Credit: NASA/JPL/Space Science Institute
The Earth’s climate is warming. No one knows exactly what effects this gradual change will ultimately have on our planet, but that does not mean we have no way of finding out. In fact, there are several logical places to look for what the end result of global warming, and the future of our planet, might be. We need search no further than the other planets and moons in our solar system.
Much of our fundamental knowledge about climate and its potential for change or stability comes from comparative planetology. Astrobiologists are constantly thinking about what it takes for a planet to maintain a livable climate like our own over the long haul, which for us means billions of years. Spacecraft studies of Earth’s neighbors in the solar system have revealed that both of our near-neighbor planets, Venus and Mars, once enjoyed Earthlike environments at some time in their past, until each underwent a change of climate that would have been fatal for any surface life. What we have learned from our celestial investigations is that there are many processes that can doom a once mild planet to an eternity of fire or ice. As we come to know more about these planetary histories, and as we now incorporate new information about recent space probes, Earth’s relative climate stability over billions of years stands out as an anomaly.
Climate is hard to predict because planetary climate systems are full of nonlinear feedback effects that tend to dampen (negative feedback) or amplify (positive feedback) any possible changes. Even though the sun has grown steadily brighter since its birth 4.6 billion years ago, Earth’s climate has stayed within the range of liquid water and “life as we know it” due to a negative feedback loop that sucks carbon out of atmospheric CO2, producing carbonate rock, and volcanoes that pump CO2 back into the air. This process creates a natural thermostat. Regardless of climate, the Earth exhales CO2. But the rate of removal of atmospheric CO2 is highly sensitive to this climate, speeding up exponentially when it is hotter and grinding to a halt when the continents freeze over. A lengthy ice age will always cause a buildup of warming CO2, and a hot period will eventually lower the CO2, cooling our planet again. As the sun slowly brightens over billions of years (by 30 percent so far over its lifetime), this thermostat gradually lowers the overall CO2 content of the atmosphere. Given enough time to respond to any provocation, our climate is stable.
When we compare Earth with other planets, we see how easy it is for a planet to permanently lose a pleasant climate. Venus and Mars each started out with warm oceans and volcanoes, and the same carbonate thermostat that keeps Earth temperate was once also operating on our planetary siblings. Yet on each of these other planets, the feedback loop broke down, and the climate veered off toward an uninhabitable state. For each, the reasons behind the collapse are different yet illustrate two destinies that could have been ordained for our planet.
Venus shows us what happens if an Earth is made too close to its star. As the brightening sun warmed Venus’s oceans, more water evaporated. Water vapor is a powerful greenhouse gas, causing still more water to evaporate by raising temperatures. This powerful positive feedback leads to a runaway greenhouse effect. As all the water boiled off Venus, its atmosphere’s hydrogen seeped into space. With the water gone, carbonate rocks could no longer form. Nothing could remove the CO2 from the atmosphere, yet the volcanoes kept pumping it out. This is why Venus today has an almost pure CO2 atmosphere 100 times as thick as Earth’s, and a much higher temperature. It’s now hot enough there to fry our spacecraft and destroy all forms of earthly life.
Mars shows us what happens when an otherwise earthlike planet is made too small. Mars had an early billion-year spree of rivers, rainfall, warmer climate, and possibly even life. Then it all ended dramatically. What happened? With only one-third of Earth’s gravity, Mars couldn’t hold its own air. Asteroids and comets repeatedly pummeled Mars, blowing its atmosphere off the planet and cooling it. Any water that survived this onslaught lies frozen in the ground. Even worse, because of Mars’s small size, it long ago lost most of its internal heat and the CO2-belching volcanoes that once kept the carbonate thermostat working. Little Mars is doubly damned to be a frozen desert.
Venus became a desiccated oven, Mars a stale freezer. Comparative planetology teaches us that Earth is balanced between these fates. A stable, comfortable planetary climate, especially in the face of increasing industrial provocation, is not something one can take for granted.
While the most obvious planetary climate comparisons are made with our nearby gas-shrouded neighbors, recently we have focused our attention a billion miles farther from the sun, on Titan, an icy moon orbiting Saturn. Titan is the only body in the solar system, other than Earth, with an atmosphere composed mostly of nitrogen (N2). The surface temperature is -180°C—remarkably warm for its distance from the sun. The atmosphere is also roughly 5 percent methane (CH4), a potent greenhouse gas. Titan is completely shrouded in brown haze, which turns out to be a rich smog of organic molecules created by the action of sunlight on methane. Organic sludge rains down on the surface, evoking visions of young Earth, where a similarly fecund, organic goo self-assembled into our earliest ancestors.
But there is something else even more earthlike about Titan. It appears to foreshadow the fate of our planet. On Earth the temperature and pressure at the surface hover around the triple point of water: It exists simultaneously as ice, liquid, and vapor. Titan’s atmosphere similarly hovers near the triple point of methane. This analogy caused great speculation that oceans or lakes of liquid methane might be found on the surface of Titan when the Cassini/Huygens mission arrived in Saturn’s orbit in 2004. Cassini has since made 27 close passes by Titan, opportunistically taking swaths of radar and infrared images of the moon. The Huygens probe, which entered the upper atmosphere of Titan on January 14, 2005, snapping pictures and sniffing the air as it plunged toward the icy surface, was even designed so that it would float if it landed in liquid. Huygens landed not with a splash but with a slightly squishy thud, in moist sand composed of ices and organics and suffused with methane and ethane. Off in the distance, Huygens observed eroded cliffs carved by river valleys. Cassini and Huygens have discovered that Titan is full of familiar-looking features and processes: rivers, clouds, lakes, and storms, all using methane instead of water. We see all the signs of a complex “methalogical” system that resembles Earth’s hydrological system, complete with evaporation, rainfall, seasons, and monsoons.
Titan, for all its natural riches, lacks one fundamental feature. It does not have large oceans, only scattered methane lakes. Rather than a vision of Earth’s past, Titan shows us our future, when the oceans are gone and Earth is well on its way to resembling Venus. Chemically, Titan gives us a window into our organic origins, but climatically it presages our planet’s ultimate fate. Such a transition is almost inevitable, more than a billion years hence, when the brightening sun overwhelms the cooling capacity of Earth’s carbonate thermostat.
Of course, an important difference is that life itself is intimately involved in terrestrial climate cycles, though it is conceivable that some exotic form of life is involved in the methane cycle on Titan. My colleague Dirk Schulze-Makuch and I have recently proposed a methane-based metabolism for creatures that could live off of the energy and carbon cycling through Titan’s atmosphere. Maybe a biosphere could maintain the climate in a range that facilitates its own survival. It is worth remembering that we are still profoundly ignorant about life, and its possibilities, in the universe. Clearly, complex feedbacks, which we are just beginning to elucidate, have played a role in the evolution of Titan’s climate and will help us to understand those on Earth.
How do we know that our climate models will really work? This isn’t the kind of science where you can do controlled experiments, varying one parameter at a time, and we cannot simply wait to see if our projections turn out to be accurate. When we include other planets, our data set expands and gives us more certainty in our conclusions. Using the same equations with which we model possible future Earths, we have been able to successfully duplicate the present-day climates of Venus, Mars, and Titan. We really can look into our future.
Poking around the solar system, we see the power of climate feedbacks. They can both maintain and destroy climate stability. As we struggle to better understand the balance of these processes on Earth, and how our own actions are perturbing them, we must look to our once habitable neighbors for clues. At the present time, they are the only way we can understand our fate. — David Grinspoon, curator of astrobiology at the Denver Museum of Nature & Science and author of Lonely Planets: The Natural Philosophy of Alien Life, was awarded the 2006 Carl Sagan Medal from the American Astronomical Society.
Originally published June 6, 2007