Capability Development for Simulation of Titan's Methane Convection, 15-R9560

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Principal Investigators
Scot C.R. Rafkin
Erika L. Barth

Inclusive Dates:  07/01/05 – Current

Background - Recent discoveries from NASA's multi-billion dollar Cassini mission have sparked a renewed interest in Titan, Saturn's largest moon. Images of possible storm clouds at Titan's South Pole, river-like surface features, and evidence for surface moisture at the Huygens landing site call into question the nature of Titan's methane cycle, which has been likened to the Earth's hydrological cycle. Convective methane clouds may play an important role in the methane cycle and may produce precipitation that alters the surface, just as convective water clouds do on Earth.

Approach - This project was undertaken to establish the SwRI Space Science and Engineering Division's Planetary Atmospheres group in the Titan community. We do so by developing a new state-of-the-art Titan atmospheric model. The goal of this research is to adapt an existing SwRI atmospheric model to simulate convective methane clouds in Titan's atmosphere. The model, called the Regional Atmospheric Modeling System (RAMS), is a numerical code designed to simulate the local and regional (but not global) weather on Earth and Mars. It can be used to simulate phenomena such as convective clouds that are too small to be modeled with the large grid sizes of the perhaps better known General Circulation Models (GCMs). The nested grid structure of RAMS allows for the resolution of small-scale (kilometer-sized) phenomena such as thunderstorms embedded in the larger-scale environment modeled on a coarser grid. The model utilizes nonhydrostatic physics and contains more sophisticated microphysical routines than a GCM. For Titan's clouds, we have added methane and ethane to the model. Additionally, we have added the physics for freezing cloud droplets and melting ice crystals as Titan's troposphere spans a range of temperatures that allows for both phases of methane.

Accomplishments - We have shown that the abundance of methane near the surface is a key factor in the formation of convective clouds in Titan's atmosphere. Initiating the model with a surface humidity of 60 percent (an estimation derived from the Voyager flyby results), produces large convective clouds with vertical extents from about 3 to 35 kilometers. The clouds span a range of temperatures such that below about 16 km the particles are liquid droplets composed of a solution of methane and nitrogen, and above this altitude they are methane ice crystals. The clouds formed in this scenario could be similar to the south polar clouds seen by the Cassini spacecraft and ground-based observers.

Another scenario uses the data from the Huygens probe to simulate a much dryer environment (40 percent surface humidity). In this scenario, the model does not produce convective clouds, rather a thinner stratiform cloud composed of methane and nitrogen droplets between altitudes of 5 and 10 km. The absence of convection is consistent with thermodynamic diagrams of the region, and an optically thin stratiform cloud composed of liquid droplets below 15 km was seen by the Huygens probe during its descent.

Figure 1. Methane convective cloud formed in an environment with 60% surface humidity. The cloud is shown at a time of 3 hours after initial cloud formation occurs. Latent heat release from methane condensation drives large updrafts of 10-20 m/s. Shaded contours are log10 of the number of cloud particles (per cm3) for both droplet and ice clouds and contour lines indicate vertical velocities (m/s).
Figure 2. Stratiform cloud formed in an environment similar to that seen by the Huygens probe, 3 hours after initial cloud formation occurs. In this dryer environment, there is insufficient energy to promote convection nor does the cloud reach altitudes high enough for the cloud droplets to freeze. Shaded contour and contour lines have the same meaning as in Figure 1.

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