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ScienceWeek
ASTRONOMY: LIQUID SURFACES ON THE MOON TITAN
The following points are made by D.B. Campbell et al (Science 2003 302:431):
1) As the largest satellite of Saturn and the only one in the Solar System with a substantial atmosphere, Titan is of considerable interest, an interest heightened by the Cassini mission's rendezvous with the Saturn system in 2004.
2) Photochemically produced haze layers in Titan's upper atmosphere have made it difficult to investigate its lower atmosphere and surface at optical wavelengths. However, it is possible to observe the surface with Earth- and spacecraft-based radar systems, because the atmosphere is transparent at radio wavelengths.
3) The strength and variability of the radar backscatter cross sections measured at 3.5-cm wavelength (1) indicated that Titan's surface is not homogeneous and cast doubt on models for Titan's atmosphere and surface that suggested the presence of a deep hydrocarbon ocean (2). Observations in the near infrared (IR) with the Hubble Space Telescope and ground-based telescopes using speckle imaging and adaptive optics techniques (3-5) have provided coarse surface maps of Titan. Especially notable is a bright, high-albedo region centered near 110 deg longitude and extending over 90 deg in longitude. Near-IR spectroscopic observations of the bright region suggest that its composition is primarily that of water ice.
4) The authors report on observations with the recently upgraded Arecibo 13-cm-wavelength radar system. The authors observed Titan on 16 nights in November and December 2001 and on 9 nights in November and December 2002, transmitting at 13-cm wavelength with the 305-m Arecibo telescope and receiving the echo with Arecibo. Titan's rotational and orbital periods are 15.9 days, and the 2001 observations were obtained at a uniform 22.6 deg (800 km) interval in longitude. The 9 observations in 2002 did not provide uniform coverage.
5) In summary: 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 sub-earth 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.
References (abridged):
1. D. O. Muhleman, A. W. Grossman, B. J. Butler, M. A. Slade, Science 248, 975 (1990)
2. J. I. Lunine, D. J. Stevenson, Y. L. Yung, Science 222, 1229 (1983)
3. P. H. Smith, M. T. Lemmon, R. D. Lorenz, L. A. Sromovsky, J. J. Calwell, M. D. Allison, Icarus 119, 336 (1996)
4. S. G. Gibbard et al., Icarus 139, 189 (1999)
5. R. Meier, B. A. Smith, T. C. Owen, R. J. Terrile, Icarus 145, 462 (2000)
Science http://www.sciencemag.org
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ON THE ATMOSPHERE OF SATURN'S MOON TITAN
The following points are made by P. Rannou et al (Nature 2002 418:853):
1) Titan, the largest moon of Saturn, is the only satellite in the Solar System with a dense atmosphere. Titan's atmosphere is mainly nitrogen with a surface pressure of 1.5 atmospheres and a temperature of 95 kelvins(1). A seasonally varying(2) haze, which appears to be the main source of heating and cooling that drives atmospheric circulation(3,4), shrouds the moon. The haze has numerous features that have remained unexplained. There are several layers(5), including a "polar hood", and a pronounced hemispheric asymmetry(2). The upper atmosphere rotates much faster than the surface of the moon, and there is a significant latitudinal temperature asymmetry at the equinoxes.
2) Earth-based observations have long indicated the presence of CH4 in Titan's atmosphere and the possibility of a rich organic chemistry. The complexity of Titan's atmospheric photochemistry was confirmed by Voyager observations in 1980 which revealed the presence of nine gaseous organic molecules other than CH4. In addition, laboratory simulations using gas mixtures identical to Titan's atmosphere have shown that the solid organic material produced in the laboratory has the same optical properties as the haze determined from Titan's geometric albedo. Radiative transfer calculations show that this haze is the dominant absorber of sunlight in Titan's atmosphere, particularly in the ultraviolet and blue regions of the spectrum, with the result that only 10% of the solar flux at the top of Titan's atmosphere reaches the surface. A significant fraction of sunlight (40%) is absorbed by the haze in the stratosphere, creating an anti-greenhouse effect. In addition, the haze is also the dominant source of infrared cooling in the stratosphere, and therefore it is not surprising that the stratospheric circulation is dominated by radiative balance due to the haze.
3) The authors describe a numerical simulation of Titan's atmosphere, which appears to explain the observed features of the haze. The critical new factor in the model is the coupling of haze formation with atmospheric dynamics, which includes a component of strong positive feedback between the haze and the winds.
References (abridged):
1. Lellouch, E. et al. Titan's atmosphere and hypothesis ocean: a re-analysis of the Voyager 1 radio-occultation and IRIS 7.7-m data. Icarus 79, 328-349 (1989)
2. Sromovsky, L. A. et al. Implication of Titan's north-south brightness asymmetry. Nature 292, 698-702 (1981)
3. Hourdin, F. et al. Numerical simulation of the general circulation of the atmosphere of Titan. Icarus 117, 358-374 (1995)
4. Tokano, T., Neubauer, F. M., Laube, M. & McKay, C. P. Seasonal variation of Titan's atmospheric structure simulated by a general circulation model. Icarus 47, 493-520 (1999)
5. Rages, K. & Pollack, J. B. Vertical distribution of scattering haze in Titan's upper atmosphere. Icarus 55, 50-62 (1983)
Nature http://www.nature.com/nature
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NUMERICAL SIMULATION OF THE GENERAL CIRCULATION OF THE ATMOSPHERE OF TITAN.
The following points are made by F. Hourdin et al (Icarus 1995 117:358):
1) The authors report the atmospheric circulation of Titan was investigated with a general circulation model. The representation of the large-scale dynamics is based on a grid point model developed and used at Laboratoire de Meteorologie Dynamique for climate studies. The code also includes an accurate representation of radiative heating and cooling by molecular gases and haze as well as a parametrization of the vertical turbulent mixing of momentum and potential temperature.
2) Long-term simulations of the atmospheric circulation are presented. Starting from a state of rest, the model spontaneously produces a strong superrotation with prograde equatorial winds (i.e., in the same sense as the assumed rotation of the solid body) increasing from the surface to reach 100 m/sec near the 1-mbar pressure level. Those equatorial winds are in very good agreement with some indirect observations, especially those of the 1989 occultation of Star 28-Sgr by Titan. On the other hand, the model simulates latitudinal temperature contrasts in the stratosphere that are significantly weaker than those observed by Voyager 1 which, the authors suggest, may be partly due to the nonrepresentation of the spatial and temporal variations of the abundances of molecular species and haze.
3) The authors present diagnostics of the simulated atmospheric circulation underlying the importance of the seasonal cycle and a tentative explanation for the creation and maintenance of the atmospheric superrotation based on a careful angular momentum budget.
Icarus http://icarus.cornell.edu/
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THE THERMAL STRUCTURE OF TITAN'S ATMOSPHERE.
The following points are made by C.P. McKay et al (Icarus 1989 80:23):
1) The authors report they have developed a radiative-convective model of the thermal structure of Titan's atmosphere. The model computes the solar and infrared radiation in a series of spectral intervals with vertical resolution. Sources of opacity in the visible and near infrared include stratospheric haze particles, methane cloud particles, and gaseous methane; sources of opacity in the thermal infrared include the pressure-induced opacity of N2, CH4, and H2, the permitted transitions of C2H2 and C2H6, and particulate opacity.
The haze properties are determined with a simple microphysics model. The model contains a minimum of free parameters and the authors attempt to determine these by fits to independent data sets. The authors find that gas and haze opacity alone, with the temperatures fixed by Voyager observations, produces a model that is within a few percent of radiative convective balance everywhere in the atmosphere. In a self-consistent computation of temperatures, the authors find that their model calculation for the surface temperature is, in general, colder than the observed value by 5-10 kelvins.
The presence or absence of methane condensation clouds only slightly alters the results. Good agreement can be obtained by adjusting the parameters in the model. The model parameters in these optimized cases are typically within 15% of the baseline values and within the limits allowed by observations. The authors conclude that the most important factors controlling Titan's thermal structure are absorption of sunlight by the stratospheric haze and the pressure-induced gas opacity in the infrared. Within the uncertainties of the model, these effects can explain the observed temperature profile. Condensation clouds play a minor role, if any.
Icarus http://icarus.cornell.edu/
ScienceWeek http://www.scienceweek.com
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