Composition, Physical State, and Distribution of Ices at the Surface of Triton
Composition, Physical State, and Distribution of Ices at the Surface of Triton
复制标题
海卫一表面冰的成分、物理状态和分布
DOI：
10.1006/icar.1999.6111
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发表时间：
1999
期刊：
影响因子：
3.2
通讯作者：
T. Roush
中科院分区：
文献类型：
--
作者：
E. Quirico;S. Douté;B. Schmitt;C. Bergh;D. Cruikshank;T. Owen;T. Geballe;T. Roush
本文分析了1995年9月7日用英国红外望远镜（Mauna Kea，HI）的冷却光栅光谱仪CGS 4对海卫一冰表面进行的近红外观测。该分析分两步进行。第一步是通过将观测结果与实验室透射光谱（直接光谱分析）进行比较来识别构成海卫一表面的分子;这也提供了关于组分物理状态的信息。海卫一光谱中的大多数谱带被分配给先前发现的CH 4、N2、CO和CO2分子的特定振动谱带。对CH 4谱带频率的详细比较确信地表明，该分子以稀释状态存在于固体β-N2中。另外还观测到三个新的谱带，峰值分别位于5717、5943和6480 cm−1（分别为1.749、1.683和1.543 μm）。实验室实验表明，在固体N2中分离出的C2 H6与第二条谱带非常吻合，但这意味着出现了未观察到的谱带，因此排除了这一归属。然而，C2 H6可能以另一种物理状态存在，需要更多的实验。当与9个分子（C2 H2，C2 H4，C3 H8，NH3，SO2，HC 3 N，CH 3OH，NO，NO2）的光谱比较时，未发现这3个谱带的合理候选者。
根据D. P. Cruikshank et al.（1993，Science 261，742; in preparation），这里提出的工作导致Triton表面的两种可能的表示。首先，由N2：CH 4：CO地形组成的两区表面，N2：CH 4：CO由固溶体组成，其中N2是主要分子，并且由纯结晶H2O和CO2颗粒的混合物组成的H2O+CO2地形。第二种表示是由N2：CH 4：CO地形和两个地理上分离的H2O和CO2地形组成的三区域表面。
分析的第二步包括使用双向反射模型（S。Doute和B。施密特1998，J.地球物理学。Res. Planets103，31367）。建模首先证实了直接光谱分析，因为CH 4被稀释在固体β-N2中，从而高度置信地得出N2：CH 4：CO地形实际上是固体溶液的结论。它还提供了关于这一地形的数字信息，即颗粒大小、地理丰度以及CH 4和CO浓度。大的颗粒尺寸（约10厘米）意味着该地形的结构是致密的结晶固体而不是颗粒状，这与J. Eluszkiewicz（1991，J. Geophys. Res.96，19，217）。此外，N2带的精确建模可能表明温度大于或等于35.6 K。
虽然在光谱上无法区分，但海卫一表面最多可以存在10%的纯甲烷冰表面积，从而解释了旅行者2号观测到的高大气甲烷丰度。
最后，建模表明，没有一个两个或三个区域的表示能够同时拟合Triton光谱的K和H区域。这种不匹配的起源尚未阐明，但怀疑是一种工具效应。因此，提出了一些关于H2O和CO2分子物理状态的问题，但不幸的是，观测约束条件缺失。新的近红外观测可以部分地提供这些缺失的限制，并且对于探测海卫一表面上的新分子非常重要。这样的新数据对于识别5717、5943和6480 cm−1（1.749、1.683和1.543 μm）的三个波段特别有用。
This paper presents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September 7, with the cooled grating spectrometer CGS4 at the United Kingdom Infrared Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules composing Triton's surface by comparing the observations with laboratory transmission spectra (direct spectral analysis); this also gives information on the physical state of the components. Most of the bands in Triton's spectrum were assigned to specific vibration bands of the CH4, N2, CO, and CO2 molecules previously discovered. A detailed comparison of the frequencies of the CH4 bands confidently indicated that this molecule exists in a diluted state in solid β-N2. Three new bands peaking at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm, respectively) were also observed. Laboratory experiments have shown that C2H6 isolated in solid N2 fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6 may exist in another physical state, and more experiments are necessary. No plausible candidate was found for these three bands when comparing with the spectra of nine molecules (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2).
In view of the results of D. P. Cruikshank et al. (1993, Science261, 742; in preparation), the work presented here leads to two possible representations of the surface of Triton. First, a two-region surface composed of a N2:CH4:CO terrain, N2:CH4:CO consisting of a solid solution in which N2 is the dominant molecule, and of a H2O+CO2 terrain, composed of a mixture of pure crystalline H2O and CO2 grains. The second representation is a three-region surface composed of a N2:CH4:CO terrain and two geographically separated H2O and CO2 terrains.
The second step of the analysis consisted of using a bidirectionnal reflectance model (S. Doute and B. Schmitt 1998, J. Geophys. Res. Planets103, 31367). The modeling first confirms the direct spectral analysis in that CH4 is diluted in solid β-N2, giving a high degree of confidence to the conclusion that the N2:CH4:CO terrain is in fact a solid solution. It also provides numerical information on this terrain, namely the size of the grains, the geographical abundance, and the CH4 and CO concentrations. The large grain size (around 10 cm) would mean that the texture of this terrain is a compact crystalline solid rather than granular, which is in agreement with calculations from J. Eluszkiewicz (1991, J. Geophys. Res.96, 19,217). In addition, an accurate modeling of the N2 band could suggest that the temperature is greater or equal to 35.6 K.
Although undistinguishable in the spectra, a maximum of 10% surface area of pure CH4 ice can be present at the surface of Triton, thus explaining the high atmospheric CH4 abundance observed by Voyager 2.
Finally, the modeling showed that none of the two- or three-region representations was able to fit simultaneously the K and H regions of the spectrum of Triton. The origin of this misfit is not yet elucidated, but an instrumental effect is suspected. Some questions about the physical state of the H2O and CO2 molecules are thus raised, but unfortunately observational constraints are missing. New near-infrared observations could partly provide these missing constraints, and would be important for detecting new molecules on Triton's surface. Such new data would be especially useful to identify the three bands at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm).