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Titan’s Methane Cycle

Liquid is a thermodynamically stable phase for CH4 at the surface of Titan. It has long been proposed that lakes or seas exists on Titan.


The  Cassini Titan Radar Mapper instruments had its sixth radar pass of Titan (T16) on 22 July 2006 (UTC). The synthetic aperture radar (SAR) arc-shaped imaging swath extends from mid-northern latitudes to near the north pole and back, and is 6, 130 km long with spatial resolutions of 300-1, 200 m  (figure on right). The portion of the swath that extended from 70o to 83o north contained more than 75 radar dark patches, from 3km to more than 75 km across.


The radar backscatter of the dark patches is consistent with that expected from a very smooth surface. Radiometric brightness temperature that obtained along with the SAR swaths indicated the patches are flat surfaces of a low-dielectric material, such as liquid CH4.


There are some interesting features for these dark patches. At several of the dark patches, sinuous features lead into the dark patch (figure a below), which resemble channels elsewhere on Titan interpreted to be fluvial in origin. Two of the dark patches are connected by a narrow channel (figure b below). However, no extensive network of channels is seen, suggesting that the channels are not the only source for infilling the dark patches.

Circumstantial evidence for liquid on the surface come in the form of the so-called lakes, which are features of varying shapes and sizes that have extremely low reflectivity in Cassini RADAR data.

Lakes on Titan

North portion of the T16 swath. Dark patches are interpreted as lakes. 1o of latitude is about 45 km.

Several of the depressions seem to be only partly filled with liquid. These depressions may never have filled fully, or may have partly evaporated in the past (figure c on right). Other depressions appear dehydrated (figure d on right).  These lakes in possible varying states of fill suggest that the lakes in northern high-latitude region might be ephemeral on some unknown time scale. It is plausible that they are in the middle of some hydrological cycle of liquid CH4 . If such lakes cover 0.2 ~ 4% of Titan’s surface, they will buffer the atmospheric CH4 ‘s relative humidity at its observed value, removing the requirement for a putative steady drizzle at the equator. If the abundance of lakes seen Cassini SAR swath T16 are typical of their coverage poleward of about 70o in both hemisphere, the fraction of  Titian’s surface covered by lakes is within this range.





Stofan, E. R. et al. The Lakes of Titan. Nature. Vol. 445. 2007.

Lunine J.I. and Atreya S.K. The methane cycle on Titan. Nature Geoscience. Vol. 1. 2008


Fifteen dark patches are in relatively steep-sided, rimmed, circular depressions, in contrast with other dark patches that exhibit no such topography at this scale. These depression seem to have exited in this form before being filled with dark material, and do not show clear evidence of erosion. The dark patches in depression resemble terrestrial lakes confined within impact basins, volcanic calderas or karst dolines or sinkholes. Impact basin is not very likely due to the nested nature and limited size of the depression. A volcanic origin is possible given their morphology and the previous identification of cryovolcanic calderas on Titan.


According to the radiometric and morphological characteristics, these dark patches are either liquid-filled lakes or depressions and channels formed in the past which have now been infilled by a very low-density deposit—most likely, liquid hydrocarbon. The liquids most likely contain CH4 and C2H6 because pure CH4 lakes have sufficiently high vapor pressure to cool and freeze, which would potentially lead to rough surfaces and temperatures inconsistent with the elevated emissions relative to the surroundings at radio wavelengths. With significant quantities (>50%) of C2H6 present,  the vapor pressure is suppressed to maintain a liquid fill.

A partly-filled depression

A dehydrated depression

a. Channels leads to the depression 

b. Channels connecting two depressions