The reason for Mars' tumultuous past.

• Position: Topography

Deep below the surface of the withered, dead, and barren world that today is Mars lies evidence of a much more tumultuous upbringing of the Red Planet than scientists had expected. Research by an international team of planetary scientists, including Isamu Matsuyama of the Lunar and Planetary Laboratory at the University of Arizona, Tucson, solves some of the biggest puzzles surrounding Mars' mysterious infancy.

The scientists provide an elegant and simple explanation of the geological features that long have vexed astronomists. Between 3,000,000,000 and 3,500,000,000 years ago, when life made its first appearance on Earth in the form of single-celled bacteria, dramatic changes happened on our neighboring planet.

One of them caused the entire planet to tip over, bringing regions that once were closer to the poles toward the equator. The cause were giant eruptions that threw up the largest volcanoes in the solar system and formed a bulge known as the Tharsis region, home of Olympus Mons. Towering 16 miles above the Martian surface, Olympus Mons is tall enough to eclipse three Mount Everests stacked on top of one another.

When such a massive volcanic bulge forms in one area, it throws the entire planet out of kilter, according to Matsuyama, assistant professor in the Department of Planetary Sciences, who first presented evidence for the "great Martian tilt" in 2010 using gravity observations. "Any major shift of planetary mass--on the...