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Nixon CA. The Composition and Chemistry of Titan's Atmosphere. ACS EARTH & SPACE CHEMISTRY 2024; 8:406-456. [PMID: 38533193 PMCID: PMC10961852 DOI: 10.1021/acsearthspacechem.2c00041] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/07/2022] [Revised: 11/02/2023] [Accepted: 02/02/2024] [Indexed: 03/28/2024]
Abstract
In this review I summarize the current state of knowledge about the composition of Titan's atmosphere and our current understanding of the suggested chemistry that leads to that observed composition. I begin with our present knowledge of the atmospheric composition, garnered from a variety of measurements including Cassini-Huygens, the Atacama Large Millimeter/submillimeter Array, and other ground- and space-based telescopes. This review focuses on the typical vertical profiles of gases at low latitudes rather than global and temporal variations. The main body of the review presents a chemical description of how complex molecules are believed to arise from simpler species, considering all known "stable" molecules-those that have been uniquely identified in the neutral atmosphere. The last section of the review is devoted to the gaps in our present knowledge of Titan's chemical composition and how further work may fill those gaps.
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Affiliation(s)
- Conor A. Nixon
- Planetary Systems Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, United
States
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2
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Proton and Electron Irradiations of CH4:H2O Mixed Ices. ATOMS 2023. [DOI: 10.3390/atoms11020019] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
The organic chemistry occurring in interstellar environments may lead to the production of complex molecules that are relevant to the emergence of life. Therefore, in order to understand the origins of life itself, it is necessary to probe the chemistry of carbon-bearing molecules under conditions that simulate interstellar space. Several of these regions, such as dense molecular cores, are exposed to ionizing radiation in the form of galactic cosmic rays, which may act as an important driver of molecular destruction and synthesis. In this paper, we report the results of a comparative and systematic study of the irradiation of CH4:H2O ice mixtures by 1 MeV protons and 2 keV electrons at 20 K. We demonstrate that our irradiations result in the formation of a number of new products, including both simple and complex daughter molecules such as C2H6, C3H8, C2H2, CH3OH, CO, CO2, and probably also H2CO. A comparison of the different irradiation regimes has also revealed that proton irradiation resulted in a greater abundance of radiolytic daughter molecules compared to electron irradiation, despite a lower radiation dose having been administered. These results are important in the context of the radiation astrochemistry occurring within the molecular cores of dense interstellar clouds, as well as on outer Solar System objects.
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3
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Taguchi K, Gilbert A, Sherwood Lollar B, Giunta T, Boreham CJ, Liu Q, Horita J, Ueno Y. Low 13C- 13C abundances in abiotic ethane. Nat Commun 2022; 13:5790. [PMID: 36184637 PMCID: PMC9527245 DOI: 10.1038/s41467-022-33538-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2022] [Accepted: 09/22/2022] [Indexed: 11/09/2022] Open
Abstract
Distinguishing biotic compounds from abiotic ones is important in resource geology, biogeochemistry, and the search for life in the universe. Stable isotopes have traditionally been used to discriminate the origins of organic materials, with particular focus on hydrocarbons. However, despite extensive efforts, unequivocal distinction of abiotic hydrocarbons remains challenging. Recent development of clumped-isotope analysis provides more robust information because it is independent of the stable isotopic composition of the starting material. Here, we report data from a 13C-13C clumped-isotope analysis of ethane and demonstrate that the abiotically-synthesized ethane shows distinctively low 13C-13C abundances compared to thermogenic ethane. A collision frequency model predicts the observed low 13C-13C abundances (anti-clumping) in ethane produced from methyl radical recombination. In contrast, thermogenic ethane presumably exhibits near stochastic 13C-13C distribution inherited from the biological precursor, which undergoes C-C bond cleavage/recombination during metabolism. Further, we find an exceptionally high 13C-13C signature in ethane remaining after microbial oxidation. In summary, the approach distinguishes between thermogenic, microbially altered, and abiotic hydrocarbons. The 13C-13C signature can provide an important step forward for discrimination of the origin of organic molecules on Earth and in extra-terrestrial environments.
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Affiliation(s)
- Koudai Taguchi
- Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551, Japan.
| | - Alexis Gilbert
- Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551, Japan. .,Earth-Life Science Institute (WPI-ELSI), Tokyo Institute of Technology, Meguro, Tokyo, 152-8550, Japan.
| | - Barbara Sherwood Lollar
- Department of Earth Sciences, University of Toronto, Toronto, ON, M5S 3B1, Canada.,Institut de physique du globe de Paris (IPGP), Université Paris Cité, Paris, France
| | - Thomas Giunta
- Department of Earth Sciences, University of Toronto, Toronto, ON, M5S 3B1, Canada.,Univ Brest, CNRS, Ifremer, Geo-Ocean, F-29280, Plouzané, France
| | | | - Qi Liu
- State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China
| | - Juske Horita
- Department of Geosciences, Texas Tech University, Lubbock, TX, 79409, USA
| | - Yuichiro Ueno
- Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551, Japan. .,Earth-Life Science Institute (WPI-ELSI), Tokyo Institute of Technology, Meguro, Tokyo, 152-8550, Japan. .,Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima-cho, Yokosuka, 237-0061, Japan.
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4
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Steckloff JK, Soderblom JM, Farnsworth KK, Chevrier VF, Hanley J, Soto A, Groven JJ, Grundy WM, Pearce LA, Tegler SC, Engle A. Stratification Dynamics of Titan's Lakes via Methane Evaporation. THE PLANETARY SCIENCE JOURNAL 2020; 1:26. [PMID: 32905475 PMCID: PMC7473120 DOI: 10.3847/psj/ab974e] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Saturn's moon Titan is the only extraterrestrial body known to host stable lakes and a hydrological cycle. Titan's lakes predominantly contain liquid methane, ethane, and nitrogen, with methane evaporation driving its hydrological cycle. Molecular interactions between these three species lead to non-ideal behavior that causes Titan's lakes to behave differently than Earth's lakes. Here, we numerically investigate how methane evaporation and non-ideal interactions affect the physical properties, structure, dynamics, and evolution of shallow lakes on Titan. We find that, under certain temperature regimes, methane-rich mixtures are denser than relatively ethane-rich mixtures. This allows methane evaporation to stratify Titan's lakes into ethane-rich upper layers and methane-rich lower layers, separated by a strong compositional gradient. At temperatures above 86K, lakes remain well-mixed and unstratified. Between 84 and 86K, lakes can stratify episodically. Below 84K, lakes permanently stratify, and develop very methane-depleted epilimnia. Despite small seasonal and diurnal deviations (<5K) from typical surface temperatures, Titan's rain-filled ephemeral lakes and "phantom lakes" may nevertheless experience significantly larger temperature fluctuations, resulting in polymictic or even meromictic stratification, which may trigger ethane ice precipitation.
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Affiliation(s)
- Jordan K Steckloff
- Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 77 Massachusetts Ave, Cambridge, MA 02139
- Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719
- University of Texas at Austin, Department of Aerospace Engineering and Engineering Mechanics, Aerospace Engineering (ASE) Building 2617 Wichita Street, C0600, Austin, Texas 78712
| | - Jason M Soderblom
- Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 77 Massachusetts Ave, Cambridge, MA 02139
| | - Kendra K Farnsworth
- University of Arkansas, Arkansas Center for Space and Planetary Sciences, F47 Stone House North, Fayetteville, AR 72701
| | - Vincent F Chevrier
- University of Arkansas, Arkansas Center for Space and Planetary Sciences, F47 Stone House North, Fayetteville, AR 72701
| | - Jennifer Hanley
- Lowell Observatory, 1400 W Mars Hill Rd, Flagstaff, AZ 86001
- Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011
| | - Alejandro Soto
- Southwest Research Institute, 1050 Walnut St. Suite 300, Boulder, CO 80302
| | - Jessica J Groven
- Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011
- Washington State University, Institute for Shock Physics, Pullman WA 99164
| | - William M Grundy
- Lowell Observatory, 1400 W Mars Hill Rd, Flagstaff, AZ 86001
- Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011
| | - Logan A Pearce
- University of Texas at Austin, Department of Aerospace Engineering and Engineering Mechanics, Aerospace Engineering (ASE) Building 2617 Wichita Street, C0600, Austin, Texas 78712
- Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011
- Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721
| | - Stephen C Tegler
- Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011
| | - Anna Engle
- Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011
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5
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Dimitrov V. Kinetic Modelling of Global Evolution of Titan's Atmosphere. PROGRESS IN REACTION KINETICS AND MECHANISM 2019. [DOI: 10.3184/007967405779134001] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Methane CH4 is the only highly reactive and short-lived background component in Titan's atmosphere, so its overall reserve predetermines both features and duration of atmospheric chemical activity. Current methane atmospheric abundance is provided by its global circulation. There are two sources of methane replenishment, i.e. recycling of the primordial reserve trapped in Titan's interior and reconversion of non-saturated final products of the atmospheric photochemical process, reconversion being the minor constituent in the global methane balance. The total bulk of primordial methane gas hydrate depends on the packing index (cage-filling efficiency) α, the latter being limited to 7.2 × 10−4< α < 5 × 10−2 {kg CH4/kg clathrate}. The specification of α seems to be one of the most relevant problems of the experimental modelling of Titan's chemistry. The total number of methane renewal cycles so far equals Np ∼ 200.
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Affiliation(s)
- Vasili Dimitrov
- Department of Geophysics and Planetary Sciences, Tel-Aviv University, Tel-Aviv, 69978, Israel
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6
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Radebaugh J, Ventra D, Lorenz RD, Farr T, Kirk R, Hayes A, Malaska MJ, Birch S, Liu ZYC, Lunine J, Barnes J, Le Gall A, Lopes R, Stofan E, Wall S, Paillou P. Alluvial and fluvial fans on Saturn's moon Titan reveal processes, materials and regional geology. ACTA ACUST UNITED AC 2016. [DOI: 10.1144/sp440.6] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
Abstract
AbstractFans, landforms that record the storage and transport of sediment from uplands to depositional basins, are found on Saturn's moon Titan, a body of significantly different process rates and material compositions from Earth. Images obtained by the Cassini spacecraft's synthetic aperture radar reveal morphologies, roughness, textural patterns and other properties consistent with fan analogues on Earth also viewed by synthetic aperture radar. The observed fan characteristics on Titan reveal some regions of high relative relief and others with gentle slopes over hundreds of kilometres, exposing topographic variations and influences on fan formation. There is evidence for a range of particle sizes across proximal to distal fan regions, from c. 2 cm or more to fine-grained, which can provide details on sedimentary processes. Some features are best described as alluvial fans, which implies their proximity to high-relief source areas, while others are more likely to be fluvial fans, drawing from larger catchment areas and frequently characterized by more prolonged runoff events. The presence of fans corroborates the vast liquid storage capacity of the atmosphere and the resultant episodic behaviour. Fans join the growing list of landforms on Titan derived from atmospheric and fluvial processes similar to those on Earth, strengthening comparisons between these two planetary bodies.
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Affiliation(s)
- Jani Radebaugh
- Brigham Young University, S-389 ESC, Provo, UT 84601, USA
| | | | - Ralph D. Lorenz
- Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
| | - Tom Farr
- NASA Jet Propulsion Laboratory, Pasadena, CA 91109, USA
| | - Randy Kirk
- US Geological Survey, Astrogeology Division, Flagstaff AZ 86001, USA
| | - Alex Hayes
- Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
| | | | - Sam Birch
- Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
| | - Zac Yung-Chun Liu
- School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
| | - Jonathan Lunine
- Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
| | - Jason Barnes
- Department of Physics, University of Idaho, Moscow, ID 83844, USA
| | - Alice Le Gall
- LATMOS Observatoire de Versailles Saint-Quentin-en-Yvelines (OVSQ), Paris, France
| | - Rosaly Lopes
- NASA Jet Propulsion Laboratory, Pasadena, CA 91109, USA
| | | | - Steve Wall
- NASA Jet Propulsion Laboratory, Pasadena, CA 91109, USA
| | - Philippe Paillou
- Observatoire Aquitain des Sciences de l'Univers, Universite de Bordeaux, Floirac, France
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7
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Autonomous real-time landing site selection for Venus and Titan using Evolutionary Fuzzy Cognitive Maps. Appl Soft Comput 2012. [DOI: 10.1016/j.asoc.2012.01.014] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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8
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Cornet T, Bourgeois O, Le Mouélic S, Rodriguez S, Sotin C, Barnes JW, Brown RH, Baines KH, Buratti BJ, Clark RN, Nicholson PD. Edge detection applied to Cassini images reveals no measurable displacement of Ontario Lacus' margin between 2005 and 2010. ACTA ACUST UNITED AC 2012. [DOI: 10.1029/2012je004073] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
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9
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Schulze-Makuch D, Méndez A, Fairén AG, von Paris P, Turse C, Boyer G, Davila AF, António MRDS, Catling D, Irwin LN. A two-tiered approach to assessing the habitability of exoplanets. ASTROBIOLOGY 2011; 11:1041-1052. [PMID: 22017274 DOI: 10.1089/ast.2010.0592] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability. The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time. The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature. For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions. As such, the PHI requires more detailed knowledge than is available for any exoplanet at this time. However, future missions such as the Terrestrial Planet Finder will collect this information and advance the PHI. Both indices are formulated in a way that enables their values to be updated as technology and our knowledge about habitable planets, moons, and life advances. Applying the proposed metrics to bodies within our Solar System for comparison reveals two planets in the Gliese 581 system, GJ 581 c and d, with an ESI comparable to that of Mars and a PHI between that of Europa and Enceladus.
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Affiliation(s)
- Dirk Schulze-Makuch
- School of Earth and Environmental Sciences, Washington State University, Pullman, Washington, USA.
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10
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Schulze-Makuch D, Haque S, de Sousa Antonio MR, Ali D, Hosein R, Song YC, Yang J, Zaikova E, Beckles DM, Guinan E, Lehto HJ, Hallam SJ. Microbial life in a liquid asphalt desert. ASTROBIOLOGY 2011; 11:241-258. [PMID: 21480792 DOI: 10.1089/ast.2010.0488] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Pitch Lake in Trinidad and Tobago is a natural asphalt reservoir nourished by pitch seepage, a form of petroleum that consists of mostly asphaltines, from the surrounding oil-rich region. During upward seepage, pitch mixes with mud and gases under high pressure, and the lighter portion evaporates or is volatilized, which produces a liquid asphalt residue characterized by low water activity, recalcitrant carbon substrates, and noxious chemical compounds. An active microbial community of archaea and bacteria, many of them novel strains (particularly from the new Tar ARC groups), totaling a biomass of up to 10(7) cells per gram, was found to inhabit the liquid hydrocarbon matrix of Pitch Lake. Geochemical and molecular taxonomic approaches revealed diverse, novel, and deeply branching microbial lineages with the potential to mediate anaerobic hydrocarbon degradation processes in different parts of the asphalt column. In addition, we found markers for archaeal methane metabolism and specific gene sequences affiliated with facultative and obligate anaerobic sulfur- and nitrite-oxidizing bacteria. The microbial diversity at Pitch Lake was found to be unique when compared to microbial communities analyzed at other hydrocarbon-rich environments, which included Rancho Le Brea, a natural asphalt environment in California, USA, and an oil well and a mud volcano in Trinidad and Tobago, among other sites. These results open a window into the microbial ecology and biogeochemistry of recalcitrant hydrocarbon matrices and establish the site as a terrestrial analogue for modeling the biotic potential of hydrocarbon lakes such as those found on Saturn's largest moon Titan.
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Affiliation(s)
- Dirk Schulze-Makuch
- School of Earth and Environmental Sciences, Washington State University, Pullman, 99164-6376, USA.
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11
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Hayes AG, Wolf AS, Aharonson O, Zebker H, Lorenz R, Kirk RL, Paillou P, Lunine J, Wye L, Callahan P, Wall S, Elachi C. Bathymetry and absorptivity of Titan's Ontario Lacus. ACTA ACUST UNITED AC 2010. [DOI: 10.1029/2009je003557] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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12
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Wilson EH, Atreya SK. Titan’s Carbon Budget and the Case of the Missing Ethane. J Phys Chem A 2009; 113:11221-6. [DOI: 10.1021/jp905535a] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Eric H. Wilson
- Jet Propulsion Laboratory, 4800 Oak Grove Drive M/S 169-237, Pasadena, California 91109-8099, Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143
| | - Sushil K. Atreya
- Jet Propulsion Laboratory, 4800 Oak Grove Drive M/S 169-237, Pasadena, California 91109-8099, Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan 48109-2143
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13
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Tokano T. Limnological structure of Titan's hydrocarbon lakes and its astrobiological implication. ASTROBIOLOGY 2009; 9:147-164. [PMID: 19371158 DOI: 10.1089/ast.2007.0220] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
Cassini radar recently detected several putative liquid hydrocarbon lakes in the polar region of Saturn's moon Titan. Such lakes may contain organic sediments deposited from the atmosphere that would promote prebiotic-type chemistry driven by cosmic rays, the result of which could be the production of more complex molecules such as nitrogen-bearing organic polymer or azides. The physical properties of the lake and their temporal evolution under Titan's present climatic setting were investigated by means of a one-dimensional lake thermal stratification model. Lakes can undergo various evolutions, depending on the initial composition and depth of the lake and hydrocarbon abundance in the near-surface atmosphere. Pure methane ponds, which may occasionally form when heavy methane hailstones reach the surface, would be transitory in that they would evaporate, freeze up, and eventually dry up. On the other hand, lakes filled with a mixture of methane, ethane, and nitrogen would be more stable; and freezing or drying would not necessarily occur in most cases. Such lakes undergo a seasonal cycle of thermal stratification in spring and early summer and convective overturning in other seasons. The summer thermal stratification near the lake surface could be destabilized by bottom heating as a result of an enhanced geothermal heat flux, e.g., in the vicinity of cryovolcanoes. Most likely the composition of the lake and atmosphere would come to equilibrium by way of a small amount of evaporation, but the lake-atmosphere system could be repeatedly brought out of equilibrium by irregular precipitation. The viability of prebiotic-like chemistry in the lake may depend on many lake parameters, such as temperature, liquid or frozen state, and convective mixing. Moreover, convective mixing may drive suspension of solid acetylene and other sediments on the lake bottom and redistribution of dissolved gases, which might be relevant for putative life-forms that consume hydrogen and solid acetylene.
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Affiliation(s)
- Tetsuya Tokano
- Institut für Geophysik und Meteorologie, Universität zu Köln, Köln, Germany.
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14
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Griffith CA. Storms, polar deposits and the methane cycle in Titan's atmosphere. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2009; 367:713-728. [PMID: 19073459 DOI: 10.1098/rsta.2008.0245] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
In Titan's atmosphere, the second most abundant constituent, methane, exists as a gas, liquid and solid, and cycles between the atmosphere and the surface. Similar to the Earth's hydrological cycle, Titan sports clouds, rain and lakes. Yet, Titan's cycle differs dramatically from its terrestrial counterpart, and reveals the workings of weather in an atmosphere that is 10 times thicker than the Earth's atmosphere, that is two orders of magnitude less illuminated, and that involves a different condensable. While ongoing measurements by the Cassini-Huygens mission are revealing the intricacies of the moon's weather, circulation, lake coverage and geology, knowledge is still limited by the paucity of observations. This review of Titan's methane cycle therefore focuses on measured characteristics of the lower atmosphere and surface that appear particularly perplexing or alien.
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Affiliation(s)
- Caitlin Ann Griffith
- Department of Planetary Sciences, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85721, USA.
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15
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Tobie G, Choukroun M, Grasset O, Le Mouélic S, Lunine JI, Sotin C, Bourgeois O, Gautier D, Hirtzig M, Lebonnois S, Le Corre L. Evolution of Titan and implications for its hydrocarbon cycle. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2009; 367:617-631. [PMID: 19073458 DOI: 10.1098/rsta.2008.0246] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
Measurements of the carbon and nitrogen isotopic ratios as well as the detection of 40Ar and 36Ar by the gas chromatograph mass spectrometer (GCMS) instrument on board the Huygens probe have provided key constraints on the origin and evolution of Titan's atmosphere, and indirectly on the evolution of its interior. Those data combined with models of Titan's interior can be used to determine the story of volatile outgassing since Titan's formation. In the absence of an internal source, methane, which is irreversibly photodissociated in Titan's stratosphere, should be removed entirely from the atmosphere in a time-span of a few tens of millions of years. The episodic destabilization of methane clathrate reservoir stored within Titan's crust and subsequent methane outgassing could explain the present atmospheric abundance of methane, as well as the presence of argon in the atmosphere. The idea that methane is released from the interior through eruptive processes is also supported by the observations of several cryovolcanic-like features on Titan's surface by the mapping spectrometer (VIMS) and the radar on board Cassini. Thermal instabilities within the icy crust, possibly favoured by the presence of ammonia, may explain the observed features and provide the conditions for eruption of methane and other volatiles. Episodic resurfacing events associated with thermal and compositional instabilities in the icy crust can have major consequences on the hydrocarbon budget on Titan's surface and atmosphere.
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Affiliation(s)
- G Tobie
- Laboratoire de Planétologie et Géodynamique de Nantes, Université Nantes Atlantique, 2, rue de la Houssinière, 44322 Nantes Cedex 03, France CNRS, UMR-6112, 2, rue de la Houssinière, 44322 Nantes Cedex 03, France.
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16
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The identification of liquid ethane in Titan's Ontario Lacus. Nature 2008; 454:607-10. [PMID: 18668101 DOI: 10.1038/nature07100] [Citation(s) in RCA: 224] [Impact Index Per Article: 14.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2008] [Accepted: 05/19/2008] [Indexed: 11/08/2022]
Abstract
Titan was once thought to have global oceans of light hydrocarbons on its surface, but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear that no such oceans exist. There are, however, features similar to terrestrial lakes and seas, and widespread evidence for fluvial erosion, presumably driven by precipitation of liquid methane from Titan's dense, nitrogen-dominated atmosphere. Here we report infrared spectroscopic data, obtained by the Visual and Infrared Mapping Spectrometer (VIMS) on board the Cassini spacecraft, that strongly indicate that ethane, probably in liquid solution with methane, nitrogen and other low-molecular-mass hydrocarbons, is contained within Titan's Ontario Lacus.
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17
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18
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Oremland RS, Voytek MA. Acetylene as fast food: implications for development of life on anoxic primordial Earth and in the outer solar system. ASTROBIOLOGY 2008; 8:45-58. [PMID: 18199006 DOI: 10.1089/ast.2007.0183] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
Acetylene occurs, by photolysis of methane, in the atmospheres of jovian planets and Titan. In contrast, acetylene is only a trace component of Earth's current atmosphere. Nonetheless, a methane-rich atmosphere has been hypothesized for early Earth; this atmosphere would also have been rich in acetylene. This poses a paradox, because acetylene is a potent inhibitor of many key anaerobic microbial processes, including methanogenesis, anaerobic methane oxidation, nitrogen fixation, and hydrogen oxidation. Fermentation of acetylene was discovered approximately 25 years ago, and Pelobacter acetylenicus was shown to grow on acetylene by virtue of acetylene hydratase, which results in the formation of acetaldehyde. Acetaldehyde subsequently dismutates to ethanol and acetate (plus some hydrogen). However, acetylene hydratase is specific for acetylene and does not react with any analogous compounds. We hypothesize that microbes with acetylene hydratase played a key role in the evolution of Earth's early biosphere by exploiting an available source of carbon from the atmosphere and in so doing formed protective niches that allowed for other microbial processes to flourish. Furthermore, the presence of acetylene in the atmosphere of a planet or planetoid could possibly represent evidence for an extraterrestrial anaerobic ecosystem.
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19
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Stofan ER, Elachi C, Lunine JI, Lorenz RD, Stiles B, Mitchell KL, Ostro S, Soderblom L, Wood C, Zebker H, Wall S, Janssen M, Kirk R, Lopes R, Paganelli F, Radebaugh J, Wye L, Anderson Y, Allison M, Boehmer R, Callahan P, Encrenaz P, Flamini E, Francescetti G, Gim Y, Hamilton G, Hensley S, Johnson WTK, Kelleher K, Muhleman D, Paillou P, Picardi G, Posa F, Roth L, Seu R, Shaffer S, Vetrella S, West R. The lakes of Titan. Nature 2007; 445:61-4. [PMID: 17203056 DOI: 10.1038/nature05438] [Citation(s) in RCA: 440] [Impact Index Per Article: 25.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2006] [Accepted: 11/09/2006] [Indexed: 11/09/2022]
Abstract
The surface of Saturn's haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70 degrees north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan's surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface 'liquid methane' table.
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Affiliation(s)
- E R Stofan
- Proxemy Research, Rectortown, Virginia 20140, USA.
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20
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21
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Hunten DM. The sequestration of ethane on Titan in smog particles. Nature 2006; 443:669-70. [PMID: 17035997 DOI: 10.1038/nature05157] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2006] [Accepted: 08/01/2006] [Indexed: 11/09/2022]
Abstract
Saturn's largest satellite, Titan, has a dense atmosphere of nitrogen with a few per cent of methane. At visible wavelengths its surface is hidden by dense orange-brown smog, which is produced in the stratosphere by photochemical reactions following the dissociation of methane by solar ultraviolet light. The most abundant of the products of these reactions is ethane, and enough of it should have been generated over the life of the Solar System to form a satellite-wide ocean one kilometre deep. Radar observations have found specular reflections in 75 per cent of the surface spots observed, but optical searches for a sun-glint off an ocean have been negative. Here I explain the mysterious absence or rarity of liquid ethane: it condenses onto the smog particles, instead of into liquid drops, at the cold temperatures in Titan's atmosphere. This dusty combination of smog and ethane, forming deposits several kilometres thick on the surface, including the observed dunes and dark areas, could be named 'smust'. This satellite-wide deposit replaces the ocean long thought to be an important feature of Titan.
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Affiliation(s)
- D M Hunten
- Lunar and Planetary Laboratory, The University of Arizona, Tucson, Arizona 85721, USA.
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22
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Griffith CA, Penteado P, Rannou P, Brown R, Boudon V, Baines KH, Clark R, Drossart P, Buratti B, Nicholson P, McKay CP, Coustenis A, Negrao A, Jaumann R. Evidence for a Polar Ethane Cloud on Titan. Science 2006; 313:1620-2. [PMID: 16973876 DOI: 10.1126/science.1128245] [Citation(s) in RCA: 147] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal the presence of a vast tropospheric cloud on Titan at latitudes 51 degrees to 68 degrees north and all longitudes observed (10 degrees to 190 degrees west). The derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon's north pole. Preferential condensation of ethane, perhaps as ice, at Titan's poles during the winters may partially explain the lack of liquid ethane oceans on Titan's surface at middle and lower latitudes.
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Affiliation(s)
- C A Griffith
- Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 85721 USA
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23
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Tobie G, Lunine JI, Sotin C. Episodic outgassing as the origin of atmospheric methane on Titan. Nature 2006; 440:61-4. [PMID: 16511489 DOI: 10.1038/nature04497] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2005] [Accepted: 11/28/2005] [Indexed: 11/09/2022]
Abstract
Saturn's largest satellite, Titan, has a massive nitrogen atmosphere containing up to 5 per cent methane near its surface. Photochemistry in the stratosphere would remove the present-day atmospheric methane in a few tens of millions of years. Before the Cassini-Huygens mission arrived at Saturn, widespread liquid methane or mixed hydrocarbon seas hundreds of metres in thickness were proposed as reservoirs from which methane could be resupplied to the atmosphere over geologic time. Titan fly-by observations and ground-based observations rule out the presence of extensive bodies of liquid hydrocarbons at present, which means that methane must be derived from another source over Titan's history. Here we show that episodic outgassing of methane stored as clathrate hydrates within an icy shell above an ammonia-enriched water ocean is the most likely explanation for Titan's atmospheric methane. The other possible explanations all fail because they cannot explain the absence of surface liquid reservoirs and/or the low dissipative state of the interior. On the basis of our models, we predict that future fly-bys should reveal the existence of both a subsurface water ocean and a rocky core, and should detect more cryovolcanic edifices.
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Affiliation(s)
- Gabriel Tobie
- Laboratoire de Planétologie et Géodynamique, UMR-CNRS 6112, Université de Nantes, 44322 Nantes cedex 03, France.
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24
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Fulchignoni M, Ferri F, Angrilli F, Ball AJ, Bar-Nun A, Barucci MA, Bettanini C, Bianchini G, Borucki W, Colombatti G, Coradini M, Coustenis A, Debei S, Falkner P, Fanti G, Flamini E, Gaborit V, Grard R, Hamelin M, Harri AM, Hathi B, Jernej I, Leese MR, Lehto A, Lion Stoppato PF, López-Moreno JJ, Mäkinen T, McDonnell JAM, McKay CP, Molina-Cuberos G, Neubauer FM, Pirronello V, Rodrigo R, Saggin B, Schwingenschuh K, Seiff A, Simões F, Svedhem H, Tokano T, Towner MC, Trautner R, Withers P, Zarnecki JC. In situ measurements of the physical characteristics of Titan's environment. Nature 2005; 438:785-91. [PMID: 16319827 DOI: 10.1038/nature04314] [Citation(s) in RCA: 545] [Impact Index Per Article: 28.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2005] [Accepted: 10/11/2005] [Indexed: 11/08/2022]
Abstract
On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 +/- 0.25 K, and the pressure was 1,467 +/- 1 hPa.
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Affiliation(s)
- M Fulchignoni
- LESIA, Observatoire de Paris, 5 Place Janssen, 92195 Meudon, France. [2
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25
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Roe HG, Brown ME, Schaller EL, Bouchez AH, Trujillo CA. Geographic control of Titan's mid-latitude clouds. Science 2005; 310:477-9. [PMID: 16239473 DOI: 10.1126/science.1116760] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Observations of Titan's mid-latitude clouds from the W. M. Keck and Gemini Observatories show that they cluster near 350 degrees W longitude, 40 degrees S latitude. These clouds cannot be explained by a seasonal shift in global circulation and thus presumably reflect a mechanism on Titan such as geysering or cryovolcanism in this region. The rate of volatile release necessary to trigger cloud formation could easily supply enough methane to balance the loss to photolysis in the upper atmosphere.
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Affiliation(s)
- Henry G Roe
- Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.
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26
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West RA, Brown ME, Salinas SV, Bouchez AH, Roe HG. No oceans on Titan from the absence of a near-infrared specular reflection. Nature 2005; 436:670-2. [PMID: 16079839 DOI: 10.1038/nature03824] [Citation(s) in RCA: 49] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/10/2005] [Accepted: 05/18/2005] [Indexed: 11/08/2022]
Abstract
With its substantial atmosphere of nitrogen, hydrocarbons and nitriles, Saturn's moon Titan is a unique planetary satellite. Photochemical processing of the gaseous constituents produces an extended haze that obscures the surface. Soon after the Voyager fly-bys in 1980 and 1981 photochemical models led to the conclusion that there should be enough liquid methane/ethane/nitrogen to cover the surface to a depth of several hundred metres. Recent Earth-based radar echoes imply that surface liquid may be present at a significant fraction of the locations sampled. Here we present ground-based observations (at near-infrared wavelengths) and calculations showing that there is no evidence thus far for surface liquid. Combined with the specular signatures from radar observations, we infer mechanisms that produce very flat solid surfaces, involving a substance that was liquid in the past but is not in liquid form at the locations we studied.
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Affiliation(s)
- R A West
- MS 169-237 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA.
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27
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Sotin C, Jaumann R, Buratti BJ, Brown RH, Clark RN, Soderblom LA, Baines KH, Bellucci G, Bibring JP, Capaccioni F, Cerroni P, Combes M, Coradini A, Cruikshank DP, Drossart P, Formisano V, Langevin Y, Matson DL, McCord TB, Nelson RM, Nicholson PD, Sicardy B, LeMouelic S, Rodriguez S, Stephan K, Scholz CK. Release of volatiles from a possible cryovolcano from near-infrared imaging of Titan. Nature 2005; 435:786-9. [PMID: 15944697 DOI: 10.1038/nature03596] [Citation(s) in RCA: 174] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/27/2004] [Accepted: 03/24/2005] [Indexed: 11/09/2022]
Abstract
Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar (ref. 1) and, similar to the Earth, N2 is the main component of the atmosphere. Methane is the second most important component, but it is photodissociated on a timescale of 10(7) years (ref. 3). This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for a more solid (perhaps icy) surface. We also find a circular structure approximately 30 km in diameter that does not resemble any features seen on other icy satellites. We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titan's atmosphere.
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Affiliation(s)
- C Sotin
- Laboratoire de Planétologie et Géodynamique, UMR CNRS 6112, Université de Nantes, Nantes, 44100, France.
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28
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Porco CC, Baker E, Barbara J, Beurle K, Brahic A, Burns JA, Charnoz S, Cooper N, Dawson DD, Del Genio AD, Denk T, Dones L, Dyudina U, Evans MW, Fussner S, Giese B, Grazier K, Helfenstein P, Ingersoll AP, Jacobson RA, Johnson TV, McEwen A, Murray CD, Neukum G, Owen WM, Perry J, Roatsch T, Spitale J, Squyres S, Thomas P, Tiscareno M, Turtle EP, Vasavada AR, Veverka J, Wagner R, West R. Imaging of Titan from the Cassini spacecraft. Nature 2005; 434:159-68. [PMID: 15758990 DOI: 10.1038/nature03436] [Citation(s) in RCA: 334] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/11/2005] [Accepted: 02/07/2005] [Indexed: 11/08/2022]
Abstract
Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titan's nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titan's history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150-200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved.
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Affiliation(s)
- Carolyn C Porco
- Cassini Imaging Central Laboratory for Operations, Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA.
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29
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Curtis DB, Glandorf DL, Toon OB, Tolbert MA, McKay CP, Khare BN. Laboratory Studies of Butane Nucleation on Organic Haze Particles: Application to Titan's Clouds. J Phys Chem A 2005; 109:1382-90. [PMID: 16833455 DOI: 10.1021/jp045596h] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Titan, Saturn's largest satellite, has a thick nitrogen/methane atmosphere with various hydrocarbons present in minor amounts. Recent observations suggest that CH4 may condense to form clouds near the moon's tropopause. Titan's methane cloud formation is probably triggered by a sequential nucleation of hydrocarbons onto Titan's haze material as tropospheric convection occurs due to differential heating of the surface or as the haze settles through the lower stratosphere. To better constrain Titan's cloud formation mechanism, investigations of the nucleation of several hydrocarbons will be necessary. Butane was chosen for this study because it has a relatively high freezing point and is estimated to be present at 200 part per billion levels. If this amount of butane were to condense on each haze particle, a visible cloud would be observed. Laboratory measurements at T = 125 K were performed to determine the relative ease of solid butane nucleation onto laboratory-produced tholin particles having an elemental composition of C5H5N, and solid films of hexane and acetonitrile. We find that butane nucleation onto the haze particles requires a relatively high saturation ratio of S > 1.30. Because butane nucleation is difficult, it may occur on only a very small subset of the total haze particles available. Such selective nucleation of butane would lead to those particles becoming coated with significant amounts of butane. Requiring a high saturation ratio for butane nucleation will reduce the optical depth of butane clouds by a factor of 100 because the particles will be fewer in number for a given condensed mass.
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Affiliation(s)
- Daniel B Curtis
- Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA
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30
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Abstract
It has long been known that Saturn's largest moon, Titan, has a thick nitrogen atmosphere, which obscures the underlying surface. In his Perspective, Lorenz highlights the report by Campbell et al., who have used the giant Arecibo and Green Bank radio telescopes as a radar to probe Titan's hidden surface. The surface appears to be distinct from those of the icy satellites of Jupiter, in both brightness and polarization. The new data show sharp spikes in the reflected microwave spectrum, indicating large, smooth areas of radar-dark material. These features suggest the widespread existence of lakes or seas of liquid hydrocarbons on Titan.
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Affiliation(s)
- Ralph Lorenz
- Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
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31
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Abstract
Arecibo radar observations of Titan at 13-centimeter wavelength indicate that most of the echo power is in a diffusely scattered component but that a small specular component is present for about 75% of the subearth locations observed. These specular echoes have properties consistent with those expected for areas of liquid hydrocarbons. Knowledge of the areal extent and depth of any deposits of liquid hydrocarbons could strongly constrain the history of Titan's atmosphere and surface.
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Affiliation(s)
- Donald B Campbell
- National Astronomy and Ionosphere Center and Department of Astronomy, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA.
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32
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Griffith CA, Owen T, Geballe TR, Rayner J, Rannou P. Evidence for the exposure of water ice on Titan's surface. Science 2003; 300:628-30. [PMID: 12714742 DOI: 10.1126/science.1081897] [Citation(s) in RCA: 124] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Abstract
The smoggy stratosphere of Saturn's largest moon, Titan, veils its surface from view, except at narrow wavelengths centered at 0.83, 0.94, 1.07, 1.28, 1.58, 2.0, 2.9, and 5.0 micrometers. We derived a spectrum of Titan's surface within these "windows" and detected features characteristic of water ice. Therefore, despite the hundreds of meters of organic liquids and solids hypothesized to exist on Titan's surface, its icy bedrock lies extensively exposed.
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Affiliation(s)
- Caitlin A Griffith
- Department of Planetary Sciences, University of Arizona, 1629 East University Boulevard, Tucson, AZ 85721-0092, USA.
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33
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Abstract
We have discovered frequent variations in the near-infrared spectrum of Titan, Saturn's largest moon, which are indicative of the daily presence of sparse clouds covering less than 1% of the area of the satellite. The thermodynamics of Titan's atmosphere and the clouds' altitudes suggest that convection governs their evolutions. Their short lives point to the presence of rain. We propose that Titan's atmosphere resembles Earth's, with clouds, rain, and an active weather cycle, driven by latent heat release from the primary condensible species.
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Affiliation(s)
- C A Griffith
- Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86011-6010, USA
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35
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Lorenz RD, McKay CP, Lunine JI. Analytic investigation of climate stability on Titan: sensitivity to volatile inventory. PLANETARY AND SPACE SCIENCE 1999; 47:1503-1515. [PMID: 11543195 DOI: 10.1016/s0032-0633(99)00038-0] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
We develop a semiempirical grey radiative model to quantify Titan's surface temperature as a function of pressure and composition of a nitrogen-methane-hydrogen atmosphere, solar flux and atmospheric haze. We then use this model, together with non-ideal gas-liquid equilibrium theory to investigate the behavior of the coupled surface-atmosphere system on Titan. We find that a volatile-rich Titan is unstable with respect to a runaway greenhouse-small increases in solar luminosity from the present value can lead to massive increases in surface temperature. If methane has been photolyzed throughout Titan's history, then this runaway can only be avoided if the photolytic ethane is removed from the surface-atmosphere system.
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Affiliation(s)
- R D Lorenz
- Lunar and Planetary Laboratory, University of Arizona, Tucson 85721-0092, USA.
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36
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Abstract
The 1980 encounter by the Voyager 1 spacecraft with Titan, Saturn's largest moon, revealed the presence of a thick atmosphere containing nitrogen and methane (1.4 and approximately 0.05 bar, respectively). Methane was found to be nearly saturated at Titan's tropopause, which, with other considerations, led to the hypothesis that Titan might experience a methane analogue of Earth's vigorous hydrological cycle, with clouds, rain and seas. Yet recent analyses of Voyager data indicate large areas of super-saturated methane, more indicative of dry and stagnant conditions. A resolution to this apparent contradiction requires observations of Titan's lower atmosphere, which was hidden from the Voyager cameras by the photochemical haze (or smog) in Titan's stratosphere. Here we report near-infrared spectroscopic observations of Titan within four narrow spectral windows where the moon's atmosphere is ostensibly transparent. We detect pronounced flux enhancements that indicate the presence of reflective methane condensation clouds in the troposphere. These clouds occur at a relatively low altitude (15+/-10 km), at low latitudes, and appear to cover approximately 9 per cent of Titan's disk.
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Affiliation(s)
- C A Griffith
- Department of Physics and Astronomy, Northern Arizona University, Flagstaff 86001-6010, USA.
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37
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McKay CP, Chau Martin S, Griffith CA, Keller RM. Temperature lapse rate and methane in Titan's troposphere. ICARUS 1997; 129:498-505. [PMID: 11541736 DOI: 10.1006/icar.1997.5751] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
We have reanalyzed the Voyager radio occultation data for Titan, examining two alternative approaches to methane condensation. In one approach, methane condensation is facilitated by the presence of nitrogen because nitrogen lowers the condensation level of a methane/nitrogen mixture. The resulting enhancement in methane condensation lowers the upper limit on surface relative humidity of methane obtained from the Voyager occultation data from 0.7 to 0.6. We conclude that in this case the surface relative humidity of methane lies between 0.08 and 0.6, with values close to 0.6 indicated. In the other approach, methane is allowed to become supersaturated and reaches 1.4 times saturation in the troposphere. In this case, surface humidities up to 100% are allowed by the Voyager occultation data, and thus the upper limit must be set by other considerations. We conclude that if supersaturation is included, then the surface relative humidity of methane can be any value greater than 0.08--unless a deep ocean is present, in which case the surface relative humidity is limited to less than 0.85. Again, values close to 0.6 are indicated. Overall, the tropospheric lapse rate on Titan appears to be determined by radiative equilibrium. The lapse rate is everywhere stable against dry convection, but is unstable to moist convection. This finding is consistent with a supersaturated atmosphere in which condensation-and hence moist convection-is inhibited.
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Affiliation(s)
- C P McKay
- Space Science Division, NASA Ames Research Center, Moffett Field, California 94035, USA.
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38
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Lara LM, Lellouch E, López-Moreno JJ, Rodrigo R. Vertical distribution of Titan's atmospheric neutral constituents. ACTA ACUST UNITED AC 1996. [DOI: 10.1029/96je02036] [Citation(s) in RCA: 261] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
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39
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Griffith CA, Zahnle K. Influx of cometary volatiles to planetary moons: the atmospheres of 1000 possible Titans. JOURNAL OF GEOPHYSICAL RESEARCH 1995; 100:16907-22. [PMID: 11539417 DOI: 10.1029/95je01135] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
We use a Monte Carlo model to simulate impact histories of possible Titans, Callistos, and Ganymedes. Comets create or erode satellite atmospheres, depending on their mass and velocity distributions: faster and bigger comets remove atmophiles; slower or smaller comets supply them. Mass distributions and the minimum total mass of comets passing through the Saturn system were derived from the crater records of Rhea and Iapetus. These were then scaled to give a minimum impact history for Titan. From this cometary population, of 1000 initially airless Titans, 16% acquired atmospheres larger than Titan's present atmosphere (9 x 10(21) g), and more than half accumulated atmospheres larger than 10(21) g. In contrasts to the work of Zahnle et al. (1992), we find that, in most trials, Callisto acquires comet-based atmospheres. Atmospheres acquired by Callisto and, especially, Ganymede are sensitive to assumptions regarding energy partitioning into the ejecta plume. If we assume that only the normal velocity component heats the plume, the majority of Ganymedes and half of the Callistos accreted atmospheres smaller than 10(20) g. If all the impactor's velocity heats the plume, Callisto's most likely atmosphere is 10(17) g and Ganymede's is negligible. The true cometary flux was most likely larger than that derived from crater records, which raises the probability that Titan, Ganymede, and Callisto acquired substantial atmospheres. However, other loss processes (e.g., sputtering by ions swept up by the planetary magnetic field, solar UV photolysis of hydrocarbons) are potentially capable of eliminating small atmospheres over the age of the solar system. The dark material on Callisto's surface may be a remnant of an earlier, now vanished atmosphere.
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Affiliation(s)
- C A Griffith
- Department of Physics and Astronomy, Northern Arizona University, Flagstaff, USA
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40
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Abstract
Thermodynamic and photochemical arguments suggest that Titan, the largest satellite of Saturn, has a deep ocean of liquid hydrocarbons. At visible wavelengths, Titan's surface is obscured by a thick stratospheric haze, but radar observations have revealed large regions of high surface reflectivity that are inconsistent with a global hydrocarbon ocean. Titan's surface has also been imaged at infrared wavelengths, and the highest-resolution data (obtained by the Hubble Space Telescope) show clear variations in surface albedo and/or topography. The natural interpretation of these observations is that Titan, like the Earth, has continents and oceans. But Titan's high orbital eccentricity poses a problem for this interpretation, as the effects of oceanic tidal friction would have circularized Titan's orbit for most configurations of oceans and continents. Here we argue that a more realistic topography, in which liquid hydrocarbons are confined to a number of disconnected seas or crater lakes, may satisfy both the dynamical and observational constraints.
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Affiliation(s)
- S F Dermott
- Department of Astronomy, University of Florida, Gainesville 32611-2055
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41
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Gautier D. Titan's atmosphere composition: certainties and speculations. ADVANCES IN SPACE RESEARCH : THE OFFICIAL JOURNAL OF THE COMMITTEE ON SPACE RESEARCH (COSPAR) 1995; 15:295-301. [PMID: 11539241 DOI: 10.1016/s0273-1177(99)80100-8] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
Firm results concerning the thermal structure, the composition, the seasonal effects of the atmosphere of Titan, as well as the superotation of its stratosphere are reviewed. The nature of the surface of the satellite, the possible presence of argon in the atmosphere and the structure and composition of clouds and aerosols are, among other topics, still speculative. The implications of the observed deuterium enrichment on the origin of ices in the outer part of the nebula are controversial.
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Affiliation(s)
- D Gautier
- Dèpartement de Recherches Spatiales, Observatoire de Paris-Meudon, France
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Raulin F, Bruston P, Coll P, Coscia D, Gazeau MC, Guez L, de Vanssay E. Exobiology on Titan. J Biol Phys 1995. [DOI: 10.1007/bf00700419] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
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Lunine JI, McKay CP. Surface-atmosphere interactions on Titan compared with those on the pre-biotic Earth. ADVANCES IN SPACE RESEARCH : THE OFFICIAL JOURNAL OF THE COMMITTEE ON SPACE RESEARCH (COSPAR) 1995; 15:303-311. [PMID: 11539243 DOI: 10.1016/s0273-1177(99)80101-x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
The surface and atmosphere of Titan constitute a system which is potentially as complex as that of the Earth, with the possibility of precipitation, surface erosion due to liquids, chemistry in large surface or subsurface hydrocarbon reservoirs, surface expressions of internal activity, and occasional major impacts leading to crustal melting. While none of the above have been observed as yet, the composition, density and thermal properties of Titan's atmosphere make it uniquely suited in the outer solar system as a place where such processes may occur. The one attribute of the Earth not expected on Titan is biological activity, which has had a profound effect on the evolution of the Earth's surface-atmosphere system. The earliest environment of Titan could have been warm enough for liquid ammonia-water solutions to exist on or near surface; pre-biotic organic processes may have taken place in such an environment. After a few hundred million years surface ammonia-water would have disappeared. Therefore, study of Titan through the Cassini-Huygens mission, planned for launch in 1997, primarily affords the opportunity to understand planet-wide surface-atmosphere interactions in the presence of fluids but in the absence of life. More speculative is the possibility that endogenic and exogenic heating continue to provide short-lived environments on Titan wherein pre-biotic organic processes in the presence of water happen.
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Affiliation(s)
- J I Lunine
- Lunar and Planetary Laboratory, University of Arizona, Tucson 85721, USA
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Sen AD, Anicich VG, Arakelian T. Dielectric constant of liquid alkanes and hydrocarbon mixtures. JOURNAL OF PHYSICS D: APPLIED PHYSICS 1992; 25:516-521. [PMID: 11538383 DOI: 10.1088/0022-3727/25/3/027] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
The complex dielectric constants of n-alkanes with two to seven carbon atoms have been measured. The measurements were conducted using a slotted-line technique at 1.2 GHz and at atmospheric pressure. The temperature was varied from the melting point to the boiling point of the respective alkanes. The real part of the dielectric constant was found to decrease with increasing temperature and correlate with the change in the molar volume. An upper limit to all the loss tangents was established at 0.001. The complex dielectric constants of a few mixtures of liquid alkanes were also measured at room temperature. For a pentane-octane mixture the real part of the dielectric constant could be explained by the Clausius-Mosotti theory. For the mixtures of n-hexane-ethylacetate and n-hexane-acetone the real part of the dielectric constants could be explained by the Onsager theory extended to mixtures. The dielectric constant of the n-hexane-acetone mixture displayed deviations from the Onsager theory at the highest fractions of acetone. The dipole moments of ethylacetate and acetone were determined for dilute mixtures using the Onsager theory and were found to be in agreement with their accepted gas-phase values. The loss tangents of the mixtures exhibited a linear relationship with the volume fraction for low concentrations of the polar liquids.
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Affiliation(s)
- A D Sen
- Jet Propulsion Laboratory, Pasadena, CA 91109, USA
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Abstract
There are many parallels between the atmospheric thermal structure of the Saturnian satellite Titan and the terrestrial greenhouse effect; these parallels provide a comparison for theories of the heat balance of Earth. Titan's atmosphere has a greenhouse effect caused primarily by pressure-induced opacity of N2, CH4, and H2. H2 is a key absorber because it is primarily responsible for the absorption in the wave number 400 to 600 cm-1 "window" region of Titan's infrared spectrum. The concentration of CH4, also an important absorber, is set by the saturation vapor pressure and hence is dependent on temperature. In this respect there is a similarity between the role of H2 and CH4 on Titan and that of CO2 and H2O on Earth. Titan also has an antigreenhouse effect that results from the presence of a high-altitude haze layer that is absorbing at solar wavelengths but transparent in the thermal infrared. The antigreenhouse effect on Titan reduces the surface temperature by 9 K whereas the greenhouse effect increases it by 21 K. The net effect is that the surface temperature (94 K) is 12 K warmer than the effective temperature (82 K). If the haze layer were removed, the antigreenhouse effect would be greatly reduced, the greenhouse effect would become even stronger, and the surface temperature would rise by over 20 K.
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Affiliation(s)
- C P McKay
- Space Science Division, National Aeronautics and Space Administration, Ames Research Center, Moffett Field, CA 94035, USA
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Griffith CA, Owen T, Wagener R. Titan's surface and troposphere, investigated with ground-based, near-infrared observations. ICARUS 1991; 93:362-378. [PMID: 11538707 DOI: 10.1016/0019-1035(91)90219-j] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
New observations of Titan's near-infrared spectrum (4000-5000 cm-1) combined with points taken from Fink and Larson's (1979) spectrum (4000-12500 cm-1) provide information on Titan's haze, possible clouds, surface albedo, and atmospheric abundance of H2. In the near-infrared, the main features in Titan's spectrum result from absorption of solar radiation by CH4. The strength of this absorption varies considerably with wavelength, allowing us to probe various atmospheric levels down to the surface itself by choosing specific wavelengths for analysis. At 4715 cm-1, the pressure-induced S(1) fundamental band of H2 lies in the wings of CH4 bands. Based on current values for the CH4 line parameters, Titan's spectrum can be best interpreted with a volume mixing ratio of H2 between 0.5 and 1.0%. Our observations suggest the existence of an optically thin CH4 cloud layer. The optical depths that we derive for Titan's haze and clouds are small enough to allow us to sense the surface of Titan at 4900, 6250, and 7700 cm-1. The most plausible interpretation of the albedos determined at these wavenumbers suggests a surface dominated by "dirty" water ice. A global ethane ocean is not compatible with these albedos.
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Affiliation(s)
- C A Griffith
- Physics Department, State University of New York, Stony Brook 11794, USA
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Abstract
The present understanding of the atmosphere and surface conditions on Saturn's largest moon, Titan, including the stability of methane, and an application of thermodynamics leads to a strong prediction of liquid hydrocarbons in an ethane-methane mixture on the surface. Such a surface would have nearly unique microwave reflection properties due to the low dielectric constant. Attempts were made to obtain reflections at a wavelength of 3.5 centimeters by means of a 70-meter antenna in California as the transmitter and the Very Large Array in New Mexico as the receiving instrument. Statistically significant echoes were obtained that show Titan is not covered with a deep, global ocean of ethane, as previously thought. The experiment yielded radar cross sections normalized by the Titan disk of 0.38 +/- 0.15, 0.78 +/- 0.15, and 0.25 +/- 0.15 on three consecutive nights during which the sub-Earth longitude on Titan moved 50 degrees. The result for the combined data for the entire experiment is 0.35 +/- 0.08. The cross sections are very high, most consistent with those of the Galilean satellites; no evidence of the putative liquid ethane was seen in the reflection data. A global ocean as shallow as about 200 meters would have exhibited reflectivities smaller by an order of magnitude, and below the detection limit of the experiment. The measured emissivity at similar wavelengths of about 0.9 is somewhat inconsistent with the high reflectivity.
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An occult view of Titan. Nature 1990. [DOI: 10.1038/343315a0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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Abstract
The atmospheres of bodies in the outer solar system are distinct in composition from those of the inner planets and provide a complementary set of clues to the origin of the solar system. This article reviews current understanding of the origin and evolution of these atmospheres on the basis of abundances of key molecular species. The systematic enrichment of methane and deuterated species from Jupiter to Neptune is consistent with formation models in which significant infall of icy and rocky planetesimals accompanies the formation of giant planets. The atmosphere of the Saturnian satellite Titan has been strongly modified by photochemistry and interaction with the surface over 4.5 billion years; the combined knowledge of this moon's bulk density and estimates of the composition of the surface and atmosphere provide some constraints on this body's formation. Neptune's satellite Triton is a poorly known object for which it is hoped that substantial information will be gleaned from the Voyager 2 encounter in August 1989. The mean density of the Pluto-Charon system is well known and suggests an origin in the rather water-poor solar nebula. The recent occultation of a star by Pluto provides evidence that carbon monoxide, in addition to methane, may be present in its atmosphere.
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