Aliens Could Detect Life on Earth. Here's How.

美國《國家地理》：外星人如何探測到地球生命？

As the universe's only known harbor for life, Earth is arguably one strange rock. But light-years from our solar system, other intelligent beings on a similar planetary oasis might be gazing in our direction and seeing us as a sign that they're not alone in the universe.

作為宇宙中唯一已知的生命港灣，地球可以說是一塊奇特的巖石了。但是，在距離我們太陽系光年之外的某個類地行星綠洲上，或許有其他智慧生物正在注視著我們的方向，并將我們地球生命視為他們在宇宙中并不孤單的象征。

Once spotted, our planet would likely intrigue E.T. Our sun is relatively stable, not prone to disastrous flares that'd rip our atmosphere to shreds. What's more, we fall squarely within our sun's habitable zone, the area around a star where liquid water can persist on a planet's surface. (These are just some of the things that make life as we know it possible on Earth.)

一旦被發(fā)現(xiàn)，我們的星球可能會激起外星人的興趣。因為我們的太陽相對穩(wěn)定，不容易爆發(fā)災難性的耀斑活動，將我們的大氣層撕成碎片。更重要的是，我們地球剛好落在太陽系的可居住區(qū)內(nèi)，這個區(qū)域處在恒星附近，液態(tài)水可以留存于地球表面。（這些只是我們所知道的使地球有可能存在生命的要素。）

Faraway scientists might then attempt to spot our atmosphere, to see whether life's thumb is on the chemical scales. But what would they be looking for? And could they really infer life's presence across trillions of miles?

遙遠星球的科學家可能會試圖發(fā)現(xiàn)我們的大氣層，看看在化學層面上是否有生命存在。但他們要找的是什么呢？他們真的能跨越數(shù)萬億英里推斷出地球生命的存在嗎？

Life Is Pretty Gassy

生命跟氧氣有很大關系

Terra Cognita?

發(fā)現(xiàn)新大陸？

Beyond those chemical clues, alien astronomers with truly massive telescopes might even be able to map Earth's surface from afar, down to major urban areas.

除了這些化學線索之外，外星天文學家擁有真正的大型望遠鏡，他們甚至可能繪制出地球表面的地圖，從偏僻遙遠之地覆蓋到主要的城市地區(qū)。

Advanced Intelligence

高級情報

If intelligent life is nearby, the most straightforward way to find earthlings would be to listen for us. For the last century, human civilization has been broadcasting its existence to the cosmos via our leaky radio transmissions. Occasionally, we've broadcast messages to E.T. intentionally, and sent golden records into the void on the off-chance that aliens stumble across our interplanetary spacecraft.

如果地球附近有智慧生命的話，那么找到地球人最直接的方法就是聽到我們的聲音。上個世紀，人類文明一直通過無線電傳輸向宇宙廣播我們的存在。偶爾地，我們還會故意向外星人廣播信息，將黃金唱片送到太空中播放，希望外星人會無意中偶遇到我們的星際飛船。

Of course, Earth is more than 4.5 billion years old, and life has changed a lot over the eons. What if alien astronomers had looked our way even a billion years ago?

當然了，地球已經(jīng)有超過45億年的歷史，地球上的生命在億萬年間已經(jīng)發(fā)生了很大的變化。要是外星天文學家們早在十億年前就已經(jīng)朝我們的方向看過了呢？

In a 2018 paper in Science Advances, Olson and her colleagues simulated how Earth's atmosphere has changed over time. Even three billion years ago, aliens may have been able to infer life by sniffing out methane and carbon dioxide in the early atmosphere. But our modern atmosphere—a literal beacon for life—arrived only about 500 million years ago.

在《科學進展》雜志2018年的一篇論文中，科學家Olson和她的同事模擬了地球大氣層隨時間的變化情況。就算外星人在30億年前就能通過探測早期地球大氣中的甲烷和二氧化碳來推斷生命是否存在，但現(xiàn)代的地球大氣，即真正意義上的生命港灣，也是大約5億年前才形成的。