Did you know that Mars might be the reason you look the way you do? It's not just a pretty face in the sky; it's a key player in the story of life on Earth. Without Mars, humans and animals might not exist, or might have evolved to look entirely different.
You might be thinking, 'What's Mars ever done for me?' Well, quite a lot, it turns out. Mars could be the reason you're reading this article today. A new study suggests that Mars's gravitational pull on Earth could be shaping the tilt of our planet, influencing the length of its Ice Ages, and even affecting the evolution of life as we know it.
Let's explore how Mars's effect on Earth might be more significant than we ever imagined. Mars is about half the size of Earth and about one-tenth its mass, making it a small, relatively light planet. But its gravitational influence on Earth is anything but small.
Stephen Kane, a professor of planetary astrophysics at the University of California Riverside, decided to investigate how Mars's gravitational nudges might be affecting Earth's climate patterns. Existing studies suggest that sediment layers on the ocean floor reflect how Earth's climate cycles are influenced by Mars.
"I knew Mars had some effect on Earth, but I assumed it was tiny," says Kane. "I 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."
An Ice Age is a long period on Earth when our planet has permanent ice sheets at its poles. Earth is about 4.5 billion years old and has gone through at least five major Ice Ages. The most recent began 2.6 million years ago and is still ongoing. To find out how Mars might be influencing these Ice Ages, Kane ran computer simulations of the Solar System and long-term variations in Earth's orbit and tilt, which dictate how sunlight reaches the planet's surface over tens of thousands to millions of years.
These cycles of shifting orbit and position are called Milankovitch cycles, and they're key to understanding how, when, and for how long Ice Ages occur. Scientists say one Milankovitch cycle is driven by Venus and Jupiter gravitationally tugging on our planet, and lasts 430,000 years. As this happens, Earth's orbit around the Sun shifts from nearly circular to more elongated, then back again. This change in orbit affects how much of the Sun's energy reaches Earth, and can therefore influence Ice Ages.
In his computer simulations, Kane found that the 430,000-year cycle occurred regardless of whether Mars was present. But when he removed Mars from the simulation, two other major cycles – one of 100,000 years and another of 2.3 million years – disappeared. "When you remove Mars, those cycles vanish," says Kane. "And if you increase the mass of Mars, they get shorter and shorter because Mars is having a bigger effect."
So, what would life be like without Mars? Those cycles seemingly caused by Mars's gravitational pull affect how circular or stretched Earth's orbit is. But they also affect the point at which Earth makes its closest approach to the Sun, and how much Earth is tilted. These in turn affect how much sunlight Earth receives, affecting 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," says Kane. "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."
Mars's mass also influences the rate at which Earth's tilt changes. "As the mass of Mars was increased in our simulations, the rate of change in Earth's tilt goes down," says Kane. "So increasing the mass of Mars has a kind of stabilizing effect on our tilt."
The results suggest that Earth might have evolved differently without Mars. Glacial periods affect the proliferation of regions like forests and grasslands on Earth, which drive evolutionary changes like walking upright, the use of tools, and social cooperation. "Without Mars, Earth's orbit would be missing major climate cycles," says Kane. "What would humans and other animals even look like if Mars weren't there?"
This study also suggests that Mars's affect on Earth's climate – and the development of life here – could be used to inform how astronomers look at planets orbiting stars beyond the Sun, known as exoplanets. "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," says Kane. The paper is published in Publications of the Astronomical Society of the Pacific.
