The End is Nigh: Updating and Finalizing My Thesis (A discussion of the history of methane on Titan)

Having successfully defended my thesis, I am now in the process of making a few revisions. It isn’t anything too extreme; a couple changes and a few additions to the intro is all it is. I’ve looked into getting permission for figures in my intro. It was very easy, and I’ve taken care of it for all of my existing figures. Any new figures I add won’t take any time to get permission for either. For the bulk of this blog, I’m going to present here a few of the “major” additions I’m making (since thats where my focus has been), which is essentially just more background info about Titan/Cassini.

Outgassing of Methane from Titan’s Interior

Justification for section

I chose to expand on this because there were a few questions related to the outgassing event. Dr. Tornabene wanted to understand the connection between outgassing and volcanic events. Dr. Campbell-Brown wanted to know more about the existence of Argon in the atmosphere and why it is an indicator of outgassing. Dr. Molnar wanted to better understand the origin. By delving deeper into the outgassing history, we get a better idea of Titan’s timeline/history and how it likely formed. Then I can discuss how isotopes (Argon and others) support this history while also commenting briefly on what it says about the material that sourced Titan.

This is presented in Chapter 1.2.1 Titan’s Atmosphere and Climate, now labeled Titan’s Atmosphere and Origin.

Excerpt of Original Material 

Titan’s abundance of methane and organic compounds results in a complex cycle of rain, erosion, and deposition. Titan’s surface pressure is very similar to Earth’s (1.5 bars vs 1 bar), and with temperatures at 94K, methane can be stable as both a gas and liquid (Kouvaris and Flasar, 1991).  Titan’s methane isn’t in an ocean like water on Earth, rather it is in large lakes, and the specific heat (the energy needed to raise a substance by 1K) is harder to overcome because of the lower solar flux (Lunine and Atreya, 2008). Nevertheless, rainfall and cloud events have been observed if rarely (Turtle et al., 2011; Porco et al., 2005; Griffith et al., 1998). Modeled rainfall requirements for the observed channels suggest that there should be twice as much methane vapor in the atmosphere than observed at the equator (Lunine and Atreya, 2008). Short term (100s of yrs) rainfall could be fueled by evaporation of the lakes, but methane is slowly being destroyed over a much longer lifespan of 10-100 Ma (Figure 6). Therefore, it has been suggested that outgassing event(s) have released methane from the interior in Titan’s past to resupply its atmospheric methane (Figure 7) (Choukroun et al., 2010; Choukroun and Sotin, 2012; Tobie et al., 2006).

New Material

ch1 fig

Figure 7: A theoretical evolution of Titan’s interior (b) and how it has affected the outgassing of methane throughout Titan’s history (a). The outgassing rate is controlled by the interior evolution (a). The final outgassing event is shown  with 10% and 50% of the methane reservoir outgassed since the second outgassing event. This is modified from Tobie et al. (2006).

Tobie et al. (2006) present a theoretical evolution of Titan’s interior that composes of three major outgassing events. The first event begins with the quick overturn of Titan’s initial core. The outpouring of methane produces a thick layer of methane clathrate above the ocean because of how methane interacts with liquid water under Titan’s temperatures and pressures (Tobie et al., 2006; Sloan 1998). Clathrates are compounds that are trapped within the ice lattice. These can alter the rheological and thermal properties ice as well (Durham et al., 2010). The low viscosity and low conductivity acts as an insulator, warming the ocean which releases methane. After differentiation, the silicate core begins to convect; the heat flux thins the clathrate layer with the buoyant methane accumulating at the base and escaping through cracks (Tobie et al., 2006; Lunine and Stevenson 1987). As the interior cools, the liquid ocean begins to freeze to form a layer of ice I. The thick ice layer convects, forming warm plumes (likely from tidal dissipation, Sotin et al., 2002) that breaks through the clathrate layer making it unstable. Tobie et al. (2006) suggest the last stage occurred perhaps ~1.0 Ga. This model explores a range of parameters but finds that the change in these cycles is the length and intensity. Furthermore, it is consistent with the signs of volcanic features that would form during this event (Elachi et al., 2005) and with isotopic signatures in the atmosphere.

Isotopic measurements help to verify the outgassing events but also constrains the origin and evolution of Titan’s volatiles as a whole. Evidence  of 40Ar in Titan’s atmosphere reflects the decay of 40K which would have been sourced from rock-water interactions, and its existence in the atmosphere suggest potassium rich water magmas reached the surface through volcanism likely fueled by the outgassing (Wait et al., 2005; Niemann et al., 2005; Tobie et al., 2006). Another line of evidence comes from the lack of enrichment of heavier carbon isotopes in methane despite seeing it in nitrogen isotopes. The 15N/14N ratio is enhanced as heavier 15N sinks below the 14N which can escape more readily (Lunine et al., 1999; Hidayat and Marten, 1998). Inversely, the ratio of 13C/12­­C in hydrocarbons is closer to the terrestrial value suggesting it has not undergone the same escape enrichment. That is to say, the carbon in the hydrocarbons (i.e. methane) have been recently sourced (~1 Ga or less). Congruently, the deuterium in methane is lightly enriched (~1.5 times) which is still significantly lower than organic molecules in organic clouds (i.e. the outer solar system) (Van Dishoeck et al., 1993; Meier et al., 1998). This reflects the slightly warmer circum-Saturnian nebula that would have undergone more processing of the volatiles that likely sourced Titans reservoir (Lunine and Tittemore, 1993).

Crater Morphometry and Scaling Laws

Justification for section

There was a good bit of confusion on the scaling laws that are shown in plots, and this is an important piece of my work. I expand on what is already there to help them understand the relationship between scaling laws and the worlds being studied. This is not as extensive and is more integrated with what was already there.

Excerpt of Original Material

The most descriptive characteristic of a crater is its diameter. The diameter of a crater can be used to predict the approximate depth and overall shape of a crater. The material properties of the planetary body also matter (see above), but the transition point from simple to complex to multiringed craters is unique for each body (Bray et al., 2008; Schenk, 2002, 1989). However, some planets are sufficiently similar to compare. Gravity is the driving factor behind the simple-complex transition diameter, but lithospheric structures also influence this transition because they can be indicative of the thermal structure in the crust (Melosh, 1989; Schenk, 2002; Turtle and Pierazzo, 2001). The thermal structure controls how material will react to the force of a shock wave; warmer material tends to act more ductile (plastically deforming) than brittle (faulting) (Turcotte and Schubert, 2014). This is especially important on icy moons where temperature is the driving factor in viscous relaxation because it controls the viscosity of the material (Durham et al., 2010; Schurmeier and Dombard, 2018).

New Material Mixed with more Original Material

Therefore, it is important to accurately characterize the diameter of a crater to accurately constrain the global trend of crater morphologies (Figure 12). These trends are known as scaling laws; this approach is often used for terrestrial worlds. Schenk (2002) discusses how it changes for the icy moons of Jupiter. Compared to the moon, the simplest craters follow the same trend, but the transition to complex craters occurs much sooner because of the differences in material strength and body gravity. For similar reasons, these icy moons reach a point where the complex craters (with central peaks) become central pits. The effect of size and strength is observed best with Europa because it is being embedded with significant tidal heating. Therefore, the transitions occur sooner, but worlds of similar size, thermal structure, and material strength often have very similar scaling laws (e.g. Ganymede and Callisto).

scaling law

Figure 12: Depth/diameter measurements for fresh impact craters on Ganymede. The thick lines are for lunar craters, and the thin lines are least-squares fits through the Ganymede data. Simple craters are solid dots, and complex craters are split into those with central peaks (open circles) and central pits and domes (crosses). The multiringed craters are shown with error bars.

Titan, like Earth, undergoes a great deal of degradation (see Section 1.2 and 1.3.5), so the crater structure becomes harder to interpret. Turtle et al. (2005) addresses the issue of crater diameters identifying the difference between the final rim diameter (rim to rim distance after forming) and the apparent crater diameter (rim to rim distance after erosion). Terrestrial (and Titan) craters undergo significant degradation which make it difficult to find the crater rim even with topography (Figure 13). Without a population of fresh craters, you are not able to develop a unique scaling law. Multiringed basins are similarly difficult to characterize even before erosion. In these cases, the best approach is to clearly identify the ambiguity required when interpreting a crater. An example of this is like Schenk (2002) showing multiringed basins with error but not a trend line if to it.

(Image of degraded craters).

Other work needed

There is a brief addition to chapter two I need to add related to data processing so it is clearer what I did in my thesis. Although, the bulf of the work is just making these changes to the intro. There are couple more “major” changes I’m still working on, 1) cratering rates (a new subsection to discuss in more detail) and 2) Discuss the degradation of titans craters from a material perspective. Elaborate on whats souring the fluid/infill. Mention Alyssa’s work and Catherine’s 2015 work.

I’m posting this now, but I’m hoping to add at least one more section before I present this at group meeting 8/15/18 1PM. For now, enjoy.


One Reply to “The End is Nigh: Updating and Finalizing My Thesis (A discussion of the history of methane on Titan)”

  1. “Compared to the moon, the simplest craters follow the same trend, but the transition to complex craters occurs much sooner because of the differences in material strength and body gravity.”

    The gravity on Titan is similar to that of the Moon, so I would guess the differences are most sensitive to material strength.

    “For similar reasons, these icy moons reach a point where the complex craters (with central peaks) become central pits.”

    This is rather vague. To read one theory for the origin of central pits, see Elder et al. 2012.

    “The effect of size and strength is observed best with Europa because it is being embedded with significant tidal heating.”

    I don’t think this is a true statement. Are you saying the ice is warmer, and so the material properties are different? I thought Europa was so different because of its extremely thin ice shell. This is not related to size or strength, but rather interior structure.


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