After writing the original post about setting sail on Ligeia Mare I could not help but feel as though I had missed something. As if I hadn’t completely accomplished what I had set out to do in that post. It came to me after a few days, but I decided to write the first two posts in the series on meteoritics before circling back to Titan. Here is what I discovered: I never really talked about how or even if someone could sail on the seas of Titan! While it was fascinating to tour Ligeia Mare and imagine the bays and rivers we might encounter, I need to step back and ask the 100 million dollar question: Is it even possible to sail the alien seas of Titan? How much do we really know about the surface conditions of Titan? The ability to sail on an alien world in the traditional sense implies a number of conditions to exist that we take for granted on Earth. Is there wind near the surface of Titan? Also, as you recall, liquid methane is really, really cold. What is its consistency? Is it like molasses or like water? How about the depth of the sea? How much do know about it? These questions will need to be answered long before someone plunks a sailboat on Ligeia Mare and starts exploring.
Figure 1.- Points of Sail
Let’s start with the wind. Without wind to fill our sails, nothing else will matter. And it has to be the right kind of wind – hurricane force winds just won’t do! There are two sources of information that can help us find out here. The first is that plucky little lander, the Huygens probe. What did it learn of the surface conditions? It turns out that during it’s descent to the surface the Huygens probe detected winds in the 10’s of kilometers per hour. Once on the surface Huygens measured wind velocities between 0.3 m/s and 1 m/s (1m/s = approx. 2.2 mi/hr). This isn’t much, but it’s something. This measurement was on land of course, and not on the surface of a sea. Fortunately, there is some indication of winds over the sea. Initially, scientists puzzled over the apparent lack of any waves in Titan’s seas. However, in 2014 researchers analyzing radar data from Cassini noticed something interesting in Kraken Mare. They noticed changes in surface roughness of all three major seas from one pass of the moon to another. The only explanation for the feature they were seeing was the presence of waves! This is a promising find. It turns out that most of the recent studies of Titan have occurred during the winter season in northern hemisphere. The northern hemisphere is now starting to move into its version of summer and the change is bringing with it more winds, thus more wave action. Scientists predict that the potential to reach up to 45 mi/hr during the long northern summer season. Given these two points of data we can put to rest this question – surface level winds do exist on Titan.
Figure 2. – Clear waters of the Caribbean. On Titan you might imagine shades of orange instead of blue.
With the question of wind settled, let’s look at the sea itself. For starters, let’s cover the depth of the seas we’re wanting to sail. Cassini answered this quite well. Cassini’s radar indicates an average depth for Ligeia Mare of about 50 meters with the deepest areas being about 200 meters . This would be plenty deep enough for a good sailboat!
Now, let’s talk about this liquid methane stuff. How is liquid methane similar, or different, from water? For starters, Wikipedia notes that liquid methane is odorless, colorless, nontoxic and noncorrosive – all good things in my book if I intend to set a boat down in it. If we are floating on a sea of colorless liquid then we have a chance of seeing what lies below the sea, just like one can see the bottom of shallows of the oceans on Earth. Wikipedia also notes that the density of liquid methane is 0.42262 g/cm3, or about 42% of the density of liquid water.
Figure 3. –Boat Displacement. The numbers on the hulls of these ships represent their weights, respectively, which the illustration is highlighting as the only difference between them. The heavier ship sits much lower in the water than the lighter one.
So what does this difference in density mean for a sail boat on Titan? To get a clear picture of the answer, let’s use an example. Imagine a ship that weighs 10,000 lbs on Earth. This ship would displace 10,000 lbs of water when set afloat. However, on Titan, where the gravity is only 0.14g, she would only weight 1,400 lbs. On Titan it would displace 1,400 lbs of Methane. To get a mental picture of this result, imagine if you were to freeze the ocean in which a boat was sitting and then remove the boat, you’d be left with a depression, like a large bathtub. The volume of this depression would be equal to the volume required for the displaced weight of liquid. In my example, a volume needed for 10,000 lbs of water is 160 ft3. On Titan, the volume needed for 1400 lbs of methane is only 53 ft3. This means our boat would float much higher on Titan than it would on Earth. If the gravity on Titan were the same as the gravity on Earth, our boat would actually sit lower into the sea. However, the lower gravity more than counter balances the difference in fluid density this such that a 10,000 lb boat only displaces a third of liquid as it would on Earth. This fact will need to be accounted for in the design of a sailboat for Titan and taken into account in ensuring it’s stability.
The Air Pressure
Figure 4. – Fast moving sailboat with a very tall mast
Now that we’ve established that a boat on Titan would have wind in it’s sail and that moving through methane is actually easier than moving through water, let’s look at other factors that might affect our boat’s performance. The surface pressure is 1.45 atm – or 45% greater than surface pressure on Earth, and with the temperature so much colder, the density of air on the surface of Titan is about 4.5 times that of Earth. Would this affect how well a boat will sail? For a minute, let’s dip our toe into the physics of a sail to understand the impact. For a given area of sail, with known drag and lift coefficients, the lift and drag forces are:
Where ρ is the density of air, and V is the velocity of the wind. A component of the lift generated by the sail is what pulls the ship forward. Here we find that a sail on Titan would have 4.5 times more lift and 4.5 times more drag, but the difference between them would also grow by 4.5 times. This would indicate that sails used on Titan can be much smaller than sails used on Earth! This is an important find because the small the sail has to be, the shorter the mast has to be and the lighter the vehicle can be. However, if the estimated wind velocities remain as low as Huygen’s felt, this benefit may get eroded away as the sail area is increased to compensate. Determining the appropriate sail area will be an interesting design challenge.
The Design Concepts
If sailing on the seas of Titan is as feasible as the above factors lead me to believe, what would such a mission look like? There have been a couple proposals for probes to Titan over the past 10 years that can be used as a starting point.
The article headlines for the Titan Mare Explorer (TiME) proposal suggest that the concept of sailing on Titan has already been studied. Read a little deeper and you find that this probe doesn’t actually ‘sail’. It floats and drifts wherever the currents and winds push it. In 2011 the PIs for TiME won a $3 Million grant from NASA to develop the concept. As a result, much of the issues related to thermal control, communications and power have been well studied. The InSight mission to Mars ultimately ended up getting selected over the TiME mission, but no good idea ever dies. I suspect we’ll see this proposal come back for another selection round. Personally, I would like to see the design of TiME be revamped to become a sailing vessel rather than a drifter. In my mind’s eye I keep seeing the picture of this wonderful probe being beached hours or days after landing, ending the mission prematurely. Here is a Wikipedia article for more information about TiME: https://en.wikipedia.org/wiki/Titan_Mare_Explorer.
Another proposal floating around the internet is that of the Titan submarine. This probe would spend it’s time below the surface, obviously, and attempt to explore the bottom of the sea. It would surface from time to time to transmit back to Earth and to image sea conditions, and nearby coastline if possible. The concept is not for a small vehicle however. Initial designs call for it weighing about 1 ton. While a solid idea for sure, my first concern for this probe is thermal. Being submerged in liquid methane is like attempting to swim in the arctic sea instead of sailing on it. How long could such a probe last in these conditions? How would it’s subsurface operations be controlled? I’m sure they’ve been chewing on these challenges and, no doubt, we’ll see more of this idea in the years to come. For more information, here is a link to a paper about the submarine concept: http://www.hou.usra.edu/meetings/lpsc2015/pdf/1259.pdf.
Figure 5. – Catamaran design with twin hulls sitting in the water and a connecting bridge above the water line.
Obviously, these two proposals do not use a sail as a means of moving around so what would a sailing version of them look like? A lot more analysis would need to be done to answer that question. Factors that will need to be addressed include vessel stability, sail design, sail trimming and reefing systems, boom control, rudder control, etc. These are challenges facing the robotic sailboat designers on Earth and it’s not easy. My imagination conjures up a catamaran style, multihull craft with a main sail and jib. Distributed within the twin hulls would be the power supply, subsurface sensors, downward facing fiberoptic camera, light source and radar. The bridge would house the primary electronics, and environmental sensors. A single, rotating camera could be mounted on the top of the mast, or mounted on the deck. As for navigation, there is a challenge here. All evidence points towards Titan not having a magnetic field of it’s own, however there is a catch. Saturn’s magnetic field appears to induce a magnetic field in Titan. It is unclear, in my look at some of this research, whether or not this induced field would be strong enough or consistent enough for navigation purposes using a magnetometer. This is all notional of course, but you can see the advantages of such a design. Lastly, in a worst case failure of the sailing system, the multihull configuration ensures that a stable drifter-style probe remains for the mission to continue.
While only a cursory look at the challenge, the initial answer to the question of the feasibility of sailing on Titan’s seas is “Yes”. We can expect wind for the sail, appreciable depth in the sea, and easy passage through the methane itself. The icing on the cake is that the higher air pressure means that each mile/per hour of wind gives a greater push than it does on Earth, which will be handy if the winds are as light as the Huygen’s probe experienced. We also discovered another design characteristic in that due to a much lower gravity than Earth, combined with a low specific gravity of liquid methane, our ship will float much higher than on Earth. We are now armed with a feasible means of propulsion to use when exploring Titan. However, recent proposals for follow-on probes from Huygens have been for difter or submarine style designs. A sail would provide a low power means of locomotion and improve the probe’s versatility and reach over the drifter design. The implementation of a sailing probe would also represent a whole new advancement in planetary exploration similar to how the wheeled rover was an advancement from the stationary lander. It also would gain public interest as many people enjoy the nostalgia of sailing. Employing this mode of travel, used for thousands of years here on Earth, on an alien word embodies with it a piece of ourselves, and our ancestors. It’s almost fitting that the means of travel used by early explorers to cross our oceans to find new lands be used to explore new worlds as well. I only have one final question – Is it too early to start a Titan Sailing Club?