"Long before it's in the papers"
November 25, 2015

RETURN TO THE WORLD SCIENCE HOME PAGE


Disintegrating Martian moon could become a ring

Nov. 25, 2015
Courtesy of University of California - Berkeley
and World Science staff

Mars’ larg­est moon is slowly fall­ing to­ward the plan­et—but rath­er than smash in­to it, the moon will probably dis­in­te­grate in­to bits that will form a ring around Mars, sci­en­tists pre­dict.

The de­mise of the moon Pho­bos will oc­cur in an es­ti­mat­ed 20 mil­lion to 40 mil­lion years, leav­ing a ring that will last for an­y­where from one mil­lion to 100 mil­lion years, ac­cord­ing to the two earth sci­en­tists at the Uni­vers­ity of Cal­i­for­nia, Berke­ley.

Reconstruction of how a future ring around Mars might appear. (Image by Tushar Mittal using Celestia 2001-2010, Celestia Development Team)


In a pa­pe­r ap­pear­ing on­line this week in the jour­nal Na­ture Ge­o­sci­ence, post­doc­tor­al fel­low Ben­ja­min Black and grad­u­ate stu­dent Tushar Mit­tal es­ti­mate the co­he­sive­ness of Pho­bos. They con­clude that it can’t re­sist tid­al forc­es that will pull it apart when it gets clos­er to the Red Plan­et.

“S­tand­ing on the sur­face of Mars a few tens of mil­lions of years from now, it would be pret­ty spec­tac­u­lar to watch,” Black said. How­ev­er, it’s not clear wheth­er the ring would be vis­i­ble from Earth, as is the case with Sat­urn’s im­pres­sive rings, Black and Mit­tal said.

Just as earth’s moon pulls on our plan­et in dif­fer­ent di­rec­tions, rais­ing tides in the oceans, for ex­am­ple, so too Mars tugs dif­fer­ently on dif­fer­ent parts of Pho­bos. As Pho­bos gets clos­er to the plan­et, the tugs are enough to pull the moon apart, sci­en­tists say. This is be­cause Pho­bos is al­ready highly frac­tured, with lots of pores and rub­ble. 

Dis­mem­ber­ing it is like pulling apart a gra­nola ba­r, Black said, scat­ter­ing crumbs and chunks eve­ry­where. The re­sult­ing rub­ble—rocks of var­i­ous sizes and a lot of dust—would, in this view, con­tin­ue to or­bit Mars and quickly spread around the plan­et in a ring.

While the larg­est chunks would even­tu­ally spir­al in­to the plan­et and col­lide at a graz­ing an­gle to pro­duce egg-shaped craters, the re­search­ers said, the ma­jor­ity of the de­bris would cir­cle the plan­et for mil­lions of years un­til these pieces, too, drop on­to the plan­et in “moon” show­ers, like me­te­or show­ers. Only Mars’ oth­er moon, Dei­mos, would re­main.

Black and Mit­tal were drawn to the ques­tion of what might hap­pen to Pho­bos be­cause its ex­pected fate is so dif­fer­ent from that of most oth­er known moons.

“While our moon is mov­ing away from earth at a few cen­time­ters per year, Pho­bos is mov­ing to­ward Mars at a few cen­time­ters per year, so it is al­most in­ev­i­ta­ble that it will ei­ther crash in­to Mars or break apart,” Black said. “One of our mo­tiva­t­ions for stu­dy­ing Pho­bos was as a test case to de­vel­op ideas of what pro­cesses a moon might un­dergo as it moves in­ward to­ward a plan­et.”

Only one oth­er moon in the so­lar sys­tem, Nep­tune’s larg­est moon, Tri­ton, is known to be mov­ing clos­er to its plan­et, Mit­tal and Black said.

Stud­y­ing such moons is rel­e­vant to con­di­tions in our early so­lar sys­tem, Mit­tal said, when it’s likely there were many more moons around the plan­ets that have since dis­in­te­grated in­to rings—the sus­pected ori­gins of the rings of the out­er plan­ets. 

Some stud­ies es­ti­mate that dur­ing plan­et forma­t­ion, 20-30 pe­rcent of plan­ets ac­quire moons mov­ing in­ward and des­tined for de­struc­tion, though they would have long since dis­ap­peared. Some of Mars’ sev­er­al thou­sand el­lip­ti­cal craters may even have been formed by rem­nants of such moon­lets crash­ing to the sur­face at a graz­ing an­gle.

The na­ture of rings re­sult­ing from a Pho­bos break­up would de­pend on where the break­up hap­pens, Black and Mit­tal said.

If it oc­curred rel­a­tively low, say 680 km (420 miles) from the sur­face, “it would form a really nar­row ring com­pa­ra­ble in dens­ity to that of one of Sat­urn’s most mas­sive rings,” Mit­tal said. “Over time it would spread out and get wid­er, reach­ing the top of the Mar­tian at­mos­phere in a few mil­lion years, when it would start los­ing ma­te­ri­al be­cause stuff would keep rain­ing down on Mars.”

If the moon breaks up far­ther from Mars, the ring could pe­rsist for 100 mil­lion years be­fore rain­ing down on Mars, they found.

Mit­tal said it’s not clear wheth­er the dust and de­bris rings would be vis­i­ble from earth, since dust does­n’t re­flect much sun­light, where­as ice in the rings of the out­er plan­ets makes them easily vis­i­ble. But Mars’ ring may re­flect enough light to make Mars slightly brighter as seen from Earth, he said, and through a tel­e­scope the shad­ows of the rings might al­so be vis­i­ble on the sur­face.


* * *

Send us a comment on this story, or send it to a friend











y Sign up for
e-newsletter

   
 
subscribe
 
cancel

On Home Page         

LATEST

  • Aster­oid min­ing could be­gin within a few de­cades, scient­ists claim

  • Fos­sil for­est may shed light on first big tre­es

EXCLUSIVES

  • Study links global warming, war for first time—in Syria

  • Smart­er mice with a “hum­anized” gene?

  • Was black­mail essen­tial for marr­iage to evolve?

  • Plu­to has even cold­er “twin” of sim­ilar size, studies find

MORE NEWS

  • F­rog said to de­scribe its home through song

  • Even r­ats will lend a help­ing paw: study

  • D­rug may undo aging-assoc­iated brain changes in ani­mals

Mars’ largest moon is slowly falling toward the planet—but rather than smash into it, the moon will probably disintegrate into bits that will form a ring around Mars, scientists predict. The demise of the moon Phobos will occur in an estimated 20 to 40 million years, leaving a ring that will last for anywhere from one million to 100 million years, according to the two earth scientists at the University of California, Berkeley. In a paper appearing online this week in the journal Nature Geoscience, postdoctoral fellow Benjamin Black and graduate student Tushar Mittal estimate the cohesiveness of Phobos and conclude that it can’t resist tidal forces that will pull it apart when it gets closer to Mars. “Standing on the surface of Mars a few tens of millions of years from now, it would be pretty spectacular to watch,” Black said. However, it’s not clear whether the ring would be visible from Earth, as is the case with Saturn’s impressive rings, Black and Mittal said. Just as earth’s moon pulls on our planet in different directions, raising tides in the oceans, for example, so too Mars tugs differently on different parts of Phobos. As Phobos gets closer to the planet, the tugs are enough to pull the moon apart, scientists say. This is because Phobos is already highly fractured, with lots of pores and rubble. Dismembering it is like pulling apart a granola bar, Black said, scattering crumbs and chunks everywhere. The resulting rubble—rocks of various sizes and a lot of dust—would, in this view, continue to orbit Mars and quickly spread around the planet in a ring. While the largest chunks would eventually spiral into the planet and collide at a grazing angle to produce egg-shaped craters, the researchers said, the majority of the debris would circle the planet for millions of years until these pieces, too, drop onto the planet in “moon” showers, like meteor showers. Only Mars’ other moon, Deimos, would remain. Black and Mittal were drawn to the question of what might happen to Phobos because its expected fate is so different from that of most other known moons. “While our moon is moving away from earth at a few centimeters per year, Phobos is moving toward Mars at a few centimeters per year, so it is almost inevitable that it will either crash into Mars or break apart,” Black said. “One of our motivations for studying Phobos was as a test case to develop ideas of what processes a moon might undergo as it moves inward toward a planet.” Only one other moon in the solar system, Neptune’s largest moon, Triton, is known to be moving closer to its planet, Mittal and Black said. Studying such moons is relevant to conditions in our early solar system, Mittal said, when it’s likely there were many more moons around the planets that have since disintegrated into rings—the suspected origins of the rings of the outer planets. Some studies estimate that during planet formation, 20-30 percent of planets acquire moons moving inward and destined for destruction, though they would have long since disappeared. Some of Mars’ several thousand elliptical craters may even have been formed by remnants of such moonlets crashing to the surface at a grazing angle. The nature of rings resulting from a Phobos breakup would depend on where the breakup happens, Black and Mittal said. If it occurred relatively low, say 680 km (420 miles) from the surface, “it would form a really narrow ring comparable in density to that of one of Saturn’s most massive rings,” Mittal said. “Over time it would spread out and get wider, reaching the top of the Martian atmosphere in a few million years, when it would start losing material because stuff would keep raining down on Mars.” If the moon breaks up farther from Mars, the ring could persist for 100 million years before raining down on Mars, they found. Mittal said it’s not clear whether the dust and debris rings would be visible from earth, since dust doesn’t reflect much sunlight, whereas ice in the rings of the outer planets makes them easily visible. But Mars’ ring may reflect enough light to make Mars slightly brighter as seen from Earth, he said, and through a telescope the shadows of the rings might also be visible on the surface.