宇宙微塵揭露遠古大氣的成分

原始網址：www.sciencedaily.com/releases/2016/05/160511154214.htm

Cosmic dust reveals Earth's ancient atmosphere

宇宙微塵揭露遠古大氣的成分

Using the oldest fossil micrometeorites -- space dust -- ever found, Monash University-led research has made a surprising discovery about the chemistry of Earth's atmosphere 2.7 billion years ago.

利用自古以來發現的最古老微隕石化石—宇宙塵，蒙納許大學領導的研究對27億年前的大氣化學成分做出了驚人的發現。

The findings of a new study published today in the journal Nature -- led by Dr Andrew Tomkins and a team from the School of Earth, Atmosphere and Environment at Monash, along with scientists from the Australian Synchrotron and Imperial College, London -- challenge the accepted view that Earth's ancient atmosphere was oxygen-poor. The findings indicate instead that the ancient Earth's upper atmosphere contained about the same amount of oxygen as today, and that a methane haze layer separated this oxygen-rich upper layer from the oxygen-starved lower atmosphere.

由蒙納許大學地球、大氣與環境學院的 Andrew Tomkins博士及團隊率領，偕同來自澳洲同步輻射中心、倫敦帝國學院的科學家，於今日刊登在期刊《自然》（Nature）的研究中，挑戰了地球遠古大氣相當缺乏氧氣的普遍看法。這項發現指出地球古代的高層大氣含氧量與現今相若，且有一層甲烷霧霾將富含氧的高層大氣從缺氧的低層大氣隔絕開來。

Dr Tomkins explained how the team extracted micrometeorites from samples of ancient limestone collected in the Pilbara region in Western Australia and examined them at the Monash Centre for Electron Microscopy (MCEM) and the Australian Synchrotron.

Tomkins博士解釋他們的團隊是如何從西澳皮爾布拉(Pilbara)地區的遠古石灰岩樣品中萃取出微隕石，並在澳洲同步輻射中心和蒙納許電子顯微鏡中心檢視它們。

"Using cutting-edge microscopes we found that most of the micrometeorites had once been particles of metallic iron -- common in meteorites -- that had been turned into iron oxide minerals in the upper atmosphere, indicating higher concentrations of oxygen than expected," Dr Tomkins said.

「我們利用最先進的顯微鏡發現這些微隕石大都曾是隕石中常見的金屬鐵微粒，它們在經過高層大氣時被轉變成氧化鐵礦物，顯示當時氧氣濃度比預期中的還要高。」Tomkins 博士說。

"This was an exciting result because it is the first time anyone has found a way to sample the chemistry of the ancient Earth's upper atmosphere," Dr Tomkins said.

「這個結果相當令人興奮。因為這是人們首度發現有一種方法可以用來取樣遠古大氣高層的化學性質。」Tomkins 博士說。

Imperial College researcher Dr Matthew Genge -- an expert in modern cosmic dust -- performed calculations that showed oxygen concentrations in the upper atmosphere would need to be close to modern day levels to explain the observations.

帝國學院的Matthew Genge博士是研究現代宇宙塵的專家，他經由計算顯示當時高層大氣的氧氣濃度必須要相當接近現今的數值才能解釋觀測結果。

"This was a surprise because it has been firmly established that the Earth's lower atmosphere was very poor in oxygen 2.7 billion years ago; how the upper atmosphere could contain so much oxygen before the appearance of photosynthetic organisms was a real puzzle," Dr Genge said.

「我對這個結果感到相當驚訝，因為已經有很確切的證據指出27億年前地球的低層大氣相當缺乏氧氣，然而高層大氣何以能在行使光合作用的生物出現之前含有那麼多氧氣，確實是個謎團。」Genge博士說。

Dr Tomkins explained that the new results suggest the Earth at this time may have had a layered atmosphere with little vertical mixing, and higher levels of oxygen in the upper atmosphere produced by the breakdown of CO 2 by ultraviolet light.

Tomkins博士解釋這項新結果顯示那時地球或許擁有分層明顯的大氣，少有垂直混和(vertical mixing)的現象，而高層大氣的高濃度氧氣應該是由紫外線分解二氧化碳所產生。

"A possible explanation for this layered atmosphere might have involved a methane haze layer at middle levels of the atmosphere. The methane in such a layer would absorb UV light, releasing heat and creating a warm zone in the atmosphere that would inhibit vertical mixing," Dr Tomkins said.

「會產生這樣分層明顯的大氣，可能之一的解釋跟大氣中間有一層甲烷霧霾有關。像這樣的甲烷層可以吸收紫外線，並釋放出熱能而在大氣當中製造出一個暖區，進而抑制垂直混和發生。」Tomkins 博士說。

"It is incredible to think that by studying fossilised particles of space dust the width of a human hair, we can gain new insights into the chemical makeup of Earth's upper atmosphere, billions of years ago." Dr Tomkins said.

「想想我們藉由研究跟人髮寬度相仿的宇宙塵微粒化石，就能得到關於上億年前地球高層大氣化學組成的新資訊，這真的是件相當驚人的事情。」

Dr Tomkins outlined next steps in the research.

Tomkins博士大致說明了他們之後的研究方向。

"The next stage of our research will be to extract micrometeorites from a series of rocks covering over a billion years of Earth's history in order to learn more about changes in atmospheric chemistry and structure across geological time. We will focus particularly on the great oxidation event, which happened 2.4 billion years ago when there was a sudden jump in oxygen concentration in the lower atmosphere."

「我們研究的下一個階段將是從各種岩石中萃取出微隕石，它們會涵蓋地球歷史數十億年以上的時間。這是為了得知在不同的地質年代，大氣成分和結構會如何變化的更多相關資訊。我們將會特別專注在發生於24億年前的大氧化事件(great oxidation event)，在當時低層大氣的氧濃度突然地躍昇。」

引用自：Monash University. "Cosmic dust reveals Earth's ancient atmosphere." ScienceDaily. ScienceDaily, 11 May 2016.