Astronomers weekly announce the discovery of new exoplanets, some similar in size or temperature to our planet –- but Earth-like worlds are not always far away.
Though Saturn's largest moon, Titan, is a small, cold world orbiting on the outskirts of the solar system, it actually boasts many familiar features.
“Titan is fascinating because it has some surprising properties so similar to Earth,” said planetary scientist Oded Aharonson from the California Institute of Technology. “It has a liquid which erodes channels, an atmosphere, a hydrologic cycle, and many other parallels.”
Chief among Titan’s interesting qualities is that it's the only body other than Earth where liquids are known to flow in large concentrations on the surface. Because average temperatures there are -300 degrees Fahrenheit, these liquids are not water. Instead, hydrocarbons such as methane and ethane rain down from clouds, course over the landscape in rivers and eventually pour out into large lakes and seas.
The presence of liquids has sparked scientists’ imaginations. If Titan has so many Earth-like features, perhaps it possesses one more terrestrial trait: the presence of life. Native organisms on Titan would be an incredible discovery, showing that life may have formed more than once and suggesting it's common in the universe.
In this gallery, Wired looks closer at the details of this strange wet world so similar and so different from our own.
Sea of Ligeia
Creatively colored in this image, Ligeia Mare looks like an inviting place for a summer vacation. Ligeia is one of Titan’s largest lakes, with a surface area bigger than Lake Superior, located in the planet’s northern high latitudes.
The radar data for this picture came from NASA’s Cassini spacecraft, which has been in orbit around Saturn since 2004. Around the edges, many channels are visible. Rivers of hydrocarbons carved these channels, in much the same way that the Colorado River etched out the Grand Canyon.
Image: Antoine Lucas, Oded Aharonson & The Cassini Radar Science Team, Caltech/JPL/NASA
The Rains in the Plains
NASA’s Cassini spacecraft has captured evidence of large weather systems on Titan. In 2010, astronomers noticed that the parts of the surface had darkened after the passing of rain clouds, suggesting the areas became wet after rainstorms.
Much like Earth, Titan has seasonal changes as well as longer-term climate cycles, which may take place over tens or hundreds of thousands of years. Researchers are now creating models of interactions between Titan’s surface, liquids, and atmosphere in order to better understand these processes.
The largest lake yet found on Titan is the gargantuan Kraken Mare. Discovered in 2007, researchers don’t yet know its full extent but it's been estimated to be about the size of Earth’s largest lake, the Caspian Sea.
Image: NASA/JPL-Caltech/Space Science Institute
Rivers of Methane
The Huygens lander detached from NASA’s Cassini satellite and plummeted toward the surface of Titan in 2005. Along the way, it captured spectacular details of the water and landscape.
This image shows a high ridge with channels bringing liquid hydrocarbons into a river. One thing that researchers hope to better understand is how such flows erode the crust of Titan. The surface is composed of rocks and water ice frozen hard as rock, and scientists don't yet know the details of how liquid hydrocarbon eats away into such material.
Image: ESA/NASA/JPL/University of Arizona
The largest lake in Titan’s Southern Hemisphere is named Ontario Lacus. Slightly smaller than its Lake Ontario namesake, Ontario Lacus boasts many features reminiscent of Earthly bodies of water.
Along the northern shoreline are flooded river valleys and hills more than half a mile high. A round-headed bay can be seen at the southeastern shore. And the middle western shoreline features the first delta observed on Titan.
The delta’s shape suggests that liquid first flowed in one channel from the higher plain and then switched at a later point. Such channel-switching deltas can be seen on Earth at the southern end of Lake Albert between Uganda and the Democratic Republic of Congo.
Titan’s Xanadu region is an Australia-sized plateau composed of water ice. Flowing over this frozen landscape are remnants of rivers networks, seen as bright cobweb-like filaments in the right side of the image.
Ever-changing plate tectonics have built continents and drowned mountains on Earth. Similarly, the surface of Titan has been remolded by geologic processes and the presence of liquid on its surface.
The shoreline in this image features many easily identifiable aspects, such as bay, channels, and islands. The islands located in the lower left are likely peaks of a mountain chain that has been flooded by Titan’s liquid hydrocarbon lake.
An enormous island lies right in the middle of a section of Kraken Mare on Titan. The landmass, surrounded on all sides by liquid, is roughly the size of the Big Island of Hawaii.
Using radar data, researchers have mapped the elevation of islands such as this on Titan. What they find is a steady rise and then a flat “bench” existing near the shoreline. This is similar to islands on Earth, which show the same type of bench topography due to erosion from liquid water.
When the European Space Agency's Huygens lander reached the surface of Titan in 2005, it captured the first-ever evidence of liquid on the surface of another planet. Landing near the edge of a dried lakeshore, it also photographed the rocks seen in the left-hand image.
With their flat shape and rounded edges, the Titan pebbles bear a very strong similarity to rocks from a terrestrial shoreline, suggesting that similar processes generated them.
Image: NASA/JPL/ESA/University of Arizona and S.M. Matheson
Saturn-rise Over the Lake
This is an artist’s depiction of a large lake on Titan, as seen from the surface. The landscape is at once alien and familiar: peaceful waves lapping at the shoreline contrast with a strangely-colored sky lit by the bright rings and body of Saturn.
While this image is fanciful, astronomers may get a chance to capture even more spectacular views if the Titan Mare Explorer (pdf) is selected to fly to Saturn. This mission would land a small boat-like probe on a large lake of Titan – likely Kraken or Ligeia Mare – in order to explore the seas of another world.
TiME is currently competing with two other proposed missions, a comet-hopping satellite and a Martian seismometer, for selection as part of NASA’s low-cost Discovery program. If selected, the boat-probe would launch from Earth in 2015 and reach Titan by 2022.
Images: 1) NASA 2) Jet Propulsion Laboratory/Corby Waste