越來越多的證據表明金星曾經適宜居住（雙語）

If you could hop in a time-travelling spacecraft, go back three billion years and land any place in our solar system, where would you want to end up? Earth, with its barren continents and unbreathable atmosphere? Or Mars, a chillier version its big brother? Wait, what about Venus?

如果你能跳進一台時間旅行機器，穿越三十億年的時光回到過去並降落在太陽系中的任何位置，你最後會停在哪裡？地球？降落在那一片無法呼吸的荒蕪大陸？還是火星？一個僅比地球溫度稍低的星球？等等，你覺得金星怎麼樣？

Did Venus (left) used to be Earth-like? Models suggests it』s possible. Image: Wikimedia

金星（左）是否曾與地球相似？模型表明它很可能是的。圖片：維基媒體

Venus has a rep for being a toxic hellscape, but three billion years ago, it may have been the best piece of real estate our solar system had to offer — or at least, a close second to Earth. This hypothesis has been around for years, but it』s gaining traction thanks to climate models developed by researchers at NASA』s Goddard Institute for Space Studies and elsewhere.

金星常被人形容為一個地獄般的場所，但三十億年前，它可能是我們太陽系提供的最好的一方樂土 - 或者至少環境能接近地球。這個假設已經存在多年，但得益於NASA戈達德太空研究所（NASA』s Goddard Institute for Space Studies）以及其他地方研究員開發的氣候模型，人們才逐漸開始關注它。

Those models show that for two billion years, Venus could have had balmy, Earth-like temperatures and liquid water oceans, despite getting dosed with 40 per cent more solar radiation than Earth is today. But it depends on whether the Venus of bygone days spun as slowly as modern Venus does.

這些模型顯示，兩十億年來，雖然金星比現在的地球要多吸收40%的太陽輻射，但它可能依然有溫和的，類似地球的溫度和液態海洋。當然這取決於以前的金星是否跟現在轉的一樣慢。

「If Venus was spinning more rapidly, all bets are off, 」 Michael Way, lead author of a new study that has been submitted for publication in Geophysical Research Letters told Gizmodo. But, under the right conditions, 「you get temperatures almost like Earth. That』s remarkable.」

「如果金星轉的速度很快，那麼所有的一切就不成立，」已在《地球物理學研究快報》提交出版的一項新研究主要作者，Michael Way告訴Gizmodo。但是，在適當的條件下，「它的溫度幾乎接近地球。那就是了不起的發現。「

The habitability of Earth and Mars have changed throughout the lifetime of the solar system. Geologic evidence suggests that Mars was much wetter in the distant past, although whether the Red Planet had liquid water oceans or was frozen under a mantle of ice is still a matter of debate. Earth, meanwhile, has swung from hothouse to icehouse and back, all the while accumulating oxygen in its atmosphere and becoming a more favourable place for complex life.

在太陽系的演化過程中，地球和火星的居住環境已經發生了改變。地質證據表明，火星在遙遠的過去更加潮濕，但這顆紅色星球否有液態海洋還是地幔下凍結的冰塊仍是一個有爭議的問題。反觀地球，從溫室到冰窖反反覆復，同時積累了大氣中的氧氣，並成為一個更有利複雜生命生存的地方。

But what about Venus? The past habitability of Earth』s nearest neighbour has received scant attention compared with that of Mars. Our bias may stem from the fact that modern day Venus is so forbidding, with its impenetrably thick atmosphere, toxic thunderclouds and atmospheric pressure nearly 100 times higher than that of Earth. When a planet turns spacecraft after spacecraft into a puddle of goo within seconds, it』s only natural for people to get frustrated and turn their attention elsewhere.

但相比較火星，金星作為過去和地球最接近的棲息地，幾乎沒有獲得多少關注。我們的偏見可能來自，現代金星是那麼可怕的，其嚴密的大氣層，有毒雷雲和比地球高出近100倍的大氣壓。當一個行星沒過幾秒就讓人們發射出一個又一個的航天器陷入泥坑，人們感到沮喪和並把注意力轉到其他地方也是自然不過。

Still, just because Venus is weird and awful today doesn』t mean it always was. The entire surface was reworked by volcanic activity some 700 million years ago, and we have no idea what Venus looked like before that. But measurements of the hydrogen isotope ratio in Venus』 atmosphere imply that the planet used to have much more water — maybe enough to support oceans.

不過，現在金星的怪異和可怕並不意味著它以前也一直也是這樣。整個表面在7億年前因火山活動而被重塑，所以我們沒法知道金星之前長什麼樣。但通過測量金星大氣中的氫同位素的比值，這個星球曾經可能有過更多的水 - 也許足以生成海洋。

Taking a first stab at the question of whether Venus was once habitable, Way and his colleagues combined a global topographic dataset collected by the Magellan spacecraft with water and solar radiation estimates for past Venus. All of this information was plugged into global climate models, similar to those used to study climate change on Earth today.

對於金星曾經是否適合居住這一問題，Way和他的同事做的第一件事就是，結合麥哲倫航天器收集的全球地形數據集和對過去金星水和太陽輻射的估算。所有這些信息被代入到全球氣候模型，類似於今天研究地球上氣候變化的方式。

The initial results were encouraging. Despite the fact that ancient Venus would have received much more sunlight than modern Earth 2.9 billion years ago, Way』s models predict an average surface temperature of just 11C. By 715 million years ago, the surface would have only warmed up four degrees — meaning Venus could have had a temperate climate for at least 2 billion years.

初步結果令人鼓舞。儘管古代金星在29億年前比現在的地球獲得更多的陽光，Way的模型預測其表面平均溫度只有11攝氏度。 截至7.15億年前，其表面溫度也只上升四度 - 這意味著金星可能有至少20億年的溫帶氣候。

Recent research suggests that a powerful 『electric wind』 on Venus could have helped strip water vapour out of the planet』s atmosphere. Image: NASA

最近的研究表明，金星上強大的「電氣風」可以幫助水汽脫離這顆行星的大氣層。圖片：NASA

There is, however, a catch: Those numbers are entirely dependent on past Venus having similar topography and orbital characteristics to modern Venus. When Way re-ran his models but gave 2.9 billion year-old Venus a facelift to look like modern day Earth, surface temperatures rose considerably.

然而，有一個難題：這些數字是完全依賴於過去金星與現在金星具有類似的地形和軌道特性。當Way針對類似地球的地形給29億歲的金星重跑了他的模型，其表面溫度有明顯的上升。

「We wanted to see what effect topography might have had on the climate state of this world,」 Way said. 「Sure enough, it had a big effect.」 The reason, he says, probably has to do with changes in the amount of reflective surface on Venus, and shifting atmospheric dynamics.

「我們想看看地形對這個世界的氣候會產生什麼樣的影響，」Way說。 「果然，它帶來很大的影響。」究其原因，他說，可能是由於金星表面的改變影響了陽光的反射，從而使得大氣層動態變化著。

Another fascinating twist has to do with Venus』 rotation. In his initial models of 2.9 billion year old Venus, Way stuck with a slow, modern-day rotational period of 243 Earth days. But when he spun Venus up to give it a 16 Earth-day period, the planet once again became a pressure cooker.

另一個奇妙的怪異點在於金星的公轉。在他對29億歲金星的最初的模型中，Way使用一個現在的緩慢公傳周期，相當於243個地球日。但是，當他把金星的公轉周期加速到16個地球日時，這顆行星再次成為一個高壓鍋。

This has to do with the impact of Venus』 rotational rate on atmospheric circulation patterns called Hadley cells.

金星的這種旋轉速率對大氣環流的影響稱為哈德利環流。

「Earth has many [circulation] cells because our planet rotates fast,」 Way said. 「But if you rotate slowly, you have one in the north and one in the south, period. That changes the atmospheric dynamics of the world by a significant amount.」

「地球上有很多種環流，是因為我們的地球旋轉快，」Way說。 「但如果你慢慢地旋轉，你只有一個北極環流和一個南極環流。這些都會對顯著改變世界上的大氣動力學「。

Specifically, slow-rotating Venus develops a gigantic cloud right at the sub-solar point, that is, where the sun』s rays are hitting the surface dead on. This basically turns the atmosphere of Venus into a giant solar reflector. When Venus rotates more quickly, this pattern doesn』t form.

具體地，緩慢旋轉的金星會在日下點產生一片巨大的雲，因此，所有照在這表面的太陽光線都無法到達行星內部。這基本上使得金星的大氣層成為了一個巨大的太陽能反射器。當金星更快速地旋轉，這種情況將不會形成。

While the study doesn』t actually confirm that Venus was once habitable, it does point to a plausible scenario in which Venus could have been. It』s worth noting that a planet』s rotation rate can change dramatically over time — Earth』s rotation is gradually slowing down because of the Moon — and some researchers suspect that Venus spun more quickly in the past. But there』s no easy way for us to find out. We could get indirect evidence by doing population studies on small, rocky exoplanets in Venus-like orbits.

雖然這項研究並沒有真正證實金星曾經是可居住的，但它指向了一個可信的情景，這個情景可能就是金星曾經經歷的。值得注意的是一顆行星的旋轉速度可隨時間發生顯著變化 - 地球因為月球的存在旋轉速度才逐漸減慢 - 一些研究人員懷疑，金星過去更迅速地旋轉。但對我們來說沒有簡單的方法。我們只能通過研究星族軌道來獲得間接證據，如系外類似金星的小型岩質行星。

And, if Venus was a balmy paradise for billions of years, one has to wonder what sort of apocalypse led to the present-day situation.

而且，如果金星在數十億年間都是一個溫和天堂，人們想知道什麼樣的災變導致了現今的局面。

「We really need more data before we can say much more,」 Way said. But, he added, the study does indicate that worlds in Venus-like orbits should not immediately be deemed uninhabitable.

「我們確實需要更多的數據才能揭示的更多，」Way說。但是，他補充說，這項研究確實表明，當發現類似金星軌道的行星，不應該立即認為其不適合居住。

「In plots of the habitable zone, you typically see Venus on the outside,」 he said. 「And for modern day Venus, that』s certainly true. But, if you have a Venus-like world around a solar-type star with a slow rotation, it could be quite a reasonable place for life to exist, especially in the oceans.」

「在對可居住對象的研究中，你通常會看到金星被排除在外，」他說。 「而對於現在的金星，這麼做毫無疑問。但是，如果你發現了一個類似太陽系中的緩慢旋轉的金星，這可能是生命存在的地方，尤其是在海洋中。「

A past-habitable Venus also opens up new possibilities about the origin of life on Earth. From meteorites, we know that Earth and Mars were swapping material in the distant past, prompting astrobiologists to wonder whether the Red Planet could have seeded our world with life. But if life was just as likely to emerge on Venus, that』s one more planet to add to mix. Incredibly, we don』t know whether there are any meteorites from Venus here on Earth, because we』ve never been able to analyse a rock on Venus for comparison.

一個過去可居住的金星同時也暗示著地球生命起源的新的可能性。從隕石中，我們知道，地球和火星都在遙遠的過去交換物質，促使太空生物學家懷疑這顆紅色星球是否可能將生命播種到我們生活的世界。但是，如果金星中可能出現生命，那麼將有多一個星球混合進來。令人難以置信的是，我們不知道是否有來自金星的隕石落在地球上，因為我們從來沒有去分析金星上的岩石從而進行比較。

Until then, we can』t deny the possibility that our distant ancestors were actually born on that torrid acid bath next door. 「It could be that life got started on Venus, and then seeded Earth,」 Way said. 「Or vice-versa.」

在此之前，我們不能否認，我們的遠祖可能實際上出生在隔壁那個炎熱的酸浴行星上。 「可能生命開始於金星，然後播種地球,」Way說。 「或者相反。」(翻譯：楊乃漳)

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