ScienceWeek

PLANETARY SCIENCE: ON THE CLOUDS OF TITAN

The following points are made by Emmanuel Lellouch (Science 2006 311:186):

1) In planetary exploration, progress does not always come where you expect. In recent years, the predominantly nitrogen atmosphere of Saturn's moon Titan, often touted as a laboratory for understanding the early Earth, has not revealed as many clues about the complexity of its chemical evolution as was hoped. On the other hand, a wealth of new data suggest that, in terms of physical processes, Titan's surface and atmosphere may be more similar to those of Earth than any other body in the Solar System. New work[1] demonstrates that many observed features of Titan's clouds can be explained in terms of a dynamically controlled methane cycle sharing essential features with the water cycle on Earth, but with important differences.

2) Given the cold temperatures prevailing in Titan's troposphere (94 K at the surface, 71 K at 40-km altitude), methane, the second most abundant gas in Titan's atmosphere (about 5% of the atmosphere at the surface), is condensable in its liquid or solid forms, as is water on Earth. Whether methane actually condenses to form clouds, however, depends on many factors. The latest analyses of the Voyager data [2] left little room for clouds, finding instead an abundance of methane in the upper stratosphere well in excess of vapor-pressure equilibrium.

3) But 10 years ago, clouds were discovered in ground-based observations [3] from their effect on the near-infrared brightness of Titan's disk and shown to be present at 15- to 25-km altitude. This breakthrough initiated a decade of uninterrupted progress. The observations could distinguish between sparse clouds, variable over time scales of hours, which were suggestive of convective evolution, and large storm systems. Since 2000, direct imaging and spectral imaging observations [4.5] provided the ultimate proof for the existence of clouds and revealed their geographic locations. Temporal monitoring of the clouds from ground-based observations and their imaging from Cassini indicate that there are currently two classes of clouds on Titan: large storms near the south pole, variable but with relatively long lifetimes of several weeks; and short-lived (typically one terrestrial day), often elongated clouds at mid-southern latitudes (40 deg S).

4) Why are the Titan clouds located at these particular latitudes? On Titan, with the exception of a surface boundary layer, perhaps a few kilometers high, the troposphere is mostly controlled by radiative, rather than convective, processes. As a result, the level of free convection, where parcels of moist air become buoyant upon condensation and can ascend in the atmosphere, is normally well above the top of the boundary layer. Convective clouds can possibly form if the thickness of this convective surface layer is increased by enhanced surface temperatures, and it was initially proposed [4] that clouds at the south pole result from the maximum insolation currently experienced by this region. This explanation is a little too simple. As is also true on Earth, even at southern summer solstice -- which for Titan occurred in October 2002 -- the south pole is probably not the point where surface temperatures are highest. Radiative time scales in Titan's troposphere are so long that the surface temperatures tend to be fixed by the annually averaged insolation. In addition, the amount of solar energy that reaches the surface is limited by absorption by the stratospheric haze, which appears to be enhanced year-round in polar regions owing to the general atmospheric circulation.

References (abridged):

1. P. Rannou et al., Science 311, 201 (2006)

2. R. E. Samuelson et al., Planet. Space Sci. 45, 959 (1997)

3. C. A. Griffith et al., Nature 395, 575 (1998)

4. M. E. Brown et al., Nature 420, 795 (2002)

5. H. G. Roe et al., Astrophys. J. 618, L49 (2005)

Science http://www.sciencemag.org

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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)

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