科學家首度對地函的內部分界提出解釋

原文網址：www.sciencedaily.com/releases/2015/12/151210144707.htm

First explanations for boundary within Earth's mantle

科學家首度對地函的內部分界提出解釋

Observed physical transition hundreds of miles below Earth's surface

觀測顯示地表數百哩之下的物理性質發生轉變

Earth's mantle, the large zone of slow-flowing rock that lies between the crust and the planet's core, powers every earthquake and volcanic eruption on the planet's surface. Evidence suggests that the mantle behaves differently below 1 megameter (1,000 kilometers, or 621 miles) in depth, but so far seismologists have not been able to explain why this boundary exists.

地函位於地殼和地核之間，是一片由緩緩流動的岩石組成的廣大區域，並驅使了所有發生在地表的地震與火山噴發。證據顯示地函的行為在深於1000公里(621哩)後會跟淺處有所差異，但至今為止地震學家仍無法解釋為何會有這個分界存在。

Two new studies co-authored by University of Maryland geologists provide different, though not necessarily incompatible, explanations. One study suggests that the mantle below 1 megameter is more viscous--meaning it flows more slowly--than the section above the boundary. The other study proposes that the section below the boundary is denser--meaning its molecules are more tightly packed--than the section above it, due to a shift in rock composition.

兩篇最新的研究各自提出了一套解釋，但兩者之間未必互相牴觸。其中一篇認為地函在1000公里之下較上方更為黏滯，意即地函在這深度以下更加難以流動；另一篇則表示由於岩石成分改變，使得在此分界下方的密度較上方要高，代表地函物質在這深度以下分子的排列更加緊密。這兩篇研究的共同作者皆包括了馬里蘭大學的地質學家。

Taken together, the studies provide the first detailed look at why large-scale geologic features within the mantle behave differently on either side of the megameter divide. The papers were published on December 11, 2015, in the journals Science and Science Advances.

總和而言，這兩篇研究為科學家首度詳細驗證以深度1000公里為界，兩側地函會出現行為差異的大尺度地質構造是如何出現。它們將會於2015年12月11日分別刊登在期刊《科學》和《科學前緣》。

"The existence of the megameter boundary has been suspected and inferred for a while," said Vedran Lekic, an assistant professor of geology at UMD and co-author of the Science paper that addresses mantle viscosity. "These papers are the first published attempts at a detailed explanation and it's possible that both explanations are correct."

「早已有人猜測且提出證據在1000公里深處有道分界存在於地函當中。」馬里蘭大學地質科學系的助理教授Vedran Lekic說。他參與在內刊登於《科學》的研究引述了地函黏滯性理論。「這兩篇研究為科學家首次發表他們試著對此現象提出的詳細解釋，而這兩者都有可能都是對的。」

Although the mantle is mostly solid, it flows very slowly in the context of geologic time. Two main sources of evidence suggest the existence of the megameter boundary and thus inspired the current studies.

雖然地函絕大部分是由固體組成，但從地質時間尺度來看它仍會緩緩流動。有兩項主要證據顯示1000公里分界確實存在，並啟發了目前正在進行的諸多研究。

First, many huge slabs of ocean crust that have been dragged down, or subducted, into the mantle can still be seen in the deep Earth. These slabs slowly sink downward toward the bottom of the mantle. A large number of these slabs have stalled out and appear to float just above the megameter boundary, indicating a notable change in physical properties below the boundary.

首先，許多巨大的海洋板塊在被拖入(隱沒)至地函後，我們仍然能看到它們存在於地球深處。這些隱沒板塊會緩緩往地函底部下沉，但其中有許多會在停滯在1000公里分界上，這讓它們看起來像在此處漂浮。這種現象意味著在此分界之下地函的物理性質勢必發生了顯著改變。

Second, large plumes of hot rock rise from the deepest reaches of the mantle, and the outlines of these structures can be seen in the deep Earth as well. As the rock in these mantle plumes flows upward, many of the plumes are deflected sideways as they pass the megameter boundary. This, too, indicates a fundamental difference in physical properties on either side of the boundary.

其次，在地函最深處會有由熾熱岩石形成的地函柱往上湧動，而我們同樣能看到這種地球深處構造的外觀。當這些地函柱中的岩石往上流動，可以發現有許多地函柱在經過1000公里分界後流動方向會偏折。這一樣代表著分界兩側的物理性質一定有本質上的差異。

"Learning about the anatomy of the mantle tells us more about how the deep interior of Earth works and what mechanisms are behind mantle convection," said Nicholas Schmerr, an assistant professor of geology at UMD and co-author of the Science Advances paper that addresses mantle density and composition. "Mantle convection is the heat engine that drives plate tectonics at the surface and ultimately leads to things like volcanoes and earthquakes that affect people living on the surface."

「認識地函的基本性質可以告訴我們地球深處如何運作，以及地函的對流機制為何」馬里蘭大學地質科學系的助理教授Nicholas Schmerr說。他參與在內發表於《科學前緣》的論文則是以地函密度和成分差異來解釋1000公里分界。「地函對流是驅動地表板塊構造運動的動力來源，這最終會引發火山爆發和地震之類的種種事物而影響到生活在地表的人們。」

The physics of the deep Earth are complicated, so establishing the mantle's basic physical properties, such as density and viscosity, is an important step. Density refers to the packing of molecules within any substance (gas, liquid or solid), while viscosity is commonly described as the thickness of a fluid or semi-solid. Sometimes density and viscosity correlate with each other, while sometimes they are at odds. For example, honey is both more viscous and dense than water. Oil, on the other hand, is more viscous than water but less dense.

在地球深處進行的物理作用相當複雜，因此確立地函的基本物理性質，像是密度和黏滯性，對於我們要了解它會是相當重要的一步。任何物質(氣體、液體、固體)的密度代表了它內部分子排列的緊密程度，而黏滯性則通常用來描述液體或半液體的濃稠程度。有時密度與黏滯性會有關聯，但有時卻會互相衝突。比方說，蜂蜜跟水相比，它的黏滯性與密度都較高；另一方面，石油雖然比水更為濃稠，然而它的密度卻較低。

In their study, Schmerr, lead author Maxim Ballmer (Tokyo Institute of Technology and the University of Hawaii at Manoa) and two colleagues used a computer model of a simplified Earth. Each run of the model began with a slightly different chemical composition--and thus a different range of densities--in the mantle at various depths. The researchers then used the model to investigate how slabs of ocean crust would behave as they travel down toward the lower mantle.

在Schmerr、Maxim Ballmer(第一作者，任職於東京工業大學和夏威夷大學馬諾分校)以及另外兩名科學家同僚的研究中，他們利用了簡化的地球模型。每一次的模擬開始時，他們都會先微調地函不同深度的化學成分，因此密度也會跟著改變。接著研究人員利用模型來探討海洋隱沒板塊在往下潛至下部地函的過程中，會展現出何種行為模式。

In the real world, slabs are observed to behave in one of three ways: The slabs either stall at around 600 kilometers, stall out at the megameter boundary, or continue sinking all the way to the lower mantle. Of the many scenarios for mantle chemical composition the researchers tested, one most closely resembled the real world and included the possibility that slabs can stall at the megameter boundary. This scenario included an increased amount of dense, silicon-rich basalt rock in the lower mantle, below the megameter boundary.

現實世界中，隱沒板塊呈現出的行為模式會是下列三者其一：堆積在600公里深附近、停滯在1000公里分界、或者一路下沉至下部地函。研究人員反覆以不同地函成分測試，其中有一種呈現出來的情境與真實世界世界最為相似，且包含了隱沒板塊能停留在1000公里分界的可能性。在此情境中，1000公里分界之下的下部地函會擁有更多密度較高、富含矽的玄武岩質岩石。

Lekic, lead author Max Rudolph (Portland State University) and another colleague took a different approach, starting instead with whole-Earth satellite measurements. The team then subtracted surface features--such as mountain ranges and valleys--to better see slight differences in Earth's basic shape caused by local differences in gravity. (Imagine a slightly misshapen basketball with its outer cover removed.)

在Lekic、Max Rudolph(研究第一作者，任職於波特蘭州立大學)和另一名科學家同僚的研究中，他們則採取了不同方法。研究團隊先取得衛星對整個地球的觀測影像，接著他們減去地表特徵，像是山脈和深谷造成的影響，以更詳細的觀察因為各地重力不同，而對地球原本的外形造成的輕微起伏(想像一個外皮剝掉後稍稍變形的籃球)。

The team mapped these slight differences in Earth's idealized shape onto known shapes and locations of mantle plumes and integrated the data into a model that helped them relate the idealized shape to differences in viscosity between the layers of the mantle. Their results pointed to less viscous, more free-flowing mantle rock above the megameter boundary, transitioning to highly viscous rock below the boundary. Their results help to explain why mantle plumes are frequently deflected sideways as they extend upward beyond the megameter boundary.

研究團隊得到這些與地球理想外形的細微差異後，再將其疊合在標有已知地函柱的形狀和位置圖上。接著研究團隊把這些資料整合至模型當中，以幫助他們瞭解地球理想外型和現實的些微出入，跟地函不同層的黏滯性差異之間有何關聯。他們的結果指出原本黏滯性較低，更容易流動的地函岩石，在經過1000公里分界後會變成黏滯性較高的岩石。他們的結論有助於解釋為何地函柱往上流動經過1000公里分界後，時常會往他處偏折。

"While explaining one mystery--the behavior of rising plumes and sinking slabs--our results lead to a new conundrum," Lekic said. "What causes the rocks below the megameter boundary to become more resistant to flow? There are no obvious candidates for what is causing this change, so there is a potential for learning something fundamentally new about the materials that make up Earth."

「在解開為何上湧地函柱和隱沒板塊會有這些行為的謎題時，我們的結果卻又製造了一道新難題。」Lekic說。「是什麼造成1000公里分界下方的岩石更難以流動？目前並無有力的假說可以解答是什麼造成了這種改變，但也因此在解決這項謎題的過程中，我們可能相當有機會可以學習到一些攸關地球組成本質的新事物。」

Lekic and Schmerr plan to collaborate to see if the results of both studies are consistent with one another--in effect, whether the lower mantle is both dense and viscous, like honey, when compared with the mantle above the megameter boundary.

Lekic和Schmerr計畫要合作以觀察他們倆個團隊的研究成果是否可以彼此相容，實際上就是要確認下部地函與1000公里分界之上的地函互相比較，是否前者真的比後者更加緻密且濃稠，就像蜂蜜和水的關係一樣。

"This work can tell us a lot about where Earth has been and where it is going, in terms of heat and tectonics," Schmerr said. "When we look around our solar system, we see lots of planets at various stages of evolution. But Earth is unique, so learning what is going on deep inside its mantle is very important."

「這項工作可以告訴我們許多事物，讓我們了解地球的熱力學和構造運動一直以來是如何運作，而未來的走向又是如何。」Schmerr說。「當我們環顧我們所處的太陽系，我們可以看到處在不同演化階段的各式星體。但地球是獨一無二的，因此了解它的地函深處運作方式是件至關重要的事。」

引用自：University of Maryland. "First explanations for boundary within Earth's mantle: Observed physical transition hundreds of miles below Earth's surface." ScienceDaily. ScienceDaily, 10 December 2015.

回溯38億年直達「生命樹」的根源

原文網址：www.sciencedaily.com/releases/2015/11/151130152206.htm

 

Looking Back 3.8 Billion Years Into the Root of the 'Tree of Life'

回溯38億年直達「生命樹」的根源

NASA-funded researchers at the Georgia Institute of Technology are tapping information found in the cells of all life on Earth, and using it to trace life's evolution. They have learned that life is a master stenographer -- writing, rewriting and recording its history in elaborate biological structures.

喬治亞理工學院的研究團隊在NASA資助下，正在一一寫下從地球上所有生命的細胞中發現的資訊，並用此來追溯生命演化的過程。他們從中認識到生命實為速記高手，將它在規畫生物藍圖過程中的點點滴滴詳細記錄、修改並重新編撰。

Some of the keys to unlocking the origin of life lie encrypted in the ribosome, life's oldest and most universal assembly of molecules. Today's ribosome converts genetic information (RNA) into proteins that carry out various functions in an organism. But the ribosome itself has changed over time. Its history shows how simple molecules joined forces to invent biology, and its current structure records ancient biological processes that occurred at the root of the Tree of Life, some 3.8 billion years ago.

核醣體為生命中最古老，也是最常見的分子複合體，其中隱含著要解開生命起源這項謎題的部分關鍵。現今核醣體的主要功能是將基因含有的資訊轉譯成蛋白質，而這些蛋白質在生命體內可以展現出各式各樣的功能。但核醣體本身也會隨著時間而逐漸改變。從它的歷史中我們可以得知簡單的小分子如何齊心協力發明出生命，而它今日的構造也記載著在38億年前，生命樹的根基曾經進行過何種生物作用。

By examining variations in the ribosomal RNA contained in modern cells, scientists can visualize the timeline of life far back in history, elucidating molecular structures, reactions and events near the biochemical origins of life.

藉由檢視現今細胞中核醣體RNA的變異，科學家可以描繪出直達遙遠過去的時間線，而釐清將近生命起源時，有何生化反應發生，而參與其中的分子架構又是如何。

"Biology is a great keeper of records," said Loren Williams, a professor in the Georgia Tech School of Chemistry and Biochemistry, and principal investigator for the NASA Astrobiology Institute's Georgia Tech Center for Ribosome Adaptation and Evolution from 2009-2014. "We are figuring out how to read some of the oldest records in biology to understand pre-biological processes, the origin of life, and the evolution of life on Earth."

「生物是各種歷史紀錄的保管者。」喬治亞理工學院化學與生化學院的教授Loren Williams說，她也於2009年至2014年在NASA天體生物學研究所的喬治亞理工學院核醣體適應與演化中心擔任計畫主持人。「我們正尋找方法來解讀生命中某些最古老的資訊，以瞭解地球生命起源之前的化學反應、生命如何開始和生物如何演化。」

The study is scheduled to be reported November 30 in the Early Edition of the journal Proceedings of the National Academy of Sciences.

本研究預計於11月30日刊登在期刊《美國國家科學院院刊》的早版。

Like rings in the trunk of a tree, the ribosome contains components that functioned early on in its history. The center of the trunk records the tree's youth, and successive rings represent each year of the tree's life, with the outermost layer recording the present. Just as the core of a tree's trunk remains unchanged over time, all modern ribosomes contain a common core dating back 3.8 billion years. This common core is the same in all living organisms, including humans.

就像樹幹中的年輪一樣，核醣體仍保有某些在歷史早期擁有功能的部分。樹輪的中心記載著樹木的童年，一輪一輪往外則代表著往後的每一年，而最外層正在記錄著當下發生的種種事蹟。如同樹輪的中心在時光洪流中仍能保持原貌，現今所有的核醣體都會擁有在38億年前便已經形成的核心。包含人類在內的生物全體，核醣體共有的核心都會是一模一樣的。

"The ribosome recorded its history," said Williams. "It accreted and got bigger and bigger over time. But the older parts were continually frozen after they accreted, just like the rings of a tree. As long as that tree lives, the inner rings will not change. The very core of the ribosome is older than biology, produced by evolutionary processes that we still don't understand very well."

「核醣體會記錄自己的歷史軌跡。」Williams說。「隨著時間它會逐漸生長並越變越大。但就像年輪一樣，雖然核醣體會持續成長，其核心仍然完好如初。只要樹還活著，年輪內層就不會改變。核醣體最深的核心地帶比生命本身還要古老，而形成核心的演化過程我們仍然所知甚少。」

While exploiting this record-keeping ability of the ribosome reveals how biology has changed over time, it can also point to the environmental conditions on Earth in which that biology evolved, and help inform our search for life elsewhere in the Universe.

利用核醣體會記錄自身歷史的能力，我們不但可以知道生物如何隨著時間演變，也能了解生物出現當時地球的環境條件，而這有助於我們尋找宇宙他處的生命。

"This work enables us to look back in time past the root of the tree of life -- the ancestor of all modern cells -- to a time when proteins and nucleic acids had not yet become the basis for all biochemistry," said Carl Pilcher, interim director of the NASA Astrobiology Institute. "It helps us understand some of the earliest stages in the development of life on Earth, and can guide our search for extraterrestrial environments where life may have developed."

「這項研究讓我們能追溯過往直至生命樹的根基，也就是當今所有細胞的祖先出現的時刻。當時蛋白質和核苷酸尚未成為所有生化作用的基礎。」NASA天體生物學研究所的代理所長Carl Pilcher說。「這有利於我們了解地球生命發展過程中一些最早的階段，同時也可以指引我們尋找可能有生命發育其中的地外環境。」

By rewinding, reverse engineering, and replaying this ancient ribosomal tape, researchers are uncovering the secrets of creation and are answering foundational, existential questions about our place in the Universe.

藉著解開、還原並重播這捲記錄在核醣體中的古老錄像，研究人員能挖掘出生物創始當時的秘密，並回答這項由來已久，有關人類存在本質的問題：我們在宇宙中的定位為何？

By studying more additions to the ribosome, the research team -- with key contributions by Georgia Tech Research Scientist Anton Petrov -- found "molecular fingerprints" that show where insertions were made, allowing them to discern the rules by which it grew. Using a technique they call the Structural Comparative Method, the researchers were able to model the ribosome's development in great detail.

在喬治亞理工學院的研究員Anton Petrov做出的重要貢獻下，研究團隊得以對核醣體的增長研究得更加透徹。他們發現了可以顯示插入序列形成位置的「分子指紋」，使他們得以辨識出核醣體成長的規則。利用他們稱作比對結構法的技術，研究人員能夠非常精細的模擬核醣體的演變過程。

"By taking ribosomes from a number of species -- humans, yeast, various bacteria and archaea -- and looking at the outer portions that are variable, we saw that there were very specific rules governing how they change," said Williams. "We took those rules and applied them to the common core, which allowed us to see all the way back to the first pieces of RNA."

「我們採集了許多物種身上的核醣體，包括人類、酵母菌還有各式各樣的細菌與古菌，並且觀察這些核醣體外層有所差異的部分，我們發現這些變化遵循著特定的規則。」Williams說。「我們收集這些規則並將之套用在核醣體共同的核心上，如此我們便能從各種面向回溯至第一段RNA的誕生。」

Some clues along the way helped. For instance, though RNA is now responsible for creating proteins, the very earliest life had no proteins. By looking for regions of the ribosome that contain no proteins, the researchers could determine that those elements existed before the advent of proteins. "Once the ribosome gained a certain capability, that changed its nature," Williams said.

某些推裡邏輯至始至終都很有幫助。舉例來說，雖然RNA現今的作用是負責製造蛋白質，但十分早期的生命體內是沒有蛋白質的。因此研究人員可以觀察核醣體中並未含有蛋白質的區域，來了解蛋白質出現以前核醣體就已經擁有的零件。「核醣體一旦增加了某項新功能，它的樣貌也會隨之改變。」Williams說。

While the ribosomal core is the same across species, what's added on top differs. Humans have the largest ribosome, encompassing some 7,000 nucleotides representing dramatic growth from the hundred or so base pairs at the beginning.

雖然核醣體的核心在各種物種身上皆如出一轍，但增建在其上的構造卻大異其趣。人類擁有最大的核醣體，大致上由7000個核苷酸分子組成。這顯示從一開始僅僅數百個左右的鹼基對，歷經了相當劇烈的增長才有現今的樣貌。

"What we're talking about is going from short oligomers, short pieces of RNA, to the biology we see today," said Williams. "The increase in size and complexity is mind-boggling."

「我們正在談論的是現今我們看到的生物，是如何從那些短小的寡聚物(oligomer)和RNA碎片演變而成。」Williams說。「這些由小至大、由簡至繁的過程變化之劇烈，實在是令人難以想像。」

The researchers obtained their ribosomes from structure and sequence databases that have been produced to help scientists identify new species. Ribosomes can be crystallized, which reveals their three dimensional structures.

研究團隊從幫助科學家辨認新物種用的核醣體結構與序列資料庫，來獲取他們需要的資料。核醣體可以形成結晶，可以被用來辨識出核醣體本身的三維構造。

Beyond understanding how evolution played out over time, this knowledge of the ribosome's development could have more practical modern-day health applications.

除了可以了解隨著時間進行演化如何作用之外，這項核醣體演變的知識也能實際應用在當代醫學。

"The ribosome is one of the primary target for antibiotics, so understanding its architecture and consistently throughout biology could be of great benefit," said Williams. "By studying the ribosome, we can start thinking about biology in a different way. We can see the symbiotic relationship between RNA and proteins."

「由於核醣體是抗生素作用的主要目標之一，因此了解它的結構和它在所有生物身上皆有的共通性質會有很大的益處。」Williams說。「藉由研究核醣體，我們可以看到RNA和蛋白質之間的共生關係，而能夠從另外一個角度來思考生命科學。」

As a next step, Williams and colleagues are now using experiments to verify what their model shows.

Williams和其同僚正在進行的下一步是利用實驗來驗證他們的模型呈現出來的結果。

"We have a coherent and consistent model that accounts for all the data we have going all the way back to a form of biology that is very primitive compared to what we have now," Williams explained. "We plan to continue testing the predictions of the model."

「與現有的方式相比，我們的模型一路模擬回至非常原始的生命形式時，可以得到相當一致且合理的數據。」Williams解釋。「我們接下來的計劃是繼續去驗證模型得到的預測結果。」

In addition to those already named, the research included Burak Gulen, Ashlyn Norris, Chad Bernier, Nicholas Kovacs, Kathryn Lanier, Stephen Harvey, Roger Wartell and Nicholas Hud from Georgia Tech, and George Fox from the University of Houston.

除了以上提到的人名之外，其他的研究人員包括喬治亞理工學院的Burak Gulen、Ashlyn Norris、Chad Bernier、Nicholas Kovacs、Kathryn Lanier、Stephen Harvey、Roger Wartell和Nicholas Hud，以及休士頓大學的George Fox。

This research was funded in part by the NASA Astrobiology Institute under grant NNA09DA78A. The content is solely the responsibility of the authors and does not necessarily represent the official views of NASA.

此研究部分由NASA天體生物學研究所資助，計畫編號為NNA09DA78A。內容責任歸屬為作者獨有，並不全然代表NASA的官方立場。

引用自：Georgia Institute of Technology. "Looking Back 3.8 Billion Years Into the Root of the 'Tree of Life'." ScienceDaily. ScienceDaily, 30 November 2015.