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Mars' gravity has a surprisingly large influence on Earth, affecting our planet's tilt and orbit and thereby contributing to climate cycles lasting hundreds of thousands to millions of years, new simulations have shown.

It has long been known that Earth's long-term climate is governed by the Milankovitch cycles, long-term variations in our planet's orbit and tilt governed by the gravitational pull of other planets in the solar system. Venus, which is the closest planet to Earth, and Jupiter, the most massive planet in the solar system, are the main culprits, dragging Earth around over millennia with their gravity. Mars, too, has some effect; previous studies looking at sediment laid down on Earth's ocean floor have suggested the possibility of climate changes resulting from gravitational interactions with Mars. However, the effect of Mars' gravity on Earth had not been quantified until now.

As it turns out, Mars may be "punching above its weight," researchers say. "I knew Mars had some effect on Earth, but I assumed it was tiny," said Stephen Kane of the University of California, Riverside, in a statement. "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."

Along with Pam Vervoort of the University of Birmingham in the U.K. and Jonathan Horne, of the University of Southern Queensland in Australia, Kane ran detailed simulations of the solar system that tested the effects that the planets had on Earth.

It is important to note that the Milankovitch cycles are in no way connected to current anthropogenic global warming — rather, the cycles impact Earth's climate over geological timescales. While the cycles can trigger ice ages (defined here as when ice covers the poles for long periods), they also ensure that ice ages don't last forever as conditions change as a consequence of the cycles impacting Earth's orbit and tilt.

Specifically, the Milankovitch cycles control periodic variations in Earth's axial tilt (known as its obliquity), the eccentricity (i.e. how elongated) of Earth's orbit around the sun, and a property referred to as the precession of the equinoxes. This last one governs when Earth reaches perihelion — closest point to the sun — in its orbit. Currently, perihelion takes place in January when it is northern winter and southern summer, but the precession gradually sees perihelion move to later in the year before coming full circle.

One of the strongest Milankovitch cycles has a period of 430,000 years, making Earth's orbit slightly eccentric, or oval-shaped, and is controlled by the gravity of Venus and Jupiter. For this cycle, Kane's instincts were correct — the simulations showed that when Mars was removed, its absence had no effect on this particular cycle.

Yet remove Mars from the solar system in the simulations and the other two notable Milankovitch cycles, with periods of 100,000 years and 2.4 million years, respectively, go away.

"When you remove Mars, those cycles vanish," said Kane. "And if you increase the mass of Mars, they get shorter and shorter because Mars is having a bigger effect."

Kane, Vervoort and Horner also discovered that by changing the mass of Mars in the simulations, they could control the amount of variation in Earth's tilt relative to the ecliptic plane.

"As the mass of Mars was increased in our simulations, the rate of change in Earth's tilt goes down," said Kane. "So increasing the mass of Mars has a kind of stabilizing effect on our tilt."

Many different orbital properties affect Earth's climate. The planet's tilt determines how much sunlight the poles receive. The eccentricity of its orbit governs how close and how far away Earth gets from the sun. And the precession of the equinox controls at what point during the year Earth receives maximum insolation, which when combined with the tilt and eccentricity can alter Earth's climate significantly.

Earth's tilt can vary between 21.5 and 24.5 degrees every 41,000 years. This is relatively stable; by comparison, Mars's tilt is much more chaotic, with variations of up to 90 degrees based on ancient geologic evidence. Until now, it was thought that Earth's stable obliquity was maintained by the presence of our moon. However, Kane's simulations show that Mars' gravity also stabilizes Earth's tilt. This potentially removes the necessity for a large moon to keep an Earth-like planet from wobbling, meaning that perhaps Earth isn't so rare — at least not in this context.

Mars's location in the solar system also enhances its effect on Earth. The closer a planet is to the sun, the more that the sun's gravity dominates rather than the planet's.

"Because Mars is further from the sun, it has a larger gravitational effect on Earth than it would if it was closer," said Kane. "It punches above its weight."

And so, astronomers hunting for habitable exoplanets shouldn't just settle for finding a potential Earth-like planet; they should also look for a modest outer planet that can help stabilize the Earth analog and regulate its climate over long periods.

"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," said Kane.

Would Earth's climate be sufficiently stable for complex life to develop without Mars' presence? Perhaps we'll never know the answer to this question, but it's a fascinating 'what if?'.

"Without Mars, Earth's orbit would be missing major climate cycles," said Kane. "What would humans and other animals even look like if Mars weren't there?"

The results of the simulations were presented on Dec. 18 in Publications of the Astronomical Society of the Pacific.