Exploring clouds and gaseous abundances in the atmospheres of Uranus and Neptune

天王星和海王星，被称为“冰巨人”，是太阳系中最神秘、最不为人所知的行星之一。天王星的两极倾斜了惊人的98度（与地球的23.5度相比），导致太阳强迫的巨大年度变化，两极每年每单位面积接收的阳光比赤道多！相比之下，海王星29度的倾角显得不那么反常。旅行者2号在1986年和1989年的近距离飞掠为我们提供了这两个世界的唯一近距离照片，并揭示了天王星与太阳的辐射几乎处于完美的平衡状态，而海王星发出的热辐射是它接收的太阳辐射的2.5倍以上！也许是这种不平衡的结果，天王星的大气循环被发现是相当安静的，而海王星的大气循环是非常动态和活跃的。四分之一个多世纪后，自适应光学的发展改变了地面望远镜的空间分辨率。天王星的大气层活动在2007年春分时急剧增加，而海王星的大气层则显示出云活动的巨大变化。我们一直在通过夏威夷的双子座北望远镜和智利的欧洲南方天文台甚大望远镜进行广泛的近红外地面观测项目来监测这些发展。近红外反射光谱使我们能够确定这些大气中云和气体丰度的垂直和水平分布，使用我们世界领先的NEMESIS检索代码。最近的亮点包括：1)首次在天王星大气中检测到硫化氢（Irwin et al., 2018; https://doi.org/10.1038/s41550-018-0432-1），并可能在海王星大气中检测到（Irwin et al., 2019; https://doi.org/10.1016/j.icarus.2018.12.014）；2)首次在地面探测到海王星大气中甲烷的纬度变化（Irwin et al., 2021; https://doi.org/10.1016/j.icarus.2020.114277）；最近，一个“整体”云模型与观测到的天王星和海王星的反射率光谱在0.3到2.5微米之间相匹配（Irwin等人，2022;https://doi.org/10.1029/2022JE007189），并且可以解释这两个世界之间的颜色差异。该项目的目的是发展我们的地面可见光和近红外观测计划，将现有观测结果与詹姆斯·韦伯太空望远镜最近的观测结果结合起来，并提出和进行未来的观测。鉴于2022年NASA行星科学年代际调查建议对其中一颗冰巨星进行专门的太空任务，这个项目尤其及时。这项工作将使用Irwin等人（2019）（https://doi.org/10.1016/j.icarus.2018.12.014）新开发的Minnaert分析方案，该方案能够有效拟合目标纬度的平均光谱及其边缘暗化特性，从而更强烈地限制粒子散射特性。此外，这项工作将使用新开发的图像反卷积方案（Irwin等人，2022;https://doi.org/10.1029/2022JE007189），该方案提高了空间分辨率，可以更好地区分不同纬度，并寻找离散的大气特征。最后，在项目中有可能通过将散射代码（目前用Fortran编写）转录到python并使用NEMESIS已经用于系外行星检索的嵌套采样方法来实现对观测结果的更多贝叶斯分析。

Uranus and Neptune, known as the "Ice Giants" are amongst the most mysterious and poorly understood planets in our solar system. The poles of Uranus are tipped over by an extraordinary 98 degrees (compared with an obliquity of 23.5 degrees for the Earth) leading to enormous annual variations in solar forcing, with the poles annually receiving more sunlight per unit area than the equator! In contrast, Neptune's obliquity of 29 degrees appears much less anomalous. The Voyager 2 fly-bys in 1986 and 1989 provided our only close-up views of these worlds and revealed that Uranus is in almost perfect radiative balance with the Sun, while Neptune emits thermally more than 2.5 times the solar radiation it receives! Perhaps as a result of this imbalance, the atmospheric circulation of Uranus was found to be rather quiescent, while that of Neptune was extraordinarily dynamic and active. More than a quarter of a century later, the spatial resolution of ground-based telescopes has been transformed by the development of adaptive optics. The activity of Uranus's atmosphere has been seen to increase dramatically through its equinox in 2007, while Neptune's atmosphere shows enormous changes in cloud activity. We have been monitoring and these developments with an extensive programme of near-infrared ground-based observations at the Gemini-North Telescope in Hawai'i and ESO's Very Large Telescope in Chile. Near infrared reflectance spectroscopy enables us to determine the vertical and horizontal distribution of cloud and gaseous abundances in these atmospheres, using our world-leading NEMESIS retrieval code. Recent highlights include: 1) the first positive detection of hydrogen sulphide in the atmosphere of Uranus (Irwin et al., 2018; https://doi.org/10.1038/s41550-018-0432-1) and probable detection in Neptune's atmosphere (Irwin et al., 2019; https://doi.org/10.1016/j.icarus.2018.12.014); 2) the first ground-based detection of methane latitudinal variability in Neptune's atmosphere (Irwin et al., 2021; https://doi.org/10.1016/j.icarus.2020.114277); and most recently 3) a 'holistic' cloud model that matches the observed reflectivity spectra of both Uranus and Neptune from 0.3 to 2.5 microns (Irwin et al., 2022; https://doi.org/10.1029/2022JE007189), and can explain the difference in colour between these two worlds.The aim of this project is to develop our ground-based visible and near-infrared observing programme and combine existing observations with recent observations made with the James Webb Space Telescope, and also propose and conduct future observations. This project is particularly timely given that a dedicated space mission to one of the Ice Giants was recommended by the 2022 NASA Decadal Survey in Planetary Science. The work will use the newly-developed Minnaert analysis scheme of Irwin et al. (2019) (https://doi.org/10.1016/j.icarus.2018.12.014), which enables efficient fitting of both the mean spectra at target latitudes and their limb-darkening properties, which constrains the particle scattering properties much more strongly. In addition, the work will use a newly-developed image deconvolution scheme (Irwin et al., 2022; https://doi.org/10.1029/2022JE007189) which improves the spatial resolution and allows better discrimination between different latitudes and also the search for discrete atmospheric features. Finally, there is a possibility in the project to implement a more Bayesian analysis of the observations via transcription of the scattering code (currently written in Fortran) to python and using a nested sampling approach that is already used by NEMESIS for exoplanet retrievals.