By Jonathan Amos
Science correspondent, BBC News, San Francisco
Ligeia Mare covers about 100,000 sq km in Titan's high north
A daring proposal to try to put a "boat" down on a sea of Saturn's moon Titan is about to be submitted to Nasa.
The scientific team behind the idea is targeting Ligeia Mare, a vast body of liquid methane sited in the high north of Saturn's largest moon.
The concept will be suggested to the US space agency for one of its future mission opportunities that will test a novel power system.
It would be the first exploration of a planetary sea beyond Earth.
"It is something that would really capture the imagination," said Dr Ellen Stofan, from Proxemy Research, who leads the study team.
"The story of human exploration on Earth has been one of navigation and seafaring, and the idea that we could explore for the first time an extraterrestrial sea I think would be mind-blowing for most people," she told BBC News.
Dr Stofan, who is also an honorary professor at University College London, has been describing her group's idea here at the American Geophysical Union's (AGU) Fall Meeting, the world's largest annual gathering of Earth scientists.
The Cassini mission currently in orbit around Saturn has confirmed the haze-shrouded moon Titan to be an extraordinary place.
Great lakes exist on its surface, fed by rivers that wash down valleys whenever it rains.
TITAN MARE EXPLORER (TiME)
A Nasa Discovery Class proposal, powered by ASRG technology
Simple mission concept would hope to be low cost - <$425m
Launch 2016; Earth and Jupiter flybys; splash-down 2023
Science payload: Mass spectrometer, sonar, meteorology and imaging instruments
In many respects, it resembles Earth and the way it cycles water between the surface and the atmosphere, except in the frigid temperatures of Titan it is not water but liquid hydrocarbons that are in constant circulation.
Scientists got a few brief hours worth of data back from Titan's land surface in 2005 when the Huygens probe touched down in an equatorial region of the moon.
Now a number of those same researchers are desperate to go back for a longer-lived stay, but to investigate this time the huge pools that contain methane, ethane, propane and probably many other types of hydrocarbon (carbon-rich) compounds.
The Titan Mare Explorer (TiME) has already been under study for about two years. It is envisaged as a relatively low-cost endeavour - in the low $400m range.
It could launch in January 2016, and make some flybys of Earth and Jupiter to pick up the gravitational energy it would need to head straight at the Saturnian moon for a splash down in June 2023.
The scientists have a couple of seas in mind for their off-world maritime research vessel. Ligeia Mare and Kraken Mare are both about 500km across.
The primary objective of the mission would be to determine the precise chemistry of one of these lakes; but also to do meteorology, to help scientists better understand how the "methane-ologic cycle" on Titan actually works.
"The key instrument is a mass spectrometer because you want to know what the lake is made of, but we also want to do things like depth-sounding," said Dr Stofan.
"We suspect from Cassini radar data that the lakes are many metres deep, but we'd love to know the overall shape of the lake basins.
"Other instruments would test different properties of the lake which would give you a handle on how the density of the liquid varied as the craft drifted along."
According to team-member Dr Ralph Lorenz, what we learn from Titan's lakes could be relevant here on Earth.
It would give scientists the opportunity to study shared climate processes at work under very different conditions.
"If we have models that will work on Earth and on Titan then we can be much more confident that those models understand the fundamentals of what's going on," explained the researcher from the Johns Hopkins University Applied Physics Laboratory.
"The photogenic appeal and the mystique of exploring a sea on another world speak for themselves, but there is a genuine practical application to do with the science that will help us address problems here on Earth."
Pictures will be essential, though. The Huygens lander sent back a vista of orange pebbles - one of the most iconic images in Solar System exploration history. A view from the surface of a methane lake, looking towards the shore would be just as amazing.
Nasa and Esa (European Space agency) are currently considering a joint multi-billion-dollar mission to the outer planets, but they have the Jupiter system and not Saturn as their next priority.
Huygens pictured icy pebbles
If TiME is to make it off the launch pad it will have to grasp one of the smaller mission opportunities that Nasa periodically offers, such as the one it runs under its Discovery Class programme. Bids for this opportunity will be invited in the coming months.
The agency will be considering mission concepts that can carry an Advanced Stirling Radioisotope Generator (ASRG). This is a plutonium-driven device that produces power far more efficiently than the traditional Radioisotope Thermoelectric Generators (RTGs) used hitherto on space missions, such as on Cassini.
ASRGs would give TiME sufficient energy to support a very capable instrument suite and a direct-to-Earth communications system to get its data home. The generators - TiME would carry two - could conceivably sustain several years of service on the lake surface.
Whatever the outcome of the Discovery competition - and there will be many bids from other mission hopefuls - Dr Lorenz believes the scientific case for going to Titan is compelling, and he envisages the orangey moon becoming a popular destination in the decades ahead.
"I think the range of science questions that are there, and the methodologies and types of vehicle that we use to address that science, is going to follow very much the Mars model - we'll have one mission building on the success of another and exploring different questions," he told BBC News.
"Hopefully, TiME will just be the first of many exciting missions to Titan."
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