Mars' lumpy magnetic field
Posted: December 4, 2001

New research from the University of Colorado at Boulder reveals that areas of the surface of Mars may be protected from the full force of solar radiation by areas of intensely magnetized crust.

The startling vision of Mars' magnetosphere is being explored by David Brain, a doctoral student at CU-Boulder's Laboratory for Atmospheric and Space Physics and his research advisor, Professor Fran Bagenal, using magnetometer data from NASA's Mars Global Surveyor spacecraft.

Brain's research has implications for the escape of atmospheric gases into space and climate evolution on the red planet, as well as the radiation environment of these areas - possibly making them safer landing sites for future human expeditions.

Brain presented his research at the annual meeting of the Department of Planetary Sciences of the American Astronomical Society in New Orleans.

The planet's crust is highly magnetized in certain areas to create "mini-magnetospheres" which may shield the planet's surface in certain areas from solar and cosmic radiation, said Brain.

Before Mars Global Surveyor was launched, scientists debated whether Mars had a weak global magnetic field, or none at all. "The spacecraft showed that Mars has no global magnetic field, but it discovered the crustal sources of small magnetospheres. The strength of the crustal sources surprised everyone," said Brain.

Earth's powerful magnetic field is created by its spinning liquid metal core. Mars is thought to be mostly dead internally, since the planet has no active dynamo in its core to create a global magnetic field.

Mars Global Surveyor has found that regions of Mars' crust are at least ten times more strongly magnetized than anything measured on Earth. Brain discovered that these regions of magnetization are so strong, in fact, that they can influence magnetometer readings to altitudes of 600 to 900 miles, or about a third of the planet's radius.

"In the absence of these crustal sources, what you have is a situation similar to Venus," said Brain. "The planet's smooth ionosphere - the outer part of Earth's atmosphere beginning roughly 33 miles above the Earth's surface -- acts as a balancing force to the powerful solar wind, or on a comet, where escaping charged gas does the same thing."

According to Brain, the regions of intense magnetization occur almost completely in the planet's southern hemisphere, which is much older than the northern plains.

"It's likely that early on, traces of Mars' magnetic field were frozen in the planet's crust when its internal dynamo shut down. Later on, 'resurfacing' of the planet's northern hemisphere by some type of heating mechanism and large impact events in the southern hemisphere demagnetized much of the planet," he said. "What we are seeing may be the remnants of that process."

Brain used Mars Global Surveyor magnetometer data to calculate the altitude to which crustal sources of magnetization could be reasonably well detected, and then used the data to plot the influence of crustal sources as a function of altitude.

"At 60 to 120 miles above the planet, the crustal sources are very visible. They are still easily identifiable at 250 to 300 miles, but it is really surprising to still see their influence at 550 miles up," he said.

The second half of his research was to compute the shape of the obstacle Mars presents to the solar wind, which reaches a speed near Earth of up to 250 miles per second.

On Earth, the solar wind influences the magnetic field to generally be smooth and follow a predictable curve. The influence of local areas of magnetization on Mars is so strong that the "mini-magnetospheres" form small bumps above the ionosphere.

This surface varies over time as the solar wind changes in intensity and as the planet rotates, said Brain. "This tells us a great deal about how the sun's energy is being delivered to the upper atmosphere," he said.

Also assisting Brain in his research were Mario Acuna and Jack Connerney of NASA's Goddard Space Flight Center in Greenbelt, Md.