Experimental Studies Involving Mixtures of Methane, Ethane, Propane, and Nitrogen with Implications for Titan
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Titan is a cryogenic geochemical laboratory. With global surface temperatures of 89–95 K and a pressure of 1.47 bar, methane (CH4) and ethane (C2H6) are the primary surface liquid constituents and the dominant species that compose the polar lakes and seas on Titan. Atmospheric nitrogen (N2) can dissolve into the liquid, which introduces a layer of complexity to the system due to differences in miscibilities between the methane–nitrogen and ethane–nitrogen systems. It is highly likely that propane (C3H8) is also part of the surface liquid composition, as it is a common by-product of the atmospheric photochemical process and has a low freezing point of 85.5 K.
The geochemistry of these organic materials creates landscapes similar to those seen on Earth. However, the surface conditions plus the parameters for the solid-liquid-vapor equilibria of these exotic mixtures are likely to cause the environment to behave in unfamiliar ways. Thus, I used the Astrophysical Materials Laboratory at Northern Arizona University to experimentally characterize the phase transitions and behaviors of mixtures involving methane, ethane, propane, and nitrogen at conditions relevant to Titan’s surface and considered how the results may be relevant to ongoing cryogenic geochemical processes occurring on Titan. This work was comprised of four primary studies: 1) identifying solid-solid phase transitions of pure ethane and mapping the liquidus and solidus boundaries of the methane–ethane system at low pressures [paper]; 2) considering the effect of nitrogen on the phase behaviors the methane– ethane system between 80–95 K at 1.5 bar [paper]; 3) exploring a ‘freezing-induced outburst’ phenomenon witnessed in the ethane–nitrogen system [paper]; and 4) examining how additions of ≤0.1 propane mole fraction further impacts the behaviors documented in mixtures of methane, ethane, and nitrogen. |
Cooling paths for three different C2H6–N2 mixtures where outbursts occurred. N2 mole fractions were derived at the temperatures shortly before the outbursts occurred (from top to bottom: 0.034, 0.024, 0.013) and the corresponding pictures were taken after each sample had stabilized post-burst. [paper]
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Tuning DraGNS' Interpretations to Titan's Expected
Surface Environment
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As a Dragonfly Student and Early Career Investigator, I worked with Drs. Patrick Peplowski (JHU Applied Physics Lab) and Ann Parsons (NASA Goddard Space Flight Center) to establish benchmarks for models of various Titan surface environments and to build our intuition for the DraGNS (Dragonfly Gamma-ray and Neutron Spectrometer) response to the possible elemental combinations on Titan.
The Dragonfly mission marks the first time a gamma-ray and neutron spectrometer suite will be sent to a water-dominated world. Previously, nuclear spectroscopy had only been used to study terrestrial bodies, where the focus was on detecting heavier rock-forming elements and hydrogen (an indicator of the presence of H2O and OH). Given the marked difference between rocky and icy worlds, we created Titan surface simulant samples and performed a series of experiments with DraGNS-like instrumentation to get a glimpse into this unique environment before Dragonfly's arrival to the satellite in the mid-2030's. For more information on this project, check out the LPSC abstracts from 2022, 2023, and 2024. |
Using Neutron Diffraction to Investigate the Ices of Pure Propane and Combinations of Methane and Ethane
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Teaming up with Dr. Helen Maynard-Casely of the Australian Nuclear Science and Technology Organisation (ANSTO), Jennifer and I have continued exploring the solid phases of methane, ethane, and propane using neutron diffraction (with the wonderfully named WOMBAT instrument).
Pure ethane is known to have three solid phases at conditions relevant to Titan's surface. Phase II has been somewhat elusive in its metastability when cooling and narrow temperature range of approximately 89.7-89.9 K. While forming phase II when cooling has still eluded us, we have seen evidence of it in the Raman spectra while warming and have gotten further confirmation through neutron diffraction. Thus, the experiments concerning the methane–ethane binary system considered the effects that small quantities of methane may have on the solid phases of ethane. Our primary questions are: 1) Does the addition of methane alter the temperature at which the solid-solid transitions occur? 2) Do we see the presence of phase II in both cooling and warming sequences, or at all? Propane is one of the most common by-products of methane photo- chemistry in Titan’s atmosphere and is likely pervasive on its surface and in its lakes. Beyond the Saturnian satellite, propane may also reside on the surfaces of Pluto and other TNOs, Ceres, Iapetus, and Comet 67P. Despite its applications to icy solar system bodies, there are limited studies concerning the behaviors of solid propane at low temperatures and pressures. Given this, we have initiated experiments that we hope will elucidate the crystal structure behavior of propane between 10–86 K. |
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