May 3, 2018 | David F. Coppedge

After Cassini, Titan Still a Problem for Long Ages

The scientist who predicted a global ocean on Titan 35 years ago is still baffled over why Cassini did not find it.

In 1981, when Voyagers 1 and 2 found Titan shrouded in methane haze, they went to work on calculations. Because Titan spends up to 20% of its orbit outside Saturn’s protective magnetic field, Titan’s predominantly-nitrogen atmosphere is exposed to the solar wind during those times. Particles in the solar wind ionize the methane (CH4) into charged forms (CH3) that quickly recombine into other hydrocarbons, mostly ethane (C2H6), because the free hydrogen ions (H+) escape into space. Ethane is a more stable molecule that is liquid at Titan temperatures. Having nowhere else to go, the ethane rains down on the surface and accumulates.

A little math shows that so much liquid ethane should have fallen in 4.5 billion years (the assumed age of the solar system), that there should be a huge quantity by now, based on how much methane is found in the atmosphere today (there could have been more in the past). “Titan is covered by an ocean one to several kilometers deep consisting mainly of ethane,” predicted Dr Jonathan Lunine in 1983 with two colleagues. Lunine subsequently reduced that estimate somewhat, but still claimed that a global ocean should be there. He continued working as an important scientist on the Cassini-Huygens mission until its conclusion last September.

Radar on the Cassini orbiter failed to find oceans of liquid, except for some large lakes near the poles. Most of the largely-flat, crater-sparse, US-diameter globe is dominated by sand dunes and dry riverbeds and playas, with very few mountains exceeding 1,000 feet. The “ground truth” came in January 2005, when the European Space Agency’s Huygens Probe landed on a dry lakebed with just some hints of methane moisture in the soil. Now, with Alexander Hayes and Ralph Lorentz, Lunine looks back at “A post-Cassini view of Titan’s methane-based hydrologic cycle.” in a new open-access paper in Nature Geoscience. What are his feelings about his failed prediction?

….With a surface pressure of 1.5 bar and temperatures of 90 to 95 K, methane and ethane condense out of a nitrogen-based atmosphere and flow as liquids on the moon’s surface. Exchange processes between atmospheric, surface and subsurface reservoirs produce methane and ethane cloud systems, as well as erosional and depositional landscapes that have strikingly similar forms to their terrestrial counterparts. Over its 13-year exploration of the Saturn system, the Cassini–Huygens mission revealed that Titan’s hydrocarbon-based hydrology is driven by nested methane cycles that operate over a range of timescales, including geologic, orbital (for example, Croll–Milankovitch cycles), seasonal and that of a single convective storm. In this Review Article, we describe the dominant exchange processes that operate over these timescales and present a post-Cassini view of Titan’s methane-based hydrologic system.

Can a cyclic view of Titan save long ages? Ethane, remember, has nowhere else to go than down. Is it building up underneath Titan instead of accumulating in oceans on the surface? The scientists found that the lakes at the poles are dominated by methane (CH4), not ethane (C2H6), by about 71% to 21%. “The fact that Titan’s seas are methane dominated further exacerbates the long-standing mystery of Titan’s missing ethane.

The mystery could be solved if Titan is young, but that possibility is out of the question for scientific materialists. They require a billions-of-years-old solar system in order for life to evolve on Earth. So what can they do about this “long-standing mystery” facing them? Incidentally, CEH predicted right before the Huygens Probe landed that there would be very little ethane found at the landing site (3/22/2005 ref. to 1/15/2005). Without empirical observations to help them, Lunine and friends speculate about what they cannot observe.

A hydrologic cycle by definition entails the exchange of material between reservoirs, most obviously between the surface and atmosphere, but also between the surface-atmosphere environment and the deep interior. On Titan, the latter interactions can only be inferred but, by analogy with the Earth, they can be expected to be important over geologic time.

And yet if there is some kind of hypothetical ethane “cycle” in operation, it should appear, shouldn’t it? It sounds too convenient to hide vast quantities of missing ethane in the “deep interior” where it remains locked up, never to be observed, just because it is “inferred” to be there. Titan has no plate tectonics, so the “analogy with the Earth” is limited. On what basis can Lunine say that “exchange of material” with the “deep interior” can be “expected to be important over geologic time”? Long ages are not observable by humans. Lunine’s whole theory rescue relies on inference from unobservable factors because he will not consider the possibility that Titan is young. The ethane should be there, but it is not. If you can bear with this lengthy quote, you will see that he has no answer.

Interaction of the methane cycle with Titan’s atmosphere occurs on a variety of timescales. The longest involves the gradual leakage of methane through the tropopause into the stratosphere and mesosphere, where methane is exposed to ultraviolet (UV) light and energetic particles. The wavelength-dependent absorbance of UV radiation, as well as the variety and variable flux of high-energy particles, drive a complex chemistry in Titan’s atmosphere. Galactic cosmic rays can even induce a small amount of energetic chemistry at the surface. The net result of this factory is the destruction of methane, loss of hydrogen by escape, and production of heavier hydrocarbons and a variety of nitrogen-bearing species, including nitriles and imines, that rain out onto the surface. While Titan’s atmosphere makes identifying specific surface compounds difficult, VIMS analysis has firmly identified ethane and benzene, and possible detections of cyanoacetylene, acetylene, toluene and acetonitrile have been reported. Methane may also escape directly, with loss rates that are potentially competitive with photochemical destruction. The approximate timescale for complete depletion of atmospheric methane—the dominant reservoir of this molecule—is tens of millions of years, dependent on the particular photochemical model and relative importance of direct escape. Isotopic ratios of carbon and deuterium in atmospheric methane are crudely consistent with this loss rate in their lower, but not upper, limit. The ultimate source of replenishment, possibility of variable loss over time, volume and location of reservoirs of photochemical products, and potential recycling mechanisms all remain unknown.

By hiding behind passive-voice verbs, he can say the “mechanisms all remain unknown” instead of “I have no idea what happened to the ethane.” Notice that the problem is worse than first stated, because the paragraph states that methane escapes from the atmosphere at loss rates comparable to UV conversion into ethane. He gives “tens of millions of years” for complete depletion, which sounds like a lot of time, but is actually on the order of just hundredths of the assumed age of Titan. Obviously the methane is not near complete depletion yet, so that reduces the age estimate even further. Incidentally, as a leading astrobiologist, Lunine was wrong about life on Titan, too. There’s no evidence of prebiotic chemistry going on down there.

Fig. 4 from Lunine et al. (2018 Apr 30) shows processes at work destroying Titan’s methane. The two outgassing processes are hypothetical. Notice the question marks for “Cryovolcanism?” and “Subsurface methane/ethane reservoirs?” The liquid ethane is falling; where did it go?

Scientists would know if the methane were depleted, because Titan would have little or no atmosphere left. Because methane is a potent greenhouse gas, the methane acts like a “space blanket” to keep the nitrogen from freezing out. Were methane loss to hit a critical low, the entire atmosphere would collapse to the surface, leaving just a thin veneer of gas. The paper acknowledges this fact, but tries to hide it by visualizing reservoirs of methane out of sight underground that periodically replenish the atmosphere by outgassing. Does he have evidence for this? Only by inference. He appeals to ratios of argon and potassium ions that “provide evidence for at least some outgassing from the interior.” The evidence, however, is all inferential:

The episodic nature of outgassing mechanisms provides the possibility that there are periods in Titan’s history when atmospheric methane was fully consumed by photolysis. In the absence of methane, the atmosphere would be thinner but would not fully collapse, although pools and perhaps even polar seas of nitrogen are possible during the first billion years of Titan’s history.

Where would the methane come out from the interior? Candidates for cryovolcanoes on Titan are few and far between, and no clear sources of methane emissions were detected by Cassini. Lunine adds another theory-rescue device: the Burp Theory:

It is important to recognize that, interesting as Titan’s hydrology is today, the inventory of methane accessible to the cycle may have fluctuated substantially over geologic time, if its delivery has any stochastic component (for example, ‘belches’ of methane from the interior via cryovolcanism).

With that final theoretical belch, Lunine and friends quickly change the subject. They talk about “Titan as a window into Earth’s future,” as if the two bodies are really comparable just because they have atmospheres predominantly of nitrogen. The differences, however, are far more profound: differences in size, distance from the sun, crystal composition, plate tectonics, presence of a satellite, rotation rate, temperature, and (of course) life. One can compare this to an embarrassed guest at a party: “(Burp!) Say, what do you think of the wine? I hear it’s vintage 1859 Dar-wine.”

We have followed this story for years (see list) because it allows secular moyboy scientists to make bold predictions and test them, and this one failed spectacularly. If this is the best Lunine can do after 35 years of speculation and 13 years of Cassini data, we declare victory. Titan’s old age is falsified. Our prediction was right. Bold predictions that come true should get the honors in science.

 

Leave a Reply