美科学家发现新证据 证实地球水来自彗星风暴

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核心提示：最近，美国科学家发现强有力的证据，证明地球上的水来自于彗星。根据他们的研究发现，乌鸦座厄塔星周围曾发生一场“彗星风暴”，正是这场风暴充当了“送水工”的角色。

                               
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艺术概念图，展现了乌鸦座厄塔星周围发生的一场“彗星风暴”。

网易探索10月21日报道  地球如何成为一个如此湿润的世界？一直以来，这便是行星学研究领域的最大谜团之一。最近，美国科学家发现强有力的证据，证明地球上的水来自于彗星。根据他们的研究发现，乌鸦座厄塔星周围曾发生一场“彗星风暴”，正是这场风暴充当了“送水工”的角色。
在大约45亿年前我们居住的这颗星球形成时，太阳的热量已经将太阳系内绝大多数水驱赶到靠近边缘的区域。大部分水仍留在太阳系，或以液态或以固态存在于土星环、木卫二“欧罗巴”、海王星、天王星以及数十亿颗彗星之上。但地球上仍有很多水，科学家多年来一直无法揭开水如何来到地球之谜。一种主流理论认为，地球上的水来自于形成后大约5亿年的一次“彗星齐射”。上周，科学家发现一些彗星拥有与地球上的水同样的化学信号。这一发现无疑为“彗星送水论”提供了强有力的支持。
就在公布这项研究发现后不久，美国天文学家又发现一个重要证据，证明这一理论。这个证据来自于距地球近400万亿英里（约合643万亿公里）的乌鸦座厄塔星，一颗出现在北半球夜空中的明亮恒星。首席研究员、美国约翰斯·霍普金斯大学的凯里·利瑟表示：“我们观察到一场原始恒星风暴，撞向距离这颗恒星较近的天体。”研究论文刊登在即将出版的《天文学杂志》上。
研究过程中，利瑟和同事发现了从乌鸦座厄塔星系统距地球大约3个天文单位（地日距离3倍）的尘粒红外信号。利用斯皮策太空望远镜对这些尘粒进行详细分析后发现，它们来自于与大型多岩天体发生的巨大撞击。利瑟说：“我们发现了纳米钻石和无定形二氧化硅，说明与彗星相撞的天体最小相当于小行星谷神星，最大则是地球的几倍。如果彗星之间彼此撞击，则更像是粉扑之间的相撞。”
但利瑟也承认这一观测发现并不具有决定性，除了一场小彗星风暴外，这些碎片也可能来自一颗大彗星。他说：“我们并不确定。我们所知道的一切就是喷射出大量物质。”研究过程中，这种物质的类型引起了利瑟的注意。它们含有冰颗粒和有机化学物质，正是来自被撞成粉末的彗星的物质。此外，这些遥远尘粒的化学信号也与2008年撞击苏丹的陨石Almahata Sitta十分类似。Almahata Sitta可能来自于海王星轨道之外的柯伊伯带。这一区域潜伏着数十亿颗彗星。实际上，矮行星冥王星和阋神星本质上也是彗星。
将这些观测发现结合在一起，利瑟等人认为这些彗星不仅在太阳系诞生后10亿年为地球带来了水，同时也可能带来构成生命的基本要素。乌鸦座厄塔星的年龄大约在10亿岁左右，是否有可能存在生命？对于这个问题，我们的第一反应可能是“不”。在乌鸦座厄塔星系统，和这些彗星相撞的行星与母星之间的距离超过火星与太阳间的距离，如果按照太阳系的情况推断，在其所处的轨道，水将永远处于冻结状态。
但乌鸦座厄塔星系统并非太阳系。这颗恒星的亮度明显高于太阳，它的适居区（允许生命之源液态水存在的区域）也相应地距离母星更远。利瑟形象地说：“这个系统的中心存在一个温度更高的燃烧室，所以你必须再往后退。”

另一个问题是，是否存在任何证据证明其他年轻恒星系统也发生类似彗星风暴？或者说，拥有适于生命存在的环境？答案是：目前尚未发现任何证据。利瑟说：“我们研究了大约1000个恒星系统，满足这个条件的就只有乌鸦座厄塔星。”但他同时也强调这并未意味着其他区域没有这种证据。如果得到美国国会批准，詹姆斯·韦伯太空望远镜最早将于2018年发射升空。这架望远镜灵敏度更高，能够提供科学家更急于了解的线索。也就是说，在得出新的观测发现前就断定地球上的生命源于“太空相撞事故”还为时尚早。（来源：《时代》 编译：shooter）
(本文来源：网易探索
NASA's Spitzer Detects Comet Storm In Nearby Solar System


WASHINGTON -- NASA's Spitzer Space Telescope has detected signs of icy bodies raining down in an alien solar system. The downpour resembles our own solar system several billion years ago during a period known as the "Late Heavy Bombardment," which may have brought water and other life-forming ingredients to Earth.

During this epoch, comets and other frosty objects flung from the outer solar system pummeled the inner planets. The barrage scarred our moon and produced large amounts of dust.

Now Spitzer has spotted a band of dust around a nearby bright star in the northern sky called Eta Corvi that strongly matches the contents of an obliterated giant comet. This dust is located close enough to Eta Corvi that Earth-like worlds could exist, suggesting a collision took place between a planet and one or more comets. The Eta Corvi system is approximately one billion years old, which researchers think is about the right age for such a hailstorm.

"We believe we have direct evidence for an ongoing Late Heavy Bombardment in the nearby star system Eta Corvi, occurring about the same time as in our solar system," said Carey Lisse, senior research scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and lead author of a paper detailing the findings. The findings will be published in the Astrophysical Journal. Lisse presented the results at the Signposts of Planets meeting at NASA's Goddard Space Flight Center in Greenbelt, Md., Oct. 19.

Astronomers used Spitzer's infrared detectors to analyze the light coming from the dust around Eta Corvi. Certain chemical fingerprints were observed, including water ice, organics, and rock, which indicate a giant comet source.

The light signature emitted by the dust around Eta Corvi also resembles the Almahata Sitta meteorite, which fell to Earth in fragments across Sudan in 2008. The similarities between the meteorite and the object obliterated in Eta Corvi imply a common birthplace in their respective solar systems.

A second, more massive ring of colder dust located at the far edge of the Eta Corvi system seems like the proper environment for a reservoir of cometary bodies. This bright ring, discovered in 2005, looms at about 150 times the distance from Eta Corvi as the Earth is from the sun. Our solar system has a similar region, known as the Kuiper Belt, where icy and rocky leftovers from planet formation linger. The new Spitzer data suggest that the Almahata Sitta meteorite may have originated in our own Kuiper Belt.

The Kuiper Belt was home to a vastly greater number of these frozen bodies, collectively dubbed Kuiper Belt objects. About 4 billion years ago, some 600 million years after our solar system formed, scientists think the Kuiper Belt was disturbed by a migration of the gas-giant planets Jupiter and Saturn. This jarring shift in the solar system's gravitational balance scattered the icy bodies in the Kuiper Belt, flinging the vast majority into interstellar space and producing cold dust in the belt. Some Kuiper Belt objects, however, were set on paths that crossed the orbits of the inner planets.

The resulting bombardment of comets lasted until 3.8 billion years ago. After comets impacted the side of the moon that faces Earth, magma seeped out of the lunar crust, eventually cooling into dark "seas," or maria. When viewed against the lighter surrounding areas of the lunar surface, those seas form the distinctive "Man in the Moon" visage. Comets also struck Earth or incinerated in the atmosphere, and are thought to have deposited water and carbon on our planet. This period of impacts might have helped life form by delivering its crucial ingredients.

"We think the Eta Corvi system should be studied in detail to learn more about the rain of impacting comets and other objects that may have started life on our own planet," Lisse said.

NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer mission for the agency's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

For more information about Spitzer, visit:

http://www.nasa.gov/spitzer

原始地球的火山喷发应该能补充这些水吧。