[meteorite-list] Mars' Moon Phobos is Slowly Falling Apart

[Ron Baalke]

http://www.nasa.gov/feature/goddard/phobos-is-falling-apart

Mars' Moon Phobos is Slowly Falling Apart
November 10, 2015

The long, shallow grooves lining the surface of Phobos are likely early 
signs of the structural failure that will ultimately destroy this moon 
of Mars.

Orbiting a mere 3,700 miles (6,000 kilometers) above the surface of Mars, 
Phobos is closer to its planet than any other moon in the solar system. 
Mars' gravity is drawing in Phobos, the larger of its two moons, by about 
6.6 feet (2 meters) every hundred years. Scientists expect the moon to 
be pulled apart in 30 to 50 million years.

[Image]
New modeling indicates that the grooves on Mars' moon Phobos could be 
produced by tidal forces - the mutual gravitational pull of the planet 
and the moon. Initially, scientists had thought the grooves were created 
by the massive impact that made Stickney crater (lower right).
Credits: NASA/JPL-Caltech/University of Arizona

"We think that Phobos has already started to fail, and the first sign 
of this failure is the production of these grooves," said Terry Hurford 
of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The findings by Hurford and his colleagues are being presented Nov. 10, 
2015, at the annual Meeting of the Division of Planetary Sciences of the 
American Astronomical Society at National Harbor, Maryland.

Phobos' grooves were long thought to be fractures caused by the impact 
that formed Stickney crater. That collision was so powerful, it came close 
to shattering Phobos. However, scientists eventually determined that the 
grooves don't radiate outward from the crater itself but from a focal 
point nearby.

More recently, researchers have proposed that the grooves may instead 
be produced by many smaller impacts of material ejected from Mars. But 
new modeling by Hurford and colleagues supports the view that the grooves 
are more like "stretch marks" that occur when Phobos gets deformed by 
tidal forces.

The gravitational pull between Mars and Phobos produces these tidal forces. 
Earth and our moon pull on each other in the same way, producing tides 
in the oceans and making both planet and moon slightly egg-shaped rather 
than perfectly round.

The same explanation was proposed for the grooves decades ago, after the 
Viking spacecraft sent images of Phobos to Earth. At the time, however, 
Phobos was thought to be more-or-less solid all the way through. When 
the tidal forces were calculated, the stresses were too weak to fracture 
a solid moon of that size.

The recent thinking, however, is that the interior of Phobos could be 
a rubble pile, barely holding together, surrounded by a layer of powdery 
regolith about 330 feet (100 meters) thick.

"The funny thing about the result is that it shows Phobos has a kind of 
mildly cohesive outer fabric," said Erik Asphaug of the School of Earth 
and Space Exploration at Arizona State University in Tempe and a co-investigator 
on the study. "This makes sense when you think about powdery materials 
in microgravity, but it's quite non-intuitive."

An interior like this can distort easily because it has very little strength 
and forces the outer layer to readjust. The researchers think the outer 
layer of Phobos behaves elastically and builds stress, but it's weak enough 
that these stresses can cause it to fail.

All of this means the tidal forces acting on Phobos can produce more than 
enough stress to fracture the surface. Stress fractures predicted by this 
model line up very well with the grooves seen in images of Phobos. This 
explanation also fits with the observation that some grooves are younger 
than others, which would be the case if the process that creates them 
is ongoing.

The same fate may await Neptune's moon Triton, which is also slowly falling 
inward and has a similarly fractured surface. The work also has implications 
for extrasolar planets, according to researchers.

"We can't image those distant planets to see what's going on, but this 
work can help us understand those systems, because any kind of planet 
falling into its host star could get torn apart in the same way," said 
Hurford.

 
Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Md.