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rfmcdonaldThe Wired Science article "Titan: A Wet World Not Far From Earth", by Adam Mann, makes very interesting classifications.
The below map of Ligeia Mare, a Titanian hydrocarbon sea with a greater surface area than Lake Superior, illustrates the article.
Is Titan Earth-like? Well, let's say that Titan is exactly Earth-like save in the many, many ways in which Titan is completely different. It's an outer-system moon with a mass one percent or so that of the Earth, with a surface temperature hundreds of degrees cooler than the Earth, an atmosphere substantially denser than the Earth's and composed almost entirely of nitrogen (no oxygen there), and possessing a hydrologic cycle dominated by liquid hydrocarbons, not liquid water. The suggestively Earth-like lake above is the product of the action of eons of hydrocarbon liquid erosion of a cryogenic Titanian surface made of a mixture of rock and ice, a surface apparently tectonically inactive and perhaps as much shaped by impacts as by indigenous activity. If Titan was ever warmed to Earth-like temperatures, its cryogenic atmosphere and hydrosphere would quickly evaporate, heated to the point that it could escape the world's meager gravitational pull in a few millions of years, at most. In this very non-Earth-like environment, life on Titan would be plausibly very different from life on Earth, shaped by the world's very low temperatures and perhaps using liquid hydrocarbons instead of water as its solvent of choice.
Mars has a much more Earth-like climate, one that--at most hospitable--barely overlaps with the least hospitable extremes of Earth, may have possessed (still possess?) a hydrologic cycle predicated on water, certainly possesses a rocky surface with minimal levels of tectonic activity and clear evidence of having been worked by water, and even offers the potential for secure habitats for recognizably Earth-like life. Venus' superhot and superdense carbon dioxide atmosphere does not support water anywhere near the planetary surface, but the world did support water oceans in its cooler youth while tectonic activity in forms broadly familiar to those on Earth occurs (planetary resurfacings instead of plate tectonics, but volcanoes and quakes do occur). Critically, both worlds plausibly supported very broadly Earth-like conditions for--perhaps--billions of years.
Titan's a fascinating, complex world, deserving of attention. I wonder, though, how useful it is to identify certain of its characteristics as especially Earth-like when in most respects Titan is more different from Earth than at least two other solar system worlds, its similarities being mostly involving superficial resemblances between very different realities. If Titan is Earth-like, what about high-density and rocky Mercury? Or the very tectonically active Io? Or moons like Europa and Enceladus which plausibly have tectonic activity heating subsurface water oceans? Allowing the category of "Earth-like" to be too broad risks emptying the category of meaning.
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.
The below map of Ligeia Mare, a Titanian hydrocarbon sea with a greater surface area than Lake Superior, illustrates the article.
Is Titan Earth-like? Well, let's say that Titan is exactly Earth-like save in the many, many ways in which Titan is completely different. It's an outer-system moon with a mass one percent or so that of the Earth, with a surface temperature hundreds of degrees cooler than the Earth, an atmosphere substantially denser than the Earth's and composed almost entirely of nitrogen (no oxygen there), and possessing a hydrologic cycle dominated by liquid hydrocarbons, not liquid water. The suggestively Earth-like lake above is the product of the action of eons of hydrocarbon liquid erosion of a cryogenic Titanian surface made of a mixture of rock and ice, a surface apparently tectonically inactive and perhaps as much shaped by impacts as by indigenous activity. If Titan was ever warmed to Earth-like temperatures, its cryogenic atmosphere and hydrosphere would quickly evaporate, heated to the point that it could escape the world's meager gravitational pull in a few millions of years, at most. In this very non-Earth-like environment, life on Titan would be plausibly very different from life on Earth, shaped by the world's very low temperatures and perhaps using liquid hydrocarbons instead of water as its solvent of choice.
Mars has a much more Earth-like climate, one that--at most hospitable--barely overlaps with the least hospitable extremes of Earth, may have possessed (still possess?) a hydrologic cycle predicated on water, certainly possesses a rocky surface with minimal levels of tectonic activity and clear evidence of having been worked by water, and even offers the potential for secure habitats for recognizably Earth-like life. Venus' superhot and superdense carbon dioxide atmosphere does not support water anywhere near the planetary surface, but the world did support water oceans in its cooler youth while tectonic activity in forms broadly familiar to those on Earth occurs (planetary resurfacings instead of plate tectonics, but volcanoes and quakes do occur). Critically, both worlds plausibly supported very broadly Earth-like conditions for--perhaps--billions of years.
Titan's a fascinating, complex world, deserving of attention. I wonder, though, how useful it is to identify certain of its characteristics as especially Earth-like when in most respects Titan is more different from Earth than at least two other solar system worlds, its similarities being mostly involving superficial resemblances between very different realities. If Titan is Earth-like, what about high-density and rocky Mercury? Or the very tectonically active Io? Or moons like Europa and Enceladus which plausibly have tectonic activity heating subsurface water oceans? Allowing the category of "Earth-like" to be too broad risks emptying the category of meaning.
