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A Surprise from Ligeia Mare

Interesting doings on Titan. I would guess that the odd feature that has cropped up in Ligeia Mare, a large ethane/methane sea in Titan’s northern hemisphere — has something to do with seasonal change, and that’s one possibility this JPL news release explores. After all, summer is coming to the northern hemisphere, and studying what happens during the course of a full seasonal cycle is one of Cassini’s more intriguing duties. Have a look at the image:


Image: These three images, created from Cassini Synthetic Aperture Radar (SAR) data, show the appearance and evolution of a mysterious feature in Ligeia Mare, one of the largest hydrocarbon seas on Saturn’s moon Titan. The dark areas represent the sea, which is thought to be composed of mostly methane and ethane. Most of the bright areas represent land surface above or just beneath the water line. The mysterious bright feature appears off the coast below center in the middle and right images. Credit: NASA/JPL-Caltech/ASI/Cornell.

We’re looking at a feature that covers about 260 square kilometers within the 126,000 square kilometers of Ligeia Mare (the latter is an area a bit larger than Lake Michigan and Lake Huron combined). My first thought was that this could be explained by evaporation, but JPL points out that the shoreline of Ligeia Mare has not changed noticeably in this period, which would seem to rule that out. For now, we can consider the feature an enigma and leave it interestingly unsolved, even as we continue to watch what happens as Titan continues into its cold summer.

Three different Cassini flybys were involved in the above imagery, demonstrating that whatever this is, it was not visible in 2007, appearing only in early July of 2013. The mission’s Visible and Infrared Mapping Spectrometer could not find the feature in late July and in September of that year, and low-resolution Synthetic Aperture Radar (SAR) images from October of 2013 also fail to show it. But by August of 2014 SAR can find the feature again, although it had changed in the eleven months since the last observation.

From the JPL news release:

The SAR observation from Cassini’s August 21, 2014 Titan flyby shows that the feature was still visible, although its appearance changed during the 11 months since it was last observed. The feature seems to have changed in size between the images from 2013 and 2014 — doubling from about 30 square miles (about 75 square kilometers) to about 60 square miles (about 160 square kilometers). Ongoing analyses of these data may eliminate some of the explanations previously put forward, or reveal new clues as to what is happening in Titan’s seas.

Possibilities under discussion include surface waves, rising bubbles and solids either floating on the surface or suspended just below it. Whether or not the enigmatic changes are caused by the approaching summer at the north pole of Titan, we’re learning a great deal about how the seasons impact the distant moon. The image below gives some idea of the effect.


Image: This artist’s impression of Saturn’s moon Titan shows the change in observed atmospheric effects before, during and after equinox in 2009. The Titan globes also provide an impression of the detached haze layer that extends all around the moon (blue). Credit: ESA.

Notice the high-altitude red areas at the north pole in the earlier depictions, when it was summer in the southern hemisphere. These are apparent ‘hot spots’ amidst dense haze over the pole, in a period when the north pole was pointed away from the Sun. At equinox in 2009, both hemispheres are receiving equal amounts of sunlight and the red area is almost gone. As spring arrives in the north and the south plunges toward fall and winter, some of the haze over the north pole persists, but the hot spot is now to be found over the south pole.

Nick Teanby (University of Bristol) commented on the reversal in circulation of Titan’s atmosphere back in 2012, in an ESA news release following publication of a paper for which he was lead author in Nature:

“Even though the amount of sunlight reaching the south pole was decreasing, the first thing we saw there during the six months after equinox was actually an increase in temperature at altitudes of 400–500 km, as atmospheric gases that had been lofted to these heights were compressed as they subsequently sank into a newly forming southern vortex.”

Notice, too, the bluish layer of haze at higher altitude (400-500 kilometers), which can be seen in the limb of the moon throughout these images. It’s a separate layer from the now familiar orange smog that is produced by complex molecules filtering down into the lower atmosphere. We’re looking at a place that receives about 100 times less sunlight than the Earth does. Considering that a Titan year is almost 30 Earth years, the atmospheric changes we can make out in a period of months by Cassini’s close observation are startlingly swift.

Surely the huge area seen surfacing and partially disappearing again in the latest Cassini studies is related to all of this activity? But just what it is, and how that relationship would work, remain unknown. The Daily Mail quotes Emma Bunce (University of Leicester) as speculating that we might be seeing something analogous to an iceberg. An interesting thought, but all we can do now is continue to observe what happens next.

The paper on Titan’s seasonal change is Teanby et al., “Active upper-atmosphere chemistry and dynamics from polar circulation reversal on Titan,” Nature 491 (29 November 2012), 732-735 (abstract).


Comments on this entry are closed.

  • Alex Tolley October 1, 2014, 14:23

    My first thought was that this could be explained by evaporation

    I’m not sure that this explanation (or at least a changes in sea level) should be so quickly ruled out. The shape changes of the land mass certainly looks consistent with changing sea levels. I would have guessed that the sea has a shallow bootom with gentle rises. As the surface level drops a little, the rise appears. That it looks still obvious after submersion suggests that it is only just submerged. The last frame however does look as though teh feature is larger than the one in the center frame, which would be inconsistent with a fixed feature that is emerging or sinking in response to purely sea level changes. Perhaps the feature is also being eroded down to a spit or sand bank, extending its size.

    When we see things like this, it really begs for a close up view from a probe inside the atmosphere. It is at the edge of possibility that we are not looking at a purely geologic feature either…

  • Tulse October 1, 2014, 15:45

    It is at the edge of possibility that we are not looking at a purely geologic feature either…

    A surfacing whale? :)

  • Michael October 1, 2014, 15:50

    Cryovolcano and subsequent collapse? The feature is very large 5 by 40 km and by the increase in brightness of the right hand landmass my guess is an upwelling event in Titans mantle raising the whole right hand landmass up exposing a just subsurface feature.

  • Charley October 1, 2014, 16:13

    Cyanide ice in Titan’s atmosphere

    Gigantic polar clouds of hydrogen cyanide roughly four times the area of the UK are part of the impressive atmospheric diversity of Titan, the largest moon of Saturn, a new study led by Leiden Observatory, the Netherlands Institute for Space Research and the University of Bristol has found. The research is published today in Nature.

    Titan is unique in our solar system because of its dense nitrogen-methane atmosphere, which is very similar to Earth’s in some ways, but very different in others. For example, air temperatures are around 200 degrees colder and, in contrast to the warm salt water seas of Earth, frigid hydrocarbon lakes populate Titan’s surface.

    Titan has seasons just like Earth, only each season lasts over seven years instead of three months due to its ponderous orbit around the Sun. After equinox in 2009, Titan’s south pole entered the perpetual darkness of polar winter. Soon after, instruments on NASA’s Cassini spacecraft observed the development of a gigantic polar cloud covering over one million square kilometres – roughly four times the area of the UK.

    Bristol researcher and co-author Dr Nick Teanby said: “The cloud was first seen in images from Cassini’s cameras taken in 2012. It started off quite small but soon grew to cover the entire south polar region. This was totally unexpected and set us puzzling over what the cloud could be made of. Unfortunately, while the images showed that the cloud was very high up, at over 250km above the surface, they did not allow us to figure out what the cloud was actually made of or why it was there.”

    For the next two years Cassini gathered more data including infrared spectra of the cloudy region.

    Lead author Remco de Kok said: “When we looked at the spectra, we saw two large peaks that weren’t present in spectra of other places on Titan. These peaks coincided exactly with the peaks you’d expect from ice particles of hydrogen cyanide, or ‘blauwzuur’ (blue acid) as it’s known in the Netherlands, which is highly toxic. This was very surprising to us, since we did not expect HCN ice to be able to form so high in Titan’s atmosphere.”

    This new research suggests that Titan’s south pole must be extremely cold to allow hydrogen cyanide to condense. In fact, the upper atmosphere must have cooled by over 50 degrees in less than a year to reach a blisteringly cold -150C.

    Remco de Kok concluded: “This is a very rapid change given Titan’s long annual cycle and is much colder than previously thought possible. It suggests that once the pole is in shadow the upper atmosphere acts as a very efficient radiator of heat, perhaps due to the high abundance of exotic hydrocarbon and nitrogen based compounds, which emit strongly in the infrared.

    “Cassini is set to continue observing Titan until it takes a dive into Saturn at the end of its mission in 2017. It will be fascinating to see how the cloud will develop.”

    Read more at: http://phys.org/news/2014-10-cyanide-ice-titan-atmosphere.html#jCp

  • Abelard Lindsey October 1, 2014, 18:50

    It must be those alien nanobots in action!

  • Harry R Ray October 2, 2014, 9:27

    I agree with Michael. Here’ why! What NOBODY has mentioned is that PART of the feature NEXT to the new feature is either COMPLETELY MISSING, or, has SIGNIFIGANTLY DARKENED! I’m calling this feature “Crocidile Head” because of the dark “teeth” separating the upper and lower “jaws”. This feature has remained completely unchanged UNTIL the latest image. The “upper jaw” no longer exists in its ORIGINAL configuration, as if it fell (i.e. landslide) into the sea, leaving an erosional feature commonlu seen in deltas on earth and flood plains on Mars. If I am right, this feature should completely dissapear in the next image, as all of this material will expand and sink to the bottom of the sea bed We’ll just have to wait and see.

  • Alex Tolley October 2, 2014, 13:14

    @ Michael, harry,
    Cryo-vulcanism would be very interesting. Is there any chance that this would also release gases from the interior to help replenish the atmosphere too?

    Now that we have seen one event, perhaps we will see others of a similar nature?

  • Mark October 2, 2014, 21:56

    Regarding the mysterious feature in Ligeia Mare… How far is the Huygens probe’s landing spot from this feature? Could it be that some unsterilized bacteria from the probe have mutated from exposure to high radiation and are now forming massive seething colonies of bacterial slime in the methane seas?

  • Mark Zambelli October 9, 2014, 6:07

    My first thoughts were either a drop in liquid level exposing a submerged shelf or an icesheet that formed and has subsequently sunk in place after releasing any trapped nitrogen bubbles. However, a further thought came to mind after I read that the methane in Ligeia Mare has recently be shown to be nearly pure; so pure that Cassini is now able to image the bottom and found it to be around 200m deep. Presumably, the radar team are employing refined techniques to image the bottom so either the feature was always there but invisible as too deep, or maybe the purity of the methane has increased and the feature is now visible to the radar (maybe through temperature changes that remove liquid ethane from the Mare)… this last idea may be too much of a stretch but I thought it may(?) be worth mentioning. Could the purity of the methane improve over a few years with seasonal change?

  • ljk February 11, 2015, 9:58

    We could send this to investigate the mystery islands: