Credit: NASA
Stephen Kane, a professor of planetary astrophysics at UC Riverside, was skeptical when he read recent studies that showed the gravitational pull from Mars being connected to Earth’s long-term climate patterns. These studies suggested that sediment layers on the floor of our oceans have recorded climate cycles influenced by the Red Planet despite its distance from Earth and small size. Mars measures in at only half Earth’s diameter and 10 percent of its mass.
“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.”
For his study, Kane ran computer simulations of the long-term variations in our planet’s tilt that govern at what angle sunlight reaches the surface over periods of tens of thousands to millions of years.
Cycling through time
These so-called Milankovitch cycles — named after the Serbian geophysicist, Milutin Milankovitch, who deduced them — 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 started 2.6 million years ago and continues today.
One well-known Milankovitch cycle is caused by the gravitational pull of Venus and Jupiter and lasts 430,000. During that time, Earth’s orbital path gradually shifts from nearly circular to more stretched out and back. This change determines how much energy from the Sun reaches our planet. Over long time periods, it can cause ice sheets to grow or shrink.
That 430,000-year cycle was present in Kane’s simulations, regardless of whether Mars was there or not. But when he removed Mars, two other major cycles, one that takes 100,000 years, and another stretching 2.3 million years, disappeared.
“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 the eccentricity of Earth’s orbit, the timing of Earth’s closest approach to the Sun, and how much our planet’s axis tilts. In turn, these three parameters determine how much sunlight different parts of Earth receive. And that influences glacial cycles and long-term climate patterns.
“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.
Even more
One unexpected thing Kane found was how Mars mass influences the rate at which Earth’s tilt changes. Our planet is currently tilted at about 23.44°, an angle that changes slowly 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 was published in Publications of the Astronomical Society of the Pacific. One far reaching implication is that Kane’s simulations suggest that even small outer planets in other solar systems could shape the climates 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.
Finally, without the Red Planet where it is, how might Earth have evolved differently?
“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?”
