一项新发现揭示了天王星和海王星不同颜色的原因

美国宇航局的航海者 2 号宇宙飞船在 1980 年代行星飞越期间捕捉到了天王星（左）和海王星（右）的这些视图。 图片来源：NASA/JPL-Caltech/B. Johnson

双子座天文台和其他望远镜的观测显示雾霾过多[{” attribute=””>Uranus makes it paler than Neptune.

Astronomers may now understand why the similar planets Uranus and Neptune have distinctive hues. Researchers constructed a single atmospheric model that matches observations of both planets using observations from the Gemini North telescope, the NASA Infrared Telescope Facility, and the Hubble Space Telescope. The model reveals that excess haze on Uranus accumulates in the planet’s stagnant, sluggish atmosphere, giving it a lighter hue than Neptune.

海王星和天王星有很多共同点——它们的质量、大小和大气成分相似——但它们的外观却明显不同。 在可见光波长下，海王星的颜色明显更蓝，而天王星则呈浅青色。 天文学家现在对为什么这两颗行星的颜色如此不同有了一个解释。

新的研究表明，在两颗行星上发现的浓雾层在天王星上比在海王星上的类似层厚，并且比在海王星上更能“变白”天王星的外观。^[1] 如果没有雾 气氛 从海王星和天王星来看，它们的蓝色看起来大致相同。^[2]

这个结论来自一个模型^[3] 由牛津大学行星物理学教授帕特里克·欧文领导的一个国际团队已经开发出描述海王星和天王星大气中的气溶胶层。^[4] 以前对这些行星上层大气的研究只关注特定波长的大气外观。 然而，这个由多个大气层组成的新模型与来自两颗行星的广泛波长的观测结果相匹配。 新模型还包括更深层内的模糊粒子，以前认为这些粒子只包含甲烷和硫化氢冰云。

该图显示了天王星和海王星大气中的三层气溶胶，由帕特里克·欧文领导的一组科学家设计。 图表上的高度计代表高于 10 bar 的压力。
最深层（气溶胶层-1）很厚，由硫化氢冰和行星大气与阳光相互作用产生的颗粒的混合物组成。
影响颜色的主要层是中间层，它是天王星上比海王星上厚的一层雾颗粒（在论文中称为气溶胶层-2）。 研究小组怀疑，在这两颗行星上，甲烷冰会凝结在这一层的颗粒上，随着甲烷雪的落下，将颗粒拉入大气层深处。 由于海王星的大气层比天王星的大气层更加活跃和动荡，因此研究小组认为，海王星的大气层更有效地将甲烷颗粒分流到雾霾层并产生雪。 这消除了更多的阴霾，并使海王星的阴霾层比天王星上的薄，这意味着海王星的蓝色似乎更强烈。
在这两层之上是一个扩展的雾层（气溶胶层 3），类似于下面的层，但更脆弱。 在海王星上，大的甲烷冰颗粒也在这一层之上形成。
图片来源：双子座国际天文台/NOIRLab/NSF/AURA、J. da Silva/NASA/JPL-Caltech/B. Johnson

“这是第一个同步拟合从紫外线到近红外线反射太阳光观测的模型，”欧文解释说，他是《地球物理研究杂志：行星》上一篇研究论文的主要作者，该论文发表了这一发现。 “他也是第一个解释天王星和海王星可见颜色差异的人。”

该团队的模型由不同高度的三层气溶胶组成。^[5] 影响颜色的主要层是中间层，它是一层雾颗粒（在论文中称为气溶胶层-2），它比表面更厚。 天王星 的 海王星. 研究小组怀疑，在这两颗行星上，甲烷冰会凝结在这一层的颗粒上，随着甲烷雪的落下，将颗粒拉入大气层深处。 由于海王星的大气层比天王星的大气层更加活跃和动荡，因此研究小组认为，海王星的大气层更有效地将甲烷颗粒分流到雾霾层并产生雪。 这消除了更多的阴霾，并使海王星的阴霾层比天王星上的薄，这意味着海王星的蓝色似乎更强烈。

Mike Wong，天文学家[{” attribute=””>University of California, Berkeley, and a member of the team behind this result. “Explaining the difference in color between Uranus and Neptune was an unexpected bonus!”

To create this model, Irwin’s team analyzed a set of observations of the planets encompassing ultraviolet, visible, and near-infrared wavelengths (from 0.3 to 2.5 micrometers) taken with the Near-Infrared Integral Field Spectrometer (NIFS) on the Gemini North telescope near the summit of Maunakea in Hawai‘i — which is part of the international Gemini Observatory, a Program of NSF’s NOIRLab — as well as archival data from the NASA Infrared Telescope Facility, also located in Hawai‘i, and the NASA/ESA Hubble Space Telescope.

The NIFS instrument on Gemini North was particularly important to this result as it is able to provide spectra — measurements of how bright an object is at different wavelengths — for every point in its field of view. This provided the team with detailed measurements of how reflective both planets’ atmospheres are across both the full disk of the planet and across a range of near-infrared wavelengths.

“The Gemini observatories continue to deliver new insights into the nature of our planetary neighbors,” said Martin Still, Gemini Program Officer at the National Science Foundation. “In this experiment, Gemini North provided a component within a suite of ground- and space-based facilities critical to the detection and characterization of atmospheric hazes.”

The model also helps explain the dark spots that are occasionally visible on Neptune and less commonly detected on Uranus. While astronomers were already aware of the presence of dark spots in the atmospheres of both planets, they didn’t know which aerosol layer was causing these dark spots or why the aerosols at those layers were less reflective. The team’s research sheds light on these questions by showing that a darkening of the deepest layer of their model would produce dark spots similar to those seen on Neptune and perhaps Uranus.

Notes

1. This whitening effect is similar to how clouds in exoplanet atmospheres dull or ‘flatten’ features in the spectra of exoplanets.
2. The red colors of the sunlight scattered from the haze and air molecules are more absorbed by methane molecules in the atmosphere of the planets. This process — referred to as Rayleigh scattering — is what makes skies blue here on Earth (though in Earth’s atmosphere sunlight is mostly scattered by nitrogen molecules rather than hydrogen molecules). Rayleigh scattering occurs predominantly at shorter, bluer wavelengths.
3. An aerosol is a suspension of fine droplets or particles in a gas. Common examples on Earth include mist, soot, smoke, and fog. On Neptune and Uranus, particles produced by sunlight interacting with elements in the atmosphere (photochemical reactions) are responsible for aerosol hazes in these planets’ atmospheres.
4. A scientific model is a computational tool used by scientists to test predictions about a phenomena that would be impossible to do in the real world.
5. The deepest layer (referred to in the paper as the Aerosol-1 layer) is thick and is composed of a mixture of hydrogen sulfide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The top layer is an extended layer of haze (the Aerosol-3 layer) similar to the middle layer but more tenuous. On Neptune, large methane ice particles also form above this layer.

More information

This research was presented in the paper “Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots” to appear in the Journal of Geophysical Research: Planets.

The team is composed of P.G.J. Irwin (Department of Physics, University of Oxford, UK), N.A. Teanby (School of Earth Sciences, University of Bristol, UK), L.N. Fletcher (School of Physics & Astronomy, University of Leicester, UK), D. Toledo (Instituto Nacional de Tecnica Aeroespacial, Spain), G.S. Orton (Jet Propulsion Laboratory, California Institute of Technology, USA), M.H. Wong (Center for Integrative Planetary Science, University of California, Berkeley, USA), M.T. Roman (School of Physics & Astronomy, University of Leicester, UK), S. Perez-Hoyos (University of the Basque Country, Spain), A. James (Department of Physics, University of Oxford, UK), J. Dobinson (Department of Physics, University of Oxford, UK).

NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the international Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have for the Tohono O’odham Nation, the Native Hawaiian community, and the local communities in Chile, respectively.