Titan is unique in the outer solar system in that it is the only one of the bodies outside the Earth with liquid lakes and seas on its surface. The Titanian seas, however, are not composed of water, like Earth’s seas, but are seas of liquid hydrocarbons. What lies beneath the surface of Titan’s seas? We propose to develop a conceptual design of a submersible autonomous vehicle (submarine) to explore extraterrestrial seas. Specifically, to send a submarine to Titan’s largest northern sea, Kraken Mare. This craft will autonomously carry out detailed scientific investigations under the surface of Kraken Mare, providing unprecedented knowledge of an extraterrestrial sea and expanding NASA’s existing capabilities in planetary exploration to include in situ nautical operations. Sprawling over some 1000 km, with depths estimated at 300 m, Kraken Mare is comparable in size to the Great Lakes and represents an opportunity for an unprecedented planetary exploration mission. This mission would be a logical follow-on to a Titan surface mission such as TiME (Titan Mare Explorer) or even a component of a flagship mission of multiple vehicles. The mission concept we propose to study will investigate a full spectrum of oceanographic phenomena: chemical composition of the liquid, surface and subsurface currents, mixing and layering in the “water” column, tides, wind and waves, bathymetry, and bottom features and composition. Measurements of all these aspects of Titan’s hydrocarbon ocean environment can only be made through focused in situ exploration with a well-instrumented craft.The concept is for a robotic submarine that will be about the size of a car.
Somewhat similar to an Earth-based submarine, the cylindrical vessel about the length of a car would plunge through the thick atmosphere of Titan and dive into its largest liquid hydrocarbon sea, Kraken Mare. Here, it would explore the subsurface region for 90 days, sending data and images back to Earth. It would travel at a rather sedate one meter per second (2.2 miles per hour) using four propellers at its back, enabling it to cover a planned route of 2,000 kilometers (1,200 miles). Instruments on board would include sonar, a sampling system and a camera to answer questions such as whether there could be life on Titan, which is up for debate.There are many technical challenges to overcome in such a mission, beyond simply getting a vehicle that size all the way to Saturn and delivering it to Kraken Mare. To begin with, as it says, they're looking to design a submarine to move in liquids, but they don't know what's in the Kraken sea. Is it all liquid methane? On top of that, it's very, very cold out at Titan. These hydrocarbon seas are -180°C (-300°F). Is it colder at depth? Do parts of the sea freeze out slightly below the surface and make the entire mission impossible? In their diagram, it mentions insulation, but not heating. While it's true silicon electronics often works better cold than at room temperature, that's at -55C vs +25C. I'm not aware of any class of electronic components that are specified to operate at -180C.
I see problems with the antenna concept (green area in the top illustration). An antenna of that size will be high gain, but it will be very, very narrow area coverage. I don't think a vehicle floating in an "ocean" would be stable enough to link back to earth. I think it would more likely be better to communicate with a local space craft in Titanian orbit and have that one beam back to Earth.
Some of us who worked tangentially around NASA have remarked that NASA's biggest post-Apollo accomplishment has been to get stuff done despite their leadership. I'd like to live lone enough to see something like this, but I have a hard time thinking it will happen.
The density of the liquid in the ocean could be a challenging design criterion. The vessel needs to be dense enough to submerge and light enough to surface in who-knows-what. Maybe they'd need some initial probes to figure that out?
ReplyDeleteAs for stability floating in an ocean - at those temperatures, are there winds on the top to form waves? How strong would the internal movements of the liquid be? I guess that depends on the liquid again and the thermal regimes in the layers.
What fun!
PS - I guess I'm not the only one who was quite challenged by the food puzzle that allows you to post your comment. They seem to have gotten easier.
I kind of pondered over the movement issue last night and came to the realization that I'm incapable of imagining a "sea" without waves. Comes from living within a few miles of the ocean all my life, I guess. I'll grant you that on a world where the total surface temperature is -300F, there's not much heat energy to drive things, but I kept saying all you need is a case where the heat rises from the land faster than it rises from the sea and you get an onshore sea breeze. Reverse the rate of heat rise between land and see and you get an offshore sea breeze. And if there's a breeze, there's going to be ripples.
ReplyDeleteIf the submarine surfaces to recharge, or beam data, it'll disturb the surface and create ripples. With a high gain antenna that has a half degree beam width, that will put the beam on or off earth by large amounts.
Not that an antenna on a spacecraft is all that stable, it's just that reaction wheels and the other tricks they use to make it stable make it seem like rock compared to an ocean. Even if it's an ocean of liquid methane.
"...it's just that reaction wheels and the other tricks they use to make it stable..."
ReplyDeleteThe diagram labels the antenna as a phased array. The beamwidth and broadcast & reception vectors are all a matter of dynamically programming the individual array elements for gain and phase (delay). The physical structure need not move to track earth if it is pointed only roughly in the correction direction (like plus or minus 30-40 degrees).
I see no difficulty in fast correction of the beam direction for "earth-tracking" with a PA antenna. The vessel can bob & wiggle around pretty much all it wants, within limits.
Cryo electronics (Si-based) are often designed to operate at liquid nitrogen temperatures (77K = -195 C).
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