[meteorite-list] Does Mars Have Rings? Not Right Now, But Maybe One Day

[Ron Baalke]

https://www.jpl.nasa.gov/news/news.php?feature=6781

Does Mars Have Rings? Not Right Now, But Maybe One Day
Jet Propulsion Laboratory
March 20, 2017

As children, we learned about our solar system's planets by certain characteristics 
-- Jupiter is the largest, Saturn has rings, Mercury is closest to the 
sun. Mars is red, but it's possible that one of our closest neighbors 
also had rings at one point and may have them again someday.

That's the theory put forth by NASA-funded scientists at Purdue University, 
Lafayette, Indiana, whose findings were published in the journal Nature 
Geoscience. David Minton and Andrew Hesselbrock developed a model that 
suggests that debris that was pushed into space from an asteroid or other 
body slamming into Mars around 4.3 billion years ago alternates between 
becoming a planetary ring and clumping together to form a moon.

One theory suggests that Mars' large North Polar Basin or Borealis Basin 
-- which covers about 40 percent of the planet in its northern hemisphere 
-- was created by that impact, sending debris into space.

"That large impact would have blasted enough material off the surface 
of Mars to form a ring," Hesselbrock said.

Hesselbrock and Minton's model suggests that as the ring formed, and the 
debris slowly moved away from the Red Planet and spread out, it began 
to clump and eventually formed a moon. Over time, Mars' gravitational 
pull would have pulled that moon toward the planet until it reached the 
Roche limit, the distance within which a planet's tidal forces will break 
apart a celestial body that is held together only by gravity.

Phobos, one of Mars' moons, is getting closer to the planet. According 
to the model, Phobos will break apart upon reaching the Roche limit, and 
become a set of rings in roughly 70 million years. Depending on where 
the Roche limit is, Minton and Hesselbrock believe this cycle may have 
repeated between three and seven times over billions of years. Each time 
a moon broke apart and reformed from the resulting ring, its successor 
moon would be five times smaller than the last, according to the model, 
and debris would have rained down on the planet, possibly explaining enigmatic 
sedimentary deposits found near Mars' equator.

"You could have had kilometer-thick piles of moon sediment raining down 
on Mars in the early parts of the planet's history, and there are enigmatic 
sedimentary deposits on Mars with no explanation as to how they got there," 
Minton said. "And now it's possible to study that material."

Other theories suggest that the impact with Mars that created the North 
Polar Basin led to the formation of Phobos 4.3 billion years ago, but 
Minton said it's unlikely the moon could have lasted all that time. Also, 
Phobos would have had to form far from Mars and would have had to cross 
through the resonance of Deimos, the outer of Mars' two moons. Resonance 
occurs when two moons exert gravitational influence on each other in a 
repeated periodic basis, as major moons of Jupiter do. By passing through 
its resonance, Phobos would have altered Deimos' orbit. But Deimos' orbit 
is within one degree of Mars' equator, suggesting Phobos has had no effect 
on Deimos.

"Not much has happened to Deimos' orbit since it formed," Minton said. 
"Phobos passing through these resonances would have changed that."

"This research highlights even more ways that major impacts can affect 
a planetary body," said Richard Zurek of NASA's Jet Propulsion Laboratory, 
Pasadena, California. He is the project scientist for NASA's Mars Reconnaissance 
Orbiter, whose gravity mapping provided support for the hypothesis that 
the northern lowlands were formed by a massive impact.

Minton and Hesselbrock will now focus their work on either the dynamics 
of the first set of rings that formed or the materials that have rained 
down on Mars from disintegration of moons.

For more information about NASA missions investigating Mars, visit:

https://mars.nasa.gov/

News Media Contact
Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster at jpl.nasa.gov

Laurie Cantillo / Dwayne Brown
NASA Headquarters, Washington
202-358-1077 / 202-358-1726
laura.l.cantillo at nasa.gov / dwayne.c.brown at nasa.gov

Steve Tally / Emil Venere
Purdue University, West Lafayette, Ind.
765-494-9809 / 765-494-4709
steve at purdue.edu / venere at purdue.edu

Writer: Brian Wallheimer

2017-075