火星60万年前曾有稠密大气层
火星60万年前曾有稠密大气层http://paper.sciencenet.cn/htmlpaper/201142315582253816394.shtm
美国《科学》杂志4月21日报道，火星在60万年前拥有比今天火星大气层更加稠密的二氧化碳大气层，正因为大气层密度高，当年的火星是沙尘暴的天下，尘土飞扬，风暴频繁。
美国国家航空航天局的“火星勘测轨道飞行器”借助刺地雷达技术，在火星南极附近地区发现大面积地下“干冰湖”。科学家断言，干冰湖封存的大量固体二氧化碳在60万年前曾是火星的大气层。
美国航天局喷气推进实验室火星项目研究者杰弗里·普劳特说：“那真是一个地下大宝藏。我们先前在火星地下发现一些物质，但从来没有想到会有一个干冰湖。”
地下封存大面积干冰意味着，在某个时代，这些二氧化碳可能存在于火星的大气层中。科学家分析认为，60万年前，火星大气层的密度是如今的30倍。
厚重的大气层意味着风暴可能形成。文章主要撰写者罗杰·菲利普斯说：“那时候的火星，就像上世纪30年代美国沙尘暴地区一样，风暴频繁，沙尘飞扬，但火星的程度要更加严重。”
但稠密大气层使液态水的存在成为可能。如今火星的大气层密度只有地球的百分之一，科学家认为，如果当年火星大气层密度达到如今地球的30%，流动水确实有可能在火星表面存在。
人类对认知神秘火星的渴望，部分源自火星表面那千沟万壑的山峡溪谷与河道，而更加稠密的大气层是液态水存在的必要条件。美国航天局计划2013年启动一项新的火星探测计划，命令探测器接近火星大气层顶部，寻找火星大气溢出之谜。 本帖最后由 vimb 于 2011-5-9 18:25 编辑
这篇报道是不是暗示着，将来人类移民火星的时候，也可以把大气层的密度弄高一些？
话说火星理论上能维持的大气层密度应该能达地球的百分之多少？75%？ 是可以这么推吧：把干冰释放出来，然后再移些藻类过去，就可以改造了。 后又想到：但是不是这些干冰的量还不够吧（用来改造火星）。 以前就看过些资料 说是火星曾经可能有生命. 。。。你们想的太美好了，
火星根本不适合人类移民，哪怕只去1000个人。 融化干冰放出二氧化碳，绿化火星制造氧气，住人！::070821_01.jpg:: 地球上最恶劣的“无人区”，也比火星的环境好得多啊。
试问，谁能把地球上的沙漠全改造成良田？难道仅仅是经济问题吗？
地球上的问题都难以解决，那么暂时别太乐观去改造火星的气候了。 倒不是相信火星可以居住，只是以此报道为前提“推”了一下。 什么翻译，就是为了吸引眼球吧。不过也不排除，英文原文也是这样写的，现在的文章就喜欢这样写。
“稠密”大气前怎么也该加上“相对”两个字。火星的真正稠密大气层，6亿年前都没有，更别说60万年前了，可能36亿年前确实是稠密大气层。 看电视上说是因为火星内部地核停止或者减弱了，导致火星没有足够的引力束缚大气才是关键，主要还是火星太小了 60万年前的火星大气密度居然能达到地球的30%？等我看见原文再说。
Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of CO2 ice within the south polar layered deposits of Mars with a volume of 9,500 to 12,500 km3, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO2 volatile release. If released into the atmosphere at times of high obliquity, the CO2 reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.
这里只说南极的干冰量达到9500~12500立方公里，是预期的30倍。假如二氧化碳高速释放到火星大气中，能增加大气质量80%，然后沙尘暴会更多，但也能留存更多的液态水而不是沸腾（气压太多，水就会在很低温度下沸腾）。 本帖最后由 gohomeman1 于 2011-5-9 20:11 编辑
哪位能够提供 http://www.sciencemag.org/content/early/2011/04/20/science.1203091.full.pdf的完整PDF下载？
既使原文真如翻译中的那样，我也暂时只能当做一家之言，火星大气层在60万年内有如此巨变，我根本不信。等更多的专家、学者认可这个信息后，我再认可不迟。
说实话，我很佩服翻译者的用词：“科学家断言”。像这种颠覆性的观点，除非拥有确凿不移的铁证，否则科学家一般都是很谨慎的，怎么就敢断言了？看上去倒想上次发现火星存在大型蠕虫生命一样了。
回复 11# 徐1982
::0015:: 那为什么大气中的二氧化碳会变成干冰储存在两级呢?把气态的二氧化碳凝固..应该要增大压强什么的,,话说压强跟引力有什么联系嘛? 如果太陽大2倍 火星就適居了 等45億年吧 等45億年後再上火星享受個幾萬年就爆炸了 回复 15# FoeLam
等太阳进入红巨星后适居带也许就到土星区了 回复 16# gliese581
可是土星是氣體行星~~~~~~~~~~~~~~ 回复徐1982
那为什么大气中的二氧化碳会变成干冰储存在两级呢?把气态的二氧化碳凝固..应 ...
donkeybegood 发表于 2011-5-14 23:26 http://www.astronomy.com.cn/bbs/images/common/back.gif
可能只是单纯的降温造成的吧，就像地球上，两极的降雪最多。只不过地球上的降雪是H2O，火星上是CO2。 回复 18# vimb
yct51.gif 哦。。这样的呀。所以只是在两极地区发现CO2固体？按照这样的话赤道附近乃至中低维应该没有CO2的固体？ 哪位能够提供 的完整PDF下载？
既使原文真如翻译中的那样，我也暂时只能当做一家之言，火星大气层在60万年 ...
gohomeman1 发表于 2011-5-9 20:03 http://www.astronomy.com.cn/bbs/images/common/back.gif
Cold-Trapping Mars’ Atmosphere
The Mars Reconnaissance Orbiter has observed large deposits of frozen CO2 at Mars’ southern polar region.
Peter C. Thomas
Earth’s climate is buffered by massive oceans of liquid water and by gases, such as carbon dioxide (CO2), which cycle through the atmosphere and through biologic and geologic reservoirs. Mars has no oceans to buffer its temperatures, and the thin atmosphere (95% CO2) provides little thermal buffering. The surface pressure is close to that in equilibrium with solid CO2 at Mars’ polar temperatures, and early space probes explored whether large CO2-ice deposits might buffer the atmospheric pressure of Mars ( 1). Subsequent investigations found reservoirs of frozen water in thick polar layered deposits, but revealed only thin perennial deposits of CO2 at the south pole (termed “residual cap”) in addition to the ~30% of atmospheric CO2 that is cycled through the seasonal caps. On page 838 of this issue, Phillips et al. ( 2) report the discovery of thick deposits in the south polar region that are most likely composed of solid CO2 and comparable in mass to the present Mars atmosphere.
Interest in the mechanisms of Mars’ climate comes from its position as the only other terrestrial planet with a surface and atmosphere comparable to Earth’s and from evidence that its climate has changed over time scales of billions of years, as well as over much shorter cycles ( 3). Changes in Mars’ orbital and spin characteristics likely force many climate cycles ( 4). Obliquity, the angle between the spin axis and the normal to the orbital plane, is one such climate forcing factor. Its predicted variations are particularly great for Mars (Earth’s obliquity range is restricted by the presence of the Moon). On Mars, CO2 buffering is analogous to a laboratory cold fi nger—excess atmospheric gas accumulates at the coldest spot on the planet. For high values of obliquity solar heating at the poles can exceed that on the equator, with a possible shift of any buffering deposits, including both water and CO2. At the current obliquity (25.2°), the poles should act as cold fi ngers and promote deposition of more volatile components.
Over time scales of billions of years, Mars shows evidence of periods when liquid water was available at the surface: morphology such as channels and probable standing-water deposits, as well as chemical species found in sedimentary rocks by the Mars Exploration Rovers ( 5). For time scales of only a few million years or less, distinctive stacks of layered materials at both poles ( 6) have been the focus of inquiry into cold-trapped volatiles.
A picture of the materials above this stack has emerged, with thin water-ice caps at both poles ( 7, 8), covered in the south only by a thin residual cap (<15 m) of CO2 ice. Some of the CO2 residual cap is being eroded year to year ( 9). The ongoing erosion of the CO2 residual materials raises the question of whether the residual cap is likely to last for more than a few years, and if it is part of a hierarchy of climate cycles. The amount of material on this residual cap is only a few percent of the mass of atmospheric CO2 ( 10); thus, it is unlikely to be a record of changes in the mass of Mars’ atmosphere.
A complicated cold finger. A schematic of the south polar deposits of Mars. The oldest and most voluminous materials are the “polar layered deposits” (PLD), probably waterice rich with some admixed dust, up to 3 km in thickness. The CO2 deposits in the newly found refl ection-free zones ( 2) occupy several regions within the area of the PLD. Above the CO2 deposits is the residual cap, with an upper part of CO2 with varying thicknesses of up to ~15 m, and an underlying water–ice rich layer of unknown thickness. The CO2 in the refl ection-free zones is apparently dust covered where it is exposed without the residual CO2 and water ice residual cap cover.
Prior results from SHARAD (Shallow Radar, on the Mars Reconnaissance Orbiter) and the deeper-penetrating MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding, from Mars Express) ( 11, 12) had revealed the subsurface complexity of the polar layered deposits. However, these studies gave no indication that the thick polar layered deposits were other than water-ice rich. In the south polar region, SHARAD ( 2) has revealed regions that scatter back extremely small signals. Termed refl ection-free zones, and unlike any other areas on Mars, these regions have been interpreted as CO2 ice based on modeling the variation of the calculated depths of underlying layers; the results are more consistent with radar velocities in CO2 ice than in water ice. The geographically varying calculated thicknesses reach as much as ~700 m, but generally are less than 250 m. The 700-m value is close to a predicted maximum of ~1 km based on the expected depth of liquefaction ( 13, 14). The geographical extent of one such reflection-free zone closely matches that of an outcropping layer mapped from orbital images that has distinctive collapse forms just below the thin water-ice–rich material below the CO2 residual cap (see the figure).
If released into the atmosphere, the mass of material in the main reflection-free zone would nearly double the present surface pressure; this tabulation does not include all the refl ectionfree zones near the south pole ( 2). This reservoir implies a previous state of Mars’ climate that had a higher atmospheric pressure and which then changed to conditions in which a substantial part of that atmosphere collapsed onto the pole. This possibly happened after the last maximal obliquity-driven south polar summer heating, about 600,000 years ago ( 4), although there are predicted younger obliquity excursions nearly as great .
Mars’ atmosphere at present appears to be largely vapor-pressure controlled. Although the newly found CO2 reservoir could nearly double Mars’atmospheric mass, the resulting climate alterations would be modest, and as pointed out by Phillips et al., would involve effects of dust raising and extent and longevity of seasonal frosts, as well as the enhanced CO2 pressure. These ice reservoirs are not the path to a “warm, wet” Mars. Indeed, the limitations of the depth of CO2-ice reservoirs ( 13, 14) probably require that high former CO2 pressures needed for much warmer conditions must involve carbonate or other rock reservoirs, in addition to ice deposits. The new fi ndings of large, and possibly multiple, buried CO2 reservoirs show how complex a seemingly simple cold fi nger system can be. Mars’ cold trapping is clearly affected by seasonal kinetics, changing dust loading of the atmosphere, obliquity and other orbital cycles, and longer-term evolution of Mars geology. There is much yet to learn about this simple system. The northsouth polar asymmetry is but one example of continuing puzzles.
References and Notes
1. R. B. Leighton, B. C. Murray, Science 153, 136 (1966).
2. R. J. Phillips et al., Science 332, 838 (2011); 10.1126/ science.1203091
3. F. P. Fanale, S. E. Postawko, J. B. Pollack, M. H. Carr, R. O. Pepin, in Mars, H. H. Kieffer, B. M. Jakosky, C. Snyder, M.S. Mathews, Eds. (Univ. of Arizona Press, Tucson, 1992), pp. 1135–1179
4. J. Laskar et al., Icarus 170, 343 (2004).
5. S. W. Squyres et al., Science 306, 1698 (2004).
6. B. C. Murray et al., Icarus 17, 328 (1972).
7. T. N. Titus, H. H. Kieffer, P. R. Christensen, Science 299, 1048 (2003).
8. J.-P. Bibring et al., Nature 428, 627 (2004).
9. M. C. Malin et al., Science 294, 2146 (2001).
10. P. C. Thomas, P. B. James, W. M. Calvin, R. Haberle,M. C. Malin, Icarus 203, 352 (2009).
11. J. J. Plaut et al., AGU Fall Meet. Abstr. P13D-06 (2006).
12. D. C. Nunes et al., Lunar Planet. Sci. Conf. 37, 1450 (2006).
13. C. Sagan, J. Geophys. Res. 78, 4250 (1973).
14. M. T. Mellon, Icarus 124, 268 (1996).
15. I thank P. Gierasch for discussions
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