Titan: Earth-like on the Outside, Ocean World on the Inside
Shannon M. MacKenzie, Samuel P. D. Birch, Sarah Horst, Christophe Sotin, Erika Barth, Juan M. Lora, Melissa G. Trainer, Paul Corlies, Michael J. Malaska, Ella S… 展开作者
Shannon M. MacKenzie, Samuel P. D. Birch, Sarah Horst, Christophe Sotin, Erika Barth, Juan M. Lora, Melissa G. Trainer, Paul Corlies, Michael J. Malaska, Ella Sciamma-O'Brien, Alexander E. Thelen, Elizabeth P. Turtle, Jani Radebaugh, Jennifer Hanley, Anezina Solomonidou, Claire Newman, Leonardo Regoli, Sebastien Rodriguez, Benoit Seignovert, Alexander G. Hayes, Baptiste Journaux, Jordan Steckloff, Delphine Nna-Mvondo, Thomas Cornet, Maureen Palmer, Rosaly M. C. Lopes, Sandrine Vinatier, Ralph Lorenz, Conor Nixon, Ellen Czaplinski, Jason W. Barnes, Ed Sittler, Andrew Coates
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Submitted to the PSJ Focus Issue on Ocean World Exploration
Thanks to the Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini-Huygens revolutionized our understanding of each of the three layers of Titan--the atmosphere, the surface, and the interior--we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system.
Schumann Resonances as a tool to constrain the depth of Titan's buried water ocean: Re-assessment of Huygens observations and preparation of the EFIELD/Dragonfly experiment
During the accretion of Titan, impact heating may have been sufficient to allow the global melting of water ice and the release of volatile compounds, mainly constituted of CO2, CH4 and NH3. The duration and efficiency of exchange between the primitive massive atmosphere and the global impact-induced water ocean likely play a key role in the chemical evolution of the early Titan's atmosphere. To investigate the atmospheric composition of early Titan for a wide range of global (atmosphere + ocean) composition in volatils, we first developed a gas-liquid equilibrium model of the NH3-CO2-H2O syst…
The dynamic quadrupole Love number of Titan measured by \Cassini is $k_\mathrm{2,obs}=0.616\pm 0.067$, strongly indicating a global subsurface ocean. However, the theoretical Love number due to equilibrium tides is at most $k_\mathrm{2,eq}^\mathrm{max}\approx 0.48$ in the absence of an ice shell on top of the ocean. In reality, there is an outer ice shell of thickness $ 100\,\mathrm{km}$, reducing the equilibrium-tide Love number to $k_\mathrm{2,eq}\approx 0.42$. Therefore, other types of tidal response, like dynamic tides, may be also present in Titan. We propose that the ocean is stably stra…
Librational response of a deformed 3-layer Titan perturbed by non-keplerian orbit and atmospheric couplings
卡西尼太空任务获得的土卫六重力场分析表明，在其冰冷的表面下存在内部海洋。冰壳和海洋的地球物理参数的表征是重要的约束泰坦的演化模型。振动的知识，即围绕均匀旋转运动的周期性振荡，可以带来关于内部参数的信息。本文的目的是研究天卫六在经度上的振动响应，从分析方法组成的深层大气，弹性冰壳，内部海洋，和弹性岩石核心与土星的引力相互作用扰动。我们从刚体卫星同步自旋轨道共振的振动方程出发。我们明确地介绍了由Titan IPSL GCM（Institut Pierre Simon Laplace General Circulation Model）计算的作用在表面上的大气扭矩和由于潮汐引起的弹性固体层的周期性变形。为了比较和识别地球物理参数的影响和弹性的影响，我们调查了各种内部模型的librational响应。主要的天平动出现在两个分开的强迫频率范围：低强迫频率占主导地位的土星的年度和半年的频率，和高强迫频率制度占主导地位的土卫六的轨道频率在土星周围。我们发现，内部结构模型，包括一个内部的海洋弹性固体层导致相同的顺序的天平动振幅比无海洋的模型，这使得更具有挑战性，以区分它们的解释的天平动运动。
Methanol is a potentially important impurity in subsurface oceans on Titan and Enceladus. We report measurements of the freezing of methanol-water samples at pressures up to 350~MPa using a volumetric cell with sapphire windows. For low concentrations of methanol, the liquidus temperature is typically a few degrees below the corresponding ice freezing point, while at high concentrations it follows the pure methanol trend. In the Ice-III regime, we observe several long-lived metastable states. The results suggest that methanol is a more effective antifreeze than previously estimated, and might …
Seismology was developed on Earth and shaped our model of the Earth's interior over the 20th century. With the exception of the Philae lander, all in situ extraterrestrial seismological effort to date was limited to other terrestrial planets. All have in common a rigid crust above a solid mantle. The coming years may see the installation of seismometers on Europa, Titan and Enceladus, so it is necessary to adapt seismological concepts to the setting of worlds with global oceans covered in ice. Here we use waveform analyses to identify and classify wave types, developing a lexicon for icy ocean…
Terrestrial Analogs to Titan for Geophysical Research
Brown University（布朗大学）；University of Edinburgh（爱丁堡大学）；University of Maryland, Baltimore County（马里兰大学巴尔的摩县分校）；Nantes Université, Univ. Angers, CNRS（南特大学、昂热大学、法国国家科学研究中心）；Blue Marble Space Institute of Science（蓝大理石空间科学研究所）；Southwest Research Institute（西南研究院）；Hellenic Space Center（希腊航天中心）；University of Bristol（布里斯托大学）
Under both engineering and natural conditions on Earth and in the Universe, some gas hydrates are found to be stabilised outside their window of thermodynamic stability by the formation of an ice layer-a phenomenon termed self-preservation. Low occupancy surface regions on type I CO2 clathrate structures together with the self-preserving ice layer lead to an effective buoyancy for these structures which restricts the size range of particles that float in the ocean on Enceladus, Pluto and similar oceanic worlds. Our goal here is to investigate the implications of Lifshitz forces and low occupan…
A framework for evaluating biosignature potential against the abiotic baseline on ocean worlds
Harvard University（哈佛大学）；University of Toronto（多伦多大学）；University of Ottawa（渥太华大学）；University of Washington（华盛顿大学）；Massachusetts Institute of Technology（麻省理工学院）；UK Centre for Astrobiology, University of Edinburgh（爱丁堡大学英国天体生物学中心）；Institut de Physique du Globe de Paris (IPGP), Université Paris Cité（巴黎地球物理研究所（IPGP），巴黎西岱大学）
In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and i…
The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus' five largest 'classical' moons Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon Puck, but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved datasets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, ou…
Some of the major discoveries of the recent Cassini-Huygens mission have put Titan and Enceladus firmly on the Solar System map. The mission has revolutionised our view of Solar System satellites, arguably matching their scientific importance with that of their planet. While Cassini-Huygens has made big surprises in revealing Titan's organically rich environment and Enceladus' cryovolcanism, the mission's success naturally leads us to further probe these findings. We advocate the acknowledgement of Titan and Enceladus science as highly relevant to ESA's long-term roadmap, as logical follow-on …
The presence of a liquid solvent is widely regarded as an essential prerequisite for habitability. We investigate the conditions under which worlds outside the habitable zones of stars are capable of supporting liquid solvents on their surface over geologically significant timescales via combined radiogenic and primordial heat. Our analysis suggests that super-Earths with radionuclide abundances that are $\gtrsim 10^3$ times higher than Earth can host long-lived water oceans. In contrast, the requirements for long-lived ethane oceans, which have been explored in the context of alternative bioc…
The D/H ratio in cometary water has been shown to vary between 1 and 3 times the Earth's oceans value, in both Oort cloud comets and Jupiter-family comets originating from the Kuiper belt. We present new sensitive spectroscopic observations of water isotopologues in the Jupiter-family comet 46P/Wirtanen carried out using the GREAT spectrometer aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The derived D/H ratio of $(1.61 \pm 0.65) \times 10^{-4}$ is the same as in the Earth's oceans. Although the statistics are limited, we show that interesting trends are already becoming…
Temperate terrestrial planets orbiting low-mass stars are subject to strong tidal forces. The effects of gravitational tides on the solid planet and that of atmospheric thermal tides have been studied, but the direct impact of gravitational tides on the atmosphere itself has so far been ignored. We first develop a simplified analytic theory of tides acting on the atmosphere of a planet. We then implement gravitational tides into a general circulation model of a static-ocean planet in a short-period orbit around a low-mass star -- the results agree with our analytic theory. Because atmospheric …
The Earth viewed from outside the Solar system would be identified merely like a pale blue dot, as coined by Carl Sagan. In order to detect possible signatures of the presence of life on a second earth among several terrestrial planets discovered in a habit-able zone, one has to develop and establish a methodology to characterize the planet as something beyond a mere pale blue dot. We pay particular attention to the periodic change of the color of the dot according to the rotation of the planet. Because of the large-scale inhomogeneous distribution of the planetary surface, the reflected light…
