Next Planetary Frontier: Navigating an Alien Sea
Since NASA’s skateboard-sized Sojourner rover bounced to a landing on Mars in 1997, we have been co-explorers via to a succession of rovers, culminating in the fiery touchdown of the Volkswagen-sized Curiosity last summer.
We’ve spent a lot of time and effort exploring the arid “red rock” country of Mars. But Mars is frozen in geological time. Its water is locked away as ice. The last rainfall was billions of years ago.
Curiosity’s biggest excitement recently was when it came across an ancient dried streambed. It’s reminiscent of the Neil Youg song, “A Horse With No Name:”
“I was looking at a river bed.
And the story it told, of a river that flowed,
Made me sad to think it was dead.”
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But imagine visiting a dynamic world of lakes and rivers; imagine navigating an extraterrestrial sea sprinkled with methane rain under hazy orange skies, looming anvil-shaped clouds.
Is this a science fiction fantasy from a Jules Verne novel? No, it’s the planet-sized Saturnian moon Titan, sometimes dubbed “Earth II.”
The first potential “boat” to Titan might have been the European Space Agency’s Huygens probe, which landed on a dried river shoreline in 2005. The Huygens lander was designed to float for a short period should it have wound up plopping down on a lake of liquid ethane. (Titan is far too cold for bodies of liquid water, a methane hydrologic cycle replaces the action of water.)
However, Huygens landed in the comparatively dry equatorial region of the moon, which is Titan’s equivalent of Arizona. Lakes on Titan weren’t definitively identified until the summer of 2006 when NASA’s prolific Cassini probe swept over Titan’s north polar region. Cassini’s radar uncovered dark and unusually smooth regions that could best be explained as bodies of standing liquid.
The poles are humid enough for bodies of liquid ethane to remain stable on the surface. The northern pole is peppered with over 400 small lakes, and is reminiscent of the view out of an airplane wind while flying over Minnesota, the legendary “Land of 10,000 Lakes.” There are also four large seas in the north, two in the south.
One design concept, called the Titan Lake In-situ Sampling Propelled Explorer, (TALISE) would be a boat-probe pushed by propellers or even paddleboat wheels. The TALISE concept is being developed as a partnership between the Spanish aerospace company SENER and the Centro de Astrobiología in Madrid, Spain.
The optimum target would be Ligeia Mare, a northern sea larger than Lake Superior. Radar indicates the sea is mirror smooth with no evidence of wave action. So it would be gentle cruising for the Titan boat. The extremely dense lower atmosphere should be sluggish, as the Huygens lander found on its lazy descent. But the forecast is that the winds will pick up as Titan moves into northern hemisphere summer. (Each season of Titan is seven Earth-years long.) What’s more, Saturn’s gravitational pull induces tides that only raise sea level by a few feet.
Making a splashdown on Titan would not be nearly as hair-raising as NASA’s touted “seven minutes of terror,” for the Mars rover landing. Titan’s thick atmosphere would efficiently decelerate the probe. It would then gently parachute to a splashdown.
The pancake-shaped boat would head for the shoreline on a yearlong lake cruise.
The sea has an Earthlike shoreline with bay, inlets and river deltas. The sea level must have risen because planetary scientists see what appear to be newly flooded regions.
The probe would collect and analyze liquid and solid samples of Titan from several scientifically interesting locations along its excursion. TALISE would weigh only 225 pounds. Like any good tourist it would carry a panoramic camera. In addition, because it is a boat, it would carry an acoustic sounder and Light Detection And Ranging (LiDAR) system. And perhaps maybe even a microphone to send back the gentle sound of waves lapping up onto a shoreline 1 billion miles away.
“A really simple mission can tell you big things,” says planetary geologist Ellen Stofan of the University College London. “And, our history as human explorers is really tied up in tales of navigation.” Among the question to be explored: what is the sea’s composition; is it a soup of complex organic compounds? What is the sea depth? And how often does it rain?
However, the clock is running out on the chance to go sailing on a world nearly halfway across the solar system. Because Titan is tilted like Earth the northern polar region will be plunged into ten years of darkness beginning in 2027 And Earth will be below the horizon too, out of communication with the north polar region. The next launch opportunity would not come until 2040.
Stofan’s proposed Titan boat, called the Titan Mare Explorer was turned down last summer by NASA in favor of the InSight mission (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) to do Martian seismology.
Perhaps Exxon Mobile or British Petroleum may be interested in privately funding such a mission to the tune of $400 million. That is, once they learn there are hydrocarbon seas on Titan.
Image credit: NASA/ESA