Titan Takes the Age Stage

Saturn’s giant moon has prompted a flurry of new science papers. Can anyone keep it billions of years old?

Ever since the predicted global ocean on Titan was falsified, planetary scientists have been in theory-repair mode to keep it billions of years old. Cassini arrived in 2004 and started looking at a largely crater-free surface. The Huygens Probe landed on moist sand in January 2015, further puzzling scientists about the vast lakes of liquid that had been predicted. Now, after 12 years of encounters with the larger-than-Mercury moon of Saturn, Cassini has sent enough data for a comprehensive look at the geology, atmosphere and life span of Titan.

There’s way too much verbiage to review in all these papers. What we will focus on are data and arguments that might keep this young-looking world as old as 4.5 billion years (its assumed age). First, take a look at the beautiful new global maps assembled by Cassini’s Virtual and Infrared Mapping Spectrometer (VIMS) posted by Space.com.  Titan still has “secrets”, the article says, calling it “a still-mysterious moon” after a decade of detailed study. There’s only one large crater visible, named Sinlap, “one of the youngest impact craters to dot Titan’s surface.” OK, so where are all the old ones?

Cassini Spies Titan’s Tallest Peaks (Astrobiology Magazine). Enjoy the picture of Titan’s ski resort, rising almost 11,000 feet high. Smaller Pluto has bigger mountains than that.

Discovering the bath scum on Titan (Helen Maynard-Casely on The Conversation). Features in one of the northern lakes seem to appear and disappear. Maynard-Casely helped identify one of the crystal forms of benzene and ethane in the “scum” on Titan’s lakeshores, but fails to mention the missing ethane that should have accumulated in deep deposits over billions of years. See also PhysOrg‘s writeup.

Bubblin’ crude (New Scientist): A JPL scientist surmises that “if a methane-nitrogen mix rained down on a pure ethane lake, as much as 15 times the volume of nitrogen would be released, potentially making for some very big bubbles.” This might explain the “magic islands” disappearing act (see next article).  Quote:

It just goes to show how much these alien worlds can surprise us, says Malaska. “We used to think of Titan as very Earth-like, but the more we learn about it, we learn it is really weird.”

Titan’s “Magic Islands”: Transient features in a hydrocarbon sea (Icarus). Possible waves in the few northern lakes might explain the transient features. Can this dynamic environment have existed for billions of years?

Nature, distribution, and origin of Titan’s Undifferentiated Plains (Icarus). Wind-driven materials forming “blandlands” at mid latitudes represent “an important repository of organic materials on Titan,” this paper says, claiming that depositional or sedimentary processes account for them. But why is the material not liquid, if predominantly ethane and methane?

Fluvial erosion as a mechanism for crater modification on Titan (Icarus). Where are all the craters? They’ve been eroded away by liquid, this paper claims. Processes such as weathering, mass wasting, fluvial incision and deposition “may explain Titan’s crater distribution” (i.e., the surprisingly few that have been found). Wind can’t do it all, so rainfall and rivers must be accounting for the rest of the erosion.

The fate of ethane in Titan’s hydrocarbon lakes and seas (Icarus). This paper seems tailor-made to answer the missing ethane problem. Where did it all go? A little hocus-pocus with unobservable processes might help, especially if we ignore the global-ocean deficiency and just focus on one lake, making assumptions that cannot be tested.

Ethane is expected to be the dominant photochemical product on Titan’s surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini’s 2.2 cm radar by Ligeia Mare however suggests that this north polar sea is dominated by methane. In order to explain this apparent ethane deficiency, we explore the possibility that Ligeia Mare is the visible part of an alkanofer [a layer that stores alkanes, a class of hydrocarbons] that interacted with an underlying clathrate layer and investigate the influence of this interaction on an assumed initial ethane–methane mixture in the liquid phase. We find that progressive liquid entrapment in clathrate allows the surface liquid reservoir to become methane-dominated for any initial ethane mole fraction below 0.75. If interactions between alkanofers and clathrates are common on Titan, this should lead to the emergence of many methane-dominated seas or lakes.

 Geomorphological map of the Afekan Crater region, Titan: Terrain relationships in the equatorial and mid-latitude regions (Icarus). These scientists make judgment calls on which terrains are the oldest: the undifferentiated plains, the mountains, the dunes, or the craters. They figure the dunes are the youngest, but their conclusions appear theory-dependent. “Our geomorphological mapping results are consistent with the equatorial and mid-latitudes of Titan being dominated by organic materials that have been deposited and emplaced by aeolian activity.”

Material transport map of Titan: The fate of dunes (Icarus). In the massive dunefields, fluvial processes do not dominate, these planetary scientists say.  “Our results are consistent with aeolian transport being the dominant mechanism in the equatorial and mid-latitude zones.” They claim the dunes are in steady state, with material migrating poleward from the equator to an ultimate sink at 35 degrees latitude. But then what? “This observation suggests that either dune materials are converted or modified into plains units or that the margins of dunes are transport limited.” Remember, though, they have to account for 4.5 billion years of this process.

Structure of Titan’s evaporites (Icarus). Liquid lands on the surface and evaporates. What’s left behind, and how much? They invent a model to explain the amount and distribution of evaporite deposits, needing to explain why there seems to be less in the south compared to the north. They estimate the maximum thickness of the deposits (not described in the Abstract). Why is it not liquid? Can the mass of solid deposits compensate for the expected global ocean?

Alluvial Fan Morphology, distribution and formation on Titan (Icarus). The first survey of alluvial fans on Titan identified 82 of them, primarily at polar latitudes. These imply sediment transport downhill by rivers or wind, “suggesting that fluvial sediment transport may have been concentrated in the near-polar terrains in the geologically recent past.”

Role of fluids in the tectonic evolution of Titan (Icarus). This paper tries to hide the missing liquid under the surface again. They link subsurface liquid to crustal deformations, claiming that subsurface liquid causes surface contraction, forming ridges. Once again, though, is this enough to sequester all the liquid that should have been found after billions of years? How does the liquid get under the sediments? This theory seems to raise more questions than answers.

Update 4/05/16: Cryolava flow destabilization of crustal methane clathrate hydrate on Titan (Icarus). Another paper on Titan arrived since this entry was written. Famous planetary scientists Dennis Matson, Torrence Johnson and Christophe Sotin struggle to keep Titan’s methane concentrations going for long ages. They propose periodic cryovolcanic emissions, but admit: “However, this process cannot maintain methane resupply over geological time.” Conclusion: “The origin of Titan’s atmospheric methane and current resupply mechanism remain a mystery.”

Tethys News. That’s all for Titan. Since our previous entry about Saturn’s other youthful moons (4/02/16), another Saturnian moon made the news: Tethys, third major moon out from the rings. Another paper in Icarus shares a surpising reversal in thinking: the big canyon, Ithaca Chasma—stretching nearly from pole to pole—was apparently not formed by the huge Odysseus impact on the opposite hemisphere. Although Tethys shares chemical/physical properties with Dione and Rhea (the next two big moons outward), the scientists “also identified spectral variations [on Tethys] that are unique in the Saturnian system.” Tethys is another “yold” world, bearing “geologically young surface features” made of large ice cubes and “geologically old weathered regions” made of small ice flakes. What? Tethys has weather? There’s no atmosphere to speak of. There is space weathering by the solar wind, but that should affect all surfaces equally. Unique features cause headaches for theorizers.

It’s kind of fun watching the scientists pull their hair out to keep Titan old (what little hair is left after Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, comets, asteroids, etc.). We’re very happy that intelligent engineers got us out there for a close look, so we could witness the reactions.