Titan's allure is manyfold: It has a thick atmosphere—the only moon in the solar system to have one—stable liquid on its surface, and a landscape of lakes, seas, and dunes. So it's no surprise that astronomers are keeping an eye on Saturn's largest satellite.
Scientists now say the Huygens probe that landed on Titan in 2005 did so with a bounce, slide, and wobble, yielding new clues about its Earthlike terrain.
Meanwhile, a Spanish team has proposed sending a boatlike probe that could paddle or propel itself across Ligeia Mare, a vast, liquid hydrocarbon lake near Titan's north pole. The inspiration for the probe's design includes Mississippi River paddleboats and an amphibious Soviet vehicle with screwlike propellers.
"We thought, why not be capable of moving after landing so you can study the landing site, cruise to the shore, and explore the shore?" said Igone Urdampilleta, an aerospace engineer with SENER, a private Spanish engineering firm.
SENER is developing the design—presented at the European Planetary Science Congress in Madrid in September—with Madrid's Centro de Astrobiología.
The still conceptual probe, called the Titan Lake In-situ Sampling Propelled Explorer (TALISE), would take both liquid and soil samples to learn more about Titan's organically rich environment.
Titan Atmosphere Like "Oil Refinery"
Imagine a world shrouded in an orange-brownish fog where it rains methane, the temperature is minus 290 degrees Fahrenheit (nearly 180 below zero Celsius), and a year lasts 29.5 Earth years.
"If you were to bottle some of Titan's atmosphere, then opened the bottle on Earth, it might smell a bit like an oil refinery," said Titan expert Ralph Lorenz of the University of Arizona. "Titan is so much colder, [so] what might be sticky goop on Earth is literally rock hard on Titan."
By the same token, methane and ethane—components of natural gas on Earth and thought to be prevalent on Titan—would exist in liquid form there. According to Lorenz, scientists are "99-plus percent" certain that Ligeia Mare consists mostly of liquid ethane and methane, with the lake stretching several hundred meters across and at least 33 feet (10 meters) deep.
Ligeia Mare would offer an opportunity to study the same kind of hydrologic cycle—involving evaporation and rain—that underlies Earth's climate, said Lorenz. He helped develop the European Space Agency's Huygens probe and a separate proposal to send a floating probe, the Titan Mare Explorer, to Ligeia Mare.
Knowing the lake's composition also has implications for astrobiology.
"It's a fundamentally liquid environment. Can you develop systems that self-replicate?" Lorenz noted. "We have no idea how complicated the chemistry can get."
Titan Lake Probe Tantalizing
When NASA's Cassini spacecraft dropped the Huygens probe in January 2005, its mission was to study Titan's atmosphere as it descended through the thick haze. Huygens delivered that—and more.
"We landed on a lakebed near the equator and had a wonderful view," said planetary scientist Stefan Schröder at the German Aerospace Center in Berlin. "The landscape is very similar to what we see on Earth: what looked like a lakebed, stream patterns, and a coastline."
A new analysis of data by Schröder and colleagues revealed that Huygens made a hole nearly 5 inches (12 centimeters) deep when it landed, jumped out, and slid 12 to 16 inches (30 to 40 centimeters), then rocked back and forth several times.
"We think it's a dry lakebed but still wet on the surface. The liquid could have come from below or from rain a long time ago—we don't know," said Schröder, lead author of a new study published in the journal Planetary and Space Science.
Exploring some of these questions makes sending another probe to Titan a tantalizing prospect. The Titan Mare Explorer mission was a finalist this year for funding from NASA's Discovery Program. Asked if adding paddles and screws would enhance an exploration vessel on Ligeia Mare, the University of Arizona's Lorenz replied: "Our capsule would drift in the wind and currents. Paddles and screws are a great idea if you need more control, but everything costs money."