木卫三表面成分的全球模拟：来自VLT/SPHERE的近红外映射
Global Modelling of Ganymede's Surface Composition: Near-IR Mapping from VLT/SPHERE
摘要：
我们目前的地图木卫三的表面成分与几乎完整的经度覆盖，获得高空间分辨率近红外（0.95至1.65 $μm$）观测地面VLT/SPHERE仪器。观测到的反射光谱建模使用马尔可夫链蒙特卡罗方法来估计水冰，酸，盐和光谱平坦的变暗剂的丰度和相关的不确定性。结果证实木卫三的表面主要是水冰在年轻的明亮地形（撞击坑，沟），和低吸收光谱平坦的材料在较老的黑暗地形（如伽利略区）。冰的颗粒大小有很强的纬度和经度梯度，在赤道和后半球有较大的颗粒。这些趋势与辐射驱动溅射的纬度热梯度和全球变化的影响是一致的。硫酸的丰度很低，可能与木卫三的两极暴露在木星外部磁场中的等离子体轰击有关。丰度的最佳估计表明，可能存在多种盐的混合物，尽管其丰度低、光谱简并性和相关的不确定性意味着无法有把握地检测到个别盐种。如果存在，硫酸镁钠和氯酸镁似乎暂时与外源等离子体轰击，而氯化镁和硫酸镁似乎暂时与年轻的地形，这意味着可能的内源性起源。还对Galileo/NIMS数据进行了MCMC建模，显示了可比的分布。SPHERE的高空间分辨率允许对小尺度（<150公里）表面特征进行精确测绘，可以与更高光谱分辨率的观测一起使用，以共同确认潜在物种的存在和分布。
英文摘要：
We present maps of Ganymede's surface composition with almost complete longitude coverage, acquired using high spatial resolution near-infrared (0.95 to 1.65$μm$) observations from the ground-based VLT/SPHERE instrument. Observed reflectance spectra were modelled using a Markov Chain Monte Carlo method to estimate abundances and associated uncertainties of water ices, acids, salts and a spectrally-flat darkening agent. Results confirm Ganymede's surface is dominated by water ice in young bright terrain (impact craters, sulci), and low-albedo spectrally-flat material in older dark terrain (e.g. Galileo Regio). Ice grain size has strong latitudinal and longitudinal gradients, with larger grains at the equator and on the trailing hemisphere. These trends are consistent with the effects of the latitudinal thermal gradient and global variations in radiation driven sputtering. Sulphuric acid has a low abundance and appears potentially spatially correlated with plasma bombardment, where Ganymede's poles are exposed to the external jovian magnetic field. Best-estimate abundances suggest a mixture of salts could be present, although their low abundances, spectral degeneracies and associated uncertainties mean individual salt species cannot be detected with confidence. If present, sodium magnesium sulphate and magnesium chlorate appear tentatively correlated with exogenic plasma bombardment, while magnesium chloride and sulphate appear tentatively correlated with younger terrain, implying a possible endogenic origin. MCMC modelling was also performed on Galileo/NIMS data, showing comparable distributions. The high spatial resolution of SPHERE allows the precise mapping of small scale (<150km) surface features, which could be used along with higher spectral resolution observations to jointly confirm the presence and distribution of potential species.
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