Titan is interesting 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.
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.
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.