行星碰撞後的數百萬年之間，這個數字變成約一千萬輛。」
Heck表示：「我們的假說認為至少在兩百萬年中，這些大量的地外塵埃對地球氣候發揮了重大影響，導致氣溫下降。」
「結果首次證明這類塵埃有時可以讓地球氣溫劇烈下降。」研究主要作者，瑞典隆德大學的Birger
Schmitz表示。他也是菲爾德博物館的助理研究員。「我們的研究讓我們可以基於實際發生的事物來更加詳細理解其中機制，這反過來又可以讓我們用來評估模擬成果是否合乎現實。」
為了瞭解這起事件，研究人員從年代為4.66億年的岩石中尋找宇宙塵埃的蹤跡，並且用南極找到的微隕石作為對照。「沉積岩是過往曾為海床的岩石。我們從地球的沉積紀錄中，仔細調查裏頭含有的地外物質、隕石和微隕石。」Heck表示。「接著我們把地外物質萃取出來，試著查出它們的身分與來源。」
這些地外物質是微隕石和宇宙來的少量塵埃，要把它們萃取出來需要用酸來處理古代岩石，使得石頭溶解而留下來自宇宙的物質。團隊接著分析殘留下來的塵埃化學組成是什麼；此外，他們也分析了古代海床的岩石，目的是尋找地球岩石中很少出現的元素，以及具有特徵顯示來自外太空的同位素(同一種原子的不同形式)。比方說，氦原子一般具有兩個質子、兩個中子和兩個電子，但是從太陽拋射到宇宙的某些氦原子卻會缺少一顆中子。如果找到這些特殊的氦同位素以及時常在小行星中發現的稀有金屬，就能證明這些塵埃是來自宇宙。
之前其他科學家已經確定地球當時歷經了場冰河期。海水深淺會影響海床上岩石的形成方式，而那段期間的岩石顯示海洋當時的深度較淺，意味地球的水分被封鎖在冰河以及海冰裡面。Schmitz和他的同事首度證明冰河期跟大氣中出現更多塵埃是同時發生。他說：「兩者時間配合得非常完美。」由於塵埃可以阻擋陽光，導致全球冷化，因此冰河期的成因可以用大氣出現更多塵埃來解釋。
因為塵埃至少是在兩百萬年之間緩緩飄落地球，所以冷化的速度緩慢到生物有時間可以適應，甚至能從中獲利。生物為了在溫度不同的地區存活而產生適應，造成新物種大量演化出來。
Heck強調雖然他們證明這段全球冷化對地球上的生物有所益處，但是步調相當快速的全球變遷卻會帶來巨大的災難。「我們研究這段全球冷化時，是以數百萬年的時間尺度來討論。它迥異於6500萬年前小行星撞擊，造成恐龍滅絕的氣候變遷；也和我們現在面臨的全球暖化不同。該次全球冷化的推進速度相當和緩，造成的壓力小了許多。」
複製4.66億年前引發全球冷化的落塵來解決當今的全球暖化，是個非常迷人的想法。但是Heck表示他保持著相當謹慎的態度：「地球工程方案需要經過非常嚴格且詳細的評估。如果其中出了什麼差錯，後果可能會比實施前更加糟糕。」
雖然Heck不認為我們找到了氣候變遷的解決方法，但他表示思考這些方案對我們來說是很有幫助的。
「無可否認的，我們正在歷經全球暖化。」Heck說，「我們需要思考應該怎樣才能避免浩劫發生，或者是減輕災難。因此任何合理的想法應該都要加以探討。」
Dust from a giant asteroid crash
caused an ancient ice age
About 466 million years ago, long before
the age of the dinosaurs, the Earth froze. The seas began to ice over at the
Earth’s poles, and the new range of temperatures around the planet set the
stage for a boom of new species evolving. The cause of this ice age was a
mystery, until now: a new study in Science
Advances argues that the ice age was caused by global cooling, triggered by
extra dust in the atmosphere from a giant asteroid collision in outer space.
There’s always a lot of dust from outer space
floating down to Earth, little bits of asteroids and comets, but this dust is
normally only a tiny fraction of the other dust in our atmosphere such as
volcanic ash, dust from deserts and sea salt. But when a 93-mile-wide asteroid
between Mars and Jupiter broke apart 466 million years ago, it created way more
dust than usual. “Normally, Earth gains about 40,000 tons of extraterrestrial
material every year,” says Philipp Heck, a curator at the Field Museum,
associate professor at the University of Chicago, and one of the paper’s
authors. “Imagine multiplying that by a factor of a thousand or ten thousand.”
To contextualize that, in a typical year, one thousand semi trucks’ worth of
interplanetary dust fall to Earth. In the couple million years following the
collision, it’d be more like ten million semis.
“Our hypothesis is that the large amounts of
extraterrestrial dust over a timeframe of at least two million years played an
important role in changing the climate on Earth, contributing to cooling,” says
Heck.
“Our results show for the first time that such dust,
at times, has cooled Earth dramatically,” says Birger Schmitz of Sweden’s Lund
University, the study’s lead author and a research associate at the Field
Museum. “Our studies can give a more detailed, empirical-based understanding of
how this works, and this in turn can be used to evaluate if model simulations
are realistic.”
To figure it out, researchers looked for traces of
space dust in 466-million-year-old rocks, and compared it to tiny
micrometeorites from Antarctica as a reference. “We studied extraterrestrial
matter, meteorites and micrometeorites, in the sedimentary record of Earth,
meaning rocks that were once sea floor,” says Heck. “And then we extracted the
extraterrestrial matter to discover what it was and where it came from.”
Extracting the extraterrestrial matter—the tiny
meteorites and bits of dust from outer space—involves taking the ancient rock
and treating it with acid that eats away the stone and leaves the space stuff.
The team then analyzed the chemical makeup of the remaining dust. The team also
analyzed rocks from the ancient seafloor and looked for elements that rarely
appear in Earth rocks and for isotopes—different forms of atoms—that show
hallmarks of coming from outer space. For instance, helium atoms normally have
two protons, two neutrons, and two electrons, but some that are shot out of the
Sun and into space are missing a neutron. The presence of these special helium
isotopes, along with rare metals often found in asteroids, proves that the dust
originated from space.
Other scientists had already established that our
planet was undergoing an ice age around this time. The amount of water in the
Earth’s oceans influences the way that rocks on the seabed form, and the rocks
from this time period show signs of shallower oceans—a hint that some of the
Earth’s water was trapped in glaciers and sea ice. Schmitz and his colleagues
are the first to show that this ice age syncs up with the extra dust in the
atmosphere. “The timing appears to be perfect,” he says. The extra dust in the
atmosphere helps explain the ice age—by filtering out sunlight, the dust would
have caused global cooling.
Since the dust floated down to Earth over at least
two million years, the cooling was gradual enough for life to adapt and even
benefit from the changes. An explosion of new species evolved as creatures
adapted for survival in regions with different temperatures.
Heck notes that while this period of global cooling
proved beneficial to life on Earth, fast-paced climate change can be
catastrophic. “In the global cooling we studied, we’re talking about timescales
of millions of years. It’s very different from the climate change caused by the
meteorite 65 million years ago that killed the dinosaurs, and it’s different
from the global warming today—this global cooling was a gentle nudge. There was
less stress.”
It’s tempting to think that today’s global warming
could be solved by replicating the dust shower that triggered global cooling
466 million years ago. But Heck says he would be cautious: “Geoengineering
proposals should be evaluated very critically and very carefully, because if
something goes wrong, things could become worse than before.”
While Heck isn’t convinced that we’ve found the
solution to climate change, he says it’s a good idea for us to be thinking
along these lines.
“We’re experiencing global warming, it’s undeniable,”
says Heck. “And we need to think about how we can prevent catastrophic
consequences, or minimize them. Any idea that’s reasonable should be explored.”
原始論文：Birger
Schmitz, Kenneth A. Farley, Steven Goderis, Philipp R. Heck, Stig M. Bergström,
Samuele Boschi, Philippe Claeys, Vinciane Debaille, Andrei Dronov, Matthias Van
Ginneken, David A.t. Harper, Faisal Iqbal, Johan Friberg, Shiyong Liao, Ellinor
Martin, Matthias M. M. Meier, Bernhard Peucker-Ehrenbrink, Bastien Soens,
Rainer Wieler and Fredrik Terfelt. An extraterrestrial trigger for the
mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body. Science
Advances, 2019 DOI: 10.1126/sciadv.aax4184
引用自：Field Museum. "Dust from a giant asteroid
crash caused an ancient ice age."
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