Field Guide to the Planets

Mercury is a planet of many solar system extremes. It's the smallest planet, the closest to the Sun, and it has the shortest year, most elliptical orbit, smallest axis tilt, and largest fraction of iron. Learn how these characteristics have resulted in a planet where the Sun sometimes moves backwards across the sky, and where water ice has been found at the poles.

While the Venusian carbon dioxide atmosphere has resulted in a runaway greenhouse effect and the hottest surface temperature in the solar system, the Earth and Venus actually contain about the same amount of carbon. Explore the forces that resulted in the extreme atmospheric differences between these two otherwise-similar planets.

Given the striking similarities between the four terrestrial planets, why is Earth the only one teeming with life? Proposed as a bold theory less than 70 years ago, could plate tectonics be a main driver of life on Earth? Explore the fascinating movement of our planet's surface and the many ways in which a geologically active Earth has sustained our biologically active planet.

Compared to Venus or the giant planets, Earth has a relatively thin atmosphere. And yet, without this single, fragile layer, life would not have evolved and thrived. Discover the unique properties of each atmospheric layer, and encounter specific ways we've explored each layer as a springboard to exploring the rest of our solar system.

Our Moon, formed from the Earth about 4.5 billion years ago, is by far the largest moon in the solar system relative to its planet's size. Explore the many ways in which this uniquely coupled system affects the tides on Earth and on the Moon, our rotation and revolution, the process of tidal locking, and even the planetary stability that has allowed for the development of life on Earth.

Even before the invention of telescopes, humans were familiar with the dark lunar highlands and bright maria on the Moon's surface. But now, with knowledge gained from both robotic and crewed missions, you can also explore fascinating and complex lunar swirls, sinuous rilles, and the lava tubes that hold promise as ideal locations for future lunar bases.

Humanity's fascination with Mars is never-ending, from the days when we posited a planet covered in straight-line canals and vegetation to NASA's current Moon to Mars program. Learn how the intriguing similarities and differences between Earth and Mars have resulted in Mars' planet-wide dust storms, migrating polar ice caps, and 3.9-billion-year-old impact craters.

Recent robotic exploration provides tantalizing evidence: Mars' barren landscape could have been much more Earth-like in the past. With warmer temperatures, a thicker atmosphere, and the possibility of water oceans and tsunamis, could Mars have been an Earth sibling that supported life? Learn about recent discoveries that will guide future exploration and scientific inquiry on the Red Planet.

Fans of science fiction know that a collision with an asteroid has the potential to obliterate civilization as we know it. With 20,000 asteroids identified in near-Earth orbit, how can collision be avoided? Learn why these rocky bodies, and those in the Asteroid Belt between Mars and Jupiter, never accreted into planets and how we might harness their resources for future space travel.