At half the size of Earth and one-tenth its mass, Mars is a featherweight as far as planets go. And yet, new research reveals the extent to which Mars is quietly tugging on Earth's orbit and shaping the cycles that drive long-term climate patterns here, including ice ages.
Stephen Kane, a professor of planetary astrophysics at UC Riverside, began this project with doubts about recent studies tying Earth's ancient climate patterns to gravitational nudges from Mars. These studies suggest that sediment layers on the ocean floor reflect climate cycles influenced by the red planet despite its distance from Earth and small size.
"I knew Mars had some effect on Earth, but I assumed it was tiny," Kane said. "I'd thought its gravitational influence would be too small to easily observe within Earth's geologic history. I kind of set out to check my own assumptions."
To do so, Kane ran computer simulations of the solar system's behavior and of the long-term variations in Earth's orbit and tilt that govern how sunlight reaches the surface over tens of thousands to millions of years.
These cycles of shifting orbit and position, called Milankovitch cycles, are central to understanding how and when ice ages begin and end. An ice age is a long period when the planet has permanent ice sheets at the poles. Earth has gone through at least five major ice ages over its 4.5-billion-year history. The most recent began around 2.6 million years ago and is still ongoing.
One Milankovitch cycle is driven mainly by the gravitational pull of Venus and Jupiter and takes 430,000 years to complete. Over that time span, Earth's path around the Sun gradually shifts from nearly circular to more elongated and then back again. This change in orbital shape affects how much solar energy reaches the planet and can influence the advance or retreat of ice sheets.
That 430,000-year cycle stayed intact in Kane's simulations, regardless of whether Mars was present. But when Mars was removed, two other major cycles, one that takes 100,000 years to complete, and another stretching 2.3 million years, disappeared entirely.
"When you remove Mars, those cycles vanish," Kane said. "And if you increase the mass of Mars, they get shorter and shorter because Mars is having a bigger effect."
These cycles affect how circular or stretched Earth's orbit is (its eccentricity), the timing of Earth's closest approach to the Sun, and the tilt of its rotational axis, (its obliquity). These influence how much sunlight different parts of the Earth receive, which in turn affects glacial cycles and long-term climate patterns. Kane's results show that Mars plays a measurable role in both.
"The closer it is to the sun, the more a planet becomes dominated by the sun's gravity. Because Mars is further from the sun, it has a larger gravitational effect on Earth than it would if it was closer. It punches above its weight," Kane said.
One of the more unexpected findings was how the mass of Mars influences the rate at which Earth's tilt changes. Earth is currently tilted at about 23.5 degrees, and that angle varies slightly over time.
"As the mass of Mars was increased in our simulations, the rate of change in Earth's tilt goes down," Kane said. "So increasing the mass of Mars has a kind of stabilizing effect on our tilt."
The paper , published in Publications of the Astronomical Society of the Pacific, not only quantifies Mars' influence on Earth's orbit, but also hints at broader implications. Kane's simulations suggest that even small outer planets in other solar systems could be quietly shaping the stability of worlds that might host life.
"When I look at other planetary systems and find an Earth-sized planet in the habitable zone, the planets further out in the system could have an effect on that Earth-like planet's climate," Kane said.
The results also raise other questions about how Earth might have evolved differently. Glacial periods caused forests to shrink and grasslands to expand in shifts that drove key evolutionary changes like walking upright, tool use, and social cooperation.
"Without Mars, Earth's orbit would be missing major climate cycles," Kane added. "What would humans and other animals even look like if Mars weren't there?"
