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Corridor of Ice Identified on Titan

What an interesting thing Titan’s atmosphere turns out to be. A fine haze produced by sunlight breaking apart methane molecules settles continuously to the surface, leaving organic liquid and solid sediments. Titan also has large lakes, but these contain about a third of the necessary methane, available through evaporation, to replenish that atmosphere, which should be depleted over geological time scales. What produces Titan’s supply of methane?

It was to answer that question that Caitlin Griffith (University of Arizona Lunar and Planetary Laboratory) and colleagues embarked on a study of cryovolcano activity on Titan. Cryovolcanoes erupt not with molten rock but volatiles like water or methane, and thus could provide an answer if they are venting methane found in subsurface reservoirs. A feature on Titan’s surface called Sotra shows cryovolcanic features that imply past icy flows.

Image: A giant of a moon appears before a giant of a planet. Titan, Saturn’s largest moon, measures 5,150 km across and is larger than the planet Mercury. Credit: NASA/JPL-Caltech/Space Science Institute.

Thus the need for a study of Titan’s surface focusing on potential cryovolcano candidates. But this is tricky work, with weaker spectral features difficult to tease out because of the moon’s dense atmosphere. The work draws on tens of thousands of spectral images produced by Cassini’s Visible and Infrared Mapping Spectrometer. The team developed a principal components analysis (PCA) which allowed a more fine-grained breakdown of surface ice and sediments at the four wavelength bands that most clearly view Titan’s surface from orbit.

Out of this came the detection of an icy linear feature, a ‘corridor’ in Griffith’s words, and one that raises questions:

“This icy corridor is puzzling, because it doesn’t correlate with any surface features nor measurements of the subsurface. Given that our study and past work indicate that Titan is currently not volcanically active, the trace of the corridor is likely a vestige of the past. We detect this feature on steep slopes, but not on all slopes. This suggests that the icy corridor is currently eroding, potentially unveiling [the] presence of ice and organic strata.”

Based on the settling of sediments from the atmosphere, scientists expect the surface of Titan to be an ice bedrock coated with organic materials. What the team has found is that water ice is unevenly exposed in Titan’s tropical latitudes. The exposed icy materials do indeed appear as a long, linear corridor that stretches 6,300 kilometers, with the aforementioned Sotra region especially rich in water ice, and positioned roughly in the middle of the icy corridor.

Image: Three orientations of Titan’s globe: the icy corridor is mapped in blue. Credit: NASA/JPL-Caltech/Space Science Institute.

All of this relates to the methane question, because the distribution of organic sediments on the surface is, the researchers believe, related to the history of Titan’s interior. We seem to be seeing evidence of a past, geologically active period. As Griffith notes, the icy feature does not correlate with topography as measured by Cassini’s radar measurements, nor does it seem to correlate with gravitational field anomalies likewise noted by Cassini, which offer a limited measure of subsurface composition. Just when did it appear, and how? From the paper:

The linearity of the icy corridor over a global scale presents the question of whether tectonic processes shaped this feature, thereby manifesting the processes that mould Titan’s surface and subsurface on a global scale. However, we find no evidence that Titan is geologically active, consistent with Titan’s long-wave topography and gravity field, which indicate a thick ice shell that is conductive rather than convective.

The answer may lie in the past, with methane surging up from below:

Measurements of the 13C/12C ratio in Titan’s atmosphere indicate that methane was injected into the atmosphere not more than 0.5–1 billion years ago. This age is consistent with the volume of dunes that would have accumulated since then, and suggest, consistent with evolution models of Titan’s interior, that a major cryovolcanic event occurred several million years ago. Potentially the topography that established the steep slopes of the icy corridor is a remnant of the time when Titan was geologically active. One example is the Sotra region, which displays cryovolcanic features, the steep terrain of which exhibits the strongest water-ice features.

Thus a past cryovolcanic event could explain Titan’s methane, with the ice corridor a telling reminder of these processes. Icy features in other parts of Titan are found only in local regions exposed by erosion or cratering, which suggests that if cryovolcanism remains an active process on Titan, it is not widespread. The team plans to use the same techniques to examine Titan’s poles, where the moon’s methane seas are prominent, to explore these ideas further.

The paper is Griffith et al., “A corridor of exposed ice-rich bedrock across Titan’s tropical region,” Nature Astronomy 29 April 2019. Abstract.

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{ 12 comments… add one }
  • Charley May 1, 2019, 15:21

    I never thought about this before, but where would the methane come from that originally settled on Titan? Is it a natural byproduct of some kind of reaction between say carbon dioxide and between primordial hydrogen, triggered by diffuse sunlight? Also, I’m not understanding the significance here of this chasm that exist on the surface of the planet, what is this supposed to mean?

    • Paul Gilster May 1, 2019, 15:31

      The ice corridor is evidence of past cryovolcanic activity, meaning it’s also evidence for a methane source below the surface. Right now Titan is geologically quiet, and it’s known that the lakes don’t provide enough methane through evaporation to account for the amount in the atmosphere. This provides a mechanism, or is evidence for one.

      • ljk May 1, 2019, 16:23

        How do we really know that Titan is geologically quiet?

  • Michael C. Fidler May 1, 2019, 18:07

    The dunes are in the same area, how does that relate? Could this be base rock exposed from the erosion of the dunes? Two outside influences that might be important, Saturn’s rings are water ice, how about impacts from when the satellite broke up that formed them? The other is not tectonics but slow gravitational convection of the interior with the equatorial regions having water ice rising and poles having methane rising or subduction from interior heat source. I guess it would be a slow version of tectonic like the ocean ridges here but no continents and run parallel with the equator to poles.

  • Rob Flores May 1, 2019, 19:26

    This a bit of bad news for fans of Titan colonization. It means methane methane is not sourced from the interior anymore. They are limited resources.(admittedly huge in reserves)
    Also with extensive cryovulcanism other dissolved minerals become easily available. Not so much now. Titan colonists will have to forage on asteroids

  • Laintal May 1, 2019, 21:47

    Thanks Paul

    An interesting article, I’ll have to read the paper.

    I too was looking for further information on the “Corridor of Ice”

    Charlie the methane accreted with the formation of the moon and through differentiation ended up close to the surface where through the ice corridor erupted to the surface.

    Now to read the paper

  • Tony Mach May 2, 2019, 2:33

    I might be doing these good scientists an grave injustice, but when I read principal components analysis I have to wonder if this could be an artifact of the method used…

    Unfortunately currently the only way to find out, is for someone experienced in the methods (not me) to put in an amount of work comparable to the original authors (again, alas, I can not deliver this) and redo their analysis.

    I hate having my “skeptical me” shouting in my ear like that, but until we land instruments on Titan and provide some ground truth to such findings, I will hedge my bet (not that it matters much what I think), and be open to the possibility that this is a real feature on Titan, or that it might be a fluke – either way, this calls for more science to be done!

    • ljk May 2, 2019, 9:35

      When they first started doing remote radar mapping of Titan from Earth, the conclusions wavered between a moon with dry land and one totally covered in liquid. That they could detect anything via radar from so far away is an amazing feat in itself, but they still came to widely spaced conclusions.

      Even when Cassini got there in 2004 and could image Titan’s surface, astronomers were frankly confused at first as to what they were looking at. So in conclusion, yes, we need dedicated orbiters, landers, drones, and submarines exploring this fascinating world in directly in detail.

  • ljk May 3, 2019, 12:46

    http://astrobiology.com/2019/05/the-floatability-of-aerosols-and-waves-damping-on-titans-seas.html

    The Floatability of Aerosols and Waves Damping on Titan’s Seas

    Press Release – Source: astro-ph.EP

    Posted May 2, 2019 9:31 PM

    Titan, Saturn’s largest moon, has a dense atmosphere, together with lakes and seas of liquid hydrocarbons.

    These liquid bodies, which are in polar regions and up to several hundred kilometres in diameter, generally have smooth surfaces despite evidence of near-surface winds. Photochemically generated organic aerosols form a haze that can settle and potentially interact with the liquid surface.

    Here we investigate the floatability of these aerosols on Titan’s seas and their potential to dampen waves. We find that the majority of aerosols are denser than the liquid hydrocarbons, but that some could have liquid-repelling properties. From calculation of the capillary forces, we propose that these ‘liquidophobic’ aerosols could float and form a persistent film on Titan’s seas.

    We numerically model the wave damping efficiency of such a film under the conditions on Titan, demonstrating that even a film one molecule thick may inhibit formation of waves larger than a few centimetres in wavelength. We conclude that the presence of a floating film of aerosols deposited on Titan’s lakes and seas could explain the remarkable smoothness of their surfaces.

    Daniel Cordier, Nathalie Carrasco

    (Submitted on 2 May 2019)

    Comments: Accepted in Nature Geoscience

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

    DOI: 10.1038/s41561-019-0344-4

    Cite as: arXiv:1905.00760 [astro-ph.EP] (or arXiv:1905.00760v1 [astro-ph.EP] for this version)

    Submission history

    From: Daniel Cordier

    [v1] Thu, 2 May 2019 14:13:10 UTC (495 KB)

    https://arxiv.org/abs/1905.00760

    Astrobiology

  • Alan Flores May 11, 2019, 5:28

    I don’t have access to the paper, but if I’m not mistaken, does this imply that Perhaps Titan has gone through a cycles of cryovolcanic activity that resurface the organic dunes and restores Titan’s surface methane, until the dunes start building up again from photolysis and the methane is destroyed? Could that have been going on for all of Titan’s history? Perhaps there is just tons of layers of sedimentary organic sand and cryo-igneous rock. Damn I wanna drill on Titan now.

  • Harry R Ray May 13, 2019, 12:36

    Now, like Pluto, it is ALMOST CERTAIN that TITAN has a global ocean. ArXiv: 1905.03802. “Titan’s Dynamic Love Number Implies Stably-Stratified Ocean.” by Jing Luon. Seas, lakes, and rivers ON TOP OF oceans. HOW EXOTIC CAN YOU GET?!

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