板塊構(gòu)造是行星上生命形成的必要條件嗎？

 在太陽系中，有兩樣?xùn)|西是我們的地球所獨(dú)有的，一個是板塊構(gòu)造，地表由多個漂移的板塊構(gòu)造組成，漂移的大陸和引發(fā)的地震，另一個是生命。
導(dǎo)讀：根據(jù)本月發(fā)表在《地球和行星內(nèi)部物理學(xué)》期刊上的一項最新研究，板塊構(gòu)造可能是行星進(jìn)化成系外宜居行星的一個階段。

根據(jù)本月發(fā)表在《地球和行星內(nèi)部物理學(xué)》期刊上的一項最新研究，板塊構(gòu)造可能是行星進(jìn)化成系外宜居行星的一個階段。

在太陽系中，有兩樣?xùn)|西是我們的地球所獨(dú)有的，一個是板塊構(gòu)造，地表由多個漂移的板塊構(gòu)造組成，漂移的大陸和引發(fā)的地震，另一個是生命。

而且學(xué)院派認(rèn)為這兩者之間不無關(guān)系。

地球上復(fù)雜生命的進(jìn)化花費(fèi)了相當(dāng)長的時間，根據(jù)現(xiàn)在的估計，足有35億年的時間。使得地球地表適宜居住，達(dá)到了液態(tài)水可以存在的溫度范圍。

板塊的移動

板塊構(gòu)造是全球溫度引擎的調(diào)節(jié)機(jī)制。地球上的火山活動大都發(fā)生于板塊邊界。最重要的火山產(chǎn)品（產(chǎn)量最大的）是兩種溫室氣體：二氧化碳和水。

隨著它們在地球表面移動，一些板塊會返回地幔，導(dǎo)致板塊消失，比如太平洋的馬里亞納海溝。

大量的水和碳酸鹽（二氧化碳的礦物形式）重新回到地下。

板塊構(gòu)造也形成了山脈，在整個地質(zhì)時期組成山脈的巖石，遭受持續(xù)不斷的化學(xué)風(fēng)化作用，同時大量消耗大氣中的二氧化碳，在風(fēng)化過程中二氧化碳溶解在雨中與硅酸鹽礦物反應(yīng)，形成新的礦物，降低了大氣層二氧化碳的水平。

這些機(jī)制如同調(diào)節(jié)器。如果地球溫度升高，降雨和風(fēng)化侵蝕作用的增加使二氧化碳水平下降。如果地球溫度降低發(fā)生冰凍，侵蝕機(jī)制就會停止。

由于板塊構(gòu)造引發(fā)的火山活動，會繼續(xù)將二氧化碳排放到大氣中，使二氧化碳水平升高，最終融化冰雪。正是這種機(jī)制，讓地球在大約6億年前，從新元古代的全球冰期逐漸變暖。

宜居行星

宜居性和板塊構(gòu)造之間的關(guān)聯(lián)性已經(jīng)變得根深蒂固，在搜索宜居系外行星時重點(diǎn)都放在了尋找超級地球上。那些比地球大的巖石行星被認(rèn)為存在板塊構(gòu)造的幾率更高。

但情況并不是那么明顯。在過去的十年中，這些超級地球模擬研究表明，它們可能沒有板塊構(gòu)造，而是處于靜止蓋層模式下，內(nèi)部熱量只能通過火山作用釋放，沒有移動的板塊。

我們最近的工作已經(jīng)轉(zhuǎn)向從進(jìn)化的角度看問題。與地球類似的行星如何從形成之初的高溫、劇烈的狀態(tài)進(jìn)化到最后的涼爽、平靜，將它們最后的熱量輻射到太空？

我們發(fā)現(xiàn)，一個行星的進(jìn)化軌跡不僅取決于它的大小，還取決于它是如何開始進(jìn)化的。例如，兩個行星在每一個方面都相同，但具有不同的起始溫度，可能就會有非常不同的進(jìn)化路徑。

我們還發(fā)現(xiàn)，板塊構(gòu)造可能只是行星進(jìn)化的一個階段，行星可能開始和結(jié)束都處于靜止蓋層模式下。

長期以來，行星研究界都認(rèn)同這一點(diǎn)，當(dāng)?shù)厍蚴�去了它的�?nèi)部熱量，最終會變成靜態(tài)停滯的狀態(tài)，就像今天的火星或月亮一樣。

直觀地說，對一個星球來說，失去熱量這似乎是一個低效的方式。今天的板塊再循環(huán)在冷卻地幔方面是非常有效的。然而，地球的熱演化研究的主要問題之一是，地球在過去一定是以低效的方式失去熱量的，這樣才能解釋其目前的內(nèi)部溫度。

了解木星的衛(wèi)星

早期地球的靜止蓋層模式為此提供了一種機(jī)制。今天木星的衛(wèi)星，木衛(wèi)一與早期地球的行為類似。

在太陽系中，木衛(wèi)一是火山體最多的星球，受到木星潮汐力的影響，木衛(wèi)一上劇烈的火山活動是其散熱的主要方式，而不是板塊構(gòu)造。

在2013年的一項研究中，美國科學(xué)家威廉·摩爾和亞力山大·韋伯證實，這種狀態(tài)是由靜止蓋層模式引起的，與早期地球一樣。

對地球來講，弄清楚這個問題是不可能了，因為5億年前的地質(zhì)記錄–冥古代–是缺失的。

鋯石能夠為古礦石的組成和44億年前地表水的存在提供解釋，但涉及到構(gòu)造狀態(tài)的確定，它們就力不從心了。

最新的研究表明它們可能是熔片結(jié)晶的產(chǎn)物，而熔片是由于隕石撞擊地球形成的。

相反，地幔中保存下來的冥古代同位素特征已經(jīng)存在了數(shù)十億年，存在于火山巖中。這些證據(jù)表明，地球在很長時期都是靜止蓋層模式。

如果我們的結(jié)論是正確的，板塊構(gòu)造是類地行星進(jìn)化的一個必經(jīng)階段，那么這一說法對宜居性有著重大意義。

地球上的生命

生命很早就在地球上開始進(jìn)化了。來自古老礦物中的碳同位素和35億年前的固體化石都能為這一說法提供證據(jù)。 可能當(dāng)時地球上生命進(jìn)化時，行星的巖石圈正處于靜止蓋層模式下，不是板塊構(gòu)造。

盡管大氣壓力很低，但是火山排氣顯然提供了足夠的溫室效應(yīng)來保持地球不受凍結(jié)。這可能得益于低水平的造山運(yùn)動和碳酸鹽物質(zhì)的減少，這兩者都需要板塊構(gòu)造。

我們所提出的證據(jù)表明，在30億年前，大部分的地球大陸位于海平面以下，因此地球大氣中的二氧化碳無處可去。

這些結(jié)論也影響著宜居系外行星的搜索。很長時間以來都有一種根深蒂固的假設(shè)，宜居系外行星必須具備類似地球的板塊構(gòu)造。

對金星的簡單看法會支持這一觀點(diǎn)，因為它沒有板塊構(gòu)造，而且它及其不適于居住，金星表面沒有生命。然而，在它們的早期歷史中，由于各自不同原因，金星和地球可能有著完全相異的情況。

與早期地球類似的星球，在它上面生命進(jìn)化，它是遙遠(yuǎn)恒星系統(tǒng)中一個溫暖的、處于靜止蓋層模式下的行星，這是完全有可能的。這就增加了宜居行星的探索空間，如果這情況成立，那么宇宙中沒有板塊構(gòu)造的行星上也有存在生命的可能。

以下為英文原文：

Does a planet need plate tectonics to develop life?

Plate tectonics may be a phase in the evolution of planets that has implications for the habitability of exoplanets, according to new research published this month in the journal Physics of the Earth and Planetary Interiors.

Two of the things that make Earth unique in our solar system are that it has plate tectonics – with the surface broken up into a number of tectonic plates that drift around, moving continents and causing earthquakes – and life.

And there is a school of thought that these two are not unrelated.

Complex life on Earth took a long time to evolve; about 3.5 billion years by current estimates. This was possible as the Earth's surface has been habitable and in the temperature range for liquid water.

Plate movements

Plate tectonics provides a mechanism for this global thermostat. Most volcanism on the Earth occurs at plate boundaries in response to plate tectonics. And the most important volcanic products by mass – by a large amount – are two greenhouse gases: carbon dioxide and water.

As they move over the Earth's surface, some plates get recycled back into the mantle, at places like the Marianas Trench in the Pacific Ocean.

Enormous amounts of water and carbonate (the mineral form of CO2) get recycled back into the interior as they do.

Plate tectonics also form mountains, and one of the major sinks of CO2 over geological time periods is weathering of mountains, where CO2 dissolved in rainwater reacts with silicate minerals, forming new minerals, and drawing down atmospheric CO2 levels.

In concert, these mechanisms act as a thermostat. If the Earth gets too hot, high levels of rainfall and erosion start bringing CO2 levels down. If the Earth gets too cold and freezes over, the erosion mechanism stops.

But volcanism, due to plate tectonics, continues pumping CO2 into the atmosphere, and levels build up, eventually melting the icecaps. It was this mechanism that allowed Earth to recover from a global ice age in the Neoproterozoic, about 600 million years ago.

Habitable planets

This association between habitability, and plate tectonics, has become so entrenched that the search for habitable exosolar planets has focused on super earths. These are rocky planets larger than Earth where the odds for plate tectonics were thought to be higher.

But the case is not so clear cut. Over the past decade, simulations of these super earths suggested that they may not haveplate tectonics, but rather be in a stagnant-lid state, where a hot interior powers high levels of volcanism, but without moving plates.

Our recent work has looked at the question from an evolutionary viewpoint. How do Earth-like planets evolve from their hot, violent beginnings to their eventual cool, quiescent twilights, radiating their last heat to space?

We found that the evolutionary track a planet takes depends not only on its size, but on how it starts. For example, two planets identical in every other way, but with different starting temperatures, may evolve down very different evolutionary paths.

We also found that plate tectonics may simply be a phase in the evolution of planets, and that planets may begin and end with stagnant lids.

The planetary community has long accepted that as the Earth lost its internal heat, it would eventually settle into a quiescent stagnant state much like Mars or the Moon today.

The idea that planets may begin in a stagnant lid, though, is more surprising.

Intuitively, this seems an inefficient way for a planet to lose heat. Recycling of plates today is extremely effective at cooling the mantle. Yet one of the main issues in the study of Earth's thermal evolution is that Earth must have lost its heat less efficiently in the past, to explain its current internal temperatures.

Look to Jupiter's moon Io

An early stagnant lid on Earth provides a mechanism for that. We even have an analogue for this behaviour in Jupiter's moon Io today.

Io is the most volcanic body in the solar system, a result of Jupiter's tidal influence, and it operates in a stagnant heat-pipe mode, where it loses its heat primarily through volcanic heat pipes rather than plates.

In a 2013 study, US scientists William Moore and Alexander Webb demonstrated that this regime may have operated under the conditions of the early stagnant Earth.

Resolving the issue for Earth is tricky, as the geological record for the first 500 million years – the Hadean Eon – is missing.

Zircons have provided incredible insights into the makeup of Hadean rocks, and the existence of surface water 4.4 billion years ago, but they are equivocal when it comes to determining tectonic state.

The most recent work suggests they may be crystallising from melt sheets formed by meteorite impacts on the early Earth.

In contrast, the long-lived isotopic signatures of Hadean processes survived for billions of years in Earth's mantle, and are recorded in ancient volcanic rocks. The mixing of this material provides an important constraint for the tectonics of the Earth, and supports the idea that the Earth was largely stagnant.

If our conclusions are right, and plate tectonics is an adolescent phase in the evolution of Earth-like planets, then this has big implications for habitability.

Life on Earth

Life evolved on the Earth very early. There is evidence in carbon isotopes from Hadean zircons, and solid fossil evidence from 3.5 billion years ago. It probably evolved on a planet with a stagnant lid, not plate tectonics.

Volcanic degassing evidently provided enough of a greenhouse effect to keep the planet from freezing, despite lower atmospheric pressures. This was probably helped by low levels of mountain building and subduction of carbonate material, both of which need tectonics.

The evidence we have suggests most of the Earth's continents were below sea-level before 3 billion years ago, and so Earth's atmospheric CO2 had nowhere to go.

These conclusions also impact the search for habitable exoplanets. For a long time there has been an ingrained assumptionthat habitable exoplanets must possess plate tectonics like the Earth.

The simplistic view of Venus supports this, as it does not have plate tectonics, and is extremely inhospitable to surface life. Yet Venus and Earth may have diverged for very different reasons early in their history.

It is entirely possible that the best analogue for early Earth, on which life evolved, is a warm, stagnant-lid planet in a distant star system. This increases the exploration space for habitable planets, and in doing so, the chances of life elsewhere in the universe.