SKY_September2014.pdf

(Axel Boer) #1
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

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