Sun, Nov 24, 2019 page9

• “Possibility of life”— scientists map Saturn’s exotic moon Titan 科學家公布「泰坦星」地圖　探索生命存在可能性

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  Scientists on Monday unveiled the first global geological map of Saturn’s moon Titan including vast plains and dunes of frozen organic material and lakes of liquid methane, illuminating an exotic world considered a strong candidate in the search for life beyond Earth.

  The map was based on radar, infrared and other data collected by NASA’s Cassini spacecraft, which studied Saturn and its moons from 2004 to 2017. Titan, with a diameter of 5,150km, is the solar system’s second-biggest moon behind Jupiter’s Ganymede. It is larger than the planet Mercury.

  Organic materials — carbon-based compounds critical for fostering living organisms — play a leading role on Titan. “Organics are very important for the possibility of life on Titan, which many of us think likely would have evolved in the liquid water ocean under Titan’s icy crust,” said planetary geologist Rosaly Lopes of NASA’s Jet Propulsion Laboratory in California. “Organic materials can, we think, penetrate down to the liquid water ocean and this can provide nutrients necessary for life, if it evolved there,” added Lopes, who led the research published in the journal Nature Astronomy.

  

  NASA’s planned Dragonfly rotorcraft lander approaches a site to explore on Saturn’s moon, Titan, in an Illustration released on June 27. 這張發布於六月二十七日的模擬圖，展示美國太空總署計畫發射的「蜻蜓號」旋翼機登陸器，正在接近土星衛星「泰坦」表面一處地點，準備進行探勘。

  Photo: Reuters
  照片：路透

  On Earth, water rains down from clouds and fills rivers, lakes and oceans. On Titan, clouds spew hydrocarbons like methane and ethane — which are gases on Earth — in liquid form due to the moon’s frigid climate. Rainfall occurs everywhere on Titan, but the equatorial regions are drier than the poles, said study co-author Anezina Solomonidou, a European Space Agency research fellow.

  Plains (covering 65 percent of the surface) and dunes (covering 17 percent of the surface) made up of frozen bits of methane and other hydrocarbons dominate Titan’s mid-latitudes and equatorial regions, respectively.

  Titan is the only solar system object other than Earth boasting stable liquids on the surface, with lakes and seas full of methane being major features at its polar regions. Hilly and mountainous areas, thought to represent exposed portions of Titan’s crust of water ice, represent 14 percent of the surface.

  

  This artist’s concept envisions what hydrocarbon ice forming on a liquid hydrocarbon sea of Saturn’s moon Titan might look like in this NASA image released on Jan. 8, 2013. 美國太空總署於二○一三一月八日發布的這張概念圖，模擬出土星衛星「泰坦」表面的碳氫化合物，在液態碳氫化合物構成的海中逐漸結冰的樣貌。

  Photo: Reuters
  照片：路透

  “What is really fun to think about is if there are any ways that those more complex organics can go down and mix with water in the deep icy crust or deep subsurface ocean,” JPL scientist and study co-author Michael Malaska said. Noting that on Earth there is a bacterium that can survive just on a hydrocarbon called acetylene and water, Malaska asked, “Could it or something like it live in Titan deep in the crust or ocean where temperatures are a little warmer?”

  The map was created seven years before the US space agency is set to launch its Dragonfly mission to dispatch a multi-rotor drone to study Titan’s chemistry and suitability for life. Dragonfly is scheduled to reach Titan in 2034. “It is not only scientifically important but also really cool — a drone flying around on Titan,” Lopes said. “It will be really exciting.”

  (Reuters)

  科學家在週一公布史上第一張土星衛星「泰坦」的全球地質圖，其中包含許多廣闊的平原，冰凍有機物質構成的沙丘，以及液態甲烷形成的湖泊，揭露這片咸認為是地球以外生命搜尋重要候選地點的奇特世界。

  該地質圖是根據美國國家航空暨太空總署的卡西尼太空探測船，在二○○四年到二○一七年間研究土星及其衛星時，收集到的雷達、紅外線等資料。直徑達五千一百五十公里的泰坦星，是我們太陽系第二大的衛星，僅次於木星的衛星蓋尼米德。泰坦甚至比水星還大。

  有機物質──也就是對於培養生物體而言非常關鍵的碳基化合物──在泰坦星上扮演重要角色。美國太空總署加州噴射推進實驗室的行星地質學家羅莎莉‧洛佩茲指出：「有機物對於泰坦形成生命的可能性至關重要，我們有許多人認為，生命可能在泰坦結冰地殼底下的液態海洋中進行演化。」這篇研究發表於《自然‧天文》期刊，主持該研究的洛佩茲補充說：「我們認為，有機物質會向下滲透到液態海洋，如果其中有生命演化，就能為生命提供必要營養物。」

  在地球上，水從雲層降下，注入河流、湖泊和海洋。在泰坦星上，雲層則會噴出液態的甲烷或乙烷等碳氫化合物。這類碳氫化合物在地球上以氣態存在，但在泰坦星上卻因為嚴寒氣候而呈現液態。這份研究的共同作者是歐洲太空總署的研究員安聶琦娜‧所羅門妮朵，她表示泰坦星上各處都有降雨，不過赤道地區比起兩極較為乾燥。

  由結冰的甲烷和其他碳氫化合物形成的碎屑組成的平原（佔其地表百分之六十五）與沙丘（佔其表面百分之十七），分別分布於泰坦星的中緯度和赤道地區。

  除了地球以外，泰坦星是太陽系中唯一表面擁有穩定液體的星球，充滿甲烷的湖泊和海洋是兩極地區的主要特徵。丘陵和高山區域則被認為是泰坦地殼部分裸露出來的冰塊，佔其表面百分之十四。

  噴射推進實驗室的科學家麥可‧馬拉斯加是這篇研究的另一位共同作者，他指出：「真正有趣的是，我們可以想像那些更複雜的有機物，有什麼方法能夠滲入冰凍的地殼深處、或是沉入地殼下的深層海洋，和水混合。馬拉斯加提到，地球上有一種細菌只靠乙炔這種碳氫化合物跟水就可以生存，於是他問：「這種細菌，或是類似的有機物，會不會也存在於泰坦地殼深處或是底下溫度較暖的海洋中呢？」

  這張地圖的誕生將為美國太空總署即將於七年後啟動的「蜻蜓號」任務做好準備。該任務將派遣一架多旋翼無人機，前往泰坦星探索其化學性質和適合生命存在的可能性。「蜻蜓號」預計於二○三四年抵達泰坦星。洛佩茲表示：「這項任務不只具有重要的科學意義，同時也很酷──派一架無人機在泰坦星上四處盤旋，這將會非常讓人興奮。」

  （台北時報章厚明譯）
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