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Titan and Earth

Titan, Saturn largest moon, far from the sun, a water planet with a frigid surface temperature of -170°C, shrouded by a dense atmosphere of nitrogen and methane, is exotic by any definition. But despite this, astronomers have long seen a resemblance between Titan and Earth. This resemblance is in the processes as work that shape Titan's surface.

Like Earth, Titan's atmosphere is right for weather; it has a temperature that is right for precipitation. In Titan's atmosphere, methane can exist in either a liquid state or a gaseous state, just as water can exist in both of these states in Earth's atmosphere. This gives Titan a methane cycle of cloud formation, precipitation, and evaporation. The recent images of the Huygens probe, which landed on Titan on January 14, 2005, shows evidence for this: a network of river channels snake through the highlands of Titan. The weather on Titan is able to shape, erode, and modify Titan's water-ice landscape in the same ways that Earth's weather does. Pictures from Titan strongly resemble images of the American southwest.

Photograph of Titan's surface.

A 360° panorama of Titan's surface as seen by the Huygens probe at 8 km altitude as it drifts to the right at 6 to 7 meters per second. The probe's landing spot is the dark region to the right in the image. The white wisps on the plain are thought to be an ethane and methane fog. (Courtesy ESA/NASA/JPL/University of Arizona).

This occurrence of the same processes in radically different corners of the universe is a commonplace. The physical principles governing stars a tenth the size of our Sun is largely the same physics principles governing stars one hundred times larger than the Sun. The same laws of thermodynamics apply to the coldest regions of open space as to the hottest cores of compact stars. The same laws of gravity and conservation of angular momentum apply to the rings of Saturn and as to the plane of our Galaxy. And often, as the images of Titan show, these various principles produces very similar structures under dramatically different conditions.

Photograph of Titan's surface.

An image of Titan's surface taken by the Huygens probe as it descends to the surface. River channels are clearly visible, as is a coast line of a dry basin. The fluid responsible for these features is liquid methane, which precipitates from methane clouds in Titan's atmosphere. (Courtesy ESA/NASA/JPL/University of Arizona)

But to me, the greater lesson to take from Titan is not its similarity to Earth, but its closer resemblance to all other places in our Solar System but Earth. Look at Mars, look at Titan, and we come back to the similarity of the landscape. But look to Earth, and the similarity is always to the most barren parts of our world. Like the previous worlds we have explored, Titan lacks any sign of life, and so once again we find that life requires an Earth to exist and thrive.

Earth is unique in its sheltering of life. Unlike the other bodies in this Solar System, Earth has the right combination of size, composition, and distance from the Sun that allow water to exist and remain liquid. A little closer to the Sun, and its intensity would have dissociated our water into oxygen and hydrogen, and then blown the hydrogen off into space, a fate suffered by the water of Venus. Farther from the Sun, and our world would be frozen. In our varied universe we continue to see that life exists only in a green house.

The space agencies continue to flog the idea that Titan is interesting because it has organic compounds such as methane, but I regard this claim as hype. To understand life on Earth, we must look at worlds around other stars with conditions close to those on Earth. Earth is special, but we do not know how special. The factors that make Titan interesting, the similarity of its surface features under exotic conditions, are the very factors that limit its usefulness in understanding Earth.

Freddie Wilkinson

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