Six or seven billion years from now the sun will begin to die, but it will not die alone. In its death throes it will swell into a red giant star, engulfing and incinerating Mercury, Venus, and Earth. Although life on Earth—if indeed any still exists—will surely end, a new theory suggests that the sun’s final agony may make possible the evolution of life on another world in the solar system—Saturn’s moon Titan.
Unlike any other moon in the solar system, Titan has a dense atmosphere composed mostly of nitrogen, as is Earth’s, and about 2 to 10 percent methane. Ultraviolet light from the sun breaks the methane apart, creating a thick, smoggy haze of organic compounds that hides Titan’s surface. And on that surface, some astronomers believe, lie seas of methane. Titan is also an extremely cold place. Besides being 900 million miles from the sun, the moon has a dark reddish haze that blocks about 90 percent of the sun’s light. As a result, the surface is a frigid -290 degrees, far too cold for life.
At least, that is Titan now. But in about 6 billion years, say planetary physicists Ralph Lorenz and Jonathan Lunine of the University of Arizona’s Lunar and Planetary Laboratory, Titan’s weather will improve dramatically. When the sun runs out of hydrogen fuel at its core and begins burning the hydrogen, helium, and heavier elements in its outer layers, it will swell tremendously, expanding out to Earth’s orbit. What happens first is that the sun simply gets brighter, and the amount of sunlight falling on Titan increases, says Lorenz. It turns out that that doesn’t increase the surface temperature as much as you’d expect, because most of this increased sunlight gets absorbed into the haze and causes it to puff up and become thicker.
But as the sun gradually changes, the spectrum of light it emits also changes—it gives off more red light and less ultraviolet. It is the ultraviolet light that makes the haze, so the haze is not produced as much, says Lorenz. More sunlight thus reaches the surface, where it will be trapped by Titan’s methane-rich lower atmosphere. (Methane is a greenhouse gas.)
Titan will never warm to Earth-like temperatures. But Lorenz and Lunine think things could get interesting when the surface temperature climbs from 290 degrees below zero to about -143 degrees. Then the surface ices start to melt, Lorenz says. The most important of those ices is ammonia, which when liquid acts like an antifreeze on water ice, lowering its melting point. Instead of melting at 32 degrees, water might melt at -100 or even -143, depending on the amount of ammonia present. And with water, even ammonia-rich water, there exists the possibility of life.
Titan has another feature favorable to life’s gestation: its haze of organic molecules. When the haze dissipates, those substances would rain onto the surface. Because you’ve got all this organic stuff, you’ve got a head start on making life, says Lorenz, and you can react these compounds with water to make amino acids, the building blocks of proteins.
If life does develop on Titan, it won’t have billions of years to evolve, as life did on Earth. When the sun is about 11.6 billion years old, around 7 billion years from now, it will start to pulsate and lose mass rapidly. This will dramatically increase the intensity of the solar wind, which will strip away Titan’s protective atmosphere. But before that, there is a good 500 million years during which the surface temperatures will be above water’s melting point, Lorenz says. Life evolved on Earth in less time. So it is certainly possible that Titan could become habitable.
What’s possible on Titan, says Lorenz, might also be possible on worlds orbiting other red giant stars. People who think about the evolution of life tend to think in terms of places like Earth or Mars, where if there is a greenhouse effect, it is due to water vapor and carbon dioxide. But maybe a lower-temperature version of the same thing, with methane instead of carbon dioxide, would allow water-rich liquids on the surface. And that is the real criterion for life.