An unexpected cooling effect in Saturn's upper atmosphere
An unexpected cooling effect in Saturn's upper atmosphere
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DOI：
10.1038/nature05518
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发表时间：
2007-01-25
期刊：
影响因子：
64.8
通讯作者：
Moore, L. E.
中科院分区：
文献类型：
--
作者：
Smith, C. G. A.;Aylward, A. D.;Moore, L. E.
太阳系四颗巨行星的上层大气呈现出高温（1，2），这不能用太阳光的吸收来解释（2，3）。在土星的情况下，太阳加热模型预测的温度为200 K（2，4），而在极地地区（5）和纬度30度（6）独立观察到的温度接近400 K。这种无法解释的“能源危机”代表了我们对这些行星大气层理解的一个主要空白。缺失能量来源的一个重要候选者是磁层（1，2，4，7 -9），它主要在地球的极地地区注入能量。这种极地能量输入被认为足以解释观测到的温度（9），前提是它被风（4，8）有效地重新分配到全球，这是一个不太清楚的过程。在这里，我们使用数值模型（4）表明，由极地能量输入驱动的风的净效应不是加热而是冷却低纬度热层。这一令人惊讶的结果使我们能够排除已知的极地能量输入作为土星能源危机的解决方案。要么有一个未知的--而且是巨大的--极地能量来源，要么更有可能是其他一些过程直接加热了低纬度地区。
The upper atmospheres of the four Solar System giant planets exhibit high temperatures(1,2) that cannot be explained by the absorption of sunlight(2,3). In the case of Saturn the temperatures predicted by models of solar heating(2,4) are 200 K, compared to temperatures of similar to 400 K observed independently in the polar regions(5) and at 30 degrees latitude(6). This unexplained 'energy crisis' represents a major gap in our understanding of these planets' atmospheres. An important candidate for the source of the missing energy is the magnetosphere(1,2,4,7-9), which injects energy mostly in the polar regions of the planet. This polar energy input is believed to be sufficient to explain the observed temperatures(9), provided that it is efficiently redistributed globally by winds(4,8), a process that is not well understood. Here we show, using a numerical model(4), that the net effect of the winds driven by the polar energy inputs is not to heat but to cool the low-latitude thermosphere. This surprising result allows us to rule out known polar energy inputs as the solution to the energy crisis at Saturn. There is either an unknown - and large - source of polar energy, or, more probably, some other process heats low latitudes directly.