During the New Horizons spacecraft's encounter with Pluto, the Alice ultraviolet spectrograph conducted a series of observations that detected emissions from both the interplanetary medium (IPM) and Pluto. In the direction of Pluto, the IPM was found to be 133.4$\pm$0.6R at Lyman $\alpha$, 0.24$\pm$0.02R at Lyman $\beta$, and <0.10R at He I 584A. We analyzed 3,900s of data obtained shortly before closest approach to Pluto and detect airglow emissions from H I, N I, N II, N$_2$, and CO above the disk of Pluto. We find Pluto's brightness at Lyman $\alpha$ to be $29.3\pm1.9$R, in good agreement with pre-encounter estimates. The detection of the N II multiplet at 1085A marks the first direct detection of ions in Pluto's atmosphere. We do not detect any emissions from noble gasses and place a 3$\sigma$ upper limit of 0.14 R on the brightness of the Ar I 1048A line. We compare pre-encounter model predictions and predictions from our own airglow model, based on atmospheric profiles derived from the solar occultation observed by New Horizons, to the observed brightness of Pluto's airglow. Although completely opaque at Lyman $\alpha$, Pluto's atmosphere is optically thin at wavelengths longer than 1425A. Consequently, a significant amount of solar FUV light reaches the surface, where it can participate in space weathering processes. From the brightness of sunlight reflected from Pluto, we find the surface has a reflectance factor (I/F) of 17% between 1400-1850A. We also report the first detection of an C$_3$ hydrocarbon molecule, methylacetylene, in absorption, at a column density of ~5$\times10^{15}$ cm$^{-2}$, corresponding to a column-integrated mixing ratio of $1.6\times10^{-6}$.

中文翻译：

---

冥王星的紫外线光谱、表面反射率和气辉辐射

在新视野号航天器与冥王星相遇期间，爱丽丝紫外光谱仪进行了一系列观测，探测到了行星际介质 (IPM) 和冥王星的辐射。在冥王星方向，发现 IPM 在 Lyman $\alpha$ 处为 133.4$\pm$0.6R，在 Lyman $\beta$ 处为 0.24$\pm$0.02R，在 He I 584A 处为 <0.10R。我们分析了在最接近冥王星之前不久获得的 3,900 秒数据，并检测了冥王星圆盘上方 HI、NI、N II、N$_2$ 和 CO 的气辉排放。我们发现冥王星在莱曼 $\alpha$ 的亮度为 $29.3\pm1.9$R，与遇到前的估计非常吻合。在 1085A 处检测到 N II 多重峰标志着首次直接检测到冥王星大气中的离子。我们没有检测到来自惰性气体的任何排放，并将 3$\sigma$ 上限设为 0。14 R 在 Ar I 1048A 线的亮度上。我们将遭遇前模型的预测和来自我们自己的气辉模型的预测（基于从新视野号观测到的太阳掩星得出的大气剖面）与冥王星气辉的观测亮度进行比较。尽管在莱曼 $\alpha$ 处完全不透明，但冥王星的大气在波长超过 1425A 时光学上很薄。因此，大量的太阳 FUV 光到达地表，在那里它可以参与空间风化过程。从冥王星反射的阳光亮度，我们发现表面在 1400-1850A 之间的反射系数 (I/F) 为 17%。我们还报告了在吸收中首次检测到 C$_3$ 碳氢化合物分子甲基乙炔，柱密度为 ~5$\times10^{15}$ cm$^{-2}$，