Titan

Voyager 1 revealed Titan's atmosphere to be predominantly molecular nitrogen with methane as the secondary constituent. The atmosphere is both dense and deep. The surface pressure is 1.5 bars. As a result of reactions in the upper atmosphere driven by solar ultraviolet and magnetospheric charged particles, a wide variety of complex hydrocarbons are produced. It is these that bestow the orangey hue on the optically

As Voyager 1 flew by Rhea it assembled this mosaic of the north polar region (the pole in on right of frame, on the terminator) displaying a variety of terrain types.

As Voyager 1 left Rhea behind, it snapped this view showing what appears to be a multiple-ringed impact structure on the terminator. The north pole is located on the terminator at the top.

thick haze that forms the visible 'surface' at an altitude of 200 kilometres. A number of thin layers of semi-transparent haze exist above this on a global basis, at altitudes ranging up to about 750 kilometres.57 While this was fascinating for the atmospheric specialists,58 the fact that the surface was obscured made the fly-by rather frustrating for the geologists. Nevertheless, the atmospheric composition prompted speculation as to the nature of the hidden surface.

Carl Sagan and Stanley Dermott at Cornell suggested in 1982 that Titan might be completely covered with an ocean of methane.59 As methane is continuously being dissociated in the upper atmosphere, it must be being replenished from the surface, and it was argued that this source must be in a liquid state. Because Titan's orbit is slightly eccentric, the resulting tide in this methane ocean would generate friction on the sea floor, which would tend to slow the moon's rotation. By the conservation of angular momentum, the moon's orbit would become perfectly circular. However, the fact that the orbit is still eccentric implied that there could not be shallow seas; there must either be no ocean, or a very deep one (a depth of 300 metres was suggested) in which the surface tides are decoupled from the ocean floor.60

With a diameter of 5,150 kilometres and a density of 1.88 g/cm3, Titan is broadly comparable with Ganymede. It should have inherited enough radioactive elements

Voyager 1's infrared spectrometer was able to identify a variety of hydrocarbons in Titan's upper atmospheric temperature inversion by virtue of their emission lines.

from the solar nebula to have kept its interior sufficiently warm for differentiation to have occurred.61 This would have driven the early devolatisation which formed the atmosphere. When the Infrared Space Observatory accurately measured the ratio of deuterium to hydrogen in Titan's stratosphere, it found it to be four times less than in comets. This implied that the atmosphere is not of cometary origin, and therefore had to be derived from outgassing.62 Recent millimetre-wavelength measurements of the ratio of nitrogen isotopes in Titan's atmosphere suggest that the 'air' was once 30 times thicker than it is now. The lighter isotope has been depleted.63,64,65,66 The fact that such an imbalance is not present in the carbon isotopes of methane implies that the methane content of the atmosphere is ephemeral. As the rate at which methane is currently being lost to photochemical reactions will erode the observed concentration in 10 million years, the atmosphere must be being replenished - if a global ocean is present this would be by evaporation, and if not then as a result of cryovolcanism. If this is done only on an intermittent basis, the atmosphere may occasionally thin and chill as methane's greenhouse effect is depleted.67 The cryovolcanically released methane might be vented directly into the atmosphere, but it might also be extruded as liquid and then drain into low-lying areas and become dissolved in ethane and higher hydrocarbons such as propane and acetylene, whose vapour pressure is too low for them to evaporate, and so must be accumulating on the surface in liquid form. Because the tropospheric methane concentration is close to its saturation point (it cannot condense in the lower troposphere, it can do so only by rising to the 'cold trap' of the tropopause), it will evaporate slowly.68

Surface structures will have been physically eroded by the hydrocarbon rainfall, and chemically eroded as this drained off into the low-lying areas. If there is a near-global ocean, the several-metre-amplitude tides will erode the icy shoreline. In the absence of ongoing tectonism to rebuild topography the icy surface might have been more or less levelled, leaving a predominantly oceanic environment. Such a reservoir of hydrocarbons on the surface would draw a significant amount of nitrogen from

Temperature IK)

Once the radio data from Voyager l's Titan occultation had been analysed, it gave a profile of the atmosphere. Above the surface, the temperature decreases towards the tropopause at an altitude of about 45 kilometres. There is a temperature inversion in the stratosphere. The ultraviolet and infrared instruments detected several 'detached' layers of haze far above the optically-thick orangey haze that forms the visible limb, and a layer of cold methane cloud near the surface.

Temperature IK)

Once the radio data from Voyager l's Titan occultation had been analysed, it gave a profile of the atmosphere. Above the surface, the temperature decreases towards the tropopause at an altitude of about 45 kilometres. There is a temperature inversion in the stratosphere. The ultraviolet and infrared instruments detected several 'detached' layers of haze far above the optically-thick orangey haze that forms the visible limb, and a layer of cold methane cloud near the surface.

the atmosphere.69 If the atmosphere is controlled by its being in thermodynamic equilibrium with the condensed volatiles on the surface, then the climate may be subjected to strong positive feedback mechanisms, and the atmosphere's density and temperature might be extremely changeable over geologically brief timescales. But is Titan really a vast ocean?

A glimpse of the surface

In the aftermath of the Voyager fly-by, it was presumed that Titan's orangey haze was optically thick across the visual and infrared, and that we would therefore be denied a direct view of the surface until a microwave imaging radar mission could be mounted. However, it was found that methane absorption is weak in parts of the near-infrared, and in the narrow band in the region between the Voyagers' ISS and IRIS instruments the atmosphere is semi-transparent.

In 1994, a team led by P.H. Smith of the Lunar and Planetary Laboratory at the University of Arizona observed Titan in the 0.94-micron 'window' using the Hubble Space Telescope's Wide Field Planetary Camera, and characterised its surface in terms of reflectivity.70 In fact, they had been hoping to see tropospheric

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