研究火星得到的線索指出生命搖籃的可能場所

原文網址：https://www.jpl.nasa.gov/news/news.php?feature=6966

研究火星得到的線索指出生命搖籃的可能場所

在火星發現古代海床熱液沉積物的證據，證實火星某處或許可以提供地球生命起源線索的想法。

最近一則國際報告檢視了美國太空總署的火星偵察軌道器(MRO)，對火星南部一處盆地中的大量沉積物的觀察結果。作者解讀資料後，認為其證實了這些沉積物是在許久以前，火星地殼中火山活動頻繁之處的熱水從海底流出而形成的說法。

「就算我們從未在火星找到生命曾經存於其上的證據，此處還是能告訴我們地球上的生命可能在何種環境誕生的線索。」美國休士頓NASA詹森太空中心的Paul Niles表示，「火山活動跟靜止水體的組合，提供的環境條件跟大約同一時間存在於地球上的十分類似——早期生命便是於此時此地演化出來。」

今日的火星既無靜止水體也無火山活動。研究人員估計這些火星沉積物是在37億年前左右由海底熱液活動造成。地球大約同時間的海底熱液環境則是地球生命起源的熱門候選時間地點之一。現今地球仍有此種環境存在，其中繁榮生長著各式各樣完全不需陽光，而是從岩石取出化學能的生物。在木衛二和土衛二這類冰質衛星內部有海底熱液活動的可能性，使得它們成為尋找地外生命任務時深感興趣的目標。

火星艾瑞達尼亞盆地(Eridania basin)的所在區域有部分是這顆紅色行星表面最古老的地殼。MRO的火星專用小型偵察影像頻譜儀(CRISM)提供的觀測資料可以用來辨識盆地內部堆積的大量沉積物含有何種礦物。

「此處呈現的故事使我們相信當時這裡有座存在許久、深度頗深的海洋，且底部擁有熱液環境。」Niles表示，「這讓我們想起地球上的深海熱液環境，或許其他世界的生命也會在類似環境發現。生命不需要有宜人的大氣層或是溫暖的地表才能生存，只需要岩石、熱和水即可。」

Niles是這篇近期發表於《自然通訊》(Nature Communications)的共同作者之一，主要作者Joseph Michalski於倫敦自然史博物館時最先開始分析這些資料，其他共同作者則是美國亞利桑那州圖森市行星科學研究所和倫敦自然史博物館的人員。

研究人員估計古代艾瑞達尼亞盆地擁有大約21萬立方公里的水量，多達古代火星其他所有湖泊和海洋的總和，同時也是美國五大湖總體積的9倍之餘。從頻譜儀的數據可以辨識出這些礦物係由蛇紋石、滑石和碳酸鹽等混雜而成，加上厚層岩床的形狀與紋理，使得研究人員認定其可能為海床熱液沉積物。在海洋消失之後，該區域轉而被岩漿覆蓋。研究人員以此為證，表示該區域為火星地殼中受到火山活動影響的區域，也同時解釋了當海洋還在時為何會有前述的熱液沉積物產生。

這項新成果增添了一筆新證據顯示火星曾經擁有相當多樣的有水環境，包括河流、湖泊、三角洲、海洋、溫泉、地下水、以及冰下噴發的火山。

報告聲稱「艾瑞達尼亞盆地的古代深海熱液沉積物代表了新的一種在火星上尋找地外生物的目標。」並表示，「艾瑞達尼亞盆地的底部不只讓探勘火星的研究人員深感興趣，它也讓我們得以一窺早期地球的樣貌。」這是因為地球上最古老的生命證據來自於類似年代與成因的海洋沉積物，但是地球早期環境的地質紀錄卻保存得十分差勁。

Mars Study Yields Clues to Possible Cradle of Life

The discovery of evidence for ancient sea-floor hydrothermal deposits on Mars identifies an area on the planet that may offer clues about the origin of life on Earth.

A recent international report examines observations by NASA's Mars Reconnaissance Orbiter (MRO) of massive deposits in a basin on southern Mars. The authors interpret the data as evidence that these deposits were formed by heated water from a volcanically active part of the planet's crust entering the bottom of a large sea long ago.

"Even if we never find evidence that there's been life on Mars, this site can tell us about the type of environment where life may have begun on Earth," said Paul Niles of NASA's Johnson Space Center, Houston. "Volcanic activity combined with standing water provided conditions that were likely similar to conditions that existed on Earth at about the same time -- when early life was evolving here."

Mars today has neither standing water nor volcanic activity. Researchers estimate an age of about 3.7 billion years for the Martian deposits attributed to seafloor hydrothermal activity. Undersea hydrothermal conditions on Earth at about that same time are a strong candidate for where and when life on Earth began. Earth still has such conditions, where many forms of life thrive on chemical energy extracted from rocks, without sunlight. But due to Earth's active crust, our planet holds little direct geological evidence preserved from the time when life began. The possibility of undersea hydrothermal activity inside icy moons such as Europa at Jupiter and Enceladus at Saturn feeds interest in them as destinations in the quest to find extraterrestrial life.

Observations by MRO's Compact Reconnaissance Spectrometer for Mars (CRISM) provided the data for identifying minerals in massive deposits within Mars' Eridania basin, which lies in a region with some of the Red Planet's most ancient exposed crust.

"This site gives us a compelling story for a deep, long-lived sea and a deep-sea hydrothermal environment," Niles said. "It is evocative of the deep-sea hydrothermal environments on Earth, similar to environments where life might be found on other worlds -- life that doesn't need a nice atmosphere or temperate surface, but just rocks, heat and water."

Niles co-authored the recent report in the journal Nature Communications with lead author Joseph Michalski, who began the analysis while at the Natural History Museum, London, and co-authors at the Planetary Science Institute in Tucson, Arizona, and the Natural History Museum.

The researchers estimate the ancient Eridania sea held about 50,000 cubic miles (210,000 cubic kilometers) of water. That is as much as all other lakes and seas on ancient Mars combined and about nine times more than the combined volume of all of North America's Great Lakes. The mix of minerals identified from the spectrometer data, including serpentine, talc and carbonate, and the shape and texture of the thick bedrock layers, led to identifying possible seafloor hydrothermal deposits. The area has lava flows that post-date the disappearance of the sea. The researchers cite these as evidence that this is an area of Mars' crust with a volcanic susceptibility that also could have produced effects earlier, when the sea was present.

The new work adds to the diversity of types of wet environments for which evidence exists on Mars, including rivers, lakes, deltas, seas, hot springs, groundwater, and volcanic eruptions beneath ice.

"Ancient, deep-water hydrothermal deposits in Eridania basin represent a new category of astrobiological target on Mars," the report states. It also says, "Eridania seafloor deposits are not only of interest for Mars exploration, they represent a window into early Earth." That is because the earliest evidence of life on Earth comes from seafloor deposits of similar origin and age, but the geological record of those early-Earth environments is poorly preserved.

原始論文Joseph R. Michalski, Eldar Z. Noe Dobrea, Paul B. Niles, Javier Cuadros. Ancient hydrothermal seafloor deposits in Eridania basin on Mars. Nature Communications, 2017; 8: 15978 DOI: 10.1038/ncomms15978

引用自：NASA/Jet Propulsion Laboratory. "Mars study yields clues to possible cradle of life."