No Magnetic Field (to Speak of) For Mars: Asteroid Implicated

" 'Supergiant' Asteroid Shut Down Mars's Magnetic Field"
National Geographic (May 11, 2009)

"A 'supergiant' asteroid several times larger than the one that likely killed the dinosaurs struck Mars with such force that it shut down the planet's magnetic field, scientists say.

"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..."

We're learning more every year, about the inner planets of the Solar System. And, scientists are using that knowledge to make more detailed - and probably more accurate - models about how things got to be the way they are.

This article is a pretty good overview of a recent explanation for why we're on Earth: and don't have neighbors on Mars.