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Project Progress

Deputy Principal Investigator Jonathan Lunine considered the degree to which Titan might be habitable under various conditions. Titan’s surface is blanketed in the equatorial regions by dunes composed of solid organics, sculpted by wind and fluvial erosion, and dotted at the poles with lakes and seas of liquid methane and ethane. The processes that work the surface of Titan resemble in their overall balance no other moon in the solar system; instead, they are most like that of the Earth. The presence of methane in place of water, however, means that in any particular planetary system, a body like Titan will always be outside the orbit of an Earth-type planet. We show that around M-dwarfs, planets with a Titan-like climate will sit at 1 AU – a far more stable environment than the 0.1 AU where Earth-like planets sit. However, an observable Titan-like exoplanet might have to be much larger than Titan itself to be observable, increasing the ratio of heat contributed to the surface atmosphere system from internal (geologic) processes versus photons from the parent star. Lunine also evaluated phosphorus chemistry on Titan using a combination of modeling and laboratory techniques. He showed that phosphorus chemistry on Titan consists of exogenous phosphates and reduced oxidation state phosphorus compounds, and accretionary phosphine. Accretionary phosphorus is shown to be delivered primarily by rocks and ices in the Saturnian sub-nebula, and heating during accretion concentrates phosphine in the crust of Titan. The exogenous compounds are capable of performing biologically-relevant chemistry, however they are active only in environments with substantial liquid water, either pure, or as a mixture with NH3 or nitrile compounds. In contrast, he showed that phosphine is soluble in methane and ethane on Titan’s surface, hence phosphine likely participates in the hydrocarbon cycle on Titan. The lack of mobility of phosphate compounds on Titan’s surface suggests that if life is present on Titan, it must have a fundamentally different biochemistry than does terrestrial life.

Pasek, M., Mousis, O. and Lunine, J.I. Phosphorous chemistry on Titan. Icarus, submitted.