Juno/JIRAM对木卫二NIR观测的贝叶斯分析
Bayesian analysis of Juno/JIRAM's NIR observations of Europa
摘要：
朱诺号航天器的光谱仪JIRAM最近在2-5 μm波长区域观测了木卫二。在这里，我们分析了20°N和40°W附近的一组观测结果的平均光谱，重点是水冰的两种形式-无定形和结晶。我们也以此为契机，提出了一种新的贝叶斯光谱反演反射光谱框架。我们首先验证这个框架使用模拟光谱的无定形和结晶冰的混合物和实验室光谱的结晶冰。我们接下来分析JIRAM数据，通过贝叶斯模型比较，发现无定形冰和结晶冰的双组分紧密混合模型（TC-IM模型）比相同物种的双组分模型更受欢迎（26σ置信度），但它们的光谱是区域/线性混合的。我们还发现，TC-IM模型是强烈的首选（在> 30σ置信度）比单组分模型，只有无定形或结晶冰，这表明这两个阶段的水冰的数据中的存在。对于JIRAM数据的最高SNR估计，TC-IM模型解对应于具有非常大的数密度分数的混合物（99.952 +/- 0.001 \%）（23.12 +/- 1.01微米）非晶冰粒，以及非常小部分（0.048 +/-0.001%）的大（565.34 +/- 1.01微米）结晶冰粒。我们发现的大量小的无定形冰粒与以前的研究一致。然而，TC-IM模型的最大似然谱与2.5和3.6 μm附近的数据存在矛盾，表明存在目前未包括在我们的模型中的非冰成分，这主要是由于低温光学常数的可用性有限。
英文摘要：
Juno spacecraft's spectrometer JIRAM recently observed the moon Europa in the 2-5 μm wavelength region. Here we present analysis of the average spectrum of a set of observations near 20°N and 40°W, focusing on the two forms of water-ice - amorphous and crystalline. We also take this as an opportunity to present a novel Bayesian spectral inversion framework for reflectance spectroscopy. We first validate this framework using simulated spectra of amorphous and crystalline ice mixtures and a laboratory spectrum of crystalline ice. We next analyze the JIRAM data and, through Bayesian model comparisons, find that a two-component intimately mixed model (TC-IM model) of amorphous and crystalline ice is strongly preferred (at 26σ confidence) over a two-component model of the same species but where their spectra are areally/linearly mixed. We also find that the TC-IM model is strongly preferred (at > 30σ confidence) over single-component models with only amorphous or crystalline ice, indicating the presence of both these phases of water ice in the data. For the highest SNR estimates of the JIRAM data, the TC-IM model solution corresponds to a mixture with a very large number density fraction (99.952 +/- 0.001 \%) of small (23.12 +/- 1.01 microns) amorphous ice grains, and a very small fraction (0.048 +/- 0.001 \%) of large (565.34 +/- 1.01 microns) crystalline ice grains. The overabundance of small amorphous ice grains we find is consistent with previous studies. The maximum-likelihood spectrum of the TC-IM model, however, is in tension with the data in the regions around 2.5 and 3.6 μm, and indicates the presence of non-ice components not currently included in our model, primarily due to the limited availability of cryogenic optical constants.
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