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Mars has two moons,
Phobos and Deimos.

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Both are small, airless
bodies with irregular shapes.

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To better understand these
moons, scientists at NASA's

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Goddard Space Flight
Center simulated the solar wind

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environment at Phobos.

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Phobos orbits incredibly
close to Mars, only thirty-seven

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hundred miles above the surface.

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In fact, it whips around the
planet in less than eight hours!

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Because it has no atmosphere
or magnetosphere, Phobos plows

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directly into the solar
wind for part of its orbit.

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The solar wind consists of
negatively charged electrons,

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which are light, and positively
charged ions, which are heavy.

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Normally they exist in equal
numbers, so the solar wind is

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electrically neutral.

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Phobos, however, absorbs the
solar wind on its dayside,

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leaving a void
over its night side.

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Because the electrons are
lighter than the ions, they rush

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in to fill the void.

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This creates a field of negative
electric potential over Phobos

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and statically
charges its night side.

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The ions are attracted to the
field and pulled in farther

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downstream, restoring the
solar wind's density and neutral

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charge.

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The simulation also looked at a
massive crater called Stickney.

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It showed that when Stickney
falls into shadow, electrons

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initially move into the crater,
and the associated electric

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potential forces
the ions to keep up.

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On the smaller scale, this
electrically charges the crater

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interior through the same
process that charges the larger

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night side.

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Phobos is often cited as a
target for future exploration,

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but roving around on the
night side, or within shadowed

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craters, could build up
static electric charge, possibly

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affecting sensitive equipment.

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Mission planners will have to
face this challenge as they set

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their sights on
the moons of Mars.

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[ Music ]

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[ Satellite beeping ]