5.3 居住区

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  宜居区

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  One of the attributes that makes Earth a habitable planet is the presence of oceans of water.  When the solar system formed, Earth and Venus were similar in size and composition, but today the surface of Venus is about 700 K, hot enough to melt lead with a staggering surface pressure that is 92 times that of Earth from the weight of a massive carbon dioxide atmosphere.  Venus has 10,000 times more carbon dioxide in its atmosphere than the Earth and three times as much nitrogen.  Earth stores carbon dioxide and nitrogen in surface rocks - after accounting for these geological reserves on Earth, the two planets have about the same abundance of both elements.  
  ::使地球成为可居住地球的特性之一是存在水的海洋。当太阳系形成时,地球和金星的大小和构成相似,但今天金星表面大约为700K,热到足以熔化铅,其表层压力惊人,比地球的二氧化碳大气重量高出92倍。金星在大气层中二氧化碳比地球多10 000倍,氮含量是地球的三倍。地球在地表岩石中储存二氧化碳和氮含量是地球的三倍。在计算了地球的这些地质保留量之后,这两个行星都拥有同样多的两种元素。

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  It is quite possible that for the first billion years, Venus had a moderate temperature atmosphere, surface water, and perhaps even life. However, because Venus is closer to the Sun, it intercepts more energy than the Earth. Liquid water would have gradually evaporated, forming a potent greenhouse gas that trapped more solar energy, leading to a positive feedback loop that further warmed the planet, driving water out of the  crust of Venus. Ultraviolet radiation dissociates water molecules into its component atoms of hydrogen and oxygen. The Venus Express spacecraft, launched in 2005 by the European Space Agency (ESA), has measured the escape of hydrogen and oxygen (roughly in a ratio of 2:1) from the upper atmosphere of Venus.  Over the 4.56 billion year lifetime of the planet, Venus has lost its water. Carbon dioxide is chemically bound in the crust of Earth rocks. However, in the presence of a hot, dry Venusian climate, carbon dioxide would  evaporate out of  the mantle of the planet, producing the heavy CO ₂ atmosphere observed today. The current condition of  Venus is the fate of an Earth-like planet that  is too close to its star to retain liquid surface water. 
  ::在最初的10亿年中,金星很可能拥有温和的大气、地表水,甚至生命。然而,由于金星离太阳较近,它截获的能量会比地球更多。液态水会逐渐蒸发,形成一种强大的温室气体,使更多的太阳能被困住,从而形成一个积极的反馈循环,使地球更加暖和,将水从金星的地壳中排出。紫外线辐射将水分子从氢和氧的原子中分离出来。2005年由欧洲航天局(欧空局)发射的金星快车航天器测量了氢和氧从金星高层大气中流出的情况(比例为2:1 ) 。在行星的45.6亿年中,金星失去了水。二氧化碳在地球岩石的地壳中被化学界束缚着。然而,在热干燥的金星气候下,二氧化碳会蒸发出行星的地壳,产生今天观察到的重CO2大气。金星的现状是一颗离其恒星太近而不能保留水的地球的命运。

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  Planets or moons that are very far away from the Sun have their reservoirs of water stored as frozen ice. If liquid water is a requirement for life, then a zone around the Sun can be defined such that it it not too hot and not too cold for liquid water to exist on the surface of a planet. This zone is called the "habitable zone." The location of the habitable zone depends on a number of factors including the luminosity of the host star, the mass of the planet, and the composition and thickness of the planet's atmosphere.  For the Sun, a conservative  range for the habitable zone has been calculated  as 0.95 - 1.37 AU.  
  ::离太阳很远的行星或卫星储有冷冻冰的储水库。如果液体水是生命的必要条件,那么太阳周围的一个区域可以被确定为不会太热,也不会太冷,以免流水存在于行星表面。这个区域被称为“可居住区”。可居住区的位置取决于若干因素,包括宿主恒星的光度、行星的质量、地球大气层的构成和厚度。对于太阳来说,可居住区的保守范围计算为0.95-1.37AU。

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  Like all stars, the Sun started out fainter, and has gradually increased in size and brightness over its lifetime. The faint, young Sun would have had habitable zone boundaries that were closer to the star and the boundaries of the habitable zone are pushed outward as the Sun evolves.  Over the 4.56-billion year lifetime of the Earth, there is a narrower region that would have been a continuously habitable zone (CHZ), and this has been estimated to be 0.95 - 1.15 AU. 
  ::与所有恒星一样,太阳开始变弱,在其寿命期内,其体积和亮度逐渐增加。 弱小的年轻太阳本来会拥有离恒星更近的可居住区界线,随着太阳的演化,可居住区的边界会推向外。 在地球的4.56亿年寿命中,有一个更窄的区域,这个区域本来会是一个可持续居住的区(CHZ),估计这个区域会为0.95 - 1.15 AU。

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  Brightness matters
  ::亮度事项

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  The location of the habitable zone depends on the brightness of the star.  For fainter stars, the habitable zone moves closer in and for brighter stars the habitable zone moves farther out.  In addition , the lifetime of the star depends on its mass. Faint, low mass stars live for hundreds of billions of years while the lifetime of bright, massive stars is only millions of years long.
  ::可居住区的位置取决于恒星的亮度。对于较弱的恒星来说,可居住区更近,更亮的恒星更近,更亮的恒星更近。此外,恒星的寿命取决于其质量。 发光,低质量恒星活了数千亿年,而光亮的巨型恒星的寿命只有数百万年。

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  The seeming advantage of long lifetimes for low mass stars like M dwarfs may be tempered by other challenges. The habitable zone is so close to these stars that a year on a habitable planet around an M dwarf star is measured in (Earth) days. This proximity leads to two important issues. The first is tidal locking of the planet.  We see the phenomenon of tidal locking with  our moon: as the moon circles the Earth, the same hemisphere of the moon points toward the Earth. Habitable zone planets orbiting M dwarfs will likewise have one "day" side of the planet while the other side is in perpetual darkness. The rotation (spin) of the planet is gravitationally synchronized with the orbit. A second issue is that  M dwarfs  typically have strong magnetic fields that produce  high velocity winds of charged particles. This stellar wind  would bombard any planets in the habitable zone and  could potentially  strip away the planet atmosphere.  
  ::对像M矮星这样的低质量恒星来说,长寿命的长寿命的优势似乎可以被其他挑战所抵消。可居住区与这些恒星非常接近,因此在M矮星周围的可居住行星上,一年是在(地球)日内测量的。这种近距离导致两个重要问题。第一个是地球的悬浮锁定。我们看到了与我们的月球一起锁住潮汐现象:当月球环绕地球时,月球的同半球点向地球。可居住区行星绕着地球运行的M矮行星同样会有一个地球的“日”一侧,而另一边则处于永恒的黑暗之中。行星的旋转(斯皮)与轨道同步旋转。第二个问题是,M矮星通常有强大的磁场,产生高速度的充电颗粒。这种恒星的风会轰炸在可居住区的任何行星,并可能使地球大气层消失。

  

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  The intrinsic brightness and the evolution time for stars establish a region called the habitable zone where liquid water can survive on the surface of the planet. The amount of intercepted starlight is shown relative to that of Earth for different mass (temperature) stars.
  ::恒星的内在亮度和进化时间形成了一个区域,叫做可居住区,其中液态水可以在行星表面生存。不同质量(温度)恒星被截取的星光数量与地球相比显示。

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   A simple way to approximate the inner boundary of the habitable zone is to calculate the distance from the star where the temperature is equal to the boiling point of water.  The outer boundary of the habitable zone is calculated as the distance from the star where the temperature would equal the freezing point of water. 
  ::接近可居住区内部边界的一个简单方法是计算温度等于沸水点的恒星距离。可居住区的外部边界计算为温度等于水冷点的恒星距离。

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  Recall that  $\begin{array}{r}L=4\pi r^2\sigma T^4\end{array}$ , scaling this to values for the solar system where the distance  $\begin{array}{r}{d}_{in}=0.95AU\end{array}$  for the inner boundary and $\begin{array}{r}{T}_{in}=373K\end{array}$  by our definition:  
  ::回顾L=4r2-2-T4, 缩放为太阳系的数值,根据我们的定义,太阳系内部边界和Tin=373K的距离为0.95AU:

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               $\begin{array}{r}\frac{L_{\ast }}{L_{\odot }}=\frac{4\pi r_{in}^2\sigma T_{in}^4}{4\pi d_{in}^2\sigma T_{in}^4}\end{array}$
  ::L* 42244444444444444444444444444444444444444444444424444442424424424444444244444444444444

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  After cancelling all of the constants ( $\begin{array}{r}4,\pi ,\sigma ,T_{in}\end{array}$  ) we can estimate the distance (in units of AU) to the inner boundary of the habitable zone for a star with luminosity  $\begin{array}{r}{L}_{\ast }\end{array}$ :
  ::在取消所有常数( 4, , , , )之后, 我们可以估计( 非盟单位) 光度L* 恒星在可居住区内部边界的距离 :

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  $\begin{array}{r}{r}_{in}=0.95\sqrt{\frac{L_{\ast }}{L_{\odot }}}\end{array}$    
  ::林=0.95L*L

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  Similarly, we can calculate the outer boundary of the habitable zone (in units of AU) for a star of   luminosity  $\begin{array}{r}{L}_{\ast }\end{array}$ with  $\begin{array}{r}{d}_{out}=1.37AU\end{array}$  for the inner boundary and   $\begin{array}{r}{T}_{out}=273K\end{array}$   by our definition:    
  ::同样,我们可以计算一个光度L* 星的可居住区的外部边界(按非盟单位计算),内边界为1.37AU,根据我们的定义,图特=273K:

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  $\begin{array}{r}{r}_{out}=1.37\sqrt{\frac{L_{\ast }}{L_{\odot }}}\end{array}$
  ::区域=1.37L*L*

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   Limitations of the Habitable Zone Concept
  ::居住区概念的局限性

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  The concept of the habitable zone helps to shape strategies in the search for life in the solar system and on planets around other worlds. I t is not a definition of where life can exist because we do not yet know enough about the environmental limits for habitability. Indeed our active searches for life in the solar system do not focus on our Moon (which is in the habitable zone, but has no surface water).  Instead our searches  take place on Mars, Europa (moon of Jupiter), Titan and Enceladus (moons of Saturn). 
  ::可居住区的概念有助于制定在太阳系和其他世界的行星上寻找生命的战略。它不是关于生命可以存在何方的定义,因为我们还不太了解居住环境的限度。 事实上,我们在太阳系中积极寻找生命并不集中在我们的月球上(在可居住区,但没有地表水 ) 。 相反,我们在火星、欧罗巴(木星月 ) 、 土卫六(土星月 ) 、 土卫六(土星月 ) 、 土卫六(土星月 ) 、 土卫六(土星月 ) 、 土卫六(土星月 ) 、 土卫六(土星月 ) 、 土卫六(土星月 ) 、 土卫六(土星月 ) 。

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  Mars is a particularly interesting site for searches of ancient or subsurface life because there is evidence that liquid water existed on Mars. Today, dry fluvial features course the surface of Mars, and sedimentation and aggregates associated with water on Earth have been discovered on the surface of Mars.   These features suggest the past existence of running rivers and lake beds on young Mars. Since the Sun would have been fainter at this time, Mars must have had effective greenhouse gases like methane in the atmosphere.  
  ::火星是搜索古代或地下生命的一个特别有趣的场所,因为有证据表明火星上存在液态水。 今天,在火星表面发现了干河特征,火星表面也发现了与地球上水有关的沉积物和集合物。 这些特征表明过去在年轻的火星上曾存在流水和湖泊床。 由于太阳此时会更加衰弱,火星在大气中一定有甲烷等有效温室气体。

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  While it is understood that the concept of the habitable zone is too limiting, it is an easy and useful definition that can be applied (at least as a starting point) to planets orbiting other stars. 
  ::虽然可以理解,可居住区的概念限制性太强,但它是一个容易和有用的定义,可以(至少作为起点)适用于环绕其他恒星的行星。