OUT THERE
62 DISCOVERMAGAZINE.COMapart; self-gravity and [tensile] strength
act to hold Phobos together.”
Just as our own Moon’s gravity
raises tides in both Earth’s oceans
and landmasses, Phobos creates a tiny
tidal bulge in Mars. The bulge moves
as the moon circles Mars, just as the
Moon-caused tidal bulge moves around
Earth. Because Phobos has an orbital
period faster than the martian day, the
tidal bulge lags behind Phobos and acts
to gradually slow it in its orbit, sending
Phobos slowly spiraling in toward
the martian surface. The rate is about
0.79 inch (2 centimeters) per year. It’s
not much, but there is nothing to stop it.
“As Phobos gets closer to Mars,” says
Mittal, “the tidal stresses increase.” The
moon’s self-gravity and tensile strength
oppose the tidal forces, but eventually
Phobos will drop to a distance — called
the Roche limit — where the martian
gravity will win the tug-of-war. Several
studies confirm that Phobos’ inward
spiral will lead to its destruction in
25 million to 75 million years.
Phobos could stave off obliteration if
it were a dense body, more able to resist
the pull of gravity past the Roche limit.
Unfortunately, data from the European
Space Agency’s Mars Express orbiter
show that the moon has a density of
just 1.9 grams per cubic centimeter.
By comparison, our Moon has a
density of 3.3 g per cubic centimeter.
Even tiny Themisto, Jupiter’s smallest
regular moon at just 4.9 miles (8 km) indiameter, has an estimated density of
about 2.6 g per cubic centimeter. Mars
Express also revealed that Phobos is
porous, likely containing large voids, so
it is probably a rubble pile, like many of
Uranus’ and Neptune’s tiny moons.
In addition, Phobos’ makeup is
similar to carbonaceous chondrite
meteorites like the ones that fell on
Canada’s Tagish Lake in 2000. These
primitive meteorites are made of a
bevy of loose, easily broken materials,
including magnetite, olivine crystals,
phyllosilicates, and complex organic
molecules such as amino acids, and
are about 10 percent water, formed in
oxygen-rich regions of the early solar
system. Phobos is not only a rubble pile
with a weak gravitational field, but the
rubble itself is somewhat brittle.
Then there’s Stickney Crater,
the giant impact basin on Phobos.
Sometime in its past, Phobos was hit
by an object that left a crater 5.6 miles
(9 km) wide — nearly half the moon’s
diameter. The impact surely
fractured much of Phobos’
interior, leaving it even
weaker. From there,
billions of years
of meteoroid
impacts have
churned the
surface of
Phobos into a
layer of fine dust
about a hundredmeters deep. Given the extremely weak
surface gravity (escape velocity for
Phobos is a mere 4.56 feet per second),
the rock making up the rubble pile just
barely stays put.
Black and Mittal used a numerical
geotechnical model designed for large
underground construction projects
to estimate what they call the moon’s
“rock mass strength.” The results place
the finale for Phobos between 20 million
and 40 million years from now, and give
more details on its violent end.
As Phobos reaches its Roche limit,
about 3,400 miles (5,470 km) from
the martian surface, tidal forces will
start pulling Phobos apart. Just as
Saturn’s tidal forces may have peeled
away the icy mantle of a Titan-sized
moon billions of years ago, the
hundred meters or so of regolith will
be stripped off Phobos. It will happen
quickly: In as little as a week, the dust
will spread into a ring circling Mars,
the moon no more.CLOCKWISE FROM TOP: NASA (2); ROEN KELLY/ASTRONOMYThe lines cutting across Phobos are caused by tidal stresses from the
gravity of Mars slowly ripping its largest moon apart.The white object in the center of Saturn’s diffuse E ring is Enceladus, a tiny
moon with a liquid ocean. Geysers at the moon’s south pole eject water
into space, forming the ring.Roche^ limitMarsPhobosAs Phobos spirals
inward, it will reach
a distance, called
the Roche limit,
where the
moon will be
pulled apart
by Mars’
gravity. The
end result
will be a
temporary
ring around
Mars.