Earth's Just Right Conditions

Courtesy of Google Images

NASA Picture of the Day: Sunrise Pacific Ocean

Courtesy Google Images

Courtesy of Google Images

Dr. James Hansen

Earth has….

...a comfortable distance from the sun. You could say we are in the “Goldilocks Zone.” Venus is only about 30% closer to the Sun than Earth and Mars is about 50% farther away. These differences matter but are not enough on their own to account for us being “in the zone.”

...a nice size for a planet to form a habitable atmosphere. Earth and Venus are similar in size, but Mars is much smaller. A planet’s size plays an important role in whether it can “hold on” to its atmosphere. Picture the planet as a big magnet pulling gases toward it – this is gravity. If a planet is too small, like Mars, it will not have enough gravitational pull to be able to hold on to enough gases needed for a habitable atmosphere. Lucky for us, earth not only is just the right size, but also just the right distance from the Sun for its heat and energy to make a difference.

...a good mix of atmospheric gases. Mars, Venus and Earth attract atmospheric gases, although in different amounts. Hydrogen and Helium are light gases that are easily drawn into these planetary atmospheres. Each planet's gravitation pull is also able to attract heavier gases, like carbon dioxide, water vapor and nitrogen. As the planets formed, these gases trapped below the rocky surface were released through heat and volcanic eruptions. It happens that Earth is just the right size to create a gravitational force strong enough to prevent just the right amount of gases from being released back to space and to build up, forming atmospheric conditions that can support life.

...just the right amount of carbon dioxide (CO2). Venus and Mars have similar amounts of CO2, the greenhouse warming gas, yet their differences in distance from the Sun produce planets that could not be more opposite. Mars is so far from the Sun that its water froze over time and left it a fridge and barren planet. Its atmosphere is almost entirely CO2 but it is so far from the Sun, CO2’s interaction with the Sun’s heat is unable to produce a significant warming effect. Venus is so close to the Sun that water on the planet evaporated. CO2 had no where to go but into the atmosphere where the Sun’s heat was absorbed by the gas, contributing to a super greenhouse effect; creating conditions just too hot for us or life we know about. Earth has less atmospheric CO2 and its distance from the sun is just right to help produce a habitable planet. This allowed Earth to form oceans from atmospheric water vapor; that absorb some of this greenhouse warming gas. The Sun’s interaction with CO2 creates an atmospheric blanket, where some CO2 is re-radiated from Earth’s surface and atmosphere back to space, and some stays near our surface to warm the planet to help create its habitable temperature.

These are some of the important conditions that must stay in balance to sustain the incredible beauty and diversity of life on the planet we call “home.” GISS scientists study all these conditions and more that make up the complex physical, chemical and biological interactions that produce Earth's environment.

James Hansen, the Director of the NASA GISS lab, began his research career studying the atmosphere of Venus and its "runaway greenhouse effect." He soon turned his attention to Earth, calculating our planet's greenhouse effect. Hansen realized, in comparing the atmospheres of Venus and Mars to Earth, scientists gained "the best proof at the time of the reality of the greenhouse effect [on the earth]. It is this research that launched his career as one of the world's most respected climate research scientists."