SKY_September2014.pdf

SkyandTelescope.com September 2014 25

One way to imagine these fi elds is to think of big mag-

netic bubbles sticking up from the surface and into the

upper atmosphere. The features look somewhat like the

localized magnetic structures rising from the Sun’s pho-

tosphere. In fact, in terms of magnetism, the Red Planet

resembles the Sun more than it does the other planets of

the solar system.

These remanent fi elds are absent from Hellas Planitia

as well as several other large impact basins. The number

of craters in Hellas’s interior pegs the basin’s formation to

about 4 billion years ago. Because of this estimate, many

scientists interpret the missing magnetism to mean that

the planet lost its global fi eld before the impact that cre-

ated Hellas.

Not everyone is convinced: some researchers argue

that the global fi eld might have survived until 3.6 billion

years ago. This argument cites remanent fi elds in regions

covered by volcanic fl ows, which have younger surfaces.

But Mars’s remanent magnetism emanates from massive,

solidifi ed fl ows below the surface and not on it, making it

diffi cult to tell whether the magnetic imprints are actually

the same age as the surface fl ows. Crater estimates don’t

have this same ambiguity, hence the popularity of the

4-billion-year cutoff.

Why Mars lost its global fi eld is an open question. To

maintain a magnetosphere, the planet would need to

rotate quickly and have lots of convection in a conducting

liquid-metallic core. This convection can occur when heat

fl ows outward from the core to the surrounding mantle

fast enough to trigger the conveyor-belt-like motion.

The most straightforward explanation for the shut-

down of Mars’s global magnetism is that the planet cooled

enough to turn off convection. On the other hand, impacts

might have stifl ed the fl ow by heating the upper mantle,

especially if the fi eld was already in decline. Mars certainly

endured its share of planetary punches in its fi rst few

Solar wind

N

N

Mars’s Crustal

Magnetic Fields

The solar wind stretches Mars’s crustal magnetic fi elds into giant

windsocks ( 1 ). Spacecraft observations suggest that the tops

of the fi elds can then snap off , carrying pockets of the planet’s

atmosphere away with them ( 2 ).

Members of the MAVEN team gather in front of the space-

craft atop its Atlas V rocket at Cape Canaveral’s Space Launch

Complex 41. The rocket launched successfully the following

afternoon on November 18, 2013.

1

2

MAVEN / CASEY A. CASS (UNIVERSITY OF COLORADO)

CASEY REED

Maven_at_Mars.indd 25 6/23/14 12:17 PM