"If the mantle becomes too hot, it's not able to cool the core as efficiently and there is no magnetic field," Roberts told National Geographic News.
Without a magnetic field, Mars was exposed to the full brunt of the solar wind, the continuous stream of charged particles emitted by the sun.
The solar wind could then have slowly eroded away Mars's atmosphere until only a wispy envelope of gas remained.
Drastic climate change would have soon followed, helping to create the desiccated Mars we know today.
While a similar impactor could conceivably shut down Earth's magnetic field, it would have to be much larger than the one that struck early Mars, Roberts said.
Earth's mantle is twice as thick as that of Mars, and its core churns much more vigorously, making it more difficult to shut down the dynamo effect that generates the field. (Learn more about Earth's dynamic processes.)
Venus also lacks a global magnetic field, but scientists think that is because its mantle is dryer and stiffer and thus less conducive to heat flow.
Sabine Stanley, a geophysicist at the University of Toronto who was not involved in the study, noted that the idea of magnetic field-killing asteroids is not new.
But the latest study, Stanley said, is the first to model the phenomena in detail and to finger a potential culprit crater on Mars.
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