SRC 2014 Research Proposal Solubility of Noble Gases Under Titan Conditions

Mentor: The Miller Lab, California Institute of Technology

Research Participants: Alice Schmitt, Eric Liu

The study of Solar System bodies serves to provide insights and clues to understanding our past. As the

prebiotic (pre-life) version of our planet no longer exists, we must look towards other planetary bodies, such as Titan

(Saturn’s moon), in order to observe processes and phenomena no longer available on Earth. Although Titan is less

than half the diameter of Earth and nearly 10 times farther from the Sun,

it is the only body other than Earth in the Solar System with a dense

atmosphere as well as the only moon with liquid bodies on its surface.3

Furthermore, Titan’s atmosphere consists of dinitrogen gas and

hydrocarbons, primarily methane and ethane.1 Titan, is the second largest

natural satellite (moon) in our solar system and the only one with a

substantial atmosphere featuring complex organic processes.2 Titan’s

atmospheric water is almost completely frozen since its equatorial

surface temperature is 93.7K, so methane may play the role on Titan that

water does on earth, forming clouds, rainfall and lakes.2 In spite of the

much lower temperature in the Saturn system, Titan holds remarkable

similarities to prebiotic Earth as its atmosphere and surface contain

complex organic chemicals in the form of gas, aerosol, and liquid phases.

Also, the processes on its surface point to the possibility of an internal water ocean.1

Further attempts to study Titan, however, are hindered by the lack of a dedicated mission after the Huygens

lander. Additionally, much of the information gathered on the Cassini-Huygens mission varies from previously

projected results.3 Many gases that were thought to be on Titan were discovered to be present in quantities that were

much smaller than previously thought.4 It appears the scaled amount of Argon-36 is much lower than anticipated,

and is nearly 250 times smaller than that of Earth’s.4 Missing gases are speculated to be trapped in the crust of

Titan; dissolution in hydrocarbon lakes and confinement in clathrate hydrate crystals are possible causes.2 Knowing

the solubilities of these gases is vital to constraining the overall noble gas budget of Titan, allowing the effective

employment of modern dating methods based on noble gas isotopic compositions.4

In the past years, teams have investigated free energies of solvation for different substances in methane and

ethane. Our goal is to provide accurate solubility data specifically for noble gases such as helium, argon, and

krypton. We will utilize the NAMD Scalable Molecular Dynamics

software5 in conjunction with the Visual Molecular Dynamics

(VMD) package6 in order to create data points and analyze them.

Molecular dynamics simulations can provide accurate projections

on the behavior of molecules under various conditions and are

more feasible alternatives to direct exploration, because it is not

financially plausible to send more probes to the Saturn system at

this time. The data points provided by the simulations can be

utilized to provide projections of dissolved noble gas

concentrations, constraining the overall budget of such gases on

Titan. Furthermore, the data can be used to assist researchers in

determining the origin and evolution of Titan through methods

such as K-Ar dating.

Under the mentorship of the Miller Lab in Summer Research Connection 2014, we hope not only to

address the data discrepancies between projected and experimental studies, but also to familiarize ourselves with

molecular dynamics simulations and analyze and interpret the resulting data.

1 F. Raulin, C. Brasse, O. Poch, P. Coll. Prebiotic-like chemistry on Titan, Chem. Soc. Rev., 2012, 41, 5380-5393 2 J. Lunine, S. Atreya. The methane cycle on Titan, Nature Geoscience, 2008, 1, 159-164 3 Facts About Titan, ESA, <http://www.esa.int/Our_Activities/Space_Science/Cassini-Huygens/Facts_about_Titan>esa.int., July

8, 2014 4 R. Hodyss, M. Choukroun, C. Sotin, P. Beauchamp. The Solubility of Ar and Kr in liquid hydrocarbons: Implications for

Titan’s geological evolution, Geophysical Research Letters, 2013, 40, 2935-2940 5 Kale, L., et al. ; Skeel, R.; Bhandarkar, M.; Brunner, R.; Gursoy, A; Krawetz, N.; Phillips, J.; Shinozaki, A.; Varadarajan, K.;

Schulten, K.; J Comp Phys 1999, 151, 283. 6 Humphrey, W.; Dalke, A.; Schulten, K. J Mol Graphics 1996, 14, 33.

Figure 1: Artist’s conception of the Huygens Lander

on Titan.2

Figure 2: VMD generated image of an argon atom within

methane molecules.