Joshua Sebree

Assistant Professor
Areas of Interest: 
I am interested in the organic processes taking place in the atmosphere of Titan and other celestial bodies. Of particular interest is how organic aerosols interact with solvents at cryogenic temperatures.
Contact Information
McCollum Science Hall 213
Phone Number: 

My research program will aims to explore new aspects of prebiotic and Titan-like aerosols and their importance both in interpreting data from missions, such as Cassini-Huygens, and in understanding how biological molecules may form in abiotic environments. This will be accomplished through three interconnected research topics: 

The effect of trace species on bulk aerosols: properties and production

The effect of extended photochemical processing on subsequent aerosol generation

The interaction of aerosols with liquids: solubility and subsequent reactions

1. The effect of trace species on bulk aerosols
The majority of Titan aerosol studies start with a premix of several percent methane in nitrogen. This premix is then exposed to an energy source to initialize reactions. The resulting aerosols are then characterized using a variety of methods including infrared (IR) spectroscopy and mass spectrometry (MS). While this “Titan Standard” mixture is a good starting point, from the Cassini mission, it is becoming evident that other atmospheric constituents should be taken into account. Data from the Cassini spacecraft has allowed for the identification of numerous trace (>1%) species in Titan’s atmosphere including acetylene (C2H2), carbon monoxide (CO), hydrogen cyanide (HCN), cyanoacetylene (HC3N) and benzene (C6H6), to name a few. Preliminary results from my current study demonstrates that the photolysis of larger aromatics (naphthalene and quinoline) at ppm levels leads to the formation of large poly-aromatic systems containing eight or more aromatic rings. We have also demonstrated that far-IR (50-600 cm-1) spectra of these new aerosols have features similar to those observed by Cassini that have not been match previously by laboratory-created aerosols. Building upon this work, my research group is focusing on how trace compounds, such as cyanoacetylene and HCN interact with these aromatic seeded aerosols and how they may lead to the incorporation of nitrogen and possible formation of PANHs within the aerosols.

2. The effect of extended photochemical processing on subsequent aerosol generations
The studies in the previous section describe work that will be carried out on 1st generation (G1) aerosols, meaning they have been exposed to a single energy source for a set amount of time. The primary purpose of the energy source is to initialize gas phase reactions that ultimately result in the formation of aerosol particles. What is important to realize when studying planetary atmospheres is that aerosol particles, once formed, may continue to be suspended in photo-reactive regions of an atmosphere for some time prior to settling out. The second aspect of my research focuses on how subsequent irradiation of suspended aerosol particles change their properties in later generations. This branch of research will aim at answering several questions. First, what changes happen to the G1 aerosol upon exposure to a second energy source? Given the relatively high density of reactive molecules in the aerosol core, polymerization reactions could dominate assuming sufficient penetration of photons. In addition, the G1 aerosols serve as a nucleation seed for further aerosol growth during subsequent irradiation steps, possibly creating a layered structure. Finally, gas-phase products formed during the first irridiation that are not be part of the G1 aerosol can react and form additional aerosols that may or may not resemble the G1 products. I postulate that these later generational aerosol types are what ultimately settles out of Titan’s atmosphere and eventually settle onto the surface and into the lakes and may be responsible for some of the unidentified features observed in the VIMS data from the Cassini Mission.

3. Aerosol Solubility
The third area of study my research group is undertaking is to perform an in-depth study on the solubility properties of aerosols in various solvent systems. It is known that Titan has lakes that are primarily composed of liquid methane and ethane at temperatures around 96 K. Exactly what happens to the aerosols when they come into contact with cryogenic-temperature hydrocarbons is unknown. Depending on the method used in generating the aerosols, they can be partially soluble in polar and/or non-polar solvents. Solubility will also depend on the amounts of heteroatoms (oxygen and nitrogen) incorporated into the aerosol. We are also investigating if the aerosols continue to be photochemically active when in contact with the solvents. Here on Earth, water serves as a medium in many chemical reactions. It is possible that liquid hydrocarbons on Titan could play a similar role. It is known from the Huygens lander that the surface of Titan does receive some UV (~300 nm) light at the surface. Many PAHs and PANHs can absorb this wavelength region, and thus may slowly react over time.

This is also an important study in terms of planet habitability. By adding compounds such as ammonia to water, it is possible to lower the freezing point of water considerably. Cryovolcanism or meteoric impacts may bring such mixtures to the surface of Titan, allowing for reactions to occur. These ammonia/water liquids could allow for extreme forms of life to exist on icy satellites at much colder temperatures then previously thought. To that end, seeing how prebiotic aerosols interact with various solvent systems at a variety of temperatures will aid in defining conditions where the seeds of life could be found in the universe