14.5 Planetary evolution  (Page 5/17)

We now come to the end of our study of the planetary system. Although we have learned a great deal about the other planets during the past few decades of spacecraft exploration, much remains unknown. Discoveries in recent years of geological activity on Titan and Enceladus were unexpected, as was the complex surface of Pluto revealed by New Horizons. The study of exoplanetary systems provides a new perspective, teaching us that there is much more variety among planetary systems than scientists had imagined a few decades ago. The exploration of the solar system is one of the greatest human adventures, and, in many ways, it has just begun.

Key concepts and summary

After their common beginning, each of the planets evolved on its own path. Different possible outcomes are illustrated by comparison of the terrestrial planets (Earth, Venus, Mars, Mercury, and the Moon). All are rocky, differentiated objects. The level of geological activity is proportional to mass: greatest for Earth and Venus, less for Mars, and absent for the Moon and Mercury. However, tides from another nearby world can also generate heat to drive geological activity, as shown by Io, Europa, and Enceladus. Pluto is also active, to the surprise of planetary scientists. On the surfaces of solid worlds, mountains can result from impacts, volcanism, or uplift. Whatever their origin, higher mountains can be supported on smaller planets that have less surface gravity. The atmospheres of the terrestrial planets may have acquired volatile materials from comet impacts. The Moon and Mercury lost their atmospheres; most volatiles on Mars are frozen due to its greater distance from the Sun and its thinner atmosphere; and Venus retained CO ₂ but lost H ₂ O when it developed a massive greenhouse effect. Only Earth still has liquid water on its surface and hence can support life.

For further exploration

Note: Resources about exoplanets are provided in The Birth of Stars and the Discovery of Planets outside the Solar System .

Articles

Meteors and Meteorites

Alper, J. “It Came from Outer Space.” Astronomy (November 2002): 36. On the analysis of organic materials in meteorites.

Beatty, J. “Catch a Fallen Star.” Sky&Telescope (August 2009): 22. On the recovery of meteorites from an impact that was seen in the sky.

Durda, D. “The Chelyabinsk Super-Meteor.” Sky&Telescope (June 2013): 24. A nice summary, with photos and eyewitness reporting.

Garcia, R.,&Notkin, G. “Touching the Stars without Leaving Home.” Sky&Telescope (October 2008): 32. Hunting and collecting meteorites.

Kring, D. “Unlocking the Solar System’s Past.” Astronomy (August 2006): 32. Part of a special issue devoted to meteorites.

Rubin, A. “Secrets of Primitive Meteorites.” Scientific American (February 2013): 36. What they can teach us about the environment in which the solar system formed.

Evolution of the Solar System and Protoplanetary Disks

Jewitt, D.,&Young, E. “Oceans from the Skies.” Scientific American (March 2015): 36–43. How did Earth and the other inner planets get their water after the initial hot period?