火星上的陡坡透露出埋藏在這顆紅色星球之下的冰層

原文網址：https://www.jpl.nasa.gov/news/news.php?feature=7038

火星上的陡坡透露出埋藏在這顆紅色星球之下的冰層

研究人員利用NASA的火星勘測軌道衛星(MRO)，發現八處遭到侵蝕的坡面露出了火星表面之下的厚重冰層。

之前科學家在火星的中緯度地區偵測到地下有冰層存在。現在，這8座最陡達到55度的斷崖透露出的最新資訊顯示該冰層內部具有層狀構造。

這些冰層在許久以前可能是以雪的形式堆積於地表。在斷面出露的冰層相對而言近乎是由水冰構成，其上方覆蓋由冰塊膠結岩石和沙塵所形成的沉積物，厚約1至2公尺。冰層不但藏有火星氣候歷史的線索，對於未來的機器或人類探勘任務來說，凍結的水可能比我們之前認為的還要容易取得。

研究人員的發現今日發表於期刊《科學》(Science)，他們運用MRO搭載的高解析度成像科學設備(HiRISE)來定位並研究這些斷崖。斷崖所在地點大約位在火星南北半球緯度55度至58度之間，在地球上等同於蘇格蘭和南美尖端所在的緯度。

「火星表面大概有三分之一的面積距地表不遠處埋有冰層，其記錄了火星的近代歷史。」研究主要作者，美國地質調查局天文地質學中心的Colin Dundas表示。該單位位於亞利桑那州的弗拉格斯塔夫。「我們在這些斷崖可以看到切穿冰層的剖面，藉此能得到冰層的三維影像以及前所未有的詳細資訊。」

一窺地下冰層

這些斷崖上露出的明亮部分即為廣大地下冰層的一角。先前NASA火星奧德賽號(MRO)上的光譜儀、MRO的透地雷達、歐洲太空總署的火星快車號衛星都偵測出火星地下有廣大的冰層，觀測剛形成的撞擊坑也指出地下有冰層存在。NASA受到奧德賽號的發現鼓舞而把鳳凰號登陸器發送至火星表面。2008年，鳳凰號任務分析並確認在北緯68度的地下埋有冰層，該緯度為這次發現的8座斷崖最北者距北極點的三分之一處。

「今日發表的這項發現讓我們驚訝的是竟然有地方可以供我們直接觀察在地下的厚重冰層。」研究共同作者，亞利桑那州立大學月球和行星實驗室的Shane Byrne表示。「就跟某種螞蟻農場一樣，你可以透過壁面的透明玻璃來認識通常掩蓋在地下的那些事物。」

科學家還不確定這些特殊的斷崖最初是如何產生。然而，曾經埋藏於地底的冰塊一旦暴露在火星的大氣當中，由於冰塊會直接從固體昇華成水蒸氣，導致這些斷崖不斷「後退」使得寬度和高度持續增加。在某些斷崖上，露出的水冰厚度可以超過100公尺。利用MRO搭載的火星專用小型偵察影像頻譜儀(CRISM)來檢視這些斷崖，可以確認露出的明亮物質為凍結的水。透過奧德賽號熱輻射成像系統(THEMIS)的攝影機再次確認，也讓科學家更加確定他們看到的並非只是覆蓋在地表的一層凍霜。

研究人員之前利用MRO的淺層雷達繪製出火星的中緯度區域地下有一大片水冰，並且估計冰層的頂端距地表大概不到10公尺。但到底有多淺？雷達測量方法並沒有足夠解析度來回答這項問題。最新研究冰層出露的斷崖確認了之前從新生隕石坑和中子光譜儀的觀察結果中做出的推論：在某些地區距地表1或2 公尺的深度之內就已經開始出現富含水冰的地層。

太空人在火星上的水源

新研究不僅指出僅被薄薄一層表土覆蓋的水冰分布範圍相當廣大，也找到可以直接取出冰層的八個地點，它們所在緯度的環境條件相對於火星極區的冰帽來說比較不那麼嚴峻。「基本上，太空人可以提著桶子和鏟子直接前往該處挖出他們需要的一切用水。」Byrne表示。

除了可以當作資源的潛在價值之外，這些露出的冰層也有科學意義，因為它們保存了火星氣候長期如何變化的證據。火星自轉軸的傾角會以數百萬年的週期變化，其幅度比地球還大上許多。雖然今日兩個星球的自轉軸傾角差不多，但在火星更加傾斜的時候，氣候條件或許有利於中緯度冰層擴張。Dundas和共同作者表示在某些斷崖中明顯的條紋和顏色變化，代表這些冰層「堆積的時候冰和沙塵的比例可能因為氣侯條件的變化而跟著改變。」

結合火星軌道探測器上諸多儀器的觀測結果，加上現已超過11年的MRO以及超過16年的奧德賽的長期觀測，這項研究才得以完成。軌道觀測還是會持續進行，不過未來在火星表面上進行的任務應該可以搜尋到更多資料。

「如果在其中一座斷崖進行任務，順著崖面往下依序取出冰層樣品就可以得到火星氣候的詳細歷史。」MRO 的計畫主持科學家，加州帕薩迪納NASA噴射推進實驗室的Leslie Tamppari表示。「火星的水分隨著時間經過發生了許多變故：它們消失去哪了？冰層什麼時候開始堆積？又是什麼時候開始消融？火星的氣候史可以告訴我們整個故事的其中一部份。」

Steep slopes on Mars reveal structure of buried ice on Red Planet

Researchers using NASA's Mars Reconnaissance Orbiter (MRO) have found eight sites where thick deposits of ice beneath Mars' surface are exposed in faces of eroding slopes.

These eight scarps, with slopes as steep as 55 degrees, reveal new information about the internal layered structure of previously detected underground ice sheets in Mars' middle latitudes.

The ice was likely deposited as snow long ago. The deposits are exposed in cross section as relatively pure water ice, capped by a layer one to two yards (or meters) thick of ice-cemented rock and dust. They hold clues about Mars' climate history. They also may make frozen water more accessible than previously thought to future robotic or human exploration missions.

Researchers who located and studied the scarp sites with the High Resolution Imaging Science Experiment (HiRISE) camera on MRO reported the findings today in the journal Science. The sites are in both northern and southern hemispheres of Mars, at latitudes from about 55 to 58 degrees, equivalent on Earth to Scotland or the tip of South America.

"There is shallow ground ice under roughly a third of the Martian surface, which records the recent history of Mars," said the study's lead author, Colin Dundas of the U.S. Geological Survey's Astrogeology Science Center in Flagstaff, Arizona. "What we've seen here are cross-sections through the ice that give us a 3-D view with more detail than ever before."

Windows into underground ice

The scarps directly expose bright glimpses into vast underground ice previously detected with spectrometers on NASA's Mars Odyssey (MRO) orbiter, with ground-penetrating radar instruments on MRO and on the European Space Agency's Mars Express orbiter, and with observations of fresh impact craters that uncover subsurface ice. NASA sent the Phoenix lander to Mars in response to the Odyssey findings; in 2008, the Phoenix mission confirmed and analyzed the buried water ice at 68 degrees north latitude, about one-third of the way to the pole from the northernmost of the eight scarp sites.

The discovery reported today gives us surprising windows where we can see right into these thick underground sheets of ice," said Shane Byrne of the University of Arizona Lunar and Planetary Laboratory, Tucson, a co-author on today's report. "It's like having one of those ant farms where you can see through the glass on the side to learn about what's usually hidden beneath the ground."

Scientists have not determined how these particular scarps initially form. However, once the buried ice becomes exposed to Mars' atmosphere, a scarp likely grows wider and taller as it "retreats," due to sublimation of the ice directly from solid form into water vapor. At some of them, the exposed deposit of water ice is more than 100 yards, or meter, thick. Examination of some of the scarps with MRO's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) confirmed that the bright material is frozen water. A check of the surface temperature using Odyssey's Thermal Emission Imaging System (THEMIS) camera helped researchers determine they're not seeing just thin frost covering the ground.

Researchers previously used MRO's Shallow Radar (SHARAD) to map extensive underground water-ice sheets in middle latitudes of Mars and estimate that the top of the ice is less than about 10 yards beneath the ground surface. How much less? The radar method did not have sufficient resolution to say. The new ice-scarp studies confirm indications from fresh-crater and neutron-spectrometer observations that a layer rich in water ice begins within just one or two yards of the surface in some areas.

Astronauts' access to Martian water

The new study not only suggests that underground water ice lies under a thin covering over wide areas, it also identifies eight sites where ice is directly accessible, at latitudes with less hostile conditions than at Mars' polar ice caps. "Astronauts could essentially just go there with a bucket and a shovel and get all the water they need," Byrne said.

The exposed ice has scientific value apart from its potential resource value because it preserves evidence about long-term patterns in Mars' climate. The tilt of Mars' axis of rotation varies much more than Earth's, over rhythms of millions of years. Today the two planets' tilts are about the same. When Mars tilts more, climate conditions may favor buildup of middle-latitude ice. Dundas and co-authors say that banding and color variations apparent in some of the scarps suggest layers "possibly deposited with changes in the proportion of ice and dust under varying climate conditions."

This research benefited from coordinated use of multiple instruments on Mars orbiters, plus the longevities at Mars now exceeding 11 years for MRO and 16 years for Odyssey. Orbital observations will continue, but future missions to the surface could seek additional information.

"If you had a mission at one of these sites, sampling the layers going down the scarp, you could get a detailed climate history of Mars," suggested MRO Deputy Project Scientist Leslie Tamppari of NASA's Jet Propulsion Laboratory, Pasadena, California. "It's part of the whole story of what happens to water on Mars over time: Where does it go? When does ice accumulate? When does it recede?"

原始論文：Colin M. Dundas et al. Exposed subsurface ice sheets in the Martian mid-latitudes. Science, 2018 DOI: 10.1126/science.aao1619

引用自：NASA/Jet Propulsion Laboratory. "Steep slopes on Mars reveal structure of buried ice on Red Planet."