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.
后续调查发现冰冻的水贮存在极区厚厚的沉积层下，但南极只覆盖着很薄的干冰（冰帽残余），而剩余的30%二氧化碳则在火星的季节性冰帽和大气层中循环。
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 reflection-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).
来自先前火星轨道勘测器上浅表雷达（SHARAD）和ESA火星快车上火星表面和电离层先进雷达（MARSIS）的深层穿透分析数据显示（文献11、12），南极表面有复杂的分层沉积。不过，这些研究除了指出厚厚的沉积层没有富含水冰外，并无新意。SHARAD的数据显示，火星南极地区的雷达散射信号极弱。这个特别的自由回波区，与火星表面其他地方迥异；这可以理解为该区域深处潜藏着干冰层，干冰的雷达回波速度明显比水冰符合得好。地质变化分析显示其厚度达700米，而通常该数据小于250米。700米的数值已接近熔解的极限——大约1千米（文献13、14，过厚的干冰会融化）。如插图所示：在轨道探测器获取的火星极区表层图像中，残余的干冰帽（CO2覆盖层）下有薄冰帽（水冰）露出，然后这个干冰自由反射区是最匹配的。
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 reflection free 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 .
假如主要自由反射区中的干冰都释放到大气中，能使当前的火星大气压强接近翻倍（~80%，见相关的另一篇PDF）；这还不包括南极的所有自由反射区（文献2）。这意味着火星以前具有较高的大气压强，后来因气候变迁，大量气体凝固在极区了。这有可能发生在火星上次自转倾角最大对应的南极夏天以后，或者说约60万年前（文献4）；虽然可能还存在更近的轨道倾角接近极大的更近时期。 |
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