Monday, February 4, 2013

Torrential Rains on Titan

Figure 1: Bright tropospheric clouds on Titan. Credit: NASA/JPL/Space Science Institute.

The thick nitrogen atmosphere of Titan supports a methane hydrological cycle that is akin to Earth’s water cycle. Like the Earth, precipitation and storm activity appear to be quite common on Titan. On Titan’s surface, numerous fluvial features point towards a hydrology based on liquid methane. Much of Titan’s surface is kept wet by a light but persistent drizzle of liquid methane which forms an enduring component of Titan’s methane hydrological cycle. Data collected by the Huygens entry probe during the descend through Titan’s atmosphere in January 2005 suggests the presence of an upper cloud layer of methane ice between 20 km to 30 km and a lower cloud layer of liquid methane-nitrogen between roughly 8 km to 16 km. The upper cloud layer of methane ice is akin to terrestrial cirrostratus clouds while the lower cloud layer of liquid methane-nitrogen is akin to terrestrial stratiform clouds. A gap between the upper and lower cloud layers exists because that region is too cold to sustain liquid clouds, but slightly too dry for pure methane to condense.

Since the upper cloud layer of methane ice is at saturation (100 percent relative humidity), methane ice crystals can grow there and eventually precipitate out. As the methane ice crystals descend down the atmosphere, they warm up and melt before reaching the surface as a light drizzle. The supply of methane for the lower cloud layer upon comes from the melting of falling methane ice crystals. Once melted, the methane droplets also allow nitrogen in the atmosphere to dissolve in them and become droplets of liquid methane with dissolved nitrogen. As the drizzle of methane-nitrogen droplets fall towards the ground, they become less enrich in nitrogen due to the preferential evaporation of dissolved nitrogen. This basic cloud structure is believed to be widespread and represents more than half of Titan’s surface. The low precipitation rate of such a drizzle means that its minuscule erosive potential cannot explain the widespread fluvial features on Titan. However, it can easily account for the generally wet character of the surface material at the Huygens landing site and possibly on most of Titan.

Figure 2: Artist’s impression of a storm on Titan. Credit: Mark A. Garlick.

Precipitation events with strong erosive potentials are required to form the widespread fluvial features on Titan and such intense precipitation events do occur on Titan. Remote observations of Titan have revealed the presence of short-lived tropospheric clouds. These clouds are likely to be convective storms that bring significant amounts of methane precipitation. Three dimensional models show that methane convection storms accompanied by heavy precipitation can occur on Titan under favourable conditions. Such favourable conditions include the presence of aerosol particles that act as cloud condensation nuclei, updrafts provided by the Hadley cell convergence or forced lifting of air masses from topographic features.

When the relatively humidity is over 80 percent, a small temperature perturbation of just 0.5 K can trigger convection, causing methane in the air mass to condense. Latent heat released from the condensation of methane powers strong updrafts with speeds of up to 20 m/s. This establishes a convective cell with cloud tops attaining altitudes of up to 30 km. Within the convective cell, the condensed methane rains out over a period of 4 to 8 hours. During such a torrential downpour, the accumulated precipitation on the surface can be as high as 110 kg/m2, comparable to severe storms on Earth. These three dimensional models show that intense downpours of methane can occur on Titan under the right environmental conditions and have the erosive power necessary to create the fluvial features on Titan’s surface. Such intense downpours are expected to leave signatures in the form of temporary liquid deposits and mild surface cooling. More observations of the cloud activity on Titan will provide a better understanding of the role convection plays in Titan’s methane hydrological cycle.

References:
1. Tetsuya Tokano, et al., “Methane drizzle on Titan”, Nature 442, 432-435 (27 July 2006)
2. R. Hueso1 and A. S├ínchez-Lavega, “Methane storms on Saturn’s moon Titan”, Nature 442, 428-431 (27 July 2006)