Volatile Transport Modeling on Triton with New Observational Constraints

Neptune’s moon Triton shares many similarities with Pluto, including volatile cycles of N2, CH4and CO, and represents a benchmark case for the study of surface-atmosphere interactions onvolatile-rich Kuiper Belt objects. The observations of Pluto by New Horizons acquired during the2015 flyby and their analysis with volatile transport models (VTMs) shed light on how volatilesublimation-condensation cycles control the climate and shape the surface of such objects. Withinthe context of New Horizons observations as well as recent Earth-based observations of Triton,we adapt a Plutonian VTM to Triton, and test its ability to simulate its volatile cycles, therebyaiding our understanding of its climate.Here we present numerical VTM simulations exploring the volatile cycles of N2, CH4 and CO onTriton over long-term and seasonal timescales (cap extent, surface temperatures, surfacepressure, sublimation rates) for varying model parameters (including the surface ice reservoir,albedo, thermal inertia, and the internal heat flux). We explore what scenarios and modelparameters allow for a best match of the available observations. In particular, our set of observational constraints include Voyager 2 observations (surface pressure and cap extent),ground-based near-infrared (0.8 to 2.4 μm) disk-integrated spectra (the relative surface area ofvolatile vs. non-volatile ice) and the evolution of surface pressure as retrieved from stellaroccultations.Our results show that Triton’s poles act as cold traps for volatile ices and favor the formation ofpolar caps extending to lower latitudes through glacial flow or through the formation of thinnerseasonal deposits. As previously evidenced by other VTMs, North-South asymmetries in surfaceproperties can favor the development of one cap over the other. Our best-case simulations areobtained for a bedrock surface albedo of 0.6-0.7, a global reservoir of N2 ice thicker than 200 m,and a bedrock thermal inertia larger than 500 SI (or smaller but with a large internal heat flux).The large N2 ice reservoir implies a permanent N2 southern cap (several 100 m thick) extendingto the equatorial regions with higher amounts of volatile ice at the south pole, which is notinconsistent with Voyager 2 images but does not fit well with observed full-disk near-infraredspectra. Our results also suggest that a small permanent polar cap exists in the northern (currentlywinter) hemisphere if the internal heat flux remains relatively low (e.g. radiogenic, < 3 mW m-2). Anon-permanent northern polar cap was only obtained in some of our simulations with high internalheat flux (30 mW m-2). The northern cap will possibly extend to 30°N in the next decade, thusbecoming visible by Earth-based telescopes. On the basis of our model results, we also discussthe composition of several surface units seen by Voyager 2 in 1989, including the bright equatorialfringe and dark surface patches.Finally, we provide predictions for the evolution of ice distribution, surface pressure and CO andCH4 atmospheric mixing ratios in the next decades. According to our model, the surface pressureshould slowly decrease but remain larger than 0.5 Pa by 2060. We also model the thermallightcurves of Triton for different climate scenarios in 2022, which serve as predictions for futureJames Webb Space Telescope observations.海卫一（Triton）作为海王星的卫星，与冥王星存在诸多相似之处，二者均存在氮（N₂）、甲烷（CH₄）与一氧化碳（CO）的挥发物循环，海卫一也是研究挥发物丰富的柯伊伯带天体表面-大气相互作用的基准案例。新视野号（New Horizons）在2015年飞掠冥王星期间获取的观测数据，结合挥发物输运模型（VTMs）开展的分析，揭示了挥发物升华-冷凝循环如何调控这类天体的气候与表面形态。结合新视野号的观测结果，以及近年基于地球的海卫一观测数据，我们将适用于冥王星的VTM改造适配至海卫一，并测试其模拟海卫一挥发物循环的能力，以此助力我们对海卫一气候的理解。 本文呈现了数值化VTM模拟结果，探究了海卫一上氮、甲烷、一氧化碳的挥发物循环在长期与季节时间尺度上的变化（包括冰盖范围、表面温度、表面压强、升华速率），同时设置了不同的模型参数（包括表面冰储量、反照率、热惯量与内热通量）。我们旨在探索哪些情景与模型参数能够最佳匹配现有观测数据。具体而言，我们的观测约束集包含旅行者2号（Voyager 2）的观测结果（表面压强与冰盖范围）、基于地球的近红外（0.8至2.4 μm）盘积分光谱（挥发冰与非挥发冰的相对表面积），以及通过恒星掩星反演得到的表面压强演化情况。 研究结果表明，海卫一的两极是挥发冰的冷阱，且通过冰川流动或薄层季节性沉积的方式，有利于极盖向更低纬度区域扩展。正如其他VTM研究此前证实的那样，表面性质的南北不对称性会使得某一极盖更易于形成。我们得到的最佳模拟情景为：基岩表面反照率为0.6~0.7、全球氮冰储量厚度超过200 m，且基岩热惯量大于500国际单位制（SI）（或热惯量较小但内热通量较高）。大规模氮冰储量意味着存在厚度达数百米的永久性南极大冰盖，其延伸至赤道区域，且南极区域的挥发冰含量更高——这一结果与旅行者2号的成像数据并不矛盾，但与观测得到的全圆盘近红外光谱吻合度欠佳。 研究结果还显示，若内热通量维持在较低水平（例如放射成因内热，小于3 mW·m⁻²），北半球（当前处于冬季）存在小型永久性极盖。仅在部分设置了高内热通量（30 mW·m⁻²）的模拟中，我们得到了非永久性的北极盖。未来十年内，北极盖可能会延伸至北纬30°，届时将可通过地面望远镜观测到。基于本模型结果，我们还对旅行者2号在1989年观测到的多个表面单元的组分进行了讨论，其中包括明亮的赤道条纹与暗色地表斑块。 最后，我们对未来数十年内的冰分布、表面压强以及一氧化碳与甲烷的大气混合比演化做出了预测。根据我们的模型，表面压强将缓慢下降，但至2060年仍将保持在0.5 Pa以上。我们还针对2022年不同气候情景下的海卫一热光变曲线进行了建模，该结果可作为未来詹姆斯·韦布空间望远镜（James Webb Space Telescope）观测的预测参考。