40億年前的火星曾什麼樣？

A new animation by NASA scientists illustrates what Mars – the fourth planet from the Sun and the second smallest planet in the Solar System – may have looked like billions of years ago.

Today, Mars is a cold, desert world. Liquid water cannot exist pervasively on its surface due to the low atmospheric pressure and surface temperature, although there is evidence for spurts of liquid flow that perhaps consist of a briny solution with reduced freezing temperature. Water under current conditions can be ice or sublimate directly into vapor without staying in a liquid phase.

Around 4 billions of years ago when the planet was young, it appears to have had a thick atmosphere that was warm enough to support oceans of liquid water – a critical ingredient for life.

The animation shows how the surface of the Red Planet might have appeared during this ancient clement period, beginning with a flyover of a lake.

“There are characteristic dendritic structured channels that, like on Earth, are consistent with surface erosion by water flows. The interiors of some impact craters have basins suggesting crater lakes, with many showing connecting channels consistent with water flows into and out of the crater,” explained Dr Joseph Grebowsky from NASA’s Goddard Space Flight Center in Greenbelt, who is the project scientist for NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission.

“Small impact craters have been removed with time and larger craters show signs of erosion by water before 3.7 billion years ago. And sedimentary layering is seen on valley walls. Minerals are present on the surface that can only be produced in the presence of liquid water.”

Estimates of the amount of water needed to explain these features have been made that equated to possibly as much as a planet-wide layer 1,640 feet deep or more. If liquid surface water existed in the past, then Mars’ atmosphere had to have had a different climate that was warmer and a pressure near or greater than the current terrestrial atmospheric pressure at the surface.

In the animation, rapidly moving clouds are employed to suggest the passage of time, and the shift from a warm and wet to a cold and dry climate is shown as the video progresses.

The lakes dry up and freeze over, while the atmosphere gradually transitions from Earthlike blue skies to the dusty pink and tan hues seen on Mars today.

It’s unknown if the habitable climate lasted long enough for life to emerge on Mars.

“The only direct evidence for life early in the history of a planet’s evolution is that on Earth,” Dr Grebowsky said.

“The earliest evidence for terrestrial life is the organic chemical structure of a rock found on the surface in Greenland. The surface was thought to be from an ancient sea floor sediment. The age of the rock was estimated to be 3.8 billion years, 700 million years from the Earth’s creation. No fossil evidence of life has yet been found from this period. The oldest claimed micro-fossils date to 3.5 billion years ago. The existence of a potential life-nurturing climate on Mars ended near these times. A comparison between the two planet’s life histories must be done with caution, due to the different chemical compositions of the surfaces and different volcanic and meteoroid impact histories. Also, the histories of life on either planet may not have been continuous. Catastrophic events could have killed off all life at one time only to have it start anew.”

The animation ends with an illustration of NASA’s MAVEN mission in orbit around present-day Mars.

MAVEN will investigate how Mars lost its atmosphere. Scheduled to be launched in November, it will arrive at Mars in September 2014.據sci-news網站11月18日報道， 今天的火星是寒冷荒蕪的世界。由於氣壓和表面溫度低，液態水根本無法存在，不是變成冰，就是直接昇華爲蒸氣。不過，40億年前的火星可不是這樣。那時它還很年輕，有着厚厚的大氣層，這讓火星溫度適宜，水也能夠存在，而水是生命的象徵。

美國航空航天局（NASA）科學家日前發佈了一個新動畫短片，推演了火星40億年前的模樣。動畫向人們展示了藍天、白雲和清澈的湖水，這似乎是地球上某處風景秀麗之地。但你絕對很難想象，這很可能就是40億年前的火星。

NASA"火星大氣和揮發性演化"(MAVEN)探測器項目科學家約瑟夫·格雷布斯基說：“火星表面有分佈着許多樹枝狀的水道，就像地球上一樣，這是水流沖刷的結果。”

而如果火星表面過去存在液態水，那麼火星的大氣層必須能營造更溫暖的氣候環境，還需要有一個與目前地球表面大氣壓相當或者更大的氣壓。

動畫中迅速移動的雲表示時間的斗轉星移，此外，動畫還演示了火星由溫溼的宜居天堂變爲乾冷的紅色星球的過程。

目前尚不清楚火星上的宜居氣候有多長，能否孕育生命。

格雷布斯基說：“在星球演變歷史早期，有直接證據證明生命存在的星球只有地球。”

“地下生物的最早證據是在格陵蘭島地表發現的有機化學結構的岩石，被認爲來自古代的海底沉積物。該岩石的年齡估計有38億年，此時地球誕生已有7億年。不過，這個時期並沒有發現生命的化石證據。最古老的微生物化石還需追溯到35億年前。比較地球和火星的生命歷史必須很謹慎，因爲各自地表化學成分、火山岩和天體衝擊影響都不一樣。同時，兩個星球上的生命歷史都可能不連續，災變事件可能一次性殺死所有生物，然後重新開啓生命歷程。”

動畫的最後顯示MAVEN探測器正在目前火星的軌道上運行，該探測器於11月18日發射，明年9月抵達火星，任務是探究火星的大氣層是如何逐漸消失的。