古代的火星上頭並沒有流動的河水，而是被冰原覆蓋

 原文網址：https://news.ubc.ca/2020/08/03/early-mars-was-covered-in-ice-sheets-not-flowing-rivers/

古代的火星上頭並沒有流動的河水，而是被冰原覆蓋

英屬哥倫比亞大學發表在《自然―地球科學》的新研究指出刻劃在火星表面的谷地網絡大多數是由冰河底下的融水雕刻而成，而非像過往認為的是由自由流動的河水切割出來的。現今主流的「溫暖潮濕的古代火星」假說推測這顆紅色行星表面曾有河流、降雨以及海洋，但這項發現無疑對此假說潑了一大盆冷水。

這張圖片是德文島上的冰帽。英屬哥倫比亞大學的研究人員認為早期的火星地貌可能就像這樣。圖片來源：Anna Grau Galofre.

之前在英屬哥倫比亞大學的地球、海洋與大氣科學系攻讀博士的Anna Grau Galofre是研究主要作者，她運用她開發的新技術探討火星表面一萬多座谷地之後得出了這項結論。此外，她和其他作者也把火星的谷地拿來和加拿大北極群島的冰下河道互相比較，結果發現兩者之間的相似程度相當驚人。

「自從首此發現火星表面的谷地之後，40多年來科學家大都推測火星表面曾有河流流動，它們侵蝕地表而形成了這些谷地，」Grau Galofre表示。「但是火星上的谷地有幾千幾百座，而它們彼此之間看起來有很大的差異。如果你觀察地球的衛星影像也可以看到許多谷地：它們有些是由河水侵蝕而成、有些是由冰河切割出來、其他的則是形成自別的作用，每種類型都有著自己獨特的樣貌。火星也是如此，由於不同谷地看起來的差異相當大，代表有許多作用參與了它們的雕刻過程。」

德文島位在加拿大的北極地區，由於島上的冰下河道跟火星的許多谷地有相似之處，激發了作者進行研究來比較它們的想法。西安大略大學地球科學系與地球及太空探測研究所的Gordon Osinski教授是共同作者，他說：「德文島是地球這裡最像火星的地點之一，島上盡是又乾又冷的極地荒漠，大多數的冰河直到底部都是凍結的。」

研究人員顯示火星的茂米谷(上半部)可以和德文島努納武特的河道(下半部)重疊。河道的形狀以及河道網絡的整體模式幾乎是如出一轍。圖片來源：Cal-Tech CTX mosaic and MAXAR/Esri.

研究人員運用最新的演算法來推估造就火星谷地的侵蝕過程，總共分析了超過一萬座谷地。「這是首次有證據顯示古代的火星冰層下方有融水形成的河道系統，它們造成的冰下侵蝕作用面積相當廣泛，」共同作者，英屬哥倫比亞大學地球、海洋與大氣科學系的Mark Jellinek教授表示。「和一般的看法相反，這項發現指出火星的谷地網絡當中只有一小部分符合地表水典型的侵蝕模式。從統計學上是否具有意義的觀點來分析火星的地表地形，藉此精確重建出火星的地形特徵與演化過程確實是種革命性的方法。」

Grau Galofre的理論也有助於解釋38億年前陽光比現在還微弱的時候，比地球還要更遠離太陽的火星如何形成這些谷地。「氣候模型預測古代火星的氣候在這些谷地網絡形成的時候比現在還要冷上許多，」Grau Galofre表示。她目前在亞利桑那州立大學地球與太空探索學院擔任博士後研究員。「我們結合了所有線索得出了一個還沒有被人認真考慮過的假說：由於自然界中一部份的流域系統是水累積在冰層底部之後所形成的，因此火星表面的河道和谷地網絡可能也是形成於冰層下方。」

如果古代火星有生命的話，這類環境也更有利於它們存活下來。冰層可以使下方的水受到更多保護因而較為穩定；另一方面，火星雖然曾經有磁場，但在幾十億年前就已經消失了，不過有冰層的話就能在沒有磁場的情況下保護生命不受太陽輻射傷害。

雖然Grau Galofre的研究是針對火星，但她開發出來的分析工具也可以用在挖掘更多關於我們星球的早期歷史。Jellinek表示他打算運用這個新的演算法來分析地球在歷史極為早期留下的侵蝕特徵。

「目前我們可以把全球冰河歷史精確地重建出來的年代，最早大約可以到一百萬到五百萬年前，」Jellinek表示。「Anna的研究可以讓我們探討冰層的前進後退歷史至少到三千五百萬年前，也就是南極剛形成的時候甚至更早，比我們現有最古老的冰芯年代還要久遠許多。這真的是件相當巧妙的分析工具。」

 

Early Mars was covered in ice sheets, not flowing rivers

A large number of the valley networks scarring Mars’s surface were carved by water melting beneath glacial ice, not by free-flowing rivers as previously thought, according to new UBC research published today in Nature Geoscience. The findings effectively throw cold water on the dominant “warm and wet ancient Mars” hypothesis, which postulates that rivers, rainfall and oceans once existed on the red planet.

To reach this conclusion, lead author Anna Grau Galofre, former PhD student in the department of earth, ocean and atmospheric sciences, developed and used new techniques to examine thousands of Martian valleys. She and her co-authors also compared the Martian valleys to the subglacial channels in the Canadian Arctic Archipelago and uncovered striking similarities.

“For the last 40 years, since Mars’s valleys were first discovered, the assumption was that rivers once flowed on Mars, eroding and originating all of these valleys,” says Grau Galofre. “But there are hundreds of valleys on Mars, and they look very different from each other. If you look at Earth from a satellite you see a lot of valleys: some of them made by rivers, some made by glaciers, some made by other processes, and each type has a distinctive shape. Mars is similar, in that valleys look very different from each other, suggesting that many processes were at play to carve them.”

The similarity between many Martian valleys and the subglacial channels on Devon Island in the Canadian Arctic motivated the authors to conduct their comparative study. “Devon Island is one of the best analogues we have for Mars here on Earth—it is a cold, dry, polar desert, and the glaciation is largely cold-based,” says co-author Gordon Osinski, professor in Western University’s department of earth sciences and Institute for Earth and Space Exploration.

In total, the researchers analyzed more than 10,000 Martian valleys, using a novel algorithm to infer their underlying erosion processes. “These results are the first evidence for extensive subglacial erosion driven by channelized meltwater drainage beneath an ancient ice sheet on Mars,” says co-author Mark Jellinek, professor in UBC’s department of earth, ocean and atmospheric sciences. “The findings demonstrate that only a fraction of valley networks match patterns typical of surface water erosion, which is in marked contrast to the conventional view. Using the geomorphology of Mars’ surface to rigorously reconstruct the character and evolution of the planet in a statistically meaningful way is, frankly, revolutionary.”

Grau Galofre’s theory also helps explain how the valleys would have formed 3.8 billion years ago on a planet that is further away from the sun than Earth, during a time when the sun was less intense. “Climate modelling predicts that Mars’ ancient climate was much cooler during the time of valley network formation,” says Grau Galofre, currently a SESE Exploration Post-doctoral Fellow at Arizona State University. “We tried to put everything together and bring up a hypothesis that hadn’t really been considered: that channels and valleys networks can form under ice sheets, as part of the drainage system that forms naturally under an ice sheet when there’s water accumulated at the base.”

These environments would also support better survival conditions for possible ancient life on Mars. A sheet of ice would lend more protection and stability of underlying water, as well as providing shelter from solar radiation in the absence of a magnetic field—something Mars once had, but which disappeared billions of years ago.

While Grau Galofre’s research was focused on Mars, the analytical tools she developed for this work can be applied to uncover more about the early history of our own planet. Jellinek says he intends to use these new algorithms to analyze and explore erosion features left over from very early Earth history.

“Currently we can reconstruct rigorously the history of global glaciation on Earth going back about a million to five million years,” says Jellinek. “Anna’s work will enable us to explore the advance and retreat of ice sheets back to at least 35 million years ago—to the beginnings of Antarctica, or earlier—back in time well before the age of our oldest ice cores. These are very elegant analytical tools.”

原始論文：Grau Galofre, A., Jellinek, A.M. & Osinski, G.R. Valley formation on early Mars by subglacial and fluvial erosion. Nat. Geosci., 2020 DOI: 10.1038/s41561-020-0618-x

引用自：University of British Columbia. "Early Mars was covered in ice sheets, not flowing rivers.”