AURA / STSCI / NASA
When the Red Planet formed, it is thought to have been much like a young Earth — hot, full of water, and roaring with a molten, churning core and mantle. The liquid rock and metal formed a magnetic dynamo that helped protect its surface and thick atmosphere from cosmic radiation.
Then, beginning around 4.2 billion years ago, it was suddenly pummeled with at least 20 asteroids between 124 and 311 miles in diameter, each leaving a crater. By contrast, the object thought to have killed of the dinosaurs on Earth is estimated to have been five to six miles wide.
"These things were enormous," James Roberts of the University of California, Santa Cruz, said. "You would not want to be around when one of these hit."
One of the last giant meteors blew a hole 1,864 miles wide in the planet, creating Utopia basin in the planet's northern hemisphere. At about 4.1 billion years old, Utopia is the oldest crust on the planet that doesn't show signs of magnetism, meaning the rocks must have cooled at a time when there was no magnetic field.
Roberts and a team of researchers calculate that the Utopia impact could have done in the magnetic dynamo, which was already flagging as the planet cooled. It injected approximately one trillion megatons of energy into the Martian mantle, or close to 10 trillion times the explosive energy of the nuclear bomb used at Hiroshima.
The heat spread in an instant as a supersonic shockwave rippled through the planet. Then, over the next 30 million years, the overheated mantle acted as a blanket for the core, preventing it from circulating enough to maintain a magnetic field.
"The core has to have organized convection to form a dynamo," Mario Acuna of NASA's Goddard Spaceflight Center in Maryland, said. "If you disturb it with an impact, it will shut off."
As the mantle cooled form the impact, there wasn't enough energy in the core to restart a dynamo from scratch, Roberts reasons, and so the magnetic field was gone forever.
"Earth probably took the same kind of punishment," Roberts said. "But it's primarily a function of size. Earth has more than 10 times more heat than Mars, and much more vigorous convection."