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November 12, 2015

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Martian moon said to be slowly falling apart

Nov. 12, 2015
Courtesy of NASA God­dard Space Flight Cen­ter 
and World Science staff

Long, shal­low grooves lin­ing the Mar­tian moon Pho­bos are prob­ably early signs of struc­tur­al fail­ure that will ul­ti­mately de­stroy it, sci­en­tists say.

Or­bit­ing a mere 3,700 miles (6,000 kilo­me­ters) above 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 2 cm, a little under an inch, per year. 

Sci­en­tists ex­pect the moon to be pulled apart in 30 to 50 mil­lion years.

New mod­el­ing sug­gests that the grooves on Mars’ moon Pho­bos could be pro­duced by tid­al forc­es. In­i­tial­ly, sci­en­tists had thought the grooves were cre­at­ed by the mas­sive im­pact that made Stick­ney crat­er (low­er right). (Cred­its: NA­SA/JPL-Caltech/U. 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, Md.

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

Pho­bos’ grooves were long thought to be frac­tures caused by the im­pact that formed a huge crat­er called Stick­ney. That crash was so pow­er­ful it al­most shat­tered Pho­bos. But sci­en­tists even­tu­ally de­ter­mined that the grooves don’t ra­di­ate out­ward from the crat­er it­self but from a 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 ma­te­ri­al 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.

An in­te­ri­or like that of Phobos can dis­tort easily be­cause it has very lit­tle strength and forc­es the out­er lay­er to re­ad­just, the re­search­ers said. They think the out­er lay­er be­haves elas­tic­ally and builds stress, but is 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, they ar­gued. Stress frac­tures pre­dicted by this mod­el line up very well with the grooves seen in im­ages of Pho­bos, they added. 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 were con­tin­u­ing.

The same fate may await Nep­tune’s moon Tri­ton, which is al­so slowly fall­ing in­ward and has a si­m­i­larly frac­tured sur­face, the sci­en­tists said. The work al­so has im­plica­t­ions for plan­ets in oth­er so­lar sys­tems, 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.


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Long, shallow grooves lining the Martian moon Phobos are likely early signs of structural failure that will ultimately destroy it, scientists say. Orbiting a mere 3,700 miles (6,000 kilometers) above 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 2 cm, or almost an inch, per year. Scientists expect the moon to be pulled apart in 30 to 50 million years. “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 were presented Nov. 10 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 a huge crater called Stickney. That crash was so powerful it almost shattered Phobos. But scientists eventually determined that the grooves don’t radiate outward from the crater itself but from a 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 material 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 [very weak gravity], 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 said. They think the outer layer behaves elastically and builds stress, but is 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, they argued. Stress fractures predicted by this model line up very well with the grooves seen in images of Phobos, they added. 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 were continuing. The same fate may await Neptune’s moon Triton, which is also slowly falling inward and has a similarly fractured surface, the scientists said. The work also has implications for planets in other solar systems, 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.