Human exploration of planets and moons in the solar system has thus far taken place on dry land, thanks largely to the lack of liquid water on most celestial bodies. The NASA Innovative Advanced Concepts program has developed a concept for a robotic submersible vehicle for planetary exploration, but it won’t be deployed in water. The aptly (though unimaginatively) named Titan Submarine Phase I Conceptual Design could some day go for a dip in the hydrocarbon lakes of Saturn’s moon Titan.

Titan has always been an interesting target for study because it’s so unusual for a moon. It’s larger than Mercury and has an atmosphere one-and-a-half times as dense as Earth’s. The atmosphere is mostly nitrogen and methane with hints of hydrogen, so certainly nothing that would support life as we know it. Titan’s temperature can reach a positively frigid -179 degrees Celsius (-290 degrees Fahrenheit). These super-low temperatures and thick atmosphere combine to create the feature scientists are so anxious to study in detail — vast lakes and rivers of liquid methane and ethane.

Data from the Voyager, Cassini, and Huygens (deployed by Cassini) probes have confirmed that there are three large polar seas on Titan’s surface, and the largest of them is known as Kraken Mare. This is the target for NASA’s Titan submarine as it would provide the most opportunities for exploration.

The conditions in Kraken Mare would be challenging to say the least. The massive hydrocarbon lake has an area of 400,000 sq km (154,000 sq miles) and is believed to be as much as 160 meters (525 feet) deep. There are also currents and tides to deal with, and it’s not like mission control could just nudge the submersible this way or that whenever it needed. Titan is about 80 light minutes away, so that means NASA would have to wait 160 minutes to get a response after each command was sent. Real time control is out of the question.

NASA engineers estimate the Titan Submarine would weigh in at about one ton and make use of electrically-powered turbines to get around the liquid methane lake. Titan is far from the Sun, making solar power inefficient, and the sub would spend most of its time beneath the surface where solar panels wouldn’t work anyway. Like many deep space probes, the sub would rely on a radiothermal Stirling generator to produce about 1kW of power. The concept submersible could reach speeds of about one meter per second, according to NASA.

There aren’t many details on exactly what sort of instruments the Titan sub would carry, but the data obtained would be transmitted back to Earth in an unusual way. Rather than introduce the complication of an orbital component to act as a link with NASA, the sub itself would have a large dorsal fin with a built-in planar phased-array antenna. It would surface for 16 hours per day to beam data back before sinking once more beneath the methane waves. Surfacing and diving will be controlled with ballast tanks, just like an Earth-bound sub, but the details have yet to be worked out. Methane and ethane don’t work quite like water, and there’s concern nitrogen gas could condense in the tanks, dropping the sub to the bottom of Kraken Mare.

This is still just an early concept of a possible future mission. NASA estimates some descendant of the Titan Submarine Phase I Conceptual Design could reach Titan around 2040, but there’s a lot of work to do before then.

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