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. 
 
 Now read: A tribute to Cassini: 10 years spent revealing the secrets of Saturn and its moons ||||| Video screenshot by Eric Mack/CNET 
 
 I think we can now officially say that NASA's real-life mission concepts have met, if not exceeded, the wildest dreams of Jules Verne and the other early futurists. Verne saw us flying around the world, traveling to the center of the Earth (not a great idea, it turns out) and to the moon, but I bet he never imagined that just 110 years after his death, we'd be working on a robot submarine to probe the flammable seas of a distant world. 
 
 Truth can be stranger than fiction, indeed. 
 
 Earlier this month, NASA published the below video introducing its concept submarine designed to explore Kraken Mare, one of the methane seas on Titan, the remarkably strange moon circling Saturn. Even if it were possible for humans to go swimming in Titan's seas, it might not be advisable. It would be something like swimming in a freezing ocean of liquefied natural gas. 
 
 Clearly, this is a mission meant for a drone, specifically a nuclear-powered, single-ton robotic submarine equipped with a seafloor camera and sampling system as described in a brief (PDF) on the concept posted online. 
 
 Getting the sub to Titan presents a bit of challenge since it won't fit into the sphere-cone shaped landers that have typically been used for planetary exploration in the past. However, it could fit into a space plane like Boeing's X-37, which was recently orbiting the Earth for some sort of classified, likely spy-ish US Air Force mission. The space plane would make a soft water landing on Kraken Mare and jettison the sub, or possibly drop it at low altitude via a parachute. 
 
 In addition to floating or diving around checking out the chemistry of Titan's sea and sea floor composition, the sub could also study the moon's tides, weather, shoreline, disappearing islands and keep an eye out for any evidence of really weird life forms that like to hang out in super chilly, gassy locales. 
 
 The Titan sub concept is very preliminary at this point, but there's plenty of time to work on it. The team behind it anticipates that it might not splashdown until sometime around 2047. ||||| ESA/NASA Acknowledgement: T. Cornet, ESA 
 
 Saturn’s largest moon could suit human settlement – provided we can keep the lights on. Thankfully, Titan has several energy sources that might one day power a colony, an analysis shows. 
 
 For all its alien strangeness, Titan is remarkably Earth-like. A thick atmosphere protects its solid surface from damaging radiation and it is the only other place in the solar system with liquid on its surface. If humans were to one day live under the yellow haze of the moon’s skies, exploring its rolling dunes and ragged peaks or settling by the side of gently stirring hydrocarbon lakes, they would need energy to power their lives. 
 
 “I think long-term, after Mars, Titan’s probably the next most important place that people will have an extended presence,” says Ralph Lorenz, a planetary scientist at Johns Hopkins University in Maryland. 
 
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 To figure out how humans might be able to survive on the distant moon far in the future, Amanda Hendrix of the Planetary Science Institute and Yuk Yung at the California Institute of Technology analysed potential energy sources. 
 
 Mining the moon 
 
 Any robotic explorers sent to initially scout out Titan would rely on nuclear power, harnessing radioactive decay to generate electricity and stay warm. Humans might be able to do the same, packing supplies from Earth to make a nuclear power plant, then mining the moon to keep it fueled. However, without investigating Titan’s interior geology, the feasibility of this idea is mostly guesswork. 
 
 But the moon is rich in easily accessible methane, making it a potential refuelling point for rockets travelling even further from Earth. “As a resource for long-term, solar-system-wide civilisation, Titan would be a pivotal place,” Lorenz says. 
 
 Although it would be inefficient to combust hydrocarbons on the moon itself due to the lack of readily available oxygen, future inhabitants of Titan could add hydrogen to acetylene to generate power. However, despite the theoretical abundance of acetylene on the moon, scientists have yet to detect it on Titan’s surface. 
 
 “It’s possible we are missing it because it’s being masked by the atmosphere,” says Sarah Hörst, a planetary scientist at Johns Hopkins University. 
 
 Carving out rivers 
 
 Hydropower might also be problematic as Titan has little rain except rare downpours of intense flash floods every few decades. “It’s not exactly ideal for hydroelectric power generation,” says Hörst. “For a short period of time, rivers would have very, very fast flow, then they’d be dry again.” 
 
 Dams or waterwheels could generate power from hydrocarbons made liquid by Titan’s extremely low temperatures, but it could be difficult to get the liquid flowing as the largest lakes and seas are lower than surrounding terrain. 
 
 “The topography doesn’t make it impossible, it just makes it a very big engineering project to carve out a river that flows downhill out of the sea,” Hendrix says. 
 
 A better option could be to put turbines in the seas because Saturn creates strong tides on Titan. Its largest sea, Kraken Mare, experiences up to a metre of tidal change each day. Those tides all flow through a narrow constriction separating the northern and southern parts of the sea, Seldon Fretum, or as it is nicknamed, the Throat of the Kraken. 
 
 “The Throat of Kraken is basically the Strait of Gibraltar,” Lorenz says. “We’re pretty sure there’s a very strong flow of liquid back and forth every Titan day. If you want reliable power that you know is going to be accessible, that’s where I would go.” 
 
 Atmospheric winds 
 
 Wind power would be equally tempting yet challenging as a long-term power source. While sand dunes indicate that Titan must have had strong surface winds at some time in the recent past, we haven’t found any evidence that such winds still blow. 
 
 But the atmospheric circulation reverses direction twice a year, and strong winds have been revealed in the upper atmosphere by cloud tracking and the brief measurements the Huygens probe made in 2005 as it descended towards the moon. 
 
 “If we had wind machines in the upper atmosphere, we could generate power ten times greater than we do with wind turbines here on Earth,” says Hendrix. “We could have some sort of airborne wind turbines tethered to the surface.” Yet this type of wind turbine is beyond the capacities of current technology. 
 
 Harness the sun 
 
 The most unexpected idea is solar power. At almost 10 times the distance from the sun as Earth, Titan receives just one hundredth the sunlight. That light gets further filtered by atmospheric haze. “The brightest it ever is on Titan is like dusk on Earth,” Hörst says. 
 
 But solar panels are getting ever-more efficient and a human civilisation on Titan would have the space to construct extensive, permanent energy infrastructure. 
 
 “If you just build solar power plants that are large enough, they’ll generate plenty of energy,” says Hendrix. She and Yung estimate that supporting 300 million people – roughly the population of the United States – would require a solar farm covering 10 per cent of Titan, or the surface area of the entire US. By contrast, generating the same amount of power on Earth would take under 10 per cent of the surface of Kansas. 
 
 As on Earth, another challenge would be keeping the solar panels clean, in this case of organic molecules created in Titan’s atmosphere that would otherwise reduce their efficiency. 
 
 “On Titan, we’d also need to think about tholin sediments settling out of the atmosphere onto the panels and having to wipe them off every so often,” Hendrix says. 
 
 Titan has the energy resources to support a human civilisation, but living there would be tricky. Squeezed by 1.5 times Earth’s atmospheric pressure yet buoyant under one-seventh Earth’s gravity, humans on its surface would feel more like divers under an ocean than astronauts on exposed airless rocks in space. 
 
 Furthermore, Titan is cold with an unbreathable atmosphere of nitrogen, methane and hydrogen, so, like divers, anyone living there would need to wrap up and carry their air with them. 
 
 Whatever power source we might one day use on Titan, we will need to perfect it on Earth first. 
 
 Journal reference: arXiv.org, DOI: 1701.00365 
 
 Correction: Titan is the only other place in the solar system apart from Earth with liquid on its surface
– Saturn's moon Titan, which happens to be larger than the planet Mercury, is cold. Very cold. As in -290 degrees Fahrenheit is not out of the ordinary. These super low temps, combined with an atmosphere roughly 1.5 times as dense as Earth's, have resulted in a few extraordinary bodies of liquid methane and ethane, reports Extreme Tech, and scientists are anxious to explore their depths. (In fact, Titan is the only body aside from Earth to boast liquid lakes on its surface.) So NASA is working up a (very early) model of a submarine drone that could do just that—one that CNET reports will likely be nuclear-powered, weigh a ton, and come equipped with a seafloor camera, sampling system, and other fancy futuristic robotic accoutrements. But don't get too excited just yet. First, this is a fully automated mission: "Even if it were possible for humans to go swimming in Titan's seas," notes CNET, it "would be something like swimming in a freezing ocean of liquefied natural gas." Hence, NASA's goal of sending the "highly capable science craft" to Titan's largest sea, Kraken Mare, around the year 2040, in what it calls "an unprecedented planetary exploration mission." Many details remain to be sorted out, including how to get the craft to Titan (Boeing's X-37 is a possibility), how to land it (parachute drop), and while there, how to communicate with Earthlings (planar phased-array antenna). (Something else on Titan has been stumping NASA.)