Latitudinal variation of methane mole fraction above clouds in Neptune's atmosphere from VLT/MUSE-NFM: Limb-darkening reanalysis
Latitudinal variation of methane mole fraction above clouds in Neptune's atmosphere from VLT/MUSE-NFM: Limb-darkening reanalysis
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来自 VLT/MUSE-NFM 的海王星大气云层上方甲烷摩尔分数的纬度变化：临边变暗再分析
DOI：
10.1016/j.icarus.2020.114277
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发表时间：
2021
期刊：
影响因子：
3.2
通讯作者：
Irwin P
中科院分区：
文献类型：
--
作者：
Irwin P
我们提出了2018年用甚大望远镜（VLT）上的多单元光谱探测器（MUSE）仪器在窄场自适应光学模式下对海王星的可见/近红外（480-930 nm）观测结果的再分析，Irwin等人，Icarus, 311, 2019。我们发现，在我们之前的分析中，推断的甲烷丰度随纬度的变化仅基于中心子午线观测，与更完整的海王星边缘变暗评估相比，低估了检索误差。此外，我们之前的分析引入了丰度及其不确定性的伪纬度变异，我们在这里重新评估。我们对这些数据的重新分析结合了基于Minnaert近似模型的边缘暗化的影响，该模型对云结构和甲烷摩尔分数提供了更强的约束，更好地利用了现有数据，并且计算效率更高。我们发现，在远离离散云特征的情况下，观测到的800 ~ 900 nm的反射率光谱非常接近于一个纬度变化但纬向对称的背景云模式，该模式由一个基于3.6 ~ 4.7 bar的不透明度和尺度高度可变的H2S云层和一个平流层雾霾组成。背景云模型与观测到的所有波长和纬度的边缘变暗相匹配，我们发现在2-4 bar，高于H2S云，但低于甲烷凝结水平，甲烷的摩尔分数在赤道为4 -6%，在南极纬度为2-4%，与之前的分析一致，赤道/极比为1.9±0.2。通过加入一个不透明度在0到0.75之间，压力小于~0.4 bar的垂直薄甲烷冰云，可以很好地拟合离散云区的光谱。
We present a reanalysis of visible/near-infrared (480–930 nm) observations of Neptune, made in 2018 with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) in Narrow Field Adaptive Optics mode, reported by Irwin et al., Icarus, 311, 2019. We find that the inferred variation of methane abundance with latitude in our previous analysis, which was based on central meridian observations only, underestimated the retrieval errors when compared with a more complete assessment of Neptune's limb darkening. In addition, our previous analysis introduced spurious latitudinal variability of both the abundance and its uncertainty, which we reassess here. Our reanalysis of these data incorporates the effects of limb-darkening based upon the Minnaert approximation model, which provides a much stronger constraint on the cloud structure and methane mole fraction, makes better use of the available data and is also more computationally efficient. We find that away from discrete cloud features, the observed reflectivity spectrum from 800 to 900 nm is very well approximated by a background cloud model that is latitudinally varying, but zonally symmetric, consisting of a H2S cloud layer, based at 3.6–4.7 bar with variable opacity and scale height, and a stratospheric haze. The background cloud model matches the observed limb darkening seen at all wavelengths and latitudes and we find that the mole fraction of methane at 2–4 bar, above the H2S cloud, but below the methane condensation level, varies from 4–---6% at the equator to 2–4% at south polar latitudes, consistent with previous analyses, with a equator/pole ratio of 1.9 ± 0.2 for our assumed cloud/methane vertical distribution model. The spectra of discrete cloudy regions are fitted, to a very good approximation, by the addition of a single vertically thin methane ice cloud with opacity ranging from 0 to 0.75 and pressure less than ~0.4 bar.
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