David Grinspoon, a planetary scientist, author, and 2006 recipient of the Carl Sagan Medal has some very interesting insights into the world outside of Earth. In “Ghosts of Climates Past,” an article published in SEED Magazine, Grinspoon presents a convincing theory on the ways we might be able to predict the environmental future of Earth by studying the environmental pasts of some of the other planets in our solar system.
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.
Greenspoon explains how both Venus and Mars started out with climates and landscapes similar to Earth’s – complete with oceans, volcanoes and carbon cycles. He then describes the conditions on Venus and Mars which catalyzed the collapse of the feedback loop maintaining each planet’s temperate climate.
Venus was doomed because of its close proximity to the Sun. As the sun grew brighter over time, it evaporated more and more of Venus’s water, which in turn warmed the atmosphere further (yes, water vapor is a potent greenhouse gas), creating a positive feedback loop and a runaway greenhouse effect. With volcanoes continuing to erupt and nothing to sequester the emitted carbon, Venus developed “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 was a totally different story. Despite its “early billion-year spree of rivers, rainfall, warmer climate, and possibly even life,” the 4th planet from the Sun was simply too small. It didn’t have enough gravitational pull to hold onto its atmosphere. Losing its atmosphere cooled Mars, freezing its water and volcanic core so that now, “Little Mars is doubly damned to be a frozen desert.”
The third comparison Greenspoon makes is not to a planet, but a moon – one of Saturn’s moons, Titan. The reason for his interest in Titan is that it is the only other body in our solar system with an atmosphere that is made of mostly nitrogen. It’s atmosphere also has about 5% methane, a potent greenhouse gas. At a “remarkably warm” -180 degrees C and a pressure around 117 millibar, Titan’s atmosphere is near the triple point of methane (meaning the methane can exist as a solid, liquid or gas) a la Earth’s atmospheric temperature and pressure hovering near the triple point of water. Subsequently, it seems, Titan has a ‘methalogical’ system (rather than a hydrological system) with rivers, clouds and rains of methane.
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.
Without much commentary on the current status of Earth’s global warming, Greenpoon subtly reminds us that “a stable, comfortable planetary climate, especially in the face of increasing industrial provocation, is not something one can take for granted.” And he says, “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.” Unfortuanately, let’s be real ninjas, we have a pretty low success rate of heeding lessons learned from our own history. Can we really expect to heed those from entirely different solar bodies?