X-Ray Emission from Jupiter’s Galilean Moons: A Tool for Determining Their Surface Composition and Particle Environment
X-Ray Emission from Jupiter’s Galilean Moons: A Tool for Determining Their Surface Composition and Particle Environment
复制标题
木星伽利略卫星发射的 X 射线：确定其表面成分和粒子环境的工具
作者：
S. Nulsen;R. Kraft;G. Germain;W. Dunn;G. Tremblay;L. Beegle;G. Branduardi‐Raymont;E. Bulbul;R. Elsner;R. Hodyss;S. Vance
我们分析了钱德拉x射线天文台对木星的观测档案，以寻找伽利略卫星的发射。以前曾报道过木卫一和木卫二使用这些数据的一个子集发射x射线。我们证实了这一发现，并从木卫三和木卫四上边际探测到x射线辐射。木卫二的x射线光谱在中性氧荧光线（525 eV）附近有很强的峰值，而木卫一的x射线光谱在氧和硫荧光线（2308 eV）都有峰值，在350到5000 eV之间有一个广泛的连续体。木卫三的光谱与木卫一相似，但没有硫峰。从木卫四上可以探测到一些事件，主要集中在氧线附近。利用伽利略任务对周围质子和电子比强度的测量，我们从两个过程中模拟了卫星的x射线光谱和通量：高能质子撞击产生的粒子诱导x射线发射（PIXE）和电子轫致辐射产生的x射线发射。由于几个因素的不确定性，电子轫致辐射和PIXE模型高估了来自木卫二的x射线通量，使我们无法对x射线发射的起源做出明确的陈述。木卫一的pxie模型预测的O和S的发射线与观测到的相似，但低估了它们的通量近两个数量级。根据pixie通量的这种差异，结合在光谱中检测到的宽带连续，我们得出结论，木卫一的x射线发射是由于电子轫致辐射。同样，由于木卫三的连续谱很宽，我们初步得出结论，它的x射线发射也是由于电子轫致辐射。木卫四在x射线下太暗，无法得出任何结论。获得卫星的现场x射线观测将提供对其元素组成的直接测量。
We analyze archival Chandra X-ray Observatory observations of Jupiter to search for emission from the Galilean moons. X-ray emission has previously been reported from Io and Europa using a subset of these data. We confirm this detection, and marginally detect X-ray emission from both Ganymede and Callisto as well. The X-ray spectrum of Europa is strongly peaked around the neutral oxygen fluorescence line (525 eV), while Io’s has peaks at both oxygen and sulfur (2308 eV) plus a broad continuum between 350 and 5000 eV. Ganymede’s spectrum is similar to Io’s, but without the sulfur peak. A few events, mostly clustered around the oxygen line, are detected from Callisto. Using measurements by the Galileo mission of the specific intensity of ambient protons and electrons, we model the X-ray spectra and flux of the moons from two processes: particle-induced X-ray emission (PIXE) from the impact of energetic protons and X-ray emission from electron bremsstrahlung. With uncertainties of a factor of a few, the electron bremsstrahlung and PIXE models overestimate the X-ray flux from Europa, preventing us from making a definitive statement about the origin of the X-ray emission. The PIXE model of Io predicts emission lines at O and S similar to those observed, but underestimates their flux by nearly two orders of magnitude. Based on this discrepancy in the PIXE flux, combined with the detected broadband continuum in the spectrum, we conclude that the X-ray emission from Io is due to electron bremsstrahlung. Likewise, because of Ganymede’s broad continuum, we tentatively conclude that its X-ray emission is also due to electron bremsstrahlung. Callisto is too faint in the X-rays to draw any conclusion. Obtaining in situ X-ray observations of the moons would provide a direct measurement of their elemental composition.