Griffith C A, Bezard B, Owen T, Gautier D
Physics Department, State University of New York, Stony Brook 11794, USA.
Icarus. 1992;98:82-93. doi: 10.1016/0019-1035(92)90209-p.
To investigate the chemistry and dynamics of Jupiter's Great Red Spot (GRS), the tropospheric abundances of NH3 and PH3 in the GRS are determined and compared to those of the surrounding region, the South Tropical Zone (STZ). These gases well up from deep in the atmosphere, and, in the upper troposphere, are depleted by condensation (in the case of NH3), chemical reactions, and UV photolysis. At Jupiter's tropopause, the chemical lifetimes of NH3 and PH3 are comparable to the time constant for vertical transport over the atmospheric scale height. The distributions of these gases are therefore diagnostic of the rate of vertical transport in the upper troposphere and lower stratosphere. Three groups of Voyager IRIS spectra are analyzed, two of the STZ and one of the GRS. The two groups of STZ spectra are defined on the basis of their radiances at 602 and 226 cm-1, which reflect, respectively, the temperature near 150 mbar and the cloud opacity in the 300-600 mbar region. One selection of STZ spectra is chosen to have the same radiance as does the GRS at 226 cm-1. The other STZ selection has a significantly greater radiance, indicative of reduced cloudiness. Variations in the abundances of NH3 and PH3 are determined within the STZ, as a background for our studies of the GRS. Within the uncertainty of our measurements (-55% and +75%), the PH3 mixing ratio at 600 mbar is 3 x 10(-7), the same for all three selections. The NH3 mixing ratio profile in the pressure region between 300 and 600 mbar is the same within error (-25% and +50% at 300 mbar) for both STZ selections. In the GRS, however, NH3 is significantly depleted at 300 mbar, with an abundance of 25% that derived for the STZ selections. Since the GRS is believed to be a region of strong vertical transport, our finding of a depletion of NH3 below the tropopause within the GRS is particularly unexpected. One of the STZ selections has a temperature-pressure profile similar to that of the GRS below the 300-mbar level; therefore, condensation at this level does not easily explain the low NH3 abundance in the GRS. All samples are taken at essentially the same latitude; photolysis and/or charged particle precipitation is probably not directly responsible either. The observed NH3 depletion may have a dynamical origin or result from some unidentified chemical processes at work in the GRS.
为了研究木星大红斑（GRS）的化学性质和动力学，我们测定了大红斑对流层中NH₃和PH₃的丰度，并将其与周围区域南热带区（STZ）的丰度进行比较。这些气体从大气深处上升，在对流层上部，由于凝结（对于NH₃而言）、化学反应和紫外光解作用而减少。在木星对流层顶，NH₃和PH₃的化学寿命与大气标高尺度上垂直输送的时间常数相当。因此，这些气体的分布是对流层上部和平流层下部垂直输送速率的诊断指标。我们分析了三组旅行者号红外光谱仪（Voyager IRIS）的光谱，两组来自南热带区，一组来自大红斑。两组南热带区光谱是根据它们在602和226厘米⁻¹处的辐射率定义的，这两个辐射率分别反映了150毫巴附近的温度和300 - 600毫巴区域的云透明度。我们选择一组南热带区光谱，使其在226厘米⁻¹处的辐射率与大红斑的相同。另一组南热带区光谱的辐射率明显更高，表明云量减少。在南热带区内确定了NH₃和PH₃丰度的变化，作为我们研究大红斑的背景。在我们测量的不确定度范围内（-55%和+75%），600毫巴处的PH₃混合比为3×10⁻⁷，三组光谱的该值相同。对于两组南热带区光谱，在300 - 600毫巴压力区域内的NH₃混合比剖面在误差范围内（300毫巴处为-25%和+50%）是相同的。然而，在大红斑中，300毫巴处的NH₃显著减少，其丰度为南热带区光谱测定值的25%。由于大红斑被认为是一个垂直输送强烈的区域，我们发现在大红斑内对流层顶以下NH₃减少这一情况尤其出乎意料。一组南热带区光谱在300毫巴以下的温度 - 压力剖面与大红斑的相似；因此，该高度处的凝结现象并不能轻易解释大红斑中NH₃丰度低的原因。所有样本基本上都在相同纬度采集；光解和/或带电粒子沉降可能也不是直接原因。观测到的NH₃减少可能源于动力学原因，或者是大红斑中某种尚未确定的化学过程导致的。
