I always find pictures of nebula and stars beautiful, but it’s new pictures of the surface of an alien world that really make me stop and stare. Most of the time it’s difficult to figure out what you are looking at, and even harder to comprehend how the landscape evolved to that state. Whenever I see an alien landscape that I can quickly relate to, it’s time to jump up and “wow” over it. I’m not talking about Mars in this post, but about another world. One that is even more wild and different, yet eerily familiar in a way – Saturn’s moon Titan.
Figure 1. – Titan (Cassini)
The Cassini Probe has sent back incredible pictures of the Saturn system, but Titan is the member of that system that truly grabs the imagination. The only moon in our solar system with a dense atmosphere, Titan was the target of the Huygens Probe that separated from Cassini and landed on this mysterious moon in 2005. While altogether an incredible feat with its own pictures to share, it was a picture sent back by Cassini itself that had me on the edge of my seat.
Figure 2.- Legeia Mare, Titan. (Cassini)
The picture was part of a post on the Casssini Mission page located here: https://www.nasa.gov/mission_pages/cassini/multimedia/pia17031.html
This is an image of Ligeia Mare. The word ‘mare’ is latin for ‘sea’. Ligeia Mare is a sea. This picture is a actually a false color radar image, meaning that the shades of yellow to black indicate the strength of radar signal reflected back to Cassini. The dark areas in this picture are where the radar encountered liquid when the beam hit the surface. One of the most interesting things about this sea is that it is not filled with water. The liquid in this sea is methan! Methane, CH4, is known to us as Natural Gas. In order for liquid methane to exist on the surface of Titan the temperature has to be very, very cold. Indeed the surface temperature of Titan is about -180C. Methane is odorless and colorless in its natural state. The familiar, and unpleasant, smell we are accustomed to is actually created by other gases added to it to aid in leak detection. Your next question might be about the flammability of the lake. Don’t worry, it’s not about to spontaneously combust. The atmosphere of Titan is primarily Nitrogen so there isn’t an oxidizer to be had, even if an ignition source, like a hot meteorite, were to contact the sea.
Ligeia Mare is located near Titan’s north pole, the 80 degree latitude passes through it’s northern region, and it occupies a 126,000 square kilometers. This is not your ordinary puddle of Methane! According to Wikipedia (yes, Wikipedia knows all) five bays have been named within Ligeia Mare. Their names are Wakasa Sinus, Puget Sinus, Patos Sinus, Rombaken Sinus and Nicoya Sinus. The word ‘sinus’ is Latin for ‘bay’ in this context.
What makes this image so magnetic is how similar the picture is to a lake you might find on Earth. Check out the Rivers flowing into it! For funzies, let’s pretend to sail this sea of methane and see the sites. We’ll start our tour at the top, in the river that empties into Puget Sinus.
Figure 3. – The North River and Old Crater
I do not know if someone out there has named these features and I’m not going to get so presumptuous as to come up with fancy names of my own. The River you see in the center we’ll simply call the North River, and the crater we’ll call Old Crater. Admittedly, Snake River and Captain Hook’s Crater would be much better names, but I’ll maintain my self-restraint.
A float down the North River would provide a nice, easy entrance into the sea. It’s a wide, gently curving river that has cut a deep channel into the surface. If I’m interpreting the image correct, the river channel descends into Ligeia Mare, and continues on as a subsurface canyon. We’ll see a few more examples of this as when we continue our tour around the lake, so there is a pattern here. It is possible that the depth of Ligeia Mare has risen to consume what was part of the North River. This could have been a long term change triggered by a warming of the region by an unknown mechanism or it could be seasonal change. When the lake is low the rivers cut channels into the exposed, dry lake bed and when the lake fills again those channels are consumed and become lake bed canyons.
Old Crater jumps out at you. It’s hard to miss, being a perfect circle with one section missing to form a perfect ‘c’. One wall of the crater has collapsed allowing it to fill with Methane and form a nice little lagoon. I wonder which formed first. Is the crater older than the lake or is the lake older than the crater? I’m going to put my money on the lake being older than the crater. It appears as if the crater cut off an older set of drainage channels evident just above it. The impact threw up material that formed the walls of the crater, but with the methane level (almost said ‘water level’ there; gotta be careful not to get too familiar here) being deeper to the right, either not enough material was there to form a wall or the wall that did form slumped away into the sea. If the depth of Ligeia Mare does exhibit a seasonal change, it would be interesting to see if the entire crater is visible at some point during the winter. For now, let’s move on.
Figure 4. – The East Rivers
Figure 5 . – The West Rivers
After arriving in Ligeia Mare on the North River, sailing in either direction along the northern shore can take you around to a few smaller draining systems that also empty into the sea. These rivers also appear to have cut subsurface channels in the shallow seabed as we saw earlier. What I’ve labeled the West Rivers (because they are on the west side of the lake) appears to meander more than most of the other rivers and empty into Wakasa Sinus. To the left-of-center in Figure 5 there appears to be an ox-bow, but it’s also possible the river is starting to braid as it reaches a more shallow slope. The river in the center of Figure 5 extends into the shallow lake almost as far as it extends onto dry land. Note that one of the East Rivers in Figure 4 extends in straight line as a subsurface channel an almost connects to the opposite shore of the bay. If this isn’t an illusion, I’d really like to know how it managed that little trick.
Figure 6. – The Vid Flumina River
In the southern region of Ligeia Mare Nicoya Sinus forms a boot-shaped gulf. At the tip of the boot is a wicked looking spike. This spike is actually the mouth of a great river named in 2012 by Cassini scientists as the Vid Flumina river. This river has been compared to the River Nile in several articles like this one: http://discovermagazine.com/2013/nov/07-titan-nile
The Vid Flumina drainage network extends for hundreds of hundreds of kilometers, right on out of the lower right corner of the image. Vid Flumina is not very wide except for the 40 km mouth that empties into Nicoya Sinus. Here the banks appear to have fairly steep walls. The view from the cliffs must be spectacular. The small dots in the mouth of the river may be floating ice, if not spotty reflections of a shallow sea bed. We see these little dots along much of the coast.
Figure 7.- The Kraken River System
In the heal of Nicoya Sinus we arrive at another interesting drainage network. There is a lot going on here. Several streams, some quite long, empty into a very wide, but shallow river I’ll call the Kraken River. This wide river meanders from the middle of the cropped image in Figure 7 towards the lower left. This wide river actually connects Ligeia Mare to the largest sea on Titan, the mighty Kraken Mare which occupies 400,000 square kilometers. Some of the straight and angular sections of this network indicate some influence from fault lines. An interesting feature in the upper left in Figure 7 consists of six connected straight lines, resembling a stick figure. Two of lines connect to drainage systems that empty in to the wide, shallow main stream. Two others may connect to a small pond at the top. There’s much more going on here, and it’s difficult to develop a complete picture of the drainage directions involved.
This completes are quick sail around Ligeia Mare. Like many vacations, we just barely touched the surface of the interesting features at this location. I hope you enjoyed the short stop at yet another fascinating site in our solar system. One could spend a lifetime sailing around and exploring this sea. As the armchair explorer, unable to ship my boat to Titan, it would be interesting to see a topographic map to better understand the terrain. A drainage analysis of such a map would help shed light on what is going on and the various directions that flow is occurring in the drainage basins and why. More radar images of the region would also help. Already researchers have spotted some changes in some of the islands over time. It would be interesting to see any changes in the coastline and methane depth over time as well. Even better, a follow up mission to Huygens Probe to land craft in Ligeia Mare would provide a challenge, and a great view, that is not found anywhere else in the solar system. Happy sailin