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  • 生物签字

    One important method for searching for life on other worlds is to search for the by-products of life that contaminate planetary atmospheres.  Oxygen alone or methane alone would not be a good biosignature, but the combination of oxygen and methane is a smoking gun for life.  At the temperatures and pressures in the atmosphere of Earth, oxygen would react quickly with methane, producing carbon dioxide and water.  The simultaneous existence of these elements on Earth occurs only because life is producing these elements and maintaining chemical disequilibrium. 
    ::在其它世界中寻找生命的一个重要方法是寻找污染行星大气的生命副产品。 光靠氧气或甲烷本身并不是一个好的生物签名,但氧气和甲烷的结合是生命的烟枪。 在地球大气中的温度和压力下,氧气会与甲烷迅速反应,产生二氧化碳和水。这些元素同时存在于地球上的唯一原因是生命正在产生这些元素并保持化学平衡。

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    A spectrum of Earth's atmosphere. From the spectrum, we can detect oxygen (O2, O3), carbon dioxide (CO2), water (H2O), and methane (CH4) that indicate the presence of life on Earth.
    ::地球大气的频谱。从频谱中,我们可以探测到氧(O2,O3),二氧化碳(CO2),水(H2O)和甲烷(CH4),表明地球上存在生命。

    The Figure below compares a  spectrum of the atmospheres of Venus, Earth, and Mars. The Earth spectrum contains a biosignature: oxygen and methane (not labeled). The presence of water also suggests that Earth is a habitable world.  In contrast, atmospheric spectra of Venus and Mars do not have these same indicators for life. 
    ::下图比较了金星、地球和火星等大气的频谱。地球频谱包含生物特征:氧气和甲烷(未贴上标签 ) 。 水的存在也表明地球是一个可居住的世界。 相反,金星和火星的大气光谱没有同样的生命指标。

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    A comparison of the atmosphere spectra from Venus, Earth, and Mars. Note how water and ozone is only present on Earth, setting it apart from our neighboring, dead planets.
    ::对比来自金星、地球和火星的大气光谱。 注意水和臭氧仅存在于地球上, 将它与我们相邻的死行星区分开来。

    Atmosphere Spectroscopy
    ::大气光谱

    When a planet transits, some of the light from the star  penetrates the atmosphere of the planet where atoms and molecules will absorb light from the star. This gives us a spectrum of the (star + planet) . When the planet is eclipsed by the star, another observation will produce a spectrum of the star-only.  We can divide the (star + planet) spectrum by the (star-only) spectrum to obtain a spectrum of the planet atmosphere. 
    ::当一个行星中转时,恒星的某些光线会穿透行星的大气层,原子和分子会吸收恒星的光。这给我们提供了一颗星光(星+行星 ) 。 当行星被恒星遮蔽时, 另一种观察将产生一颗星光。 我们可以将星光(星+行星)的光谱除以( 恒星+行星) 的光谱, 以获得行星大气的光谱。

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    A cartoon of a transiting planet. In this figure, the observer is to the right of the image. Dotted lines indicate light that has passed through the atmosphere of the planet. How does transit spectroscopy probe exclusively the atmosphere of a planet?
    ::中转行星的漫画。 在这个图中, 观察者是图像右侧的。 虚线表示穿过地球大气层的光线。 中转光谱探测如何只探测行星的大气层?

     

    The planetary spectrum is usually quite faint, so a good strategy is to photometrically observe the transit with several different filters.  This technique is called spectrophotometry and gives very low resolution information about the planetary atmosphere. Then, the planetary spectrum can be compared to synthetic models with different chemical compositions.  A poor match is rejected and a good match is retained as a plausible solution.  The Figure below (left) shows an example of transit observations taken through different filters. Each of those measurements is a measure of intensity at a particular wavelength that is plotted on the right (filled black circles with error bars for the uncertainty in the flux). The observed spectrum on the right has overplotted models: 100% methane (green), 100% water (blue), and 100% carbon dioxide (orange). The first two models can be rejected and the last model can be retained.  Further observations and additional synthetic models can be used to identify or rule out other possible atmospheric spectra. 
    ::行星频谱通常非常微弱, 因此一个很好的策略是用多个不同的过滤器对中转点进行光度观测。 这个技术叫做光谱光谱测量, 并给出关于行星大气的分辨率非常低的信息。 然后, 行星频谱可以与化学成分不同的合成模型进行比较。 匹配差的行星频谱会被否决, 并保留一个良好的匹配作为合理的解决方案。 下面的图( 左) 显示了通过不同过滤器进行中转观测的示例。 这些测量都是在右侧绘制的某个波长( 填充黑圈, 填充通通量不确定的错误条条) 的强度测量。 右侧观测频谱的模型被铺设过高: 100% 甲烷( 绿色)、 100% 水( 蓝色) 和 100% 二氧化碳( 环) 。 前两个模型可能被否决, 最后一个模型可以保留。 进一步观测和补充合成模型可以用来识别或排除其他可能的大气光谱 。

     

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    Transit photometry of GJ 1214b with different filters (left). The intensity of the transits at different wavelengths are plotted on the right, and models for 3 different synthetic model atmospheres are shown.
    ::GJ 1214b 具有不同过滤器(左)的中转光测图。不同波长的中转强度在右边绘制,并显示三种不同合成模型大气的模型。

    The    below shows the likely detection of a water feature in the atmosphere of Wasp-12b, a hot Jupiter. The next   shows  spectra from a  survey of hot Jupiters using the Hubble Space Telescope. The James Webb Space Telescope (JWST)  will be launched in 2021 and will   try to determine   the  compositions of atmospheres of  rocky planets around M dwarfs.
    ::下面显示在热木星Wasp-12b大气中可能探测到的水特征。接下来展示的是使用哈勃空间望远镜对热木星进行调查的光谱。詹姆斯·韦伯空间望远镜(JWST)将于2021年发射,并将试图确定M矮星周围岩石行星大气层的构成。

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    The atmosphere spectrum of Wasp-12b. The clear increase in transit depth at around 1.4 micrometers is a strong indication of water. The model describing a water-rich atmosphere is shown as a dark blue line and accentuated with blue squares. Results of the observations are shown as white circles. Not bad, eh?
    ::黄蜂-12b 的大气频谱。 大约1.4微米的中转深度明显增加是水的强烈迹象。 描述丰富水层的模型显示为深蓝色线, 以蓝色方块加亮。 观测结果显示为白色圆圈。 不错吧 ?

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    The atmosphere spectra of several hot Jupiters. The colored lines represent different models for the possible chemical composition of the different atmospheres of these planets. What are common features among the different planets? How do they differ?
    ::几个热木星的大气光谱。 彩色线代表着这些行星不同大气层可能化学构成的不同模型。 不同行星的共同特征是什么? 它们有什么不同?

       Question for Thought
    ::思考问题

    Recall that the transit depth is determined by the ratio of the planetary ratio to the stellar radius squared,  ( R p l a n e t R s t a r ) 2  . With this in mind, why are hot Jupiters particularly conducive to transit spectroscopy for solar type stars?  What is the advantage  offered by   M dwarfs?

     

    The Red Edge
    ::红边缘

    Another spectral feature that might be a good indicator of life is the appearance of a red edge in spectra. On Earth, the red edge  is a property of chlorophyll, which becomes abruptly reflective at infrared wavelengths.
    ::另一个光谱特征可能是生命的好指标,那就是光谱中红色边缘的外观。 在地球上,红色边缘是叶绿素的属性,它突然反射到红外波长。

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    The measured reflectance of terrestrial plants at different wavelengths. Note the abrupt increase at around 0.7 microns. This feature has been named the red edge.
    ::测量到不同波长的地面植物的反射。 注意在0. 7微米左右突增。 这个特性被称为红色边缘 。

    This property of chlorophyll keeps plants from overheating, but is also integral to plants growing efficiently through a process called shade avoidance. Plants are capable of detecting a lack of infrared light and t his triggers a response in plants that  produces longer stems and larger leaves, which allows a plant to  capture more sunlight.
    ::叶绿素的这一特性使植物避免过热,但也是植物通过一个称为避免阴影的过程有效生长的有机组成部分。 植物能够检测出红外光的缺乏,这在生产长茎和大叶的植物中引发反应,使植物能够捕捉更多的阳光。

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    A comparison of a plant grown in full sunlight (left) and a plant grown in an absence of infrared light (right). What are the most noticeable differences between the two plants?
    ::比较完全阳光下生长的植物(左)和没有红外线光(右)生长的植物。 这两个植物之间最显著的区别是什么?

    P hotosynthetic organisms were a critical step in the evolution of aerobic respiration in eukaryotes. Therefore, a similar detection in the atmospheres of other planets would be an intriguing clue for bio-activity. The red edge is a strong signal in the atmospheric spectra from Earth and an easy feature to search for in the atmospheres of other planets. 
    ::光合生物是尤卡约特体有氧呼吸进化的关键一步,因此,在其他行星的大气中进行类似的探测将是生物活动的一个引人入胜的线索。 红色边缘是来自地球的大气光谱中的强烈信号,也是其他行星大气层中容易找到的特征。