Did Mars's Magnetic Field Die With a Whimper or a Bang?
http://sciencenow.sciencemag.org/cgi...ull/2009/430/2

By Krista Zala
ScienceNOW Daily News
30 April 2009

Giant asteroids may have wiped out Mars's magnetic field. The energy released by massive
collisions upset the heat flow in the planet's iron core that produced the magnetism,
according to a new study. The finding offers a solution to the mystery ofthe disappearing
magnetic field and sheds light on early Earth conditions.

A planet's magnetic field results from a process called convection, Within the core,
molten iron rises, cools, and sinks. The convection induces a magnetic field, in a system
known as a dynamo.

Like Earth, early Mars had a magnetic field and perhaps an atmosphere conducive to liquid
water. But magnetic analysis of the martian surface indicates that when Mars was a mere
500 million years old, its magnetic field withered away. Without this shield, streams of
ionizing particles spewing from the sun strip away a planet's atmosphere,killing any life
that may have emerged or forcing it underground.

The disappearance of the martian magnetic dynamo has puzzled scientists. One theory links
it to the Late Heavy Bombardment, a period of 100 million years when asteroids--some
hundreds of kilometers across--smashed into Mars and the inner planets. Amassive
collision could warm Mars's mantle, disrupting core convection. That's because the cooling
action of the mantle draws heat from the core, keeping it churning. Without that flow,
core convection grinds to a halt.

The theory fits with the observation that the only the oldest impact craters on Mars are
magnetized. Still, all remained speculation until data came back from theMars Global
Surveyor and other recent spacecraft. Last year, planetary scientists Robert Lillis and
Michael Manga, both of the University of California, Berkeley, estimated the ages of
craters in older impact basins and showed that the previously establisheddate of heavy
bombardment, about 3.9 billion years ago, corresponds to the death of Mars's dynamo.

Could the bombardment have released enough energy to trigger the shutdown? Lillis, Manga,
and planetary geophysicist James Roberts of John Hopkins University in Baltimore,
Maryland, have modeled the effects of heat produced by impacts. When theyadded the heat
release from the biggest asteroids to models of mantle convection, they found that the
mantle became a heating blanket rather than an ice pack. The extra heat was enough to stop
core convection, the team reports in the current issue of the Journal of Geophysical
Research – Planets.

Mars was hit by at least five particularly large asteroids during the bombardment. "Any
one of the super-giant impacts could have shut off [the dynamo]," says Roberts. Earth
likely suffered the same onslaught, but at twice the radius of Mars, it probably had a
strong enough dynamo to withstand or recover from huge impacts.

Not all scientists are on board with the analysis. David Stevenson, a planetary scientist
at the California Institute of Technology in Pasadena, suggests that although the
explanation is plausible, he's not convinced the collisions released enough energy.
Furthermore, "the dynamo does not need to have an external influence to stop functioning,"
he points out, adding that without enough core convection, "it may simplydie of its own
accord."