The Habitability of the Early Titan Ocean

Dragonfly and Titan’s Astrobiology

The lead scientist on the Dragonfly Mission to Titan, Dr. Zibi Turtle, standing in front of a model of Dragonfly.

The purpose of this blog is to explore an idea I had about how we can understand the astrobiology of Titan’s early system, specifically in it’s oceans. Not a lot is known about Titan’s early system because it’s surface is very young (at most 1Gyr). Nevertheless, loose constraints have been proposed for Titan’s history, and from those, there is the potential to explore the deeper implications on the habitability of Titan. Dragonfly is planning to visit Titan in the 2030s primarily to investigate its potential to harbor, or foster, life. Titan’s conditions make it rich in organics, and when mixed with liquid water, biomolecules, like amino-acids form. I’ve talked about this before, but here I want to think about a different aspect of Titan’s habitability. That is its ocean.

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An hypothesized interior structure of Titan

Titan has great conditions for the origin of life. It does not have great conditions to sustain it. In the melt of it’s impact craters, liquid water may flourish for decades, even centuries, but it won’t last. It’s ocean is likely beneath a 100km thick ice crust. The ability for life at the surface to make it down is unlikely. On going work has shown that it is possible, but it’s still limited. However, it has been hypothesized that Titan’s shell hasn’t always been so thick. A thinner shell will likely be more apt at overturning (like say Europa). This would suggest a great deal of mixing. Unfortunately, we don’t even know if Titan had it’s large reserve of organics during this time. If it did, the higher impact rates likely facilitated a great deal of mixing. We just don’t have the data needed to know this for sure, and we may never. Nevertheless, we have made a great deal of progress at expanding the picture of Titan’s past, and despite the limitations, I think we can make that picture a little clearer.

Episodic Outgassing

Figure 2 of Tobie et al 2006 of the potential history and outgassing of methane from Titan’s interior.

A while back I wrote blog post about Titan’s likely history of outgassing driven by it’s evolving interior. This is likely the biggest hurdle in constraining Titan’s history, but it is necessary to get a good estimate of organic production. Therefore, the first step in getting that estimate is to consider the various processes that would have instigated the outgassing of methane. This can likely be modeled, assessing the stability of methane and/or other volatiles in the interior. Once the most likely causes are constrained, we next have to consider the timing of said causes. The evolution of the core is something that can be modeled. It can be constrained, at least to a range of possible scenarios. This might lead to a range of potential outgassing profiles, but as with anything, it will provide limits by which to work from. I recognize that these cannot be constrained absolutely, but we can create reasonable scenarios, based on evidence, by which to work from. This could be a project unto itself: categorizing the possible range of outgassing events in Titan’s history. From there, it’s a photochemical problem.

Photochemical Production and Deposition

Figure 4 of Krasnopolsky 2009 showing ionization rates for different sources at different depths

Models exist to predict the production rate of various organics in todays conditions. Of course, to understand production through time, these models would need to account for changing solar radiation and atmospheric conditions. The former is likely the easiest to constrain. The latter would likely entail using available information to make predictions of how atmospheres would act under various methane loads. For example, Tobie et al 2006 explains how the initial outgas would saturate the atmosphere and soak the surface with methane. Models can be used to predict the thermal profile of the atmosphere, which would effect it’s thickness. Therefore, it should be possible to ascertain an estimate of an atmospheric profile to model production under. This process would be repeated at different points in time (or possible designed to evolve with time), and, if feasible, performed for a range of outgassing profiles.

A more difficult task may be to predict deposition rates. This has been attempted to some extent (e.g. Lara et al., 1993), but this is a process that is not well understood (or at least that is my impression). Furthermore, slight fluctuations in atmospheric conditions, likely well within our ability to constrain it, may effect the process, but ultimately, we will have to make the best estimate possible.

Impact Cratering of Surface Overturn

Figure 10 from Korycansky and Zahnle 2005 of Titan cratering rates

Cratering rates are one of the more well understand aspects of Titan’s history. With this, we can use existing models (or perform our own) to predict how much overturn occurs for impactors of varying size. This will allow for estimations of total mass transfer over time. Alternatively, it may be as simple as estimating the rate of complete surface turnover. This may be one of the easier tasks. We take a given shell thickness and impact it with impactors of a range of sizes. Each impactor can be estimated to overturn some area of the surface. A Monte Carlo approach (if I am remembering that correctly) could be used to predict how long it would take for the entire surface to be overturned.

Chemical Evolution in Titan’s Ocean

With that, we would have an estimate of organic material transferred, and from there, we can predict the rate of changes and the abundance of organic material. This is necessary information to understand 1) how likely life is to arise in this environment and 2) how sustainable an environment it is through time. In my imaginary proposal, this would be less of a dedicated project and more of a large scale overview of habitability of the environment using existing information about the evolution of organics at these conditions. As for sustainability, abundance of organics may decide whether life can thrive given the resources available.

Concluding thoughts

I recognize that this post contains both a lot and very little at the same time. This is likely an impossible thing to try and accomplish, nor would many be likely to consider it a valuable use of resources given all the unknowns. I still think it presents a fascinating problem. Like many big problems, it would necessarily be incremental, either performed by many, or slowly by one. I’m positing this because I’m finishing my PhD (hopefully) in a year, and I need to start thinking about post-doc ideas. This problem stuck out to me. This blog post is my first attempt to really think through it and put pen to paper (if very loosely). This is an embarrassingly outlandish idea riddled with problems, so hopefully there is something here I can work with because I am honestly terrified to discuss it with my lab tomorrow.