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1.
New three-dimensional hydrodynamic simulations of hypervelocity impacts into the crust of Titan were undertaken to determine the fraction of liquid water generated on the surface of Saturn's largest moon over its history and, hence, the potential for surface—modification of hydrocarbons and nitriles by exposure to liquid water. We model in detail an individual impact event in terms of ejecta produced and melt generated, and use this to estimate melt production over Titan's history, taking into account the total flux of the impactors and its decay over time. Our estimates show that a global melt layer at any time after the very beginning of Titan's history is improbable; but transient melting local to newly formed craters has occurred over large parts of the surface. Local maxima of the melt are connected with the largest impact events. We also calculate the amount of volatiles delivered at the impact with various impact velocities (from 3 km/s for possible Hyperion fragments to 11 km/s for Jupiter family comets) and their retention as a possible source of Titan's atmosphere. We find the probability of impact ejecta escaping Titan with its modern dense and thick atmosphere is rather low, and dispersal of Titan organics throughout the rest of the Solar System requires impactors tens of kilometers in diameter. Water ice melting and exposure of organics to liquid water has been widespread because of impacts, but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation. The largest impactors may breach an ammonia-water mantle layer, creating a circular albedo contrast rather than a crater.  相似文献   

2.
During the descent of the Huygens probe in January 2005, its Descent Imager/Spectral Radiometer (DISR) will take the first close up images of Titan's surface. The shading imposed by the illumination of a planetary surface contains information on its topography. For planetary bodies without an optically thick atmosphere, the light can be assumed to stem from a point source. In this case, methods are available in order to estimate the shape of surface features from shading. The situation is quite different for Titan, as its atmosphere is optically thick at optical wavelengths. The sun is visible from the surface, but the illumination is dominated by diffuse radiance. In order to investigate the characteristics of shading under Titan's sky and to assess methods to retrieve the shape, different digital terrain models (DTMs) are used to simulate images according to different types of illumination. For an idealized DTM, the shape is retrieved from the shading in the simulated images. Deriving the shape from shading under Titan's sky using existing methods is only possible if the topography is relatively flat, i.e. in the absence of steep slopes.  相似文献   

3.
Methane is key to sustaining Titan's thick nitrogen atmosphere. However, methane is destroyed and converted to heavier hydrocarbons irreversibly on a relatively short timescale of approximately 10-100 million years. Without the warming provided by CH4-generated hydrocarbon hazes in the stratosphere and the pressure induced opacity in the infrared, particularly by CH4-N2 and H2-N2 collisions in the troposphere, the atmosphere could be gradually reduced to as low as tens of millibar pressure. An understanding of the source-sink cycle of methane is thus crucial to the evolutionary history of Titan and its atmosphere. In this paper we propose that a complex photochemical-meteorological-hydrogeochemical cycle of methane operates on Titan. We further suggest that although photochemistry leads to the loss of methane from the atmosphere, conversion to a global ocean of ethane is unlikely. The behavior of methane in the troposphere and the surface, as measured by the Cassini-Huygens gas chromatograph mass spectrometer, together with evidence of cryovolcanism reported by the Cassini visual and infrared mapping spectrometer, represents a “methalogical” cycle on Titan, somewhat akin to the hydrological cycle on Earth. In the absence of net loss to the interior, it would represent a closed cycle. However, a source is still needed to replenish the methane lost to photolysis. A hydrogeochemical source deep in the interior of Titan holds promise. It is well known that in serpentinization, hydration of ultramafic silicates in terrestrial oceans produces H2(aq), whose reaction with carbon grains or carbon dioxide in the crustal pores produces methane gas. Appropriate geological, thermal, and pressure conditions could have existed in and below Titan's purported water-ammonia ocean for “low-temperature” serpentinization to occur in Titan's accretionary heating phase. On the other hand, impacts could trigger the process at high temperatures. In either instance, storage of methane as a stable clathrate-hydrate in Titan's interior for later release to the atmosphere is quite plausible. There is also some likelihood that the production of methane on Titan by serpentinization is a gradual and continuous on-going process.  相似文献   

4.
Titan, Saturn's largest moon, has a thick nitrogen/methane atmosphere. The temperature and pressure conditions in Titan's atmosphere are such that the methane vapor should condense near the tropopause to form clouds. Several ground-based measurements have observed sparse cloud-like features in Titan's atmosphere, while the Cassini mission to Saturn has provided large scale images of the clouds. However, Titan's cloud formation conditions remain poorly constrained. Heterogeneous nucleation (from the vapor phase onto a solid or liquid aerosol surface) greatly enhances cloud formation relative to homogeneous nucleation. In order to elucidate the cloud formation mechanism near the tropopause, we have performed laboratory measurements of the adsorption of methane and ethane onto solid organic particles (tholins) representative of Titan's photochemical haze. We find that monolayers of methane adsorb onto tholin particles at saturation ratios less than unity. We also find that solid methane nucleates onto the adsorbed methane at a saturation ratio of S=1.07±0.008. This implies that Titan's methane clouds should form easily. This is consistent with recent measurements of the column of methane ruling out excessive methane supersaturation. In addition, we find ethane adsorbs onto tholin particles in a metastable phase prior to nucleation. However, ethane nucleation onto the adsorbed ethane occurs at a relatively high saturation ratio of S=1.36±0.08. These findings are consistent with the recent report of polar ethane clouds in Titan's lower stratosphere.  相似文献   

5.
Titan's bulk density along with Solar System formation models indicates considerable water as well as silicates as its major constituents. This satellite's dense atmosphere of nitrogen with methane is unique. Deposits or even oceans of organic compounds have been suggested to exist on Titan's solid surface due to UV-induced photochemistry in the atmosphere. Thus, the composition of the surface is a major piece of evidence needed to determine Titan's history. However, studies of the surface are hindered by the thick, absorbing, hazy and in some places cloudy atmosphere. Ground-based telescope investigations of the integral disk of Titan attempted to observe the surface albedo in spectral windows between methane absorptions by calculating and removing the haze effects. Their results were reported to be consistent with water ice on the surface that is contaminated with a small amount of dark material, perhaps organic material like tholin. We analyze here the recent Cassini Mission's visual and infrared mapping spectrometer (VIMS) observations that resolve regions on Titan. VIMS is able to see surface features and shows that there are spectral and therefore likely compositional units. By several methods, spectral albedo estimates within methane absorption windows between 0.75 and 5 μm were obtained for different surface units using VIMS image cubes from the Cassini-Huygens Titan Ta encounter. Of the spots studied, there appears to be two compositional classes present that are associated with the lower albedo and the higher albedo materials, with some variety among the brighter regions. These were compared with spectra of several different candidate materials. Our results show that the spectrum of water ice contaminated with a darker material matches the reflectance of the lower albedo Titan regions if the spectral slope from 2.71 to 2.79 μm in the poorly understood 2.8-μm methane window is ignored. The spectra for brighter regions are not matched by the spectrum of water ice or unoxidized tholin, in pure form or in mixtures with sufficient ice or tholin present to allow the water ice or tholin spectral features to be discerned. We find that the 2.8-μm methane absorption window is complex and seems to consist of two weak subwindows at 2.7 and 2.8 μm that have unknown opacities. A ratio image at these two wavelengths reveals an anomalous region on Titan that has a reflectance unlike any material so far identified, but it is unclear how much the reflectances in these two subwindows pertain to the surface.  相似文献   

6.
Our understanding of Titan, Saturn's largest satellite, has recently been consid-erably enhanced, thanks to the Cassini-Huygens mission. Since the Saturn Orbit Injection in July 2004, the probe has been harvesting new insights of the Kronian system. In par-ticular, this mission orchestrated a climax on January 14, 2005 with the descent of the Huygens probe into Titan's thick atmosphere. The orbiter and the lander have provided us with picturesque views of extraterrestrial landscapes, new in composition but reassuringly Earth-like in shape. Thus, Saturn's largest satellite displays chains of mountains, fields of dark and damp dunes, lakes and possibly geologic activity. As on Earth, landscapes on Titan are eroded and modeled by some alien hydrology: dendritic systems, hydrocarbon lakes, and methane clouds imply periods of heavy rainfalls, even though rain was never observed directly. Titan's surface also proved to be geologically active - today or in the recent past - given the small number of impact craters listed to date, as well as a few possible cryovolcanic features. We attempt hereafter a synthesis of the most significant results of the Cassini-Huygens endeavor, with emphasis on the surface.  相似文献   

7.
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8.
The Cassini Titan Radar Mapper obtained Synthetic Aperture Radar images of Titan's surface during four fly-bys during the mission's first year. These images show that Titan's surface is very complex geologically, showing evidence of major planetary geologic processes, including cryovolcanism. This paper discusses the variety of cryovolcanic features identified from SAR images, their possible origin, and their geologic context. The features which we identify as cryovolcanic in origin include a large (180 km diameter) volcanic construct (dome or shield), several extensive flows, and three calderas which appear to be the source of flows. The composition of the cryomagma on Titan is still unknown, but constraints on rheological properties can be estimated using flow thickness. Rheological properties of one flow were estimated and appear inconsistent with ammonia-water slurries, and possibly more consistent with ammonia-water-methanol slurries. The extent of cryovolcanism on Titan is still not known, as only a small fraction of the surface has been imaged at sufficient resolution. Energetic considerations suggest that cryovolcanism may have been a dominant process in the resurfacing of Titan.  相似文献   

9.
The processes of dissociation and dissociative ionization of molecular nitrogen by solar UV radiation and by the accompanying flux of photoelectrons, as well as sputtering of the atmosphere by fluxes of magnetospheric ions and pick-up ions, are the main sources of translationally excited (hot, or suprathermal) nitrogen atoms and molecules in Titan's upper atmosphere. Since Titan does not possess an intrinsic magnetic field, ions from Saturn's magnetosphere can penetrate into the outer layers of Titan's atmosphere and sputter atoms and molecules from the atmosphere in momentum-transfer and charge exchange collisions. Atmospheric sputtering by corotating nitrogen ions and carbon-containing pick-up ions, as well as photodissociation-related losses, was considered previously by Lammer and Bauer (1993) and Shematovich et al. (2001, 2003). In this paper we investigate the processes of the formation and evolution of the fraction of suprathermal nitrogen atoms and molecules in the transition region of Titan's upper atmosphere using the previously developed Monte Carlo model for hot satellite and planetary coronas (Shematovich, 1999, 2004). It is established that the suprathermal nitrogen fraction in the transition region of Titan's upper atmosphere includes nitrogen atoms and molecules but the suprathermal nitrogen concentration is relatively small owing to high rates of escape from the atmosphere and to the efficient thermalization of suprathermal nitrogen at the altitudes of the relatively dense lower thermosphere. However, the scale height for suprathermal nitrogen in the transition region is much higher than that for the ambient atmospheric gas. Therefore, suprathermal nitrogen becomes one of the dominant components in the outer exosphere.  相似文献   

10.
We present results from 14 nights of observations of Titan in 1996-1998 using near-infrared (centered at 2.1 microns) speckle imaging at the 10-meter W.M. Keck Telescope. The observations have a spatial resolution of 0.06 arcseconds. We detect bright clouds on three days in October 1998, with a brightness about 0.5% of the brightness of Titan. Using a 16-stream radiative transfer model (DISORT) to model the central equatorial longitude of each image, we construct a suite of surface albedo models parameterized by the optical depth of Titan's hydrocarbon haze layer. From this we conclude that Titan's equatorial surface albedo has plausible values in the range of 0-0.20. Titan's minimum haze optical depth cannot be constrained from this modeling, but an upper limit of 0.3 at this wavelength range is found. More accurate determination of Titan's surface albedo and haze optical depth, especially at higher latitudes, will require a model that fully considers the 3-dimensional nature of Titan's atmosphere.  相似文献   

11.
Cassini radar observations show that Titan's spin is slightly faster than synchronous spin. Angular momentum exchange between Titan's surface and the atmosphere over seasonal time scales corresponding to Saturn's orbital period of 29.5 year is the most likely cause of the observed non-synchronous rotation. We study the effect of Saturn's gravitational torque and torques between internal layers on the length-of-day (LOD) variations driven by the atmosphere. Because static tides deform Titan into an ellipsoid with the long axis approximately in the direction to Saturn, non-zero gravitational and pressure torques exist that can change the rotation rate of Titan. For the torque calculation, we estimate the flattening of Titan and its interior layers under the assumption of hydrostatic equilibrium. The gravitational forcing by Saturn, due to misalignment of the long axis of Titan with the line joining the mass centers of Titan and Saturn, reduces the LOD variations with respect to those for a spherical Titan by an order of magnitude. Internal gravitational and pressure coupling between the ice shell and the interior beneath a putative ocean tends to reduce any differential rotation between shell and interior and reduces further the LOD variations by a few times. For the current estimate of the atmospheric torque, we obtain LOD variations of a hydrostatic Titan that are more than 100 times smaller than the observations indicate when Titan has no ocean as well as when a subsurface ocean exists. Moreover, Saturn's torque causes the rotation to be slower than synchronous in contrast to the Cassini observations. The calculated LOD variations could be increased if the atmospheric torque is larger than predicted and or if fast viscous relaxation of the ice shell could reduce the gravitational coupling, but it remains to be studied if a two order of magnitude increase is possible and if these effects can explain the phase difference of the predicted rotation variations. Alternatively, the large differences with the observations may suggest that non-hydrostatic effects in Titan are important. In particular, we show that the amplitude and phase of the calculated rotation variations are similar to the observed values if non-hydrostatic effects could strongly reduce the equatorial flattening of the ice shell above an internal ocean.  相似文献   

12.
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13.
Xun Zhu  Darrell F. Strobel 《Icarus》2005,176(2):331-350
Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 m s−1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 m s−1 Titan-day−1 to maintain peak prograde winds in excess of 100 m s−1 at the 1 mbar level.  相似文献   

14.
“Water and related chemistry in the Solar System” is a Herschel Space Observatory Guaranteed-Time Key Programme. This project, approved by the European Space Agency, aims at determining the distribution, the evolution and the origin of water in Mars, the outer planets, Titan, Enceladus and the comets. It addresses the broad topic of water and its isotopologues in planetary and cometary atmospheres. The nature of cometary activity and the thermodynamics of cometary comae will be investigated by studying water excitation in a sample of comets. The D/H ratio, the key parameter for constraining the origin and evolution of Solar System species, will be measured for the first time in a Jupiter-family comet. A comparison with existing and new measurements of D/H in Oort-cloud comets will constrain the composition of pre-solar cometary grains and possibly the dynamics of the protosolar nebula. New measurements of D/H in giant planets, similarly constraining the composition of proto-planetary ices, will be obtained. The D/H and other isotopic ratios, diagnostic of Mars’ atmosphere evolution, will be accurately measured in H2O and CO. The role of water vapor in Mars’ atmospheric chemistry will be studied by monitoring vertical profiles of H2O and HDO and by searching for several other species (and CO and H2O isotopes). A detailed study of the source of water in the upper atmosphere of the Giant Planets and Titan will be performed. By monitoring the water abundance, vertical profile, and input fluxes in the various objects, and when possible with the help of mapping observations, we will discriminate between the possible sources of water in the outer planets (interplanetary dust particles, cometary impacts, and local sources). In addition to these inter-connected objectives, serendipitous searches will enhance our knowledge of the composition of planetary and cometary atmospheres.  相似文献   

15.
Hutzell WT  McKay CP  Toon OB  Hourdin F 《Icarus》1996,119(1):112-129
We have used a 2-D microphysics model to study the effects of atmospheric motions on the albedo of Titan's thick haze layer. We compare our results to the observed variations of Titan's brightness with season and latitude. We use two wind fields; the first is a simple pole-to-pole Hadley cell that reverses twice a year. The second is based on the results of a preliminary Titan GCM. Seasonally varying wind fields, with horizontal velocities of about 1 cm sec-1 at optical depth unity, are capable of producing the observed change in geometric albedo of about 10% over the Titan year. Neither of the two wind fields can adequately reproduce the latitudinal distribution of reflectivity seen by Voyager. At visible wavelengths, where only haze opacity is important, upwelling produces darkening by increasing the particle size at optical depth unity. This is due to the suspension of larger particles as well as the lateral removal of smaller particles from the top of the atmosphere. At UV wavelengths and at 0.89 micrometers the albedo is determined by the competing effects of the gas the haze material. Gas is bright in the UV and dark at 0.89 micrometers. Haze transport at high altitudes controls the UV albedo and transport at low altitude controls the 0.89 micrometers albedo. Comparisons between the hemispheric contrast at UV, visible, and IR wavelengths can be diagnostic of the vertical structure of the wind field on Titan.  相似文献   

16.
C.M. Anderson  E.F. Young  C.P. McKay 《Icarus》2008,194(2):721-745
We report on the analysis of high spatial resolution visible to near-infrared spectral images of Titan at Ls=240° in November 2000, obtained with the Space Telescope Imaging Spectrograph instrument on board the Hubble Space Telescope as part of program GO-8580. We employ a radiative transfer fractal particle aerosol model with a Bayesian parameter estimation routine that computes Titan's absolute reflectivity per pixel for 122 wavelengths by modeling the vertical distribution of the lower atmosphere haze and tropospheric methane. Analysis of these data suggests that Titan's haze concentration in the lower atmosphere varies in strength with latitude. We find Titan's tropospheric methane profile to be fairly consistent with latitude and longitude, and we find evidence for local areas of a CH4-N2 binary saturation in Titan's troposphere. Our results suggest that a methane and haze profile at one location on Titan would not be representative of global conditions.  相似文献   

17.
V. Vuitton  R.V. Yelle 《Icarus》2007,191(2):722-742
High-energy photons, electrons, and ions initiate ion-neutral chemistry in Titan's upper atmosphere by ionizing the major neutral species (nitrogen and methane). The Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft performed the first composition measurements of Titan's ionosphere. INMS revealed that Titan has the most compositionally complex ionosphere in the Solar System, with roughly 50 ions at or above the detection threshold. Modeling of the ionospheric composition constrains the density of minor neutral constituents, most of which cannot be measured with any other technique. The species identified with this approach include the most complex molecules identified so far on Titan. This confirms the long-thought idea that a very rich chemistry is actually taking place in this atmosphere. However, it appears that much of the interesting chemistry occurs in the upper atmosphere rather than at lower altitudes. The species observed by INMS are probably the first intermediates in the formation of even larger molecules. As a consequence, they affect the composition of the bulk atmosphere, the composition and optical properties of the aerosols and the flux of condensable material to the surface. In this paper, we discuss the production and loss reactions for the ions and how this affects the neutral densities. We compare our results to neutral densities measured in the stratosphere by other instruments, to production yields obtained in laboratory experiments simulating Titan's chemistry and to predictions of photochemical models. We suggest neutral formation mechanisms and highlight needs for new experimental and theoretical data.  相似文献   

18.
To explain the observed abundances of CO2 in Titan's atmosphere, a relatively high water deposition into the atmosphere needs to be invoked due to the importance of H2O photolysis in CO2 production. A likely source of H2O is icy dust particles from space. This paper considers the direct dust input to Titan's atmosphere from the interplanetary environment, and also ejecta particles from micrometeoroid impacts with the icy satellites Hyperion, Iapetus and Phoebe. It is found that the likely mass influx to Titan is 10–16 to 10–15 kg m–2 s–1. This mass influx is an order of magnitude too low to explain the observed levels of CO2 in Titan's atmosphere in the context of a recent photochemical model. This leads one to speculate as to the likelihood of one large impact to Titan in the recent past;i.e., that the atmosphere is not in equilibrium but is cnrrently losing CO2.  相似文献   

19.
Solar and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. A model of Titan's nightside ionosphere will be described and the results compared with data from the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir probe (LP) part of the Radio and Plasma Wave (RPWS) experiment for the T5 and T21 nightside encounters of the Cassini Orbiter with Titan. Electron impact ionization associated with the precipitation of magnetospheric electrons into the upper atmosphere is assumed to be the source of the nightside ionosphere, at least for altitudes above 1000 km. Magnetospheric electron fluxes measured by the Cassini electron spectrometer (CAPS ELS) are used as an input for the model. The model is used to interpret the observed composition and structure of the T5 and T21 ionospheres. The densities of many ion species (e.g., CH+5 and C2H+5) measured during T5 exhibit temporal and/or spatial variations apparently associated with variations in the fluxes of energetic electrons that precipitate into the atmosphere from Saturn's magnetosphere.  相似文献   

20.
We show that, when a natural satellite like Titan is invisible (e.g., due to an opaque atmosphere) its planetary orbit and its mass can be determined by tracking a spacecraft in close flybys. This is an important problem in the Cassini mission to the Saturnian system, which will be greatly improved by a good astrometric model for all its main components; in particular, an accuracy of a few hundred meters for the orbit of Titan is necessary to allow a measurement of its moment of inertia. The orbit of the spacecraft is the union of elliptical arcs, joined by short hyperbolic transitions: a problem of singular perturbation theory, whose solution leads to a matching condition between the inner hyperbolic orbit and the elliptical orbital elements. Since the inner elements are given in terms of the relative position and velocity of the spacecraft, accurate Doppler measurements in both regions can provide a satisfactory determination of Titan's position and velocity, hence of its Keplerian elements. The errors in this determination are discussed on the basis of the expected Allan deviation of the Doppler method; it is found that the driving errors are those in the elliptical arcs; the fractional errors in Titan's orbital elements are expected to be 10–7. It is also possible to measure the mass of the satellite; however, when the eccentricity e of the flybys is large, the mass and a scaling transformation are highly correlated and the fractional error in the mass is expected to be e times worse.  相似文献   

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