地照植被观测及其对其他行星生命探测的启示--2001-2006年工作回顾
Earthshine observation of vegetation and implication for life detection on other planets - A review of 2001 - 2006 works
摘要：
探测外星生命是未来雄心勃勃的太空任务的目标之一，这些任务旨在直接拍摄类地行星的图像。虽然行星大气层中存在的简单分子（$O_2$，$O_3$，$CO_2$等）已经被确定为可能的全球生物标志物，我们在这里审查的生命的签名从地球表面，即绿色植被的可探测性。植被反射率确实有一个特定的光谱，在700 nm左右有一个尖锐的边缘，称为“植被红边”（VRE）。此外，植被覆盖了从热带常绿森林到灌木苔原的大面积土地。因此，将其视为潜在的全球生物标志物是相关的。Earthshine允许将地球作为一个遥远的行星进行观测，即没有空间分辨率。自2001年以来，几位作者使用Earthshine观测来测试和量化VRE在地球光谱中的可探测性。egetation光谱签名被检测为一个小的“正位移”的几个波长以上的连续，开始在700 nm。这种信号出现在大多数光谱中，其强度与地球的相位（可见陆地与可见海洋）相关。观测结果表明，在地球上探测VRE需要1%或更高的光度相对精度。因此，在类似地球的行星上探测类似的东西仍然具有挑战性，此外，考虑到矿物文物的可能性和宇宙中“红边”普遍性的问题。
英文摘要：
The detection of exolife is one of the goals of very ambitious future space missions that aim to take direct images of Earth-like planets. While associations of simple molecules present in the planet's atmosphere ($O_2$, $O_3$, $CO_2$ etc.) have been identified as possible global biomarkers, we review here the detectability of a signature of life from the planet's surface, i.e. the green vegetation. The vegetation reflectance has indeed a specific spectrum, with a sharp edge around 700 nm, known as the "Vegetation Red Edge" (VRE). Moreover vegetation covers a large surface of emerged lands, from tropical evergreen forest to shrub tundra. Thus considering it as a potential global biomarker is relevant. Earthshine allows to observe the Earth as a distant planet, i.e. without spatial resolution. Since 2001, Earthshine observations have been used by several authors to test and quantify the detectability of the VRE in the Earth spectrum. The egetation spectral signature is detected as a small 'positive shift' of a few percents above the continuum, starting at 700 nm. This signature appears in most spectra, and its strength is correlated with the Earth's phase (visible land versus visible ocean). The observations show that detecting the VRE on Earth requires a photometric relative accuracy of 1% or better. Detecting something equivalent on an Earth-like planet will therefore remain challenging, moreover considering the possibility of mineral artifacts and the question of 'red edge' universality in the Universe.
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