板块构造是行星上生命形成的必要条件吗？

 在太阳系中，有两样东西是我们的地球所独有的，一个是板块构造，地表由多个漂移的板块构造组成，漂移的大陆和引发的地震，另一个是生命。
导读：根据本月发表在《地球和行星内部物理学》期刊上的一项最新研究，板块构造可能是行星进化成系外宜居行星的一个阶段。

根据本月发表在《地球和行星内部物理学》期刊上的一项最新研究，板块构造可能是行星进化成系外宜居行星的一个阶段。

在太阳系中，有两样东西是我们的地球所独有的，一个是板块构造，地表由多个漂移的板块构造组成，漂移的大陆和引发的地震，另一个是生命。

而且学院派认为这两者之间不无关系。

地球上复杂生命的进化花费了相当长的时间，根据现在的估计，足有35亿年的时间。使得地球地表适宜居住，达到了液态水可以存在的温度范围。

板块的移动

板块构造是全球温度引擎的调节机制。地球上的火山活动大都发生于板块边界。最重要的火山产品（产量最大的）是两种温室气体：二氧化碳和水。

随着它们在地球表面移动，一些板块会返回地幔，导致板块消失，比如太平洋的马里亚纳海沟。

大量的水和碳酸盐（二氧化碳的矿物形式）重新回到地下。

板块构造也形成了山脉，在整个地质时期组成山脉的岩石，遭受持续不断的化学风化作用，同时大量消耗大气中的二氧化碳，在风化过程中二氧化碳溶解在雨中与硅酸盐矿物反应，形成新的矿物，降低了大气层二氧化碳的水平。

这些机制如同调节器。如果地球温度升高，降雨和风化侵蚀作用的增加使二氧化碳水平下降。如果地球温度降低发生冰冻，侵蚀机制就会停止。

由于板块构造引发的火山活动，会继续将二氧化碳排放到大气中，使二氧化碳水平升高，最终融化冰雪。正是这种机制，让地球在大约6亿年前，从新元古代的全球冰期逐渐变暖。

宜居行星

宜居性和板块构造之间的关联性已经变得根深蒂固，在搜索宜居系外行星时重点都放在了寻找超级地球上。那些比地球大的岩石行星被认为存在板块构造的几率更高。

但情况并不是那么明显。在过去的十年中，这些超级地球模拟研究表明，它们可能没有板块构造，而是处于静止盖层模式下，内部热量只能通过火山作用释放，没有移动的板块。

我们最近的工作已经转向从进化的角度看问题。与地球类似的行星如何从形成之初的高温、剧烈的状态进化到最后的凉爽、平静，将它们最后的热量辐射到太空？

我们发现，一个行星的进化轨迹不仅取决于它的大小，还取决于它是如何开始进化的。例如，两个行星在每一个方面都相同，但具有不同的起始温度，可能就会有非常不同的进化路径。

我们还发现，板块构造可能只是行星进化的一个阶段，行星可能开始和结束都处于静止盖层模式下。

长期以来，行星研究界都认同这一点，当地球失去了它的内部热量，最终会变成静态停滞的状态，就像今天的火星或月亮一样。

直观地说，对一个星球来说，失去热量这似乎是一个低效的方式。今天的板块再循环在冷却地幔方面是非常有效的。然而，地球的热演化研究的主要问题之一是，地球在过去一定是以低效的方式失去热量的，这样才能解释其目前的内部温度。

了解木星的卫星

早期地球的静止盖层模式为此提供了一种机制。今天木星的卫星，木卫一与早期地球的行为类似。

在太阳系中，木卫一是火山体最多的星球，受到木星潮汐力的影响，木卫一上剧烈的火山活动是其散热的主要方式，而不是板块构造。

在2013年的一项研究中，美国科学家威廉·摩尔和亚力山大·韦伯证实，这种状态是由静止盖层模式引起的，与早期地球一样。

对地球来讲，弄清楚这个问题是不可能了，因为5亿年前的地质记录–冥古代–是缺失的。

锆石能够为古矿石的组成和44亿年前地表水的存在提供解释，但涉及到构造状态的确定，它们就力不从心了。

最新的研究表明它们可能是熔片结晶的产物，而熔片是由于陨石撞击地球形成的。

相反，地幔中保存下来的冥古代同位素特征已经存在了数十亿年，存在于火山岩中。这些证据表明，地球在很长时期都是静止盖层模式。

如果我们的结论是正确的，板块构造是类地行星进化的一个必经阶段，那么这一说法对宜居性有着重大意义。

地球上的生命

生命很早就在地球上开始进化了。来自古老矿物中的碳同位素和35亿年前的固体化石都能为这一说法提供证据。 可能当时地球上生命进化时，行星的岩石圈正处于静止盖层模式下，不是板块构造。

尽管大气压力很低，但是火山排气显然提供了足够的温室效应来保持地球不受冻结。这可能得益于低水平的造山运动和碳酸盐物质的减少，这两者都需要板块构造。

我们所提出的证据表明，在30亿年前，大部分的地球大陆位于海平面以下，因此地球大气中的二氧化碳无处可去。

这些结论也影响着宜居系外行星的搜索。很长时间以来都有一种根深蒂固的假设，宜居系外行星必须具备类似地球的板块构造。

对金星的简单看法会支持这一观点，因为它没有板块构造，而且它及其不适于居住，金星表面没有生命。然而，在它们的早期历史中，由于各自不同原因，金星和地球可能有着完全相异的情况。

与早期地球类似的星球，在它上面生命进化，它是遥远恒星系统中一个温暖的、处于静止盖层模式下的行星，这是完全有可能的。这就增加了宜居行星的探索空间，如果这情况成立，那么宇宙中没有板块构造的行星上也有存在生命的可能。

以下为英文原文：

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.