Mars has two moons,
Phobos and Deimos.
Both are small, airless
bodies with irregular shapes.
To better understand these
moons, scientists at NASA's
Goddard Space Flight
Center simulated the solar wind
environment at Phobos.
Phobos orbits incredibly
close to Mars, only thirty-seven
hundred miles above the surface.
In fact, it whips around the
planet in less than eight hours!
Because it has no atmosphere
or magnetosphere, Phobos plows
directly into the solar
wind for part of its orbit.
The solar wind consists of
negatively charged electrons,
which are light, and positively
charged ions, which are heavy.
Normally they exist in equal
numbers, so the solar wind is
electrically neutral.
Phobos, however, absorbs the
solar wind on its dayside,
leaving a void
over its night side.
Because the electrons are
lighter than the ions, they rush
in to fill the void.
This creates a field of negative
electric potential over Phobos
and statically
charges its night side.
The ions are attracted to the
field and pulled in farther
downstream, restoring the
solar wind's density and neutral
charge.
The simulation also looked at a
massive crater called Stickney.
It showed that when Stickney
falls into shadow, electrons
initially move into the crater,
and the associated electric
potential forces
the ions to keep up.
On the smaller scale, this
electrically charges the crater
interior through the same
process that charges the larger
night side.
Phobos is often cited as a
target for future exploration,
but roving around on the
night side, or within shadowed
craters, could build up
static electric charge, possibly
affecting sensitive equipment.
Mission planners will have to
face this challenge as they set
their sights on
the moons of Mars.
[ Music ]
[ Satellite beeping ]
