Titan: Planetary Scientists Invent Imaginary Histories
Getting from then to now is more poetic license than demonstrable fact.
Saturn’s large moon Titan has been scrutinized at multiple wavelengths by the Cassini spacecraft since it arrived in 2004: visual, infrared, ultraviolet, radio. About half of its surface has been mapped by radar (SEN). This is the Titan of 2015, but what did it used to be? For that, models are created by planetary scientists. But those models rely on assumptions that, in themselves, are philosophical, not empirical.
Imagination is a spherical shell of possibilities surrounding observable reality. If one approaches the data precommitted to scenarios from one region of that shell, then it’s possible to draw lines from the unobservable reality to the observable reality, given the freedom to tweak the initial conditions in the scenario to provide a historical path consistent with known physical processes and beliefs about the time available. The problem is that there is an infinite number of imaginary scenarios that can satisfy those constraints. Should scientists be satisfied with the publication of one plausible scenario out of infinity? A new paper in Icarus by six planetologists primarily from France gives an opportunity to watch the thinking process of secular planetary scientists working within a paradigm.
The title itself reveals the locus of their starting point in unobservable reality: “Evolution of Titan’s atmosphere during the Late Heavy Bombardment.” Their chosen scenario will assume, to the exclusion of others in the shell of imaginary possibilities, (1) evolution (not biological evolution, but the emergence of properties over time, such as an atmosphere), and (2) a Late Heavy Bombardment (LHB), a hypothetical event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago. But what if Titan is not that old? What if the LHB never happened? (see 10/08/14, 4/26/12, 9/16/10). Such questions are off the radar of these scientists. They think they have done their scientific work to imagine a scenario that draws a line from their chosen point in unobservable reality to the Titan of 2015. Watch their thinking process in the abstract, noticing how much is completely imaginary within their paradigm:
The mass and composition of Titan’s massive atmosphere, which is dominated by N2 and CH4 at present, have probably varied all along its history owing to a combination of exogenous and endogenous processes. In the present study, we investigate its fate during the Late Heavy Bombardment (LHB) by modeling the competitive loss and supply of volatiles by cometary impacts and their consequences on the atmospheric balance. For surface albedos ranging between 0.1 and 0.7, we examine the emergence of an atmosphere during the LHB as well as the evolution of a primitive atmosphere with various masses and compositions prior to this event, accounting for impact-induced crustal NH3–N2 conversion and subsequent outgassing as well as impact-induced atmospheric erosion. By considering an impactor population characteristic of the LHB, we show that the generation of a N2-rich atmosphere with a mass equivalent to the present-day one requires ammonia mass fraction of 2–5%, depending on surface albedos, in an icy layer of at least 50 km below the surface, implying an undifferentiated interior at the time of LHB. Except for high surface albedos (AS⩾0.7AS⩾0.7) where most of the released N2 remain frozen at the surface, our calculations indicate that the high-velocity impacts led to a strong atmospheric erosion. For a differentiated Titan with a thin ammonia-enriched crust (⩽5 km) and AS<0.6AS<0.6, any atmosphere preexisting before the LHB should be more than 5 times more massive than at present, in order to sustain an atmosphere equivalent to the present-day one. This implies that either a massive atmosphere was formed on Titan during its accretion or that the nitrogen-rich atmosphere was generated after the LHB.
How much of this paragraph constitutes knowledge (observable reality), and how much is imagination? Here are the factors that are not observable: (1) Titan’s history, (2) the LHB, (3) cometary impacts on Titan, (4) the quantity and nature of volatiles supplied by comets, (5) the emergence of an atmosphere, (6) the evolution of a primitive atmosphere, (7) the composition of prior atmospheres, (8) impact-induced crustal conversion from ammonia to nitrogen, (9) outgassing, (10) impact-induced atmospheric erosion, (11) an impactor population that fits the LHB hypothesis, (12) past ammonia mass fractions, (13) thickness of a proposed icy layer, (13) the nature of Titan’s interior, (14) the velocity of impactors, (15) the mass of a preexisting atmosphere, (16) the accretion of Titan.
Some of these factors can be modeled. Models, however, are simulations of reality, not reality itself. They are designed to fit the assumptions of the consensus paradigm (e.g., that Titan is 4.5 billion years old and went through the LHB). Some of the factors are tethered to observable reality (e.g., Titan’s current atmospheric composition; Titan’s surface properties observed by Cassini and the Huygens Probe). But there are enough free variables in the paradigm to imagine numerous alternative scenarios. These scientists centered on only two: the atmosphere’s evolution with and without an undifferentiated interior, given billions of years and an LHB.
What this implies is that other teams of scientists could publish other scenarios in the same journal, even within the current consensus paradigm. If the LHB were to be loosened as an assumption, the imaginary scenarios would increase. And if the paradigm were relaxed, alternative scenarios would skyrocket. How, then, can the public gain any confidence that this paper in Icarus constitutes knowledge?
Karl Popper famously proposed that the only way to advance scientific knowledge is through falsification. Scenarios such as this one could be falsified by identifying (a) evolutionary pathways that are simply too implausible given the laws of physics and chemistry, or (b) aspects of the model that contradict observations, or (c) aspects of the model that are self-contradictory. The problem is that the number of possible scenarios vastly outnumbers the scientists available to test and falsify them. And given all the free variables, proponents can always make minor tweaks to overcome falsifications.
Titan is what it is, and was what it was. We have observational access to its present, but not its past. Are planetary scientists progressing toward the truth about Titan’s past, or engaging in a random walk through infinite possibility space? Unless scientists can demonstrate to the public that their scenarios improve on empty speculations, papers like this amount to little more than jargon-rich confabulations within their chosen belief system.
Unobservable reality: how’s that for an oxymoron? Often, though, working with unobservable reality is legitimate in science. We know the sun has an interior, even though we cannot see it. Astrophysicists can model the core of the sun, even down to writing precise equations about the products of nuclear fusion. Then they can compare their model of the interior with the products emitted from the surface. The solar “neutrino deficit” was a huge problem for years, till new models accounted for it by proposing that neutrinos could change flavors en route to detectors on the Earth. The backside of the moon was an unobservable reality for thousands of years until the first orbiters took pictures of it. It wasn’t unreasonable to assume that the moon had a backside, was it? There are numerous examples of scientists assuming unobservable realities, or gaining knowledge about them by indirect means. These examples, however, mostly involve present-day phenomena.
Unobservable history is a different class. Unless its behavior is demonstrably cyclical, like the path of a comet, an object’s history is not amenable to scientific modeling. There can only be conjecture based on assumptions about initial conditions: e.g., If Titan were 4.5 billion years old, and if there was an LHB, and if its interior never differentiated, then its atmosphere might evolve along such-and-such a trajectory. Like we have shown, though, possible scenarios increase with the variables. Philosophers can prove that an infinite number of models exist that can fit the chosen assumptions and constraints. This is known as “underdetermination of theory by data.” It figured large in the writings of Duhem, Quine, Laudan and other philosophers of science (see Stanford Encyclopedia of Philosophy). Most practicing scientists have never been trained in such matters of logic. As Thomas Kuhn described, they are perfectly comfortable working within the paradigm of their peers, without even being aware they are inside a paradigm.
This entry illustrates the futility of shooting down individual models. There are too many of them. Skeptics of the current paradigm need to undermine the paradigm, not just the models it generates. When pointing out the flaws in a given model, the critic needs to prove that this undermines the whole class of models originating from that paradigm’s assumptions. One way to do that is to point out upper limits on the time available: e.g., given the observed rate of depletion of Titan’s atmosphere, it cannot be older than 100 million years. This rules out every scenario that depends on billions of years. Members of the paradigm can always posit that comets replenished the atmosphere from time to time. The critic’s job is to point out that anyone can invent ad hoc rescue devices to save a pet theory, but that is contrary to the ideals of science. Scientific claims need strong tethers to observable reality.
Exercise: Our goal at CEH is not to teach you what to think but how to think. Given the principles in this entry, analyze scientists’ behaviors and thoughts about unobservable history in these articles:
- Jupiter Moon Europa’s Dark Lines May Be Salt from Underground Sea (Space.com)
- What our solar system looked like as a ‘toddler’ (Science Daily)
- Planet formation relied on sweeping up of small glassy beads, new model suggests (PhysOrg)
- Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion (Science Magazine)
- A conversation with astronomer Dimitri Mawet (PhysOrg) – the Caltech astronomer jumps seamlessly from observable reality to an unobservable prehistory