10.6 Divergent planetary evolution

Learning objectives

By the end of this section, you will be able to:

• Compare the planetary evolution of Venus, Earth, and Mars

Venus , Mars , and our own planet Earth form a remarkably diverse triad of worlds. Although all three orbit in roughly the same inner zone around the Sun and all apparently started with about the same chemical mix of silicates and metals, their evolutionary paths have diverged. As a result, Venus became hot and dry, Mars became cold and dry, and only Earth ended up with what we consider a hospitable climate.

We have discussed the runaway greenhouse effect on Venus and the runaway refrigerator effect on Mars, but we do not understand exactly what started these two planets down these separate evolutionary paths. Was Earth ever in danger of a similar fate? Or might it still be diverted onto one of these paths, perhaps due to stress on the atmosphere generated by human pollutants? One of the reasons for studying Venus and Mars is to seek insight into these questions.

Some people have even suggested that if we understood the evolution of Mars and Venus better, we could possibly reverse their evolution and restore more earthlike environments. While it seems unlikely that humans could ever make either Mars or Venus into a replica of Earth, considering such possibilities is a useful part of our more general quest to understand the delicate environmental balance that distinguishes our planet from its two neighbors. In Cosmic Samples and the Origin of the Solar System , we return to the comparative study of the terrestrial planets and their divergent evolutionary histories.

Key concepts and summary

Earth, Venus, and Mars have diverged in their evolution from what may have been similar beginnings. We need to understand why if we are to protect the environment of Earth.

For further exploration

Articles

Venus

Dorminey, B. “Cool Science on a Hot World.” Astronomy (February 2006): 46. Five-page overview of Venus and the Venus Express mission plans.

Kargel, J. “Rivers of Venus.” Sky&Telescope (August 1997): 32. On lava channels.

Robertson, D. “Parched Planet.” Sky&Telescope (April 2008): 26. Overview of our understanding of the planet.

Robinson, C. “Magellan Reveals Venus.” Astronomy (February 1995): 32.

Stofan, E. “The New Face of Venus.” Sky&Telescope (August 1993): 22.

Zimmerman, R. “Taking Venus by Storm.” Astronomy (October 2008): 66. On results from the Venus Express mission.

Mars

Albee, A. “The Unearthly Landscapes of Mars.” Scientific American (June 2003): 44. Results from the Mars Global Surveyor and Mars Odyssey missions and an overview.

Bell, J. “A Fresh Look at Mars.” Astronomy (August 2015): 28. Nice summary of recent spacecraft results and how they are revising our understanding of Mars.

Bell, J. “Uncovering Mars’ Secret Past.” Sky&Telescope (July 2009): 22. How rovers and orbiters are helping us to understand Mars history and the role of water.

Bell, J. “The Red Planet’s Watery Past.” Scientific American (December 2006): 62. Rovers are furnishing proof that ancient Mars was wet.