Titan's atmosphere is optically thick and hides the surface and the lower layers from the view at almost all wavelengths. However, because gaseous absorptions are spectrally selective, some narrow spectral intervals are relatively transparent and allow to probe the surface. To use these intervals (called windows) a good knowledge of atmospheric absorption is necessary. Once gas spectroscopic linelists are well established, the absorption inside windows depends on the way the far wings of the methane absorption lines are cut-off. We know that the intensity in all the windows can be explained with the same cut-off parameters, except for the window at 2 μm. This discrepancy is generally treated with a workaround which consists in using a different cut-off description for this specific window. This window is relatively transparent and surface may have specific spectral signatures that could be detected. Thus, a good knowledge of atmosphere opacities is essential and our scope is to better understand what causes the difference between the 2 μm window and the other windows. In this work, we used scattered light at the limb and transmissions in occultation observed with VIMS (Visual Infrared Mapping Spectrometer) onboard Cassini, around the 2 μm window. Data shows an absorption feature that participates to the shape of this window. Our atmospheric model fits well the VIMS data at 2 μm with the same cut-off than for the other windows, provided an additional absorption is introduced in the middle of the window around ~2.065 μm. It explains well the discrepancy between the cut-off used at 2 μm, and we show that a gas with a fairly constant mixing ratio, possibly ethane, may be the cause of this absorption. Finally, we studied the impact of this absorption on the retrieval of the surface reflectivity and found that it is significant.
The Cassini-Huygens mission measured the chemical abundances of the major components of Titan's atmosphere, and analyses of the data revealed several as-yet unexplained anomalies in the methane and hydrogen profiles. We model the deceleration and ablation of meteors in Titan's atmosphere to examine whether meteor energy deposition could explain, in part, two of these anomalies. Our simulations vary meteor entry mass, trajectory angle, and velocity, and follow changes in all three as our meteors descend into a realistic Titan atmosphere. For the smallest particles, which deliver the most mass a…
The Potential Importance of Non-Local, Deep Transport on the Energetics, Momentum, Chemistry, and Aerosol Distributions in the Atmospheres of Earth, Mars and Titan
Titan harbors a dense, organic-rich atmosphere primarily composed of N$_2$ and CH$_4$, with lesser amounts of hydrocarbons and nitrogen-bearing species. As a result of high sensitivity observations by the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 6 ($\sim$230-272 GHz), we obtained the first spectroscopic detection of CH$_3$C$_3$N (methylcyanoacetylene or cyanopropyne) in Titan's atmosphere through the observation of seven transitions in the $J = 64\rightarrow63$ and $J = 62\rightarrow61$ rotational bands. The presence of CH$_3$C$_3$N on Titan was suggested by the Cassini Ion …
Seasonal variation is significant in Titan's atmosphere due to the large change of solar insolation resulting from Titan's 26.7° axial tilt relative to the plane of Saturn's orbit. Here we present an investigation of hydrocarbon and nitrile species in Titan's upper atmosphere at 400-1200 km, which includes the mesosphere and the lower thermosphere, over more than one fourth of Titan's year (2006-2014, LS=318°-60°), using eighteen stellar occultation observations obtained by Cassini/UVIS. Vertical profiles of eight chemical species (CH4, C2H2, C2H4, C2H6, C4H2, C6H6, HCN, HC3N) and haze particl…
Experimental study of heterogeneous organic chemistry induced by far ultraviolet light: Implications for growth of organic aerosols by CH3 addition in the atmospheres of Titan and early Earth
Pluto, Titan, and Triton make up a unique class of solar system bodies, with icy surfaces and chemically reducing atmospheres rich in organic photochemistry and haze formation. Hazes play important roles in these atmospheres, with physical and chemical processes highly dependent on particle sizes, but the haze size distribution in reducing atmospheres is currently poorly understood. Here we report observational evidence that Pluto's haze particles are bimodally distributed, which successfully reproduces the full phase scattering observations from New Horizons. Combined with previous simulation…
A new astrobiological model of the atmosphere of Titan
我们目前的结果的调查形成的含氮分子在土卫六的大气。我们扩展了以前的模型（Li et al. 2015，2016），以覆盖对流层顶以下的区域，因此新模型将土卫六表面的大气层处理到1500公里的高度。我们考虑冷凝和升华的影响，使用连续的，数值稳定的方法。这与气溶胶及其冷凝物的沉降和霾粒子的形成的参数化处理相结合。这些过程影响较重的物种，如含氮分子的丰度，但对较轻分子的丰度影响较小。去除分子以形成气溶胶也在确定混合比中起作用，特别是HNC、HC3N和HCN的混合比。我们发现很好的协议与最近检测到的混合比C2H5CN，与冷凝起着重要的作用，在确定该分子的丰度低于500公里。特别令人感兴趣的是丙烯腈（C2H3CN）的化学性质，Stevenson等人（2015年）提出丙烯腈是一种可以在缺氧环境中形成生物膜的分子。随着烟雾形成的列入，我们发现我们的模型预测丙烯腈与现有的观测结果很好的协议。
Amino acid precursors from a simulated lower atmosphere of Titan: Experiments of cosmic ray energy source with 13 C- and 18 O-stable isotope probing mass spectrometry
土星卫星土卫六周围的气溶胶的有机烟雾以前已经通过观测和实验室模拟实验进行了研究。在这里，我们报告的非生物形成的氨基酸前体在复杂的有机分子在实验模拟土卫六的表面附近的环境与质子照射。在600摄氏度的质子辐照下，在模拟的土卫六大气中形成的有机分子热解产生了含有HCN和NH3的化合物。这些实验结果与惠更斯号探测器对土卫六采集的样品进行热解气相色谱-质谱联用（热解GC-MS）得到的分子信息一致。辐射产物的扫描电子显微镜（SEM）和三维原子力显微镜（AFM）图像显示复杂的无定形结构（约1000 Da）中的纳米级细丝和小球。基质辅助激光解吸电离飞行时间质谱（MALDI-TOF-MS）的同位素探测实验表明，通过水解18 O标记的水，氧原子被掺入到外消旋氨基酸中。我们认为，氨基酸前体可能是在水解后形成的，正如以前的观察研究（C。A.格里菲斯，T。Owen，T. R. Geballe，J. Rayner和P. Rannou，Science，2003，300，628）。我们认为，宇宙射线是一个重要的和有效的能源生产复杂的有机物和氨基酸前体土卫六的大气雾霾。
We apply a 1D upper atmosphere model to study thermal escape of nitrogen over Titan's history. Significant thermal escape should have occurred very early for solar EUV fluxes 100 to 400 times higher than today with escape rates as high as $\approx 1.5\times 10^{28}$ s$^{-1}$ and $\approx 4.5\times 10^{29}$ s$^{-1}$, respectively, while today it is $\approx 7.5\times 10^{17}$ s$^{-1}$. Depending on whether the Sun originated as a slow, moderate or fast rotator, thermal escape was the dominant escape process for the first 100 to 1000 Myr after the formation of the solar system. If Titan's atmosp…
Annular modes explain much of the internal variability of Earth's atmosphere but have never been identified on other planets. Using reanalyses for Mars and a simulation for Titan, we demonstrate that annular modes are prominent in the atmospheres of both worlds, explaining large fractions of their respective variabilities. One mode describes latitudinal shifts of the jet on Mars, as on Earth, and vertical shifts of the jet on Titan. Another describes pulses of midlatitude eddy kinetic energy on all three worlds, albeit with somewhat different characteristics. We further demonstrate that this l…
Chemistry in Titan's N2-CH4 atmosphere produces complex organic aerosols. The chemical processes and the resulting organic compounds are still far from understood, although extensive observations, laboratory, and theoretical simulations have greatly improved physical and chemical constraints on Titan's atmosphere. Here, we conduct a series of Titan atmosphere simulation experiments with N2-CH4 gas mixtures and investigate the effect of initial CH4 ratio, pressure, and flow rate on the production rates and composition of the gas and solid products at a Titan relevant temperature (100 K) for the…
This study presents a 13 years survey of haze UV extinction profiles, monitoring the temporal evolution of the detached haze layer (DHL) in Titan's upper atmosphere (350-600 km). As reported by West et al. 2011 (GRL vol.38, L06204) at the equator, we show that the DHL is present at all latitudes below 55°N during the northern winter (2004-2009). Then, it globally sunk and disappeared in 2012. No permanent DHL was observed between 2012 and 2015. It's only in late-2015, that a new structure emerged from the Northern hemisphere and propagated to the equator. This new DHL is not as pronounced as i…
Titan's atmosphere is a natural laboratory for exploring the photochemical synthesis of organic molecules. Significant recent advances in the study of the atmosphere of Titan include: (a) detection of C$_3$ molecules: C$_3$H$_6$, CH$_2$CCH$_2$, c-C$_3$H$_2$, and (b) retrieval of C$_6$H$_6$, which is formed primarily via C$_3$ chemistry, from Cassini-UVIS data. The detection of $c$-C$_3$H$_2$ is of particular significance since ring molecules are of great astrobiological importance. Using the Caltech/JPL KINETICS code, along with the best available photochemical rate coefficients and parameteri…
Piercing through the Haze: Characterizing Titan-like terrestrial exoplanets with JWST
Johns Hopkins Applied Physics Laboratory（约翰斯·霍普金斯大学应用物理实验室）；University of Wyoming, School of Computing（怀俄明大学计算机学院）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；NASA Ames Research Center（美国宇航局艾姆斯研究中心）；Bay Area Environmental Research Institute（湾区环境研究所）
Titan's thick atmosphere is primarily composed of nitrogen and methane. Complex chemistry happening in Titan's atmosphere produces optically thick organic hazes. These hazes play significant roles in Titan's atmosphere and on its surface, and their optical properties are crucial for understanding many processes happening on Titan. Due to the lack of such information, the optical constants of laboratory prepared Titan haze analogues are essential inputs for atmospheric modeling and data analysis of remote sensing observations of Titan. Here, we perform laboratory simulations in a Titan relevant…
The TRAPPIST-1 system is home to at least seven terrestrial planets and is a target of interest for future James Webb Space Telescope (JWST) observations. Additionally, these planets will be of interest to future missions making observations in the ultraviolet (UV). Although several of these planets are located in the traditional habitable zone, where liquid water could exist on the surface, TRAPPIST-1h is interesting to explore as a potentially habitable ocean world analog. In this study, we evaluate the observability of a Titan-like atmosphere on TRAPPIST-1h. The ability of the JWST or a fut…
Context: Saturn's massive gravity is expected to causes a tide in Titan's atmosphere, producing a surface pressure variation through the orbit of Titan and tidal winds in the troposphere. The future Dragonfly mission could analyse this exotic meteorological phenomenon. Aims: We analyse the effect of Saturn's tides on Titan's atmosphere and interior to determine how pressure measurements by Dragonfly could constrain Titan's interior. Methods: We model atmospheric tides with analytical calculations and with a 3D Global Climate Model (the IPSL-Titan GCM), including the tidal response of the inter…
