卡西尼號探索泰坦星上的甲烷海洋

中國科技網4月27日報道（張微 編譯）在我們太陽系的數百顆衛星中，泰坦(土衛六)是唯一一個存在濃密大氣層和大型液態水的星球，這使其在某些方面更像類地行星。

地球和泰坦星都有一個以氮氣為主的大氣層，泰坦星大氣層的氮含量超過95%。與地球不同的是，泰坦星上的氧氣很少，其餘氣體主要是甲烷和微量的其它氣體，如乙烷。當土星距離太陽最遠時，這個星球處於極端寒冷的溫度下，這時候甲烷和乙烷以液體形態存在。

為此，科學家們一直推測泰坦星上可能存在碳氫湖和海洋，而且從NASA/ESA的卡西尼-惠更斯任務中獲取的數據不會讓你失望。自2004年抵達土星系統，卡西尼號飛船已經縱覽了土星表面62萬平方英里(160萬平方公里)的面積，其中近2%的表面被液體覆蓋。

泰坦星上有三個大型海洋，都位於這個星球的北極，被北半球上的許多小湖泊包圍。只有一個大型湖泊存在於南半球。

這些液體的確切成分還不明確，直到2014年，卡西尼號雷達儀首次顯示麗姬亞海，泰坦星上第二大海洋，面積與休倫湖和密歇根湖之和相當，富含甲烷。這項新研究發表在《地球物理研究：行星》期刊上，在不同模式下使用雷達儀器，獨立地證實了這一結果。 「在卡西尼號之前，我們本認為麗姬亞海可能主要由乙烷構成，當陽光照射導致甲烷分子分裂，大氣層中就產生了大量乙烷。與我們預計的相反，這片海主要是由純甲烷構成的，」 Alice Le Gall說。她是法國巴黎LATMOS實驗室的卡西尼雷達團隊研究院，也是這項新研究的第一作者。

這項新研究的數據是卡西尼雷達儀器在2007-2015年飛越泰坦星時收集的。

根據Le Gall的分析，幾個可能的解釋也許能夠說明泰坦海洋的主要構成成分是甲烷。「麗姬亞海可能得到了新鮮甲烷雨的補充，或者是某些物質從海洋中移除了乙烷。乙烷可能在洋底地殼終結，或者它流入了臨近海域克拉肯海，但這還需要進一步的調查研究。」

在他們的研究中，科學家們將幾個雷達觀測的，麗姬亞海釋放的熱量數據合并。他們還利用了2013年一項實驗的數據，這些數據是麗姬亞海返回的無線電信號。這個實驗的結果發表在2014年的一篇論文中，作者是雷達團隊研究員，來自紐約州伊薩卡的康奈爾大學的馬可 馬斯特羅朱塞佩，他也參與了目前這項研究。

2013年的實驗中，雷達儀器檢測到了來自海底的回波，根據卡西尼號對麗姬亞海的跟蹤監測，推斷出了麗姬亞海的深度，這也是首次探測到外星海洋的洋底。科學家們非常震驚地發現，根據雷達追蹤，海洋最深處的深度足有525英尺(160米)。

Le Gall和她的同事們利用測深信息將液體海洋和洋底的觀測溫度分開研究，這樣能夠深入地了解它們各自的組成。

「我們發現，麗姬亞海海底可能是由富含有機物的污泥層覆蓋，」Le Gall補充說 。

在泰坦的大氣中，氮和甲烷反應產生各種各樣的有機物質。科學家相信最重的物質會降落在星球表面。Le Gall和她的同事們認為，當這些化合物會進入海洋，無論是直接從空中落下，通過雨水還是通過泰坦的河流流入，有些物質溶解在液態甲烷中，不溶性的化合物，如腈和苯，則沉入海底。

該研究還發現，麗姬亞海海岸線可能是多孔的而且充滿了液態碳氫化合物。如果數據跨度是從泰坦星的冬天到春天，科學家們預計，與地球上的海邊一樣，泰坦星上海邊的固體地形會比海洋更快地回暖。

然而，根據卡西尼號的測量數據，科學家們並沒有發現，在這段時間裡，海水溫度和海岸邊有著顯著差異。這表明，湖泊和海洋周圍的地形是濕潤的，充滿了液態碳氫化合物，這讓它們的升溫和降溫速度與海洋本身一樣。

「我們正在對另一個行星的衛星上所存在外星海洋提供合理的解釋分析，這是一個非常了不起的成就，」史提夫 沃爾說，他是位於加利福尼亞州帕薩迪納，美國宇航局噴氣推進實驗室的研究人員，也是卡西尼號雷達團隊的副主任。「泰坦還會有讓我們震驚的發現。」

卡西尼-惠更斯任務是美國宇航局和ESA(歐洲航天局)，義大利航天局的一個合作項目。噴氣推進實驗室，位於帕薩迪納的加州理工大學的一個部門，負責在華盛頓的美國宇航局科學任務理事會的任務。雷達儀器是由噴氣推進實驗室和義大利航天局建造的，還有來自美國和幾個歐洲國家的團隊成員共同參與研究工作。

Cassini explores a methane sea on Titan

Of the hundreds of moons in our solar system, Titan is the only one with a dense atmosphere and large liquid reservoirs on its surface, making it in some ways more like a terrestrial planet.

Both Earth and Titan have nitrogen-dominated atmospheres—over 95 percent nitrogen in Titan s case. However, unlike Earth, Titan has very little oxygen; the rest of the atmosphere is mostly methane and trace amounts of other gases, including ethane. And at the frigid temperatures found at Saturn s great distance from the sun, the methane and ethane can exist on the surface in liquid form.

For this reason, scientists had long speculated about the possible existence of hydrocarbon lakes and seas on Titan, and data from the NASA/ESA Cassini-Huygens mission does not disappoint. Since arriving in the Saturn system in 2004, the Cassini spacecraft has revealed that more than 620,000 square miles (1.6 million square kilometers) of Titan s surface—almost two percent of the total—are covered in liquid.

There are three large seas, all located close to the moon s north pole, surrounded by numerous of smaller lakes in the northern hemisphere. Just one large lake has been found in the southern hemisphere.

The exact composition of these liquid reservoirs remained elusive until 2014, when the Cassini radar instrument was first used to show that Ligeia Mare, the second largest sea on Titan and similar in size to Lake Huron and Lake Michigan combined, is methane-rich. A new study published in the Journal of Geophysical Research: Planets, which used the radar instrument in a different mode, independently confirms this result.

"Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart. Instead, this sea is predominantly made of pure methane," said Alice Le Gall, a Cassini radar team associate at the French research laboratory LATMOS, Paris, and lead author of the new study.

The new study is based on data collected with Cassini s radar instrument during flybys of Titan between 2007 and 2015.

A number of possible explanations could account for the sea s methane composition, according to Le Gall. "Either Ligeia Mare is replenished by fresh methane rainfall, or something is removing ethane from it. It is possible that the ethane ends up in the undersea crust, or that it somehow flows into the adjacent sea, Kraken Mare, but that will require further investigation."

In their research, the scientists combined several radar observations of heat given off by Ligeia Mare. They also used data from a 2013 experiment that bounced radio signals off Ligeia. The results of that experiment were presented in a 2014 paper led by radar team associate Marco Mastrogiuseppe at Cornell University, Ithaca, New York, who also contributed to the current study.

During the 2013 experiment, the radar instrument detected echoes from the seafloor and inferred the depth of Ligeia Mare along Cassini s track over Ligeia Mare—the first-ever detection of the bottom of an extraterrestrial sea. The scientists were surprised to find depths in the sea as great as 525 feet (160 meters) at the deepest point along the radar track.

Le Gall and her colleagues used the depth-sounding information to separate the contributions made to the sea s observed temperature by the liquid sea and the seabed, which provided insights into their respective compositions.

"We found that the seabed of Ligeia Mare is likely covered by a sludge layer of organic-rich compounds," adds Le Gall.

In the atmosphere of Titan, nitrogen and methane react to produce a wide variety of organic materials. Scientists believe the heaviest materials fall to the surface. Le Gall and colleagues think that when these compounds reach the sea, either by directly falling from the air, via rain or through Titan s rivers, some are dissolved in the liquid methane. The insoluble compounds, such as nitriles and benzene, sink to the sea floor.

The study also found that the shoreline around Ligeia Mare may be porous and flooded with liquid hydrocarbons. The data span a period running from local winter to spring, and the scientists expected that—like the seaside on Earth—the surrounding solid terrains would warm more rapidly than the sea.

However, Cassini s measurements did not show any significant difference between the sea s temperature and that of the shore over this period. This suggests that the terrains surrounding the lakes and seas are wet with liquid hydrocarbons, which would make them warm up and cool down much like the sea itself.

"It s a marvelous feat of exploration that we re doing extraterrestrial oceanography on an alien moon," said Steve Wall, deputy lead of the Cassini radar team at NASA s Jet Propulsion Laboratory in Pasadena, California. "Titan just won t stop surprising us."

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA s Science Mission Directorate in Washington. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the US and several European countries.

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