[论文解读] Potential Atmospheric Compositions of TRAPPIST-1 c constrained by JWST/MIRI Observations at 15 $μ$m

Andrew Lincowski, Victoria Meadows+14 more|arXiv (Cornell University)|Aug 11, 2023

Astro and Planetary SciencePhysics and Astronomy被引用 3

一句话总结

本研究利用耦合气候-光化学模型，基于詹姆斯·韦伯太空望远镜/MIRI在15 μm波段的二次凌星观测数据，评估了TRAPPIST-1 c可能的大气成分。结果表明，在1σ置信水平内，表面压力较低（≤100 ppm）、水汽含量最高达10%的稀薄O2主导大气与观测到的421 ± 94 ppm凌星深度一致；而更厚的O2或水汽大气（≤3 bar）在1.8σ范围内也具有合理性，尽管金星型大气在3σ置信水平下被排除。

ABSTRACT

The first JWST observations of TRAPPIST-1 c showed a secondary eclipse depth of 421+/-94 ppm at 15 um, which is consistent with a bare rock surface or a thin, O2-dominated, low CO2 atmosphere (Zieba et al. 2023). Here, we further explore potential atmospheres for TRAPPIST-1 c by comparing the observed secondary eclipse depth to synthetic spectra of a broader range of plausible environments. To self-consistently incorporate the impact of photochemistry and atmospheric composition on atmospheric thermal structure and predicted eclipse depth, we use a two-column climate model coupled to a photochemical model, and simulate O2-dominated, Venus-like, and steam atmospheres. We find that a broader suite of plausible atmospheric compositions are also consistent with the data. For lower pressure atmospheres (0.1 bar), our O2-CO2 atmospheres produce eclipse depths within 1$σ$ of the data, consistent with the modeling results of Zieba et al. (2023). However, for higher-pressure atmospheres, our models produce different temperature-pressure profiles and are less pessimistic, with 1-10 bar O2, 100 ppm CO2 models within 2.0-2.2$σ$ of the measured secondary eclipse depth, and up to 0.5% CO2 within 2.9$σ$. Venus-like atmospheres are still unlikely. For thin O2 atmospheres of 0.1 bar with a low abundance of CO2 ($\sim$100 ppm), up to 10% water vapor can be present and still provide an eclipse depth within 1$σ$ of the data. We compared the TRAPPIST-1 c data to modeled steam atmospheres of $\leq$ 3 bar, which are 1.7-1.8$σ$ from the data and not conclusively ruled out. More data will be required to discriminate between possible atmospheres, or to more definitively support the bare rock hypothesis.

研究动机与目标

• 评估不同大气成分与JWST/MIRI观测到的TRAPPIST-1 c在15 μm波段的二次凌星深度之间的兼容性。
• 研究光化学过程与热结构如何影响大气消光系数和凌星深度预测。
• 确定观测到的凌星深度是否支持裸露的岩石表面，或仅支持一系列可能的大气状态。
• 在自洽的辐射-对流与光化学建模下，评估O2主导、金星型及水汽丰富大气的合理性。
• 识别未来观测中区分裸露岩石表面与不同大气状态的观测路径。

提出的方法

• 采用耦合光化学模型的双柱能量平衡模型，模拟大气热结构与成分。
• 针对O2主导、金星型及水汽主导大气，在0.1–10 bar表面压力范围内生成合成二次凌星光谱。
• 使用LBLABC与DISORT进行辐射传输计算，以确定15 μm波段的大气消光系数与热辐射。
• 将模型预测的凌星深度与JWST/MIRI F1500W观测结果（421 ± 94 ppm）进行比较。
• 在气候模型中考虑分子辐射传输与热量再分配效应，以提高凌星深度预测的准确性。
• 采用统计置信水平（σ）评估每种大气模型与观测数据的兼容性。

Figure 1: Day-side hemisphere temperature structures for all modeled atmospheres: Venus-like (left panel), steam atmospheres (middle panel) and \ce O2- \ce CO2 atmospheres (right panel). For our modeled atmospheres, we have used a thicker line to show the layers over which the 15 $\upmu$ m band reac

实验结果

研究问题

• RQ1 哪些TRAPPIST-1 c的大气成分与观测到的15 μm波段421 ± 94 ppm的二次凌星深度相兼容？
• RQ2 光化学过程与热结构如何影响富含O2或水汽的大气的预测凌星深度？
• RQ3 表面压力较低、CO2含量低且水汽含量较低的稀薄O2主导大气是否与数据相容？其最大可允许水汽混合比是多少？
• RQ4 考虑到观测到的凌星深度，厚实的金星型大气是否仍具有合理性，还是在高置信水平下被排除？
• RQ5 未来哪些观测或光谱波段最有助于区分TRAPPIST-1 c的裸露岩石表面与不同大气状态？

主要发现

• 在0.1 bar表面压力下，CO2丰度≤100 ppm的O2主导大气与观测深度在1σ范围内相容，证实了早期研究结果。
• 在15 μm波段光学薄的前提下，水汽混合比最高达10%的模型与数据在1σ范围内相容。
• 表面压力为1–10 bar的O2大气与100 ppm CO2组合产生的凌星深度位于观测值的2.0–2.2σ范围内，表明其具有合理性但不如稀薄大气可能。
• CO2含量为0.5%的大气模型与观测值相差2.9σ，表明更高CO2水平仍可能但概率较低。
• 表面压力≤3 bar的水汽大气模型与观测深度相差1.7–1.8σ，尚未被明确排除。
• 金星型大气（≥0.1 bar）在2.6–3.1σ置信水平下被排除，表明其极不可能解释观测到的凌星深度。

Figure 2: Brightness temperature spectra for the dayside hemisphere of all modeled environments, with points corresponding to the model spectra convolved to the F1500W filter band over the band’s wavelength extent (horizontal error bars show the FWHM of the filter band). We also plot lines for 340 K