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2013 Annual Science Report

NASA Jet Propulsion Laboratory - Titan Reporting  |  SEP 2012 – AUG 2013

Executive Summary

This report summarizes the 12-month period of research—September 1, 2012 through August 31, 2013—by the JPL-Titan team, hereinafter the NAI Titan team.

A series of coupled model simulations and novel laboratory experiments comprise the core research program of the NAI Titan team. The objective of this coordinated research is to understand the extent to which processes that could be active currently in Titan could lead to the formation of significant prebiotic molecular compounds, to be defined hereinafter as being composed of atoms of hydrogen, carbon, nitrogen, and oxygen. These processes might have been important in the early Earth environment and be on the path to the formation of life.

The NAI Titan research program is organized along the lines of three research themes—“Titan’s geology—places where organic chemistry can operate”, “The complexity of atmospheric organic chemistry”, and “The evolved chemical state of the Titan surface”. Note that most of the funding during this reporting period was spent on Theme 3 projects as was consistent with the selected proposal plan.

An overview of what has occurred in each of the themes is presented in what follows.

Theme 1 – Titan’s geology—places where organic chemistry can operate:

We modeled the equilibrium chemistry of potassium at high pressure in the interior aqueous media in Saturn’s moon Titan to determine the extent of potassium leaching. This, in turn, allows us to test the proposed hydrated silicate core model.

Theme 2 – The complexity of atmospheric organic chemistry:

The development of a master atmospheric model is nearing completion. Recent work involved inclusion of condensation of molecules onto grains, and sublimation back to the gas.

We analyzed the stellar occultation data from Cassini/UVIS instrument. Vertical profiles of tholin particles above ~300 km were retrieved. The profiles show two depletion regions: 750-800 km and 400-500 km ranges. A photochemistry-transport model was developed and used to interpret the production, loss, and transport in the atmosphere.

Work continued on producing realistic global Titan atmospheric profiles [winds, temperatures and densities] from the surface to ~1200km, for a variety of seasons and solar cycles for use by atmospheric models.
An energy source is needed if the organic haze sedimenting from the atmosphere comes in contact with liquid water (briefly, from cryovolcanic eruptions) to form amino acids and other molecules relevant to life. Such an energy source may be the short-wave (ultraviolet – blue) solar radiation that makes its way through Titan’s dense haze layer to the surface. We calculated the amount of UV-blue solar flux at Titan’s surface based on measurements made by the Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens Probe.

Work continued to elucidate the mechanisms and develop a quantitative understanding of particle formation and growth in the Titan atmosphere.

Theme 3 – The evolved chemical state of the Titan surface:

Work continued on understanding the reactions occurring on Titan’s surface with an emphasis on determining whether mineral deposits from meteoritic impacts can catalyze the formation of more complex molecules possessing a prebiotic character.

Work continued on understanding the condensed phase photochemistry on Titan, in particular, the longer wavelength photochemistry of solid hydrocarbons that could be due to the solar ultraviolet radiation penetration through the atmosphere down to Titan’s surface. In addition, we investigated the oxygenation chemistry involving the condensed Titan’s organic aerosols with water-ice on Titan’s surface – induced by high energy photons simulating the cosmic ray induced chemistry on Titan’s surface.

Work continued to measure the solubilities of Titan surface and atmospheric species in cryogenic liquid hydrocarbons, in order to constrain the composition of the hydrocarbon lakes, and provide an understanding into the nature of erosion and sedimentation on Titan. Relatively high organic solubilities suggest that liquid hydrocarbon-based weathering and sorting of surface organics should be occurring on Titan.

We have demonstrated that solid benzene can trap significant amounts of ethane and methane within its crystal structure at Titan surface temperatures. Experiments also suggest that liquid ethane can diffuse into solid benzene, resulting in the formation of a co-crystalline structure. This implies that lake edges and evaporite basins on Titan may hold important quantities of ethane.

We have investigated the potential of nuclear magnetic resonance spectroscopy (NMR) for the elucidation of very complex mixtures of Titan aerosol haze analogues (tholins) with identification of the major components of modest complexity using 1, 2 and 3 dimensional spectral techniques. We also have performed studies of the utility of low resolution NMR on low temperature liquid hydrocarbon mixtures analogous to Titan lake liquids towards the development of multidimensional NMR instrumentation suitable for future flight missions to solar system bodies of organic composition.

We have investigated the capability of the direct analysis in real time (DART) analysis technique, combined with an ion trap mass spectrometer having two dimensional capability, to be an enabling experimental methodology to vaporize, ionize, and structurally characterize organic components of the complex surface organic composition.

To explore the question of genetic molecules, metabolic processes, and bio-structures in a non-water-based biochemistry, we are looking in the context of Titan for genetic molecules that might support Darwinian evolution, including non-ionic polyether molecules and biopolymers linked by exotic oxyanions (such as phosphite, arsenate, arsenite, germanate). We explored options in a “warm Titan” reference case. We found that propane could be a biosolvent for certain of these “weird” alternative genetic biopolymers; propane has a huge liquid range (far larger than water).

We developed a cryogenic chamber capable of simulating Titan’s lake conditions (1.5 bar N2 atmosphere, liquid mixtures of methane-ethane-propane in variable concentrations) to provide an environment for testing components and small instruments for potential future Titan missions.