A Possible Explanation for the Sharp Decrease of Ions in Titan’s Ionosphere: Atmospheric Waves



Henry Reinhardt

Faculty Mentor(s)

Darci Snowden (Physics)


Titan, the largest of Saturn’s moons, was the target of a series of flybys from NASA’s Cassini mission. We will use the data from the 5th flyby of Cassini (T5), where the Ion and Neutral Mass Spectrometer (INMS) measured ionic and neutral particle densities within the atmosphere. The data recorded displayed a sharp decrease in densities of ions with short lifetimes, like CH5+, whereas ions with longer lifetimes did not have this same decrease. One possible explanation of this decrease is that there could be an atmospheric wave propagating, changing ion densities. If we can model this decrease, then we can not only explain the data but also have a new way of understanding the waves in Titan’s atmosphere, which will lead towards a deeper understanding of the structure and transport of energy of Titan. To explore the ionosphere of Titan, I created a model that uses photoionization to simulate the chemistry where the densities of each ion species is determined by solving the continuity equation with production and loss terms from the reaction equations. When this photochemical model is complete, a vertically propagating wave will be added to explain the sharp decrease of the ion densities. Once the vertical wave is added to the system, we can explore the effect of different wave parameters to see if the T5 data can be reproduced with this model. Should the model work as hoped, we can then look at the other flybys and analyze that data as well.

Keywords: Titan, ionosphere, photochemistry


15 thoughts on “A Possible Explanation for the Sharp Decrease of Ions in Titan’s Ionosphere: Atmospheric Waves”

  1. Nice job Henry! When you say, “add more chemistry”, what do you mean by that? I guess I’m just asking for a little more specificity.

    1. Thanks Marc! By “add more chemistry,” it pretty much boils down to adding more reactions. The reactions that I have currently (with only 19 reactions) make up the majority of the primary ions and neutral species, but by adding more chemistry the reactions for the primary ions and neutral species will become more refined. For example, if I have am looking at a particle that is moving through space and being accelerated due to gravity, I could simplify the math by saying that my initial velocity is 0 or that the acceleration is 0. For doing a lot of computations and a lot of ODE solving, that would reduce some of the time that the program takes for the sake of accuracy. By adding more reactions, it’s like I’m adding in an initial velocity or an acceleration with that example. It isn’t necessary for a simple understanding of what is going on, but by adding more chemistry and thus more math, the results will get more accurate. I hope that answers your question!

  2. Great job, Henry! One thing I’m curious about is the time dynamics of the Cassini data you were modeling. Does the overall wave pattern change dramatically over time or stay relatively stable? What types of factors in Cassini’s atmosphere might perturb the major features like the ‘bite-out’ characteristic you noted in the graph?

    1. Thank you, Dr. Craig! The wave will eventually reach a “leveling out” effect where the chemistry reaches equilibrium. The reason that we are running the code for so long is that we found that the timespan that we were looking at needed to be above 2000 seconds to reach equilibrium. So, as long as we look at a timespan that is large enough to reach chemical equilibrium, then the wave and the entire system will be stable. Smaller time spans give us more… wonky data where the impact is very large and the chemistry itself doesn’t lead to the densities that we want/expect. Within the Cassini module itself, there have been some talks about the under-estimation of gas leakage from the spacecraft causing the calculations to be incorrect. In 2015, Teolis et al. wrote up “A revised sensitivity model for Cassini INMS: Results at Titan” which introduced a sensitivity correction factor of 1.55 ±21%. This itself I don’t think could explain the bite out because there have been other methods of analyzing ions like using the torque on the spacecraft measured by the Attitude and Articulation Control System and the spacecraft atmospheric drag derived by the Navigation system even before the paper by Teolis. I hope that answers your question!

      1. Oh! In that case, there are a few interesting things going on in Titan’s atmosphere that lead to the “bite-out” effect. The biggest thing that would impact the bite-out is definitely the lifespan of the ions. Because of the shorter lifespan ions finding their way through the chemistry faster (they react with the other chemicals in the atmosphere quicker than others with longer lifespans), if there is any sort of a wave going on in the atmosphere, we’ll see a bit out where the ions go from an area of a relatively slower to a relatively faster rate of reactions. In other words, at one altitude they could be recombining at a rate of once every second, but at another they could be recombining at a rate of once ever millisecond, which would lead to a dramatic change in the densities at those different altitudes. Another thing that is going on in Titans atmosphere could be a weird amount of mixing. If there is some sort of funky mixing going on, we could also expect to see the densities have that bite-out just from some sort of mixing, without worrying about the way the chemistry acts at the different altitudes. However, the first answer of the wave carrying the short lifespan ions to areas where they react at different rates is a much cooler answer and the solution we decided to focus on here!

  3. Neat project, Henry! The altitude vs. density plot reminds me of the altitude vs. temperature plot for Earth’s atmosphere. You inspired me to look up the altitude vs. density plot for Earth’s atmosphere because I wasn’t sure what that one looked like, and yes, I see that it’s a perfectly smooth curve with no ‘bites’ taken out. I see how unique the ‘bites’ at Titan are.

    1. Thank you, Dr. Fallscheer! Yes, Titan has some sweet stuff going on and it’s especially cool to me that we can have a spacecraft fly through Titan’s atmosphere, get some data, and then work on doing some cool science with it for years to come.

  4. Henry,
    Great presentation! This project sounds incredibly interesting, and I was wondering: What kinds of implications do these findings have? Are there any other celestial bodies known to have wave-like atmospheres and what does that mean for these celestial bodies?

    1. Thanks Kendra! The really sweet thing about this model and these findings is they are applicable to other situations. Dr. Snowden was telling me about how the photoionization model itself could work for any ionosphere since we’re just looking at the chemistry. In other words, if we wanted to, we could model the Earths ionosphere with the code I’ve written and we would just need to update the reaction rates, initial densities, etc. All of the math works for any body! To answer your second question, I haven’t spent enough time studies other celestial bodies to tell you if there are other ones or not, but I would assume any atmosphere with short lifespan ions and any sort of wave would also have a similar bite-out feature going on. Since we’re just worried about the chemistry and then the wave physics, anywhere that has photoionization and wave physics (that behaves like normal physics, no science fiction allowed), then I don’t see a reason that this model couldn’t work for those situations as well.

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