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
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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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?
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？