科学美国人60秒:Chaotic Orbits Could Cause Catastrophic Collision

Isaac Newton said, “I can calculate the motion of heavenly bodies, but not the madness of people.” He meant that while people can be uNPRedictable, the planets are just the opposite—they glide through space in orbits defined by the laws of physics. But that view’s been changing and a new study bolsters a more complex outlook. 

“The big finding of this paper was the discovery of the first real firm, unambiguous geologic evidence to confirm this idea that the solar system is chaotic.”

Stephen Meyers, a geoscientist at the University of Wisconsin–Madison.

By chaotic, Meyers doesn’t mean that the planets zoom around wildly on random paths. Rather, he’s referring to the mathematical meaning of the word, in which the future state of a complex system depends strongly on its initial conditions. In this case, it means that seemingly random changes in planets’ orbits can occur because of subtle gravitational interactions between objects in the solar system.

“It’s also known as the butterfly effect. This is the exact same phenomenon. The idea that a butterfly flapping over the Indian Ocean could influence weather patterns over North America a week later.”

The idea has been proposed before, but Meyers and his team found supporting evidence in a surprising place: a rock formation in Colorado. The formation is made up of sedimentary layers deposited when a vast inland sea covered parts of North America. These layers contain a record of regularly paced climate changes that were triggered by fluctuations in the amount of sunlight hitting Earth as a result of variations in its orbit. 

For at least the last 50 million years or so, Earth’s orbit has cycled between a more circular shape and a more elliptical shape every 2.4 million years, producing climate changes with the same timing. However, by independently dating the rocks in Colorado with other methods, Meyers and his colleagues found that before about 85 million years ago, this cycle took 1.2 million years. The cycle time changed because Mars and Earth tugged on each other—just the kind of thing you would expect to see in a chaotic system. The findings are in the journal Nature. [Chao Ma, Stephen R. Meyers and Bradley B. Sageman, Theory of chaotic orbital variations confirmed by Cretaceous geological evidence]

Meyers says the results have several implications. One is that they will help scientists to date ancient rocks and understand the link between orbital changes and climate. But the other potential implication is somewhat more disturbing: billions of years from now, there’s a very small chance that Mars could crash into Earth.

“That certainly is a rather dramatic demise to the Earth that would be a consequence of the chaos.”

But that’s just life—or the end of it—in a chaotic solar system.

—Julia Rosen