Observations of Titan’s Stratosphere during Northern Summer: Temperatures, CH3CN and CH3D Abundances

IF 3.8 Q2 ASTRONOMY & ASTROPHYSICS The Planetary Science Journal Pub Date : 2024-05-29 DOI:10.3847/psj/ad47bd
Alexander E. Thelen, Conor A. Nixon, Martin A. Cordiner, Emmanuel Lellouch, Sandrine Vinatier, Nicholas A. Teanby, Bryan Butler, Steven B. Charnley, Richard G. Cosentino, Katherine de Kleer, Patrick G. J. Irwin, Mark A. Gurwell, Zbigniew Kisiel and Raphael Moreno
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Abstract

Titan’s atmospheric composition and dynamical state have previously been studied over numerous epochs by both ground- and space-based facilities. However, stratospheric measurements remain sparse during Titan’s northern summer and fall. The lack of seasonal symmetry in observations of Titan’s temperature field and chemical abundances raises questions about the nature of the middle atmosphere’s meridional circulation and evolution over Titan’s 29 yr seasonal cycle that can only be answered through long-term monitoring campaigns. Here, we present maps of Titan’s stratospheric temperature, acetonitrile (or methyl cyanide; CH3CN) abundance, and monodeuterated methane (CH3D) abundance following Titan’s northern summer solstice obtained with Band 9 (∼0.43 mm) Atacama Large Millimeter/submillimeter Array observations. We find that increasing temperatures toward high southern latitudes, currently in winter, resemble those observed during Titan’s northern winter by the Cassini mission. Acetonitrile abundances have changed significantly since previous (sub)millimeter observations, and we find that the species is now highly concentrated at high southern latitudes. The stratospheric CH3D content is found to range between 4 and 8 ppm in these observations, and we infer the CH4 abundance to vary between ∼0.9% and 1.6% through conversion with previously measured D/H values. A global value of CH4 = 1.15% was retrieved, lending further evidence to the temporal and spatial variability of Titan’s stratospheric methane when compared with previous measurements. Additional observations are required to determine the cause and magnitude of stratospheric enhancements in methane during these poorly understood seasons on Titan.
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土卫六平流层北部夏季观测:温度、CH3CN 和 CH3D 丰度
土卫六的大气成分和动力学状态以前曾由地面和空间设施进行过多次研究。然而,在土卫六北部的夏季和秋季,对平流层的测量仍然很少。对土卫六温度场和化学丰度的观测缺乏季节对称性,这就对土卫六29年季节周期中层大气经向环流和演化的性质提出了疑问,而这些问题只能通过长期的监测活动来解答。在这里,我们展示了利用阿塔卡马大毫米波/亚毫米波阵列第9波段(∼0.43毫米)观测所获得的土卫六北夏至日之后的土卫六平流层温度、乙腈(或甲基氰化物;CH3CN)丰度和单氘化甲烷(CH3D)丰度图。我们发现南方高纬度地区的温度不断升高,目前正值冬季,这与卡西尼飞行任务在土卫六北部冬季观测到的温度相似。与之前的(亚)毫米观测结果相比,乙腈的丰度发生了显著变化,我们发现该物质现在高度集中在南部高纬度地区。在这些观测中发现平流层中的 CH3D 含量在 4 到 8 ppm 之间,通过与之前测量的 D/H 值进行换算,我们推断 CH4 丰度在 0.9% 到 1.6% 之间变化。与之前的测量结果相比,我们得到了一个CH4 = 1.15%的全球值,进一步证明了土卫六平流层甲烷的时空变异性。还需要进行更多的观测,以确定土卫六上这些鲜为人知的季节平流层甲烷增加的原因和幅度。
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来源期刊
The Planetary Science Journal
The Planetary Science Journal Earth and Planetary Sciences-Geophysics
CiteScore
5.20
自引率
0.00%
发文量
249
审稿时长
15 weeks
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