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arXiv 1502.06480astro-ph.EP
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根据HST/STIS和Keck/NIRC2观测推断天王星两极区的甲烷消耗

Methane depletion in both polar regions of Uranus inferred from HST/STIS and Keck/NIRC2 observations

Lawrence Sromovsky, Erich Karkoschka, Patrick Fry, Heidi Hammel, Imke de Pater, Kathy Rages

备注：
20 pages, 19 figures, 4 tables, on-line supplemental material available
发表信息：
Icarus 238 (2014) 137-155
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摘要：

根据2012年天王星的STIS观测，我们发现甲烷体积混合比从低纬度的约4%下降到北纬60度及以上的约2%。这与2002年在南极地区发现的情况相似，尽管这两个地区的对流活动似乎截然不同。凯克和HST成像观测接近春分意味着消耗同时存在于2007年，这表明他们是持久的功能。亏损似乎主要限于对流层上部，深度从北纬30度左右向极地增加，在北纬45度达到~4巴，在北纬70度可能更深。损耗程度和深度的纬向变化是纬向环流模式的重要制约因素。我们的观测结果与先前提出的环流圈在质量上是一致的，在低纬度地区上升的富甲烷气体由于甲烷冰粒的凝结和沉积而变干，因为气体上升到甲烷凝结水平以上的高度，然后被输送到高纬度地区，在那里它下降并带来甲烷耗尽的气体。由于这个单元似乎会抑制在云实际上是从光谱模型推断的区域中形成凝结云，这表明稀疏的局部对流事件可能对云的形成很重要。我们发现在55度N和82度N之间的有效甲烷混合比的小尺度纬度变化与HST观测前后拍摄的近红外凯克图像中云反射率的纬向平均纬度变化具有显着的负相关性。如果CH 4/H2吸收比的变化被解释为当地的变化，而不是甲烷混合比，我们发现下降流与云反射率降低。

英文摘要：

From STIS observations of Uranus in 2012, we found that the methane volume mixing ratio declined from about 4% at low latitudes to about 2% at 60 deg N and beyond. This is similar to that found in the south polar regions in 2002, in spite of what appears to be strikingly different convective activity in the two regions. Keck and HST imaging observations close to equinox imply that the depletions were simultaneously present in 2007, suggesting they are persistent features. The depletions appear to be mainly restricted to the upper troposphere, with depth increasing poleward from about 30 deg N, reaching ~4 bars at 45 deg N and perhaps much deeper at 70 deg N. The latitudinal variations in degree and depth of the depletions are important constraints on models of meridional circulation. Our observations are qualitatively consistent with previously suggested circulation cells in which rising methane-rich gas at low latitudes is dried out by condensation and sedimentation of methane ice particles as the gas ascends to altitudes above the methane condensation level, then is transported to high latitudes, where it descends and brings down methane depleted gas. Since this cell would seem to inhibit formation of condensation clouds in regions where clouds are actually inferred from spectral modelling, it suggests that sparse localized convective events may be important in cloud formation. The small-scale latitudinal variations we found in the effective methane mixing ratio between 55 deg N and 82 deg N have significant inverse correlations with zonal mean latitudinal variations in cloud reflectivity in near-IR Keck images taken before and after the HST observations. If the CH4/H2 absorption ratio variations are interpreted as local variations in para fraction instead of methane mixing ratio, we find that downwelling correlates with reduced cloud reflectivity.

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