for National Geographic News
Based on the number of large craters present, scientists think very early Mars suffered 15 or so giant impacts within a span of about a hundred million years.
Now a new computer model suggests Mars's magnetic field may have been slowly weakened by four especially large impacts and then snuffed out completely by a fifth and final blow.
That impact created the 2,000-mile-wide (3,300-kilometer-wide) Utopia crater, which dates back roughly 4.1 billion years, said study team member James Roberts of the Johns Hopkins Applied Physics Lab in Maryland.
"It's possible that the four earlier impacts set everything up, and the Utopia crater was the straw that broke camel's back."
Earth has a magnetic field in part because of heat transfer between the planet's rotating molten core and the relatively cooler mantle layer above it.
This temperature difference helps create what's known as an electric dynamo, which keeps the magnetic field stable over time.
But when the solar system was first forming it went through a tumultuous time known as the Late Heavy Bombardment, in which several large asteroids—remnants of planetary formation—smashed into young Mars, Earth, Venus, and Mercury.
This is about the time the Utopia crater was formed, and roughly the same time that scientists think the Martian magnetic field shut down, said Roberts, whose research is detailed in a recent issue of the Journal of Geophysical Research.
According to the new model, the Utopia impact injected so much heat into the Martian mantle that it drastically reduced the temperature difference driving the dynamo.
SOURCES AND RELATED WEB SITES