# Earthlike Planets: Venus and Mars

## Divergent Planetary Evolution


### Learning Objectives

By the end of this section, you will be able to:
1. 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.
Burnham, R. “Red Planet Rendezvous.” Astronomy (May 2006): 68. About Mariner Valley and a flyover film constructed from many still images.
Christensen, P. “The Many Faces of Mars.” Scientific American (July 2005): 32. Results from the Rover mission; evidence that Mars was once wet in places.
Lakdawalla, E. “The History of Water on Mars.” Sky & Telescope (September 2013): 16. Clear review of our current understanding of the role of water on Mars in different epochs.
Malin, M. “Visions of Mars.” Sky & Telescope (April 1999): 42. A geological tour of the red planet, with new Mars Global Surveyor images.
McEwen, A. “Mars in Motion.” Scientific American (May 2013): 58. On gullies and other surface changes.
McKay, C. & Garcia, V. “How to Search for Life on Mars.” Scientific American (June 2014): 44. Experiments future probes could perform.
Naeye, R. “Europe’s Eye on Mars.” Sky & Telescope (December 2005): 30. On the Mars Express mission and the remarkable close-up images it is sending.
Talcott, R. “Seeking Ground Truth on Mars.” Astronomy (October 2009): 34. How rovers and orbiters are helping scientists understand the red planet’s surface.

### Websites

European Space Agency Mars Express Page: http://www.esa.int/Our_Activities/Space_Science/Mars_Express.
European Space Agency Venus Express Page: http://www.esa.int/Our_Activities/Space_Science/Venus_Express.
High Resolution Imaging Science Experiment: http://hirise.lpl.arizona.edu/.
Jet Propulsion Lab Mars Exploration Page: http://mars.jpl.nasa.gov/.
Mars Globe HD app: https://itunes.apple.com/us/app/mars-globe-hd/id376020224?mt=8.
Mars Rover 360° Panorama: http://www.360cities.net/image/curiosity-rover-martian-solar-day-2#171.10,26.50,70.0. Interactive.
NASA Center for Mars Exploration: http://www.nasa.gov/mission_pages/mars/main/index.html.
NASA Solar System Exploration Mars Page: http://solarsystem.nasa.gov/planets/mars.
NASA Solar System Exploration Venus Page: http://solarsystem.nasa.gov/planets/venus.
NASA’s Magellan Mission to Venus: http://www2.jpl.nasa.gov/magellan/.
Russian (Soviet) Venus Missions and Images: http://mentallandscape.com/C_CatalogVenus.htm.
Venus Atlas app: https://itunes.apple.com/us/app/venus-atlas/id317310503?mt=8.
Venus Express Results Article: http://www.mpg.de/798302/F002_Focus_026-033.pdf.

### Videos

50 Years of Mars Exploration: http://www.jpl.nasa.gov/video/details.php?id=1395. NASA’s summary of all missions through MAVEN; good quick overview (4:08).
Being a Mars Rover: What It’s Like to be an Interplanetary Explorer: https://www.youtube.com/watch?v=nRpCOEsPD54. 2013 talk by Dr. Lori Fenton about what it’s like on the surface of Mars (1:07:24).
Magellan Radar Maps of Venus: https://www.youtube.com/watch?v=dK4JnGTCawM. Fly-overs of six regions on Venus from the Jet Propulsion Labs.
Our Curiosity: https://www.youtube.com/watch?v=XczKXWvokm4. Mars Curiosity rover 2-year anniversary video narrated by Neil deGrasse Tyson and Felicia Day (6:01).
Planet Venus: The Deadliest Planet, Venus Surface and Atmosphere: https://www.youtube.com/watch?v=HqFVxWfVtoo. Quick tour of Venus’ atmosphere and surface (2:04).
Planetary Protection and Hitchhikers in the Solar System: The Danger of Mingling Microbes: https://www.youtube.com/watch?v=6iGC3uO7jBI. 2009 talk by Dr. Margaret Race on preventing contamination between worlds (1:28:50).

### Collaborative Group Activities

1. Your group has been asked by high NASA officials to start planning the first human colony on Mars. Begin by making a list of what sorts of things humans would need to bring along to be able to survive for years on the surface of the red planet.
2. As a publicity stunt, the mayor of Venus, Texas (there really is such a town), proposes that NASA fund a mission to Venus with humans on board. Clearly, the good mayor neglected to take an astronomy course in college. Have your group assemble a list of as many reasons as possible why it is unlikely that humans will soon land on the surface of Venus.
3. Even if humans would have trouble surviving on the surface of Venus, this does not mean we could not learn a lot more about our veiled sister planet. Have your group brainstorm a series of missions (pretend cost is no object) that would provide us with more detailed information about Venus’ atmosphere, surface, and interior.
4. Sometime late in the twenty-first century, when travel to Mars has become somewhat routine, a very wealthy couple asks you to plan a honeymoon tour of Mars that includes the most spectacular sights on the red planet. Constitute your group as the Percival Lowell Memorial Tourist Agency, and come up with a list of not-to-be missed tourist stops on Mars.
5. In the popular book and film, called The Martian, the drama really begins when our hero is knocked over and loses consciousness as he is half buried by an intense wind storm on Mars. Given what you have learned about Mars’ atmosphere in this chapter, have your group discuss how realistic that scenario is. (By the way, the author of the book has himself genially acknowledged in interviews and talks that this is a reasonable question to ask.)
6. Astronomers have been puzzled and annoyed about the extensive media publicity that was given the small group of “true believers” who claimed the “Face on Mars” was not a natural formation (see the Astronomy and Pseudoscience: The “Face on Mars” feature box). Have your group make a list of the reasons many of the media were so enchanted by this story. What do you think astronomers could or should do to get the skeptical, scientific perspective about such issues before the public?
7. Your group is a special committee of scientists set up by the United Nations to specify how any Mars samples should be returned to Earth so that possible martian microbes do not harm Earth life. What precautions would you recommend, starting at Mars and going all the way to the labs that analyze the martian samples back on Earth?
8. Have your group brainstorm about Mars in popular culture. How many movies, songs or other music, and products can you think of connected with Mars? What are some reasons that Mars would be a popular theme for filmmakers, songwriters, and product designers?

### Review Questions


### Thought Questions


### Figuring for Yourself
