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Exploring the atmosphere |
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Tianyi Fan’s Home Page |
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Titan’s Methane Cycle |
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CH4 rain is thermodynamically allowed anywhere in today’s Titan atmosphere. Some Titan GCMs suggest that seasonal clouds should form above 70o latitude, though whether they will precipitating is unknown. On the basis of morphology, the south pole clouds are convective and could be precipitating. Fluvial features, such as flooded hills and dendritic drainages, suggest there are once large body of fluid carving the Titan’s surface. Those features looks young, in the sense that there are no impact craters that overlay them. At the Huygens landing spot, channels appear very fresh on the basis of steep topography derived form stereo imagery and lack of evident degradation in the high resolution camera images. It is therefore plausible that the features are actively carved over recent geological time. Assuming the mechanical properties of water ice for crust, it is suggested only 10 parts of million of the total atmospheric content of CH4 is required in the form of a rainfall typical of a convective storm in order to carve the channels at the Huygens site. The Cassini radar data indicate that about 1% of the surface imaged is covered with fluvial features, although only 20% of the surface has been imaged. If we take 1% as typical, then the known inventory of fluvial features could be carved easily with the amount of CH4 in today’s atmosphere. |
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Evidence of CH4 clouds and rains comes from observations and general circulation models, indicating that Titan has a CH4 cycle analogue to the hydrological cycle on Earth. However, constraint by the available solar radiation, the CH4 cycle is much longer and difficult. To saturate the atmosphere with CH4, a timescale of 1000 years is needed. We call it a ‘short-term’ cycle since resupplying the CH4 loss by photolysis takes much longer timescale. |
Short-Term CH4 Cycle
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How and when do the heavy rainfall occur? Models of convective CH4 storms indicate that a relative humidity of 80% must be present. This number is not reached at Titan’s atmosphere today, which is 45% at the equator. The source of the additional CH4 might plausibly be the lakes. Evaporation of CH4 to ‘wet down’ the troposphere would take centuries, based on the limited available solar flux. Once sufficient evaporation has been achieved, the conditions at the equator would be several degrees warmer than at present because CH4 is a strong green-house gas. Convective storms would be triggered in the equatorial and mid-latitudes, scouring the surface. But the absence of and ocean and the sluggish re-evaporation associated with the small solar flux would prevent the atmosphere from recharging its supply of moisture. Eventually the atmosphere would become dry enough, except at the colder poles, that convective storms would cease, and slow recharge of the atmosphere from the lakes would begin again. How do the lakes recharged with methane? It is possible that the lakes are deep enough that only a small fraction of their methane is evaporated into the atmosphere until conditions of convective instability are reached. Alternatively, the crust might be porous and/or heavily fractured, so that the methane flows gradually back to the coldest places in the crust — the poles. Finally, direct evaporation and transport by Hadley cell circulation might occur. Interestingly, at the landing site, methane (and possibly ethane and other organics) was detected by the Huygens probe mass spectrometer vaporizing out from under the warm underbelly of the probe, and it has been suggested that a slow drizzle might occur even today at the site. Whether the detected methane is part of a larger reservoir under the plains where Huygens landed, or a local effect of drizzle, is not known. References Lunine J.I. and Atreya S.K. The methane cycle on Titan. Nature Geoscience. Vol. 1. 2008 |
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The Cassini RADAR shows details of dendritic channels in high northern latitudes. |