Trafton L M, Lester D F, Ramseyer T F, Salama F, Sandford S A, Allamandola L J
McDonald Observatory and Astronomy Department, University of Texas, Austin 78712, USA.
Icarus. 1991;89:264-76. doi: 10.1016/0019-1035(91)90178-v.
We report our discovery of an absorption feature in the infrared spectrum of Io centered at 2.1253 micrometers (4705.2 cm-1). This band is marginally resolved at resolving power 1200 with a deconvolved full width at half-maximum (FWHM) of about 4 cm-1. This contrasts with the 30- to 50-cm-1 widths of the broad absorption features previously detected at longer wavelengths which arise from mixtures of SO2 with H2S and H2O. This newly discovered feature is relatively weak, having a core only 5% below the continuum at this resolving power. Our survey from 1.98 to 2.46 micrometers (5050-4065 cm-1) at this same resolving power revealed no other feature greater than 1% of the continuum level shortward of 2.35 micrometers, and 3% elsewhere. The feature does not correspond to any gas- or solid-phase absorption that might be expected from previously identified constituents of Io's surface. No temporal or longitudinal variation has been detected in the course of 18 nights of observation over the past year and no significant variation in the strength of the feature was seen during an emergence from eclipse. These observations indicate that the source material of the feature is reasonably stable, and is more uniformly distributed in longitude than Io's hot spots. These characteristics all indicate that the feature belongs to a class different from those characterizing other known absorption features in Io's spectrum. Consequently, it should reveal important new information about Io's atmosphere-surface composition and interaction. A series of laboratory experiments of plausible surface ices indicates that (i) the band does not arise from overtones or combinations of any of the molecular vibrations associated with species already identified on Io (SO2, H2S, H2O) or from chemical complexes of these molecules, (ii) the band does not arise from H2 trapped in SO2, and (iii) the band may arise from the 2 nu3 mode of CO2. If the band arises from CO2, it is clear from its detailed shape and position that the molecules are not embedded in an SO2 matrix, as are H2S and H2O, but may be present as multimers or "clusters."
我们报告了在木卫一的红外光谱中发现的一个吸收特征，其中心波长为2.1253微米（4705.2厘米⁻¹）。在分辨能力为1200时，该谱带勉强可分辨，去卷积后的半高全宽（FWHM）约为4厘米⁻¹。这与之前在更长波长处检测到的宽吸收特征形成对比，那些宽吸收特征的宽度为30至50厘米⁻¹，是由二氧化硫与硫化氢和水的混合物产生的。这个新发现的特征相对较弱，在该分辨能力下，其核心强度仅比连续谱低5%。我们在相同分辨能力下对1.98至2.46微米（5050 - 4065厘米⁻¹）范围进行的测量显示，在2.35微米短波方向没有其他特征强度大于连续谱水平的1%，在其他位置则没有大于3%的特征。该特征与木卫一表面先前确定的成分可能产生的任何气相或固相吸收都不对应。在过去一年的18个夜晚的观测过程中，未检测到该特征的时间或经度变化，并且在一次从日食中出现的过程中，也未观察到该特征强度的显著变化。这些观测表明，该特征的源物质相当稳定，并且在经度上的分布比木卫一的热点更为均匀。这些特征都表明该特征属于与木卫一光谱中其他已知吸收特征不同的类别。因此，它应该能揭示有关木卫一大气 - 表面成分及相互作用的重要新信息。一系列关于可能的表面冰的实验室实验表明：（i）该谱带并非来自与木卫一上已识别物种（二氧化硫、硫化氢、水）相关的任何分子振动的泛音或组合，也不是来自这些分子的化学复合物；（ii）该谱带并非来自被困在二氧化硫中的氢气；（iii）该谱带可能来自二氧化碳的2ν₃模式。如果该谱带来自二氧化碳，从其详细形状和位置可以清楚地看出，这些分子不像硫化氢和水那样嵌入二氧化硫基质中，而是可能以多聚体或“团簇”形式存在。
