分享 http://blog.sciencenet.cn/u/chnweiwang 追根溯源,需要一颗执着好奇的少年心

博文

外星生命是个什么样子?

已有 3756 次阅读 2008-11-29 09:13 |系统分类:科普集锦

 

无论是在西方圣经里的上帝还是东方神话传说中的各路神仙,我们的祖先们无一例外的都把他们想象成人的样子。这倒不能说是先人们缺乏想象力,而是因为他们觉得神仙似乎跟高等动物人应该长得更相像一些罢了。

 

同样如此,科学家们在寻找地外生命的过程中,也是认为地球生物赖以生存的液态水是必须的,并将太空生命想象为也是由DNA、蛋白质和磷脂双分子层组成的有机体,甚至包括一些科幻小说和电影都将外星人比划成人的轮廓。但是,在找到真正的地外生命之前,一切都无从定论。就像我前面博文多环芳烃与地外生命中所说的那样,说不定多环芳烃才是构建地球外生物的遗传物质。

 

去年7月,美国国家学院(The National Academies,由美国科学院、工程院、医学研究院和国家研究理事会组成的非赢利非官方的学术团体)发布了一份NASA提供的报告,认为寻找太阳系和太阳系外其他星球上的生命,应该包括科学家们对怪异生命的设想和论证,也就是Life Elsewhere in Solar System Could Be Different From Life as We Know It.

 

NASA在报告中称,我们平常所知道的维持生命的必需条件,如液态水作为生物溶剂,能够新陈代谢的碳基生命,不断进化的分子系统,都不再是验证生命存在的条件。我们的调查很清楚了,外星生命可能和地球上的大相径庭,理事会主席、来自西雅图Washington大学的生物海洋学教授John Baross如是说。

   报告强调:在太阳系探险之旅中,没有什么比发现一种外星生命,即使是最原始的生命,更能够影响我们对地球在宇宙中地位的看法,更让人鼓舞的了。显然,也没有一件事会比我们在外太空和外星生命对面不相识更悲惨了。

 

报告还指出,默认外星生命要利用和地球生命相同的生物化学机制,就意味着科学家人为地限制了他们所认为地外生命存在的地域和机会。科学家们假定,地外生命也同样需要液态水,就让他们的思维被限制在火星等现在或过去曾经有液态水存在的星球;像液态氨和甲酰胺等液体,也可以作为生物溶剂,只不过生物化学反应机制不同。最新的证据表明,在土卫六(Titan,土星的卫星)内部可能存在液态氨,因此他们认为Titan是太阳系内最有可能存在怪异的外星生命的星球,应该优先增加对它的太空考察。

 

 在太阳系中寻找外星生命,关键是知道我们在寻找什么,”Baross说,至今,搜寻的目标还集中在和地球一样的生命,因为这是我们都知道的生命样本。不过,起源于其他星球的生命,或许和地球生命没有可比性。过去十年来,生物学和生物化学的进展显示,生命的基本要求可能并不是我们所想的这样。

除了生物溶剂可能被替代,研究显示,奇异的外星生命的其他一些基本原则也有可能发生变更。在地球生命中,DNA包含了四种名为核苷酸的化合物配对,但在合成生物学的实验中,科学家已经创造出包含6种甚至更多核苷酸的DNA——它们也同样能够进行基因编码,并遵循达尔文的进化论。另外,化学领域的研究也显示,生物体可以利用其他来源的能量,比如通过氢氧化钠和盐酸的反应,这意味着一个有机体可以拥有完全非碳的新陈代谢。

 

报告指出,研究者需要进一步探讨生命基本条件的各种可能变化,特别是生命起源的各种细节研究,这将有助于确定在没有水的环境中,或者水只会在极端情况下出现的环境中(太阳系的大多数行星和卫星都可以归为这一类)是否会有生命存在。研究的焦点还应该包括生物体如何突破我们所认为的关键元素,包括非碳生命的基础:能量、结构和化学反应。

 

另外,将来对外星生命的研究还应该包括地球生命。以前的研究已经确定,生命存在的基本条件是在水和能源一起存在的地方。田野研究者应该在缺乏这些资源的极端环境,如沙漠、海洋最深处等地,寻找新的生物体,以更好地了解解地球上生命的真正运作情况。这种更好的了解,将大大有助于在火星等过去环境和地球相似的星球上寻找生命。

 

太空探测计划需要调整,扩大寻找外星生命的范围。比如说,火星探测计划,就应该增加仪器监测轻元素的组成,尤其是碳、氢、氧、磷和硫,以及简单的有机官能团和有机碳。最近的证据显示,土星的另一颗卫星土卫二上,有活跃的间歇泉存在,这使生命存在的可能性大大增加,也急需研究机构优先对这颗星球进行探索。

 

 

附原文,摘自http://www.sciencedaily.com/releases/2007/07/070706135148.htm

 

WASHINGTON -- The search for life elsewhere in the solar system and beyond should include efforts to detect what scientists sometimes refer to as "weird" life -- that is, life with an alternative biochemistry to that of life on Earth -- says a new report from the National Research Council.  The committee that wrote the report found that the fundamental requirements for life as we generally know it -- a liquid water biosolvent, carbon-based metabolism, molecular system capable of evolution, and the ability to exchange energy with the environment -- are not the only ways to support phenomena recognized as life.  "Our investigation made clear that life is possible in forms different than those on Earth," said committee chair John Baross, professor of oceanography at the University of Washington, Seattle.  

 

The report emphasizes that "no discovery that we can make in our exploration of the solar system would have greater impact on our view of our position in the cosmos, or be more inspiring, than the discovery of an alien life form, even a primitive one.  At the same time, it is clear that nothing would be more tragic in the American exploration of space than to encounter alien life without recognizing it."

 

The tacit assumption that alien life would utilize the same biochemical architecture as life on Earth does means that scientists have artificially limited the scope of their thinking as to where extraterrestrial life might be found, the report says.  The assumption that life requires water, for example, has limited thinking about likely habitats on Mars to those places where liquid water is thought to be present or have once flowed, such as the deep subsurface.  However, according to the committee, liquids such as ammonia or formamide could also work as biosolvents -- liquids that dissolve substances within an organism -- albeit through a different biochemistry.  The recent evidence that liquid water-ammonia mixtures may exist in the interior of Saturn's moon Titan suggests that increased priority be given to a follow-on mission to probe Titan, a locale the committee considers the solar system's most likely home for weird life.  

 

"It is critical to know what to look for in the search for life in the solar system," said Baross.  "The search so far has focused on Earth-like life because that's all we know, but life that may have originated elsewhere could be unrecognizable compared with life here.  Advances throughout the last decade in biology and biochemistry show that the basic requirements for life might not be as concrete as we thought."

 

Besides the possibility of alternative biosolvents, studies show that variations on some of the other basic tenets for life also might be able to support weird life.  DNA on Earth works through the pairing of four chemical compounds called nucleotides, but experiments in synthetic biology have created structures with six or more nucleotides that can also encode genetic information and, potentially, support Darwinian evolution.  Additionally, studies in chemistry show that an organism could utilize energy from alternative sources, such as through a reaction of sodium hydroxide and hydrochloric acid, meaning that such an organism could have an entirely non-carbon-based metabolism.  

 

Researchers need to further explore variations of the requirements for life with particular emphasis on origin-of-life studies, which will help determine if life can exist without water or in environments where water is only present under extreme conditions, the report says.  Most planets and moons in this solar system fall into one of these categories.  Research should also focus on how organisms break down key elements, as even non-carbon-based life would need elements for energy, structure, and chemical reactions.  

 

The report also stresses that the future search for alien life should not exclude additional research into terrestrial life. Through examination of extreme environments, such as deserts and deep under the oceans, studies have determined that life exists essentially anywhere water and a source of energy are found together on Earth.  Field researchers should therefore seek out organisms with novel biochemistries and those that exist in areas where vital resources are scarce to better understand how life on Earth truly operates, the committee said.  This improved understanding will contribute greatly toward seeking Earth-like life where the conditions necessary for its existence might be met, as in the case of subsurface Mars.

 

Space missions will need adjustment to increase the breadth of their search for life.  Planned Mars missions, for example, should include instruments that detect components of light elements -- especially carbon, hydrogen, oxygen, phosphorous, and sulfur -- as well as simple organic functional groups and organic carbon.  Recent evidence indicates that another moon of Saturn, Enceladus, has active water geysers, raising the prospect that habitable environments may exist there and greatly increasing the priority of additional studies of this body.

 

NASA sponsored the report.  The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies. They are private, nonprofit institutions that provide science, technology, and health policy advice under a congressional charter. The Research Council is the principal operating agency of the National Academy of Sciences and the National Academy of Engineering.  A committee roster follows.

Copies of The Limits of Organic Life in Planetary Systems will be available from the National Academies Press; tel. 202-334-3313 or 1-800-624-6242 or on the Internet at http://www.nap.edu.  The cost of the report is $27.50 (prepaid) plus shipping charges of $4.50 for the first copy and $.95 for each additional copy.  Reporters may obtain a pre-publication copy from the Office of News and Public Information (contacts listed above).

 

 

[ This news release and report are available at http://national-academies.org ]

 

NATIONAL RESEARCH COUNCIL

Division on Engineering and Physical Sciences

Space Studies Board

 

Committee on the Limits of Organic Life in Planetary Systems


John A. Baross (chair)
Professor of Biological Oceanography
University of Washington
Seattle

Steven A. Benner
Distinguished Fellow
Foundation for Applied Molecular Evolution
Gainesville, Fla.

George D. Cody
Geologist and Member of Senior Research Staff
Geophysical Laboratory
Carnegie Institution of
Washington
Washington, D.C.

Shelley D. Copley
Professor
Department of Molecular, Cellular, and Developmental Biology
University of Colorado
Boulder

Norman R. Pace*
Professor
Department of Molecular, Cellular, and Developmental Biology
University of Colorado
Boulder

James H. Scott
Leader of the Geobiology Group
Department of Earth Sciences
Dartmouth College
Hanover, N.H.

Robert Shapiro
Professor Emeritus and Senior Research Scientist
Department of
Chemistry
New York University
New York City

Mitchell
L. Sogin
Director
Bay Paul Center
for Comparative Molecular Biology and Evolution
Marine Biological Laboratory
Woods Hole, Mass.

Jeffrey L. Stein
Kauffman Fellow
Sofinnova Ventures
San Francisco

Roger Summons
Professor
Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology
Cambridge

Jack W. Szostak*
Alexander Rich Distinguished Investigator
Massachusetts General Hospital, and
Professor of Genetics
Harvard Medical School
Boston

 

RESEARCH COUNCIL STAFF

 

David H. Smith

Study Director


* Member, National Academy of Sciences

 

 



https://blog.sciencenet.cn/blog-5545-206423.html

上一篇:生命进化时标
下一篇:土卫六电流活跃迹象意味着有生命存在?
收藏 IP: 61.167.60.*| 热度|

0

该博文允许注册用户评论 请点击登录 评论 (2 个评论)

数据加载中...
扫一扫,分享此博文

Archiver|手机版|科学网 ( 京ICP备07017567号-12 )

GMT+8, 2024-3-29 20:25

Powered by ScienceNet.cn

Copyright © 2007- 中国科学报社

返回顶部