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Projects: Observing Earth

Venus and Mars, Earth’s Sister Worlds

by Charlene M. Anderson

Our Earth is not the only child of its parent star, and as any parent or teacher knows, to understand the behavior of one child, you must consider the siblings.

Venus and Mars, worlds without oceans and that pesky phenomenon called life, provide simpler test beds for scientists to work out interactions among atmospheres and hard surfaces and, in the process, refine hypotheses about how our more complex climate behaves. By working backward from observations of the present-day states of planets, scientists can try to determine how they evolved into such inhospitable places.

Venus has become notorious for the runaway greenhouse effect created by its massive carbon dioxide atmosphere, which produces surface temperatures of nearly 900 degrees Fahrenheit (500 degrees Celsius). We’ve learned much more than that about Earth’s nearest neighbor: spacecraft have uncovered evidence of how aerosols—tiny airborne particles—helped turn the atmosphere into an opaque blanket and how sulfuric acid forms massive cloud decks that rain poison through the deadly atmosphere.

Earth is not in danger of becoming a Venus-like hellhole, but a carbon dioxide greenhouse effect is warming our climate, aerosols degrade the air we breathe, and acid rain kills forests and wears away our monuments to ourselves. What we learn from Venus can help us deal with such destructive processes on our home planet.

Although smaller than Venus and farther away from Earth, Mars also has lessons to teach us. The Red Planet lost its once-thick atmosphere, and any liquid water it possessed froze into the poles and permafrost or was lost to space. Without a protective ozone layer like Earth’s, Mars receives ultraviolet radiation from the Sun that passes unimpeded through thin air and destroys any organic compounds on the planet’s surface.

Again, Earth will never see such extreme effects, but scientists did discover an ozone hole over the Antarctic caused by atmospheric pollutants. Mars, along with Venus, has given us lessons on how fragile, precious, and unique our own world is.

—Charlene M. Anderson, Associate Director of The Planetary Society