Changing seasons on Uranus tracked across 20 years by Hubble Space Telescope

哈勃望远镜20年追踪观测揭示天王星季节变化

An analysis of two decades of data from NASA's Hubble Space Telescope has provided fresh insights into the complex atmospheric changes on Uranus that are largely driven by the effects of the sun's radiation.

通过分析美国宇航局（NASA）哈勃太空望远镜过去二十年间积累的数据，科学家们获得了关于天王星复杂大气变化的新见解。这些变化很大程度上是由太阳辐射效应驱动的。

Uranus, the seventh planet from the sun, is unique for its extreme axial tilt, with its equator and orbit making nearly at a right angle — likely the result of a collision with an Earth-size object long ago. This tilt causes the planet's poles to experience prolonged, dark winters and bright summers, leading to dramatic seasonal shifts, especially at the northern and southern poles. Despite these extreme traits, however, Uranus remains one of the least understood planets in our solar system, largely because it was only visited by a single spacecraft nearly 40 years ago, Voyager 2 — and that sole encounter coincided with an exceptional solar event, further complicating our understanding of the distant ice giant.

作为太阳系第七颗行星，天王星以其极端的轴向倾角而独树一帜——其赤道面与公转轨道面几乎成直角。这很可能是远古时期一次与地球大小天体相撞的结果。这种倾斜导致其两极经历漫长黑暗的冬季和明亮的夏季，从而引发显著的季节更替，尤其是在北极和南极地区。尽管拥有这些极端特征，天王星仍是太阳系中人类了解最少的行星之一。主要原因在于，仅有旅行者2号探测器在近40年前造访过它，而那次唯一探测恰逢异常的太阳活动，这使我们了解这颗遥远冰巨星更加困难。

Over the past two decades, a team led by astronomer Erich Karkoschka of the University of Arizona used the Space Telescope Imaging Spectrograph on the Hubble Space Telescope to track seasonal changes on Uranus. Because the ice giant takes just over 84 Earth years to complete one orbit around the sun, the researchers mainly got to observe the planet's northern spring as the sun moved from shining directly over the planet's equator to almost directly over its north pole by 2030.

在过去二十年里，由亚利桑那大学天文学家埃里克・卡尔科施卡（Erich Karkoschka）领导的团队，利用哈勃太空望远镜的空间望远镜成像光谱仪追踪天王星的季节变化。由于天王星绕太阳公转一周需 84 个地球年，研究人员主要得以观测到其北半球的春季 —— 在此期间，太阳直射点从天王星赤道附近逐渐移动，预计到2030年将几乎直射其北极。

The series of Hubble images above, seen from left to right, reveal the south polar region darkening as it enters winter shadow, while the north polar region brightens as northern summer approaches, according to a NASA statement.

根据NASA的声明，上方从左至右排列的一系列哈勃图像显示，随着南半球进入冬季，南极区域正在变暗；而随着北半球夏季临近，北极区域则逐渐变亮。

Uranus's atmosphere is primarily composed of hydrogen and helium, with a small amount of methane, which gives the planet its characteristic blue-green hue by absorbing red light from the sun and reflecting blue light.

天王星的大气层主要由氢和氦组成，并含有少量甲烷。正是甲烷通过吸收太阳红光并反射蓝光，赋予这颗行星特有的蓝绿色调。

From 2002 to 2022, Karkoschka and his colleagues observed the ice giant four times — in 2002, 2012, 2015 and 2022 — documenting a richer picture of the planet's atmospheric structure than was gathered by the single Voyager 2 flyby. The recent observations suggest complex atmospheric circulation patterns on Uranus during this period, with the data most sensitive to the methane distribution indicating a downwelling in the polar regions and upwelling in other areas, according to the NASA statement.

2002年到2022年间，Karkoschka及其同事对这颗冰巨星进行了四次观测，分别在2002年、2012年、2015年和2022年，所记录的大气结构图像比旅行者2号单次飞掠所收集的更为丰富。NASA的声明指出，最新观测显示，天王星在此期间存在复杂的大气环流模式。对甲烷分布最敏感的数据显示，其极地地区存在下沉气流，而其他区域则有上升气流。

Notably, scientists found that methane is not uniformly distributed across Uranus but is rather strongly depleted near its poles, with this depletion remaining consistent over the years.

值得注意的是，科学家们发现甲烷在天王星上并非均匀分布，而是在其极地附近显著减少，且这种减少趋势多年来保持一致。

The observations also revealed changes in aerosol concentrations, allowing scientists to map out the planet's atmospheric structure. While methane depletion and aerosol patterns remained relatively stable in the middle and low latitudes over the observed two decades, the polar regions showed more dramatic shifts.

这些观测还揭示了气溶胶浓度的变化，使科学家能够绘制出这颗行星的大气结构。在观测的20年间，中低纬度地区的甲烷减少和气溶胶模式相对稳定，而极地地区则出现了更剧烈的变化。

Notably, aerosols near the north pole became brighter, especially in recent years as the planet approached its northern summer, according to the statement. These long-term observations have provided scientists with a deeper understanding of how the ice giant's atmosphere functions and reacts to changing sunlight.

声明称，特别是北极附近的气溶胶近年来变得更明亮，这与天王星接近北半球夏季的时间点吻合。这些长期观测为科学家深入了解这颗冰巨行星的大气运作机制及其对光照变化的反应提供了依据。

They can also "serve as a proxy for studying exoplanets of similar size and composition," the Hubble team said in the statement.

哈勃团队在声明中表示，观测结果还可以“作为研究类似大小和成分系外行星的参考”。