A giant asteroid could have destroyed Mars’s chances of evolving into an Earth-like blue planet by punching a hole in its crust so large that it damaged the red planet’s magnetic field, scientists have found.

Earth’s magnetic field, generated by molten iron moving in its core, deflects radiation that would otherwise blast its atmosphere into space.

Scientists have long been puzzled why Mars lacks a similar field, but measurements from an orbiting spacecraft may have provided an answer. They have found intense magnetic anomalies affecting surface rocks all over Mars’s southern hemisphere. These appear to be remnants of a field that once embraced the whole planet.

Such anomalies are absent from the northern hemisphere, suggesting something happened to change the planet’s magnetic field in the distant past. This fits with another Mars oddity, that the rocks are much thinner in the northern hemisphere than in the south, a phenomenon known as the “crustal dichotomy”.

“The evidence suggests that a giant impact early in the planet’s history could have disrupted the molten core, changing the circulation and affecting the magnetic field,” said Sabine Stanley, assistant professor of physics at the University of Toronto, whose research was just published.

Mars is believed to have formed, along with Earth and the rest of the solar system, about 4.6 billion years ago from the clumping together of rocks and other debris left over from the formation of the sun.

As the embryonic planets grew larger, the rocks at their cores melted and fused, allowing heavier elements, especially iron, to sink to the centre. The iron, kept molten by radioactive elements, began to move, generating magnetic fields around both planets.

It had been thought that Mars’s core cooled down simply because the red planet is only half the size of Earth, but this was undermined by the recent discovery that Mercury, the innermost planet which is even smaller, has a molten core and magnetic field.

“We know Mars had a magnetic field which disappeared about 4 billion years ago and that this happened around the same time that the crustal dichotomy appeared, which is a possible link to an asteroid impact,” Stanley said.

Perhaps the biggest question is what might have happened had Mars retained its magnetic field - and whether it might have evolved life.

Monica Grady, professor of planetary and space sciences at the Open University, said: “Mars once had a much thicker atmosphere along with standing water and a magnetic field, so it would have been a very different place to the dry barren planet we see today.”

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