Mars’ gravity may help drive Earth’s cycle of ice ages

Compared to Earth, Mars is small, but it appears to have a huge influence on our planet’s climate cycles. Such small planets could affect the climate of worlds outside our solar system, something we must begin to take into account when evaluating their potential habitability.

Stephen Kane at the University of California, Riverside, and his colleagues discovered this effect by running simulations of the effect Mars would have on Earth’s orbit if it were a different mass, from 100 times its actual mass to if it were gone entirely. “I came at it from the perspective of skepticism because I had a hard time believing that Mars, which is only one-tenth the mass of Earth, could have such a profound effect on Earth’s cycles, so that’s what motivated this study to turn that Martian mass knob and see what happens,” Kane says.

Earth’s climate has many long cycles based on the eccentricity of its orbit—how stretched out its path around the Sun is—and the tilt of its axis. These orbits, driven by the gravity of the Sun and other planets in the solar system, control such fundamental events as the timing of ice ages and the intensity of seasonal changes.

One of the most influential is known as the grand cycle: over the course of 2.4 million years, the ellipse of the Earth’s orbit around the Sun lengthens and shortens again. This affects how much sunlight reaches the Earth’s surface at any given moment, regulating the timing of long-term climate change.

The scientists found that when Mars was completely removed, the great cycle disappeared along with another cycle of Earth’s eccentricity, which lasts about 100,000 years. “It doesn’t mean that if we removed Mars, Earth wouldn’t have ice ages, but it would change the entire landscape in terms of how often ice ages and associated climate effects occur,” Kane says.

As the simulated mass of Mars increased, these cycles became shorter and intensified. But the third eccentricity cycle of about 405,000 years, driven mainly by the gravitational pulls of Venus and Jupiter, remained regardless of Mars’ mass, so the Red Planet is not all-powerful, but it has more influence than anyone expected.

A more subtle effect is Mars’ influence on Earth’s tilt, which normally swings back and forth over a period of about 41,000 years. Kane and his colleagues found that Mars appears to have a stabilizing effect on this cycle, occurring less often if Mars has extra mass and more often if Mars has shrunk.

We can’t say exactly what Earth would look like if Mars wasn’t there or if it was much bigger, but there would definitely be some changes. As the search continues for Earth-like worlds with climates suitable for life as we know it, the influence of smaller planets appears to be greater than scientists realized. “We really need to know the orbital architectures of exoplanet systems really well to be able to reasonably understand the possible climate fluctuations on these planets,” he says. Sean Raymond at the University of Bordeaux in France.

However, understanding this architecture will be difficult. “It’s more of a warning than anything else: we can’t ignore the smaller objects, even though they’re quite difficult to find, because those smaller planets like Mars really have a bigger impact than we thought,” Kane says.

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