Summary
By the end of this article, you will understand how a colossal asteroid collision smashed into Mars, creating a massive ring of debris that eventually clumped together to form its two moons, Phobos and Deimos.
Quick Facts
Surprise: Phobos and Deimos were long thought to be captured asteroids, but their circular orbits prove otherwise.
Surprise: Phobos is up to 35% empty space inside, like a giant, low-density cosmic sponge.
Salient Idea: The Borealis-scale impact created a debris disk weighing 500 million trillion kilograms.
Surprise: The crater left behind covers almost the entire northern hemisphere of Mars.
The Discovery: The Great Martian Crash
For decades, the prevailing theory was that Mars’ two moons, Phobos and Deimos, were simply roaming asteroids caught by martian gravity. But recent observations revealed a Surprise: their circular, equator-aligned orbits and highly porous compositions just didn’t match the ‘captured rock’ theory. Enter the giant impact hypothesis. Researchers realized that a massive collision—specifically one that created the 7,700-kilometer Borealis basin on Mars—could have blasted enough material into space to form a moon-birthing debris ring. By tracking the mass and angular momentum of the ejected rock, scientists proved that a Borealis-scale impact would create a disk massive enough to eventually clump together and birth both Phobos and Deimos.
Original Paper: ‘Formation of Phobos and Deimos via a Giant Impact’
A Borealis-scale impact is capable of producing a disk… sufficient to form at least one of the martian moons.
— Robert I. Citron
The Science Explained Simply
This is NOT a gentle gravitational capture of a passing rock. This is a violent, planetary-scale explosion. When an asteroid carrying 3% of Mars’ total mass slams into the planet, it vaporizes rock and shoots it into a ‘circum-Mars’ orbit. The Salient Idea here is the debris disk. Instead of flying off into deep space, 1% to 4% of the asteroid’s mass gets trapped in orbit, forming a dense ring around the planet’s equator. Over thousands of years, inside a zone called the strong tidal regime, these tiny fragments of molten rock and dust cool down and crash into one another. They slowly stitch themselves together into highly porous, sponge-like moons.
Moons that formed from the same impact that produced the martian spin would be expected to orbit near the equatorial plane.
— Rosenblatt and Charnoz
The Aurora Connection
While we study this impact to understand moons, these colossal collisions also shape a planet’s ability to host auroras and protect life. Early in its history, Mars had a global magnetic field, much like Earth’s, which shielded its atmosphere from the harsh solar wind. However, giant impacts like the Borealis crash can fundamentally alter the heat dynamics inside a planet’s core, potentially shutting down the internal dynamo that generates magnetic fields. Without that magnetic shield, Mars lost its atmosphere to space weather. Studying these violent impacts helps us understand not just how moons are made, but how delicate our own planetary magnetic shield truly is.
Giant impacts do more than make moons; they alter the magnetic destiny of entire worlds.
— NorthernLightsIceland.com Team
A Peek Inside the Research
How do we study a collision that happened billions of years ago? It comes down to incredible supercomputer modeling. The research team used a method called Smoothed Particle Hydrodynamics (SPH). They programmed up to 1,000,000 digital ‘particles’ to represent Mars and the incoming asteroid, assigning them real-world physics using the Tillotson equation of state. They then smashed them together at 6 kilometers per second. By analyzing the trajectory, energy, and gravity of every single particle post-impact, they calculated exactly how much rock would stay in orbit versus fall back to the surface. It is a brilliant triumph of simulated physics.
The collisionless particle representation can accurately simulate giant impacts onto Mars to determine the inserted mass.
— Research Team SPH Data
Key Takeaways
Giant impacts were a common process in the late stages of planetary formation.
A single impact created enough debris (1 to 4% of the asteroid's mass) to form both martian moons.
Computer simulations using fluid dynamics can perfectly recreate planetary crashes from billions of years ago.
Moons formed from a debris disk will orbit around a planet's equator, unlike randomly captured asteroids.
Sources & Further Reading
Frequently Asked Questions
Q: If Mars had a ring of debris, why doesn’t it have one now?
A: The debris ring was temporary. Over millions of years, the material either clumped together to form Phobos and Deimos, fell back onto the martian surface, or was blown away by solar radiation.
Q: Why is Phobos so porous and empty inside?
A: Because it wasn’t formed as a solid rock. It formed from thousands of smaller chunks of impact debris gently clumping together in orbit, leaving large voids and empty spaces between the rocks.
