ScienceWeek

PLANETARY SCIENCE: ON SATURN'S MOON TITAN

The following points are made by J-P. Lebreton et al (Nature 2005 438:758):

1) Titan is the second-largest moon in the Solar System, after Jupiter's Ganymede, and is assumed to have formed in the Saturn subnebula about 4.5 billion years ago. One of its great mysteries is the origin of the methane in the atmosphere. With a lifetime of just 20 million years, methane must be regularly resupplied to the atmosphere to be as abundant as it is today. The surface of Titan remained hidden to the Voyager cameras, which led to speculation on its appearance and processes. The surface pressure on Titan is about 1.5 times that on Earth and the surface temperature is about minus 180 deg C. At such a low temperature, it was postulated that liquid methane might be present on Titan's surface or in underground reservoirs. Although the images returned by the Voyager spacecraft were featureless, the richness of the detected organic compounds confirmed that Titan was indeed worthy of being revisited and explored in detail.

2) The distinct orange appearance of Titan's atmosphere, as observed by the Voyagers in the early 1980s, comes from the methane-induced organic chemistry. Complex hydrocarbons and carbon-nitrogen-based compounds form high in the atmosphere, which is irradiated by solar ultraviolet rays and bombarded by energetic particles from Saturn's space environment. Methane converts to ethane, acetylene, ethylene, and so on, and when combined with nitrogen forms hydrogen cyanide and more complex nitrogen-bearing carbon and hydrocarbon compounds. These organic compounds float slowly downward in the atmosphere, condense in the stratosphere, and form the aerosols that give the well-known orange color to Titan's hazy atmosphere. The aerosols eventually rain to the surface, where they accumulate.

3) Images of Titan's surface at various resolutions were obtained by the Hubble Space Telescope[1,2] and ground-based observatories[3,4]. Early images of Titan's surface obtained by the Cassini orbiter[5] were almost as baffling as those obtained from Earth. Bright and dark patches were clearly visible on the surface. Albedo patterns suggested a heterogeneous active surface, perhaps with some fluvial processes. No direct evidence of surface liquid was found before the Huygens probe, although ground-based radar observations were interpreted as indicative of the presence of liquid surfaces near the equator.

4) An exciting scientific data set was returned by the Huygens probe, offering a new view of Titan, which appears to have an extraordinarily Earth-like meteorology, geology, and fluvial activity (in which methane would play the role of water on Earth). While many of Earth's familiar geophysical processes appear to occur on Titan, the chemistry involved is quite different. Instead of liquid water, Titan has liquid methane. Instead of silicate rocks, Titan has frozen water ice. Instead of dirt, Titan has hydrocarbon particles settling out of the atmosphere. Titan is an extraordinary world having Earth-like geophysical processes operating on exotic materials under very alien conditions.

5) In summary: Titan, Saturn's largest moon, is the only Solar System planetary body other than Earth with a thick nitrogen atmosphere. The Voyager spacecraft confirmed that methane was the second-most abundant atmospheric constituent in Titan's atmosphere, and revealed a rich organic chemistry, but its cameras could not see through the thick organic haze. After a seven-year interplanetary journey on board the Cassini orbiter, the Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January and landed softly after a parachute descent of almost 2.5 hours. The authors report an overview of the Huygens mission, which enabled studies of the atmosphere and surface, including in situ sampling of the organic chemistry, and revealed an Earth-like landscape. The probe descended over the boundary between a bright icy terrain eroded by fluvial activity probably due to methane and a darker area that looked like a river- or lake-bed. Post-landing images showed centimeter-sized surface details.

References (abridged):

1. Smith, P. H. et al. Titan's surface revealed by HST imaging. Icarus 119, 336 349 (1996)

2. Meier, R. , Smith, B. A. , Owen, T. C. & Terrile, R. J. The surface of Titan from NICMOS observations with the Hubble Space Telescope. Icarus 145, 462 473 (2000)

3. Gibbard, S. G. et al. Titan: high-resolution speckle images from the Keck telescope. Icarus 139, 189 201 (1999)

4. Coustenis, A. et al. Maps of Titan's surface from 1 to 2.5 microm. Icarus 177, 89 105 (2005)

5. Porco, C. C. et al. Imaging of Titan from the Cassini spacecraft. Nature 434, 156 165 (2005)

Nature http://www.nature.com/nature

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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)

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