Saturn’s Seasonal Variability from Four Decades of Ground-Based Mid-Infrared Observations

A multi-decade record of ground-based mid-infrared (7-25 micron) images of Saturn is used to explore seasonal and non-seasonal variability in thermal emission over more than a Saturnian year (1984-2021). Thermal emission measured by 3-m and 8-m-class observatories (notably NASA’s Infrared Telescope Facility, Subaru, and ESO’s Very Large Telescope) compares favorably with synthetic images based on both Cassini-derived temperature records and the predictions of radiative climate models. We find that 8-m class facilities are capable of resolving thermal contrasts on the scale of Saturn’s belts, zones, polar hexagon, and polar cyclones, superimposed onto large-scale seasonal asymmetries. Seasonal changes in brightness temperatures of ~30 K in the stratosphere and ~10 K in the upper troposphere are observed, as the northern and southern polar stratospheric vortices (NPSV and SPSV) form in spring and dissipate in autumn. The timings of the first appearance of the warm polar vortices is successfully reproduced by radiative climate models, confirming them to be radiative phenomena, albeit entrained within sharp boundaries influenced by dynamics. Axisymmetric thermal bands (4- 5 per hemisphere) display temperature gradients that are strongly correlated with Saturn’s zonal winds, indicating winds that decay in strength with altitude from the cloud-tops to the 1-mbar level, and implying Ferrel-like circulation cells in Saturn’s upper troposphere and stratosphere forming the system of cool zones and warm belts. Saturn’s thermal structure is largely repeatable from year to year (via comparison of infrared images in 1989 and 2018), with the exception of low latitudes. Here we find evidence of inter-annual variations because the equatorial banding at 7.9 m is inconsistent with a 15-year period for Saturn’s equatorial stratospheric oscillation, i.e., it is not strictly semi-annual. Either the oscillation has a longer period closer to 20 years, or its progression is naturally variable and interrupted by tropospheric meteorology (e.g., storms). Finally, observations between 2017-2021 extend the legacy of the Cassini mission, revealing the continued warming of the NPSV during northern summer in line with predictions of radiative climate models.

本研究采用1984-2021年（覆盖超过一个土星年）的数十年地基中红外（7-25微米）土星成像观测数据集，以探究其热辐射的季节与非季节变率。由3米及8米级观测台站——尤其是美国国家航空航天局（NASA）的红外望远镜设施、昴星团望远镜（Subaru）以及欧洲南方天文台（ESO）的甚大望远镜——所测得的热辐射，与基于卡西尼号（Cassini）温度数据及辐射气候模型预测生成的合成图像吻合度良好。研究发现，8米级观测设施能够分辨土星带区、亮区、极地六边形以及极地气旋尺度的热反差，该热反差叠加于大尺度季节不对称性之上。观测显示，平流层亮温存在约30 K的季节变化，对流层上层亮温则存在约10 K的季节变化，这与南北极平流层涡旋（NPSV和SPSV）春季形成、秋季消散的过程相对应。辐射气候模型成功复现了暖性极涡首次出现的时间，证实其本质为辐射现象，尽管其受动力学影响被限制在尖锐的边界内。每个半球存在4至5条轴对称热带，其温度梯度与土星纬向风强相关，表明风速从云顶至1毫巴高度层随高度升高而减弱，同时意味着土星对流层上层和平流层中存在费雷尔型环流圈，进而形成冷亮区与暖带的分布格局。通过对比1989年与2018年的红外图像可知，土星的热结构在多数年份间具有可重复性，仅低纬度区域例外。在低纬度区域，我们发现了年际变化的证据：7.9微米波段的赤道带分布与土星赤道平流层振荡的15年周期不符，即其并非严格的半年周期振荡。这意味着要么该振荡的周期更长、接近20年，要么其演化过程存在自然变率，并会被对流层大气现象（如风暴）打断。最后，2017至2021年间的观测延续了卡西尼号任务的观测遗产，揭示了北半球夏季期间NPSV的持续增暖，这与辐射气候模型的预测结果一致。