We’ve said it before: Mars’ moon Phobos is doomed. But a new study indicates it might be worse than we thought.
One of the most striking features we see on images of Phobos is the parallel sets of grooves on the moon’s surface. They were originally thought to be fractures caused by an impact long ago. But scientists now say the grooves are early signs of the structural failure that will ultimately destroy this moon.
“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, from NASA’s Goddard Space Flight Center.
Why is Phobos falling apart?
Two words: tidal forces.
Phobos orbits closer to its planet than any moon in the Solar System. As it orbits just 6,000 km (3,700 miles) above Mars, and the planet’s gravity is pulling Phobos in closer and closer; it is also tearing Phobos apart. Scientists estimate the ultimate destruction of this tiny moon (22 kilometers/13.5-miles in diameter) might take place in about 30 to 50 million years.
It only take about 7.5 hours for Phobos to complete an orbit around the planet, while Mars takes almost 25 hours to complete one rotation on its axis. So Phobos travels three times around the planet for every Martian day. And as Fraser explains in this video, this is a problem.
Mars’ gravity is pulling in Phobos closer by about 2 meters (6.6 feet) every hundred years. The orbit will get lower and lower until it reaches a level known as the Roche Limit. This is the point where the tidal forces between the two sides of the moon are so different that it gets torn apart.
Hurford and his colleagues, who presented their latest findings at the annual Meeting of the Division of Planetary Sciences of the American Astronomical Society this week, also delivered other bad news about the interior of Phobos – which could ultimately speed up the demise of the moon. Phobos’ insides are likely to be just a big pile of rubble — barely holding together — surrounded by a layer of powdery regolith about 100 meters (330 feet) 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.”
Phobos’ grooves have long been an issue up for debate. As mentioned previously, one idea is that the grooves were associated with the impact that formed Stickney Crater, a big 10 km-wide crater that dominates one side of Phobos. However, scientists eventually determined that the grooves don’t radiate outward from the crater itself but from a focal point nearby. Another idea is they came from Phobos moving through streams of debris thrown up from impacts 6,000 km away on the surface of Mars, with each ‘family’ of grooves corresponding to a different impact event.
But new modeling by Hurford and his team supports the idea that the grooves are more like “stretch marks” that occur when Phobos gets deformed by tidal forces.
The team said that stress fractures predicted by their model coincide 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.
Huford also said 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.
Here’s a video showing Mars Express images of Phobos over the last 10 years. The images show the grooves running across the small moon: