Titan Submarine – Close Read

Titaaaaan!

Here we go again. 😄 As has been reported, NASA has been interested in sending a robotic submarine to Saturn’s moon Titan to explore the hydrocarbon lakes near its north pole. Various dates have been mentioned and in all it seems likely the mission will be able to take off around 2040. In the 22 years we have left, we’ve got to build the submarine and make sure it can run autonomously on Titan, where the sea-surface temperature is about 95 K, whose ~~waterbodies~~ liquid-hydrocarbon-bodies are made of methane, ethane and nitrogen, and with density variations of up to 30%.

So researchers at Washington State University (WSU) tried to recreate the conditions of benthic Titan – specifically as they would be inside Kraken and Ligeia Mare – by working with the values of four variables: pressure, temperature, density and composition. Their apparatus consisted of a small, cylindrical cartridge heater submerged inside a cell containing methane, ethane and nitrogen, with controls to measure the values of the variables as well as modify conditions if needed. The scientists took a dozen readings as they varied the concentration of methane, ethane and nitrogen, the pressure, sea temperature, the heater surface temperature and the heat flux at bubble incipience.

The experimental setup used by WSU researchers to recreate the conditions inside one of Titan's liquid-hydrocarbon lakes. Source: WSU/NASA — The experimental setup used by WSU researchers to recreate the conditions inside one of Titan’s liquid-hydrocarbon lakes. Source: WSU/NASA

The data logged by WSU researchers pertaining to the conditions inside one of Titan's liquid-hydrocarbon lakes. Source: WSU/NASA — The data logged by WSU researchers pertaining to the conditions inside one of Titan’s liquid-hydrocarbon lakes. Source: Hartwig and Leachman, 2017/WSU

Based on them, they were able to conclude:

The moon’s lakes don’t freeze over even though their surface temperature is proximate to the freezing temperature of methane and ethane because of the dissolved nitrogen. The gas lowers the mixture’s freezing point (by about 16 K below the triple point), thus preventing the formation of icebergs that the robotic submarine would then have had to be designed to avoid (there’s a Titanic joke in here somewhere).
However, more nitrogen isn’t necessarily a good thing. It dissolves better in its liquid-hydrocarbon surroundings as the pressure increases and the temperature decreases – both of which will happen at lower depths. And the more nitrogen there is, the more the liquids surrounding the submarine are going to effervesce (i.e. release gas).

What issues would this pose to the vehicle? According to a conference paper authored among others by Jason Hartwig, a member of the WSU team, and presented earlier this year,

Effervescence of nitrogen gas may cause issues in two operational scenarios for any submersible on Titan. In the quiescent case, bubbles that form may interfere with sensitive science measurements, such as composition measurements, in acoustic transmission for depth sounding, and sidescan sonar imaging. In the moving case, bubbles that form along the submarine may coalesce at the aft end of the craft and cause cavitation in the propellers, impacting propulsive performance.

The quantity of effervescence and the number of sites on the submarine’s surface along which bubbles formed was observed to increase the warmer the machine’s outer surface got.

If NASA engineers get all these details right, then their submarine will work. But making sure the instruments onboard will be able to make the observations they’ll need to make and the log the data they’ll need to log presents its own challenges. When one of the members of the WSU team decided to look into the experimental cell using a borescope (which is what an endoscope is called outside a hospital) and a video recorder, this is what he got:

(Source)

Oh, Titan.

(Obligatory crib: the university press release‘s headline goes ‘WSU researchers build -300ºF alien ocean to test NASA outer space submarine’. But in the diagram of the apparatus above, note that the cartridge heater standing in for the submarine is 5 cm long. So the researchers haven’t built an alien ocean; they’ve simply reconstructed a few thimblefuls.)

Featured image: A radar image obtained by Cassini during a near-polar flyby on February 22, 2007, showing a big island in the middle of Kraken Mare on Saturn’s moon Titan. Caption and credit: NASA.

Note: This post was republished from late February 15 to the morning of February 16 because it was published too late in the night and received little traffic.

An artist's conception of the proposed Titan Submarine, which NASA could land on Titan around 2040 to explore the depths of Kraken Mare, the moon's largest hydrocarbon lake. — An artist’s conception of the proposed Titan Submarine (conceived before the latest design was released), which NASA could land on Titan around 2040 to explore the depths of Kraken Mare, the moon’s largest hydrocarbon lake. Image: NASA

Nothing bespeaks humankind’s potential more than the following statement: Around 2040, NASA plans to splash down a submarine to explore a liquid hydrocarbon lake on Titan.

Fore more than a decade now, Titan has captivated astronomers not simply by being Saturn’s largest moon by far but also with its vast seas of liquid methane and ethane. NASA has its eyes on the largest such lake, called Kraken Mare, located near the moon’s north pole. The Cassini mission helped map the lake in great detail since it reached the Saturnian system in 2004, accompanied by the Huygens probe that landed on the moon’s surface in 2005. Thanks to them, we know Kraken Mare has an intricate shoreline and deposits of water-soluble minerals around it. According to the scientists who authored the article describing the submarine, these features “hint at a rich chemistry and climate history”.

They continue: “The proposed ~1-tonne vehicle, with a radioisotope Stirling generator power source, would be delivered to splashdown circa 2040, to make a ~90-day, ~2,000 km voyage of exploration around the perimeter, and across the central depths of Kraken.” While its design is by no means final (it’s described as a “first cut”), that NASA is considering exploring Titan in great detail belies its interest in the moon as well as continued commitment to studying the Saturnian system in general. Note that the agency cancelled the development of the proposed Titan Mare Explorer – a nautical surface probe – soon after 2013 to channel the funds into developing Stirling radioisotope generators, which we now find could be used to power the submarine.

Notwithstanding future budgetary cuts, delivering such a vehicle to the surface of a faraway moon might just signify the next leap in astronautical engineering. As the scientists remark,

Even with its planetary application aside, this exercise has forced us to look at submarine vehicle design drivers in a whole new way.

The current design has been developed by scientists from the JHU Applied Physics Laboratory, the NASA Glenn Research Center, and the Penn State Applied Research Lab. It will be presented at the 46th Lunar and Planetary Science Conference in Texas, during March 16-20.

1970s Space Shuttle ditching tests at Langley show lifting bodies can make safe landing on liquid. Image: ‘Titan Submarine: Vehicle Design and Operations Concept for the Exploration of the Hydrocarbon Seas of Saturn’s Giant Moon’ by Lorenz et al

Around 2040, they expect to be able to deliver it to Titan on board a ‘spaceplane carrier’, essentially a repurposed US Air Force DARPA X-37. According to them, Titan’s thick atmosphere could allow the carrier to descend to the surface at hypersonic speeds, following which attempt a soft-landing on the Kraken Mare. Finally, “the backshell covering the submarine would be jettisoned and the lifting body would sink, leaving the submarine floating to begin operations. (Alternatively, the submersible could be extracted in low level flight by parachute).”

Once inside, it will explore tidal currents in Kraken Mare, use a camera mounted on the mast to explore the shoreline landscape, make meteorological observations, analyze sediments from the seabed, and study trace organic compounds to learn how they evolved.

The slender low-drag hull has propulsors at rear, and a large dorsal antenna at the front of which is a surface camera is mounted in a streamlined cowl. A sidescan sonar, seafloor camera, and seafloor sampling system are visible on ventral surfaces. Image: ‘Titan Submarine: Vehicle Design and Operations Concept for the Exploration of the Hydrocarbon Seas of Saturn’s Giant Moon’ by Lorenz et al

The submarine itself looks conventional apart from a large dorsal antenna and two cylindrical buoyancy tanks that jut out of the upper surface. According to its designers, the antenna was shaped so to be able to send data across billions of kilometers to Earth. And such large buoyancy tanks are necessary because the lake the submarine will explore is composed of methane and ethane, whose densities range from 450 kg/m³ to 670 kg/m³, as well as to counter the unique drag effects arising due to the dorsal antenna.

Another complication is thermodynamics. Titan has a frigid surface, cold enough to keep methane, whose boiling point is -161.5 degrees Celsius, in its liquid form. As a result, extra heat rejected from the submarine’s radioisotope power source could cause the surrounding methane and ethane to bubble. As the scientists explain, this results in “heat transfer uncertainties” as well as the potential to interfere with sonar observations. At the same time, the vessel must also be heavily insulated to allow the power source to warm its insides.

NASA first announced its intention to explore Kraken Mare with a submarine in June 2014, elaborating that the mission would help scientists learn more about the history and evolution of organic compounds in the Solar System, in turn a “critical step” along the path to understanding the formation of life. Earth and Titan are the only two objects in the System to host liquid lakes on their surfaces, albeit of different compositions.

Tag: Titan Submarine

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