Anomalous grooves on Martian moon Phobos explained by impacts

August 30, 2016
In this spacecraft image of Phobos, red arrows indicate a chain of small craters whose origin researchers were able to trace back to a primary impact at the large crater known as Grildrig. Credit: ESA/Mars Express, modified by Nayak & Asphaug

Some of the mysterious grooves on the surface of Mars' moon Phobos are the result of debris ejected by impacts eventually falling back onto the surface to form linear chains of craters, according to a new study.

One set of grooves on Phobos are thought to be stress fractures resulting from the tidal pull of Mars. The new study, published August 19 in Nature Communications, addresses another set of grooves that do not fit that explanation.

"These grooves cut across the tidal fields, so they require another mechanism. If we put the two together, we can explain most if not all of the grooves on Phobos," said first author Michael Nayak, a graduate student in Earth and planetary sciences at UC Santa Cruz.

Phobos is an unusual satellite, orbiting closer to its planet than any other moon in the solar system, with an orbital period of just 7 hours. Small and heavily cratered, with a lumpy nonspherical shape, it is only 9,000 kilometers from the of Mars (the distance from San Francisco to New York and back) and is slowly spiraling inward toward the planet. Phobos appears to have a weak interior structure covered by an elastic shell, allowing it to be deformed by tidal forces without breaking apart.

Coauthor Erik Asphaug, a planetary scientist at Arizona State University and professor emeritus at UC Santa Cruz, has been studying Phobos for many years. Recent computer simulations by him and NASA planetary scientist Terry Hurford showed how tidal stresses can cause fracturing and linear grooves in the surface layer. Although this idea was first proposed in the 1970s, the existence of so many grooves with the wrong orientation for such stress fractures had remained unexplained.

Nayak developed computer simulations showing how those anomalous grooves could result from impacts. Material ejected from the surface by an impact easily escapes the weak gravity of Phobos. But the debris remains in orbit around Mars, most of it moving either just slower or just faster than the orbital velocity of Phobos, and within a few orbits it gets recaptured and falls back onto the surface of the moon.

Nayak's simulations enabled him to track in precise detail the fate of the ejected debris. He found that recaptured debris creates distinctive linear impact patterns that match the characteristics of the anomalous grooves and chains of craters that cut across the tidal on Phobos.

"A lot of stuff gets kicked up, floats for a couple of orbits, and then gets recollected and falls back in a linear chain before it has a chance to be pulled apart and disassociated by Mars' gravity," Nayak said. "The controlling factor is where the impact occurs, and that determines where the debris falls back."

The researchers used their model to match a linear chain of small craters on Phobos to its primary source crater. They simulated an impact at the 2.6-kilometer crater called Grildrig, near the moon's north pole, and found that the pattern resulting from ejected debris falling back onto the surface in the model was a very close match to the actual crater chain observed on Phobos.

With its low mass and close orbit around Mars, Phobos is so unusual that it may be the only place in the solar system where this phenomenon occurs, Nayak said.

Explore further: Mars' moon Phobos is slowly falling apart

More information: Nature Communications, DOI: 10.1038/NCOMMS12591

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11 comments

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Solon
1.8 / 5 (10) Aug 30, 2016
Crater chains will be the eventual downfall of mechanical models of surface modification of such moons. Good riddance to bad rubbish.
Gigel
4 / 5 (12) Aug 30, 2016
A zillion asteroids out there will be the brontosaur thighbone that the EU theory will choke on.
cantdrive85
1.9 / 5 (13) Aug 30, 2016
What a pathetic attempt to explain that which can already be explained by electric discharge, without the required magic suggested above.
shavera
4.2 / 5 (10) Aug 30, 2016
No, I guess you only have to introduce a whole kind of new magic with even less plausible explanations of reality.
Phys1
4.3 / 5 (11) Aug 30, 2016
The EU zombies are here again.
matt_s
4.1 / 5 (9) Aug 30, 2016
Let's see... crater chains... something already directly observed as not an electric discharge. Yep. That's sure gonna do it.
Jonseer
not rated yet Aug 30, 2016
They need to build some nuke powered thrusters on the side of the moon opposite Mars and fire them up and force to moon to drop from orbit immediately to create a crater to mark its passing. /s
Mark Thomas
3 / 5 (5) Aug 31, 2016
"Mark's Groovy Theory" (hypothesis)

After a large, off-center impact formed Stickney Crater, Phobos received enough angular momentum (was spinning fast enough) that the secondary ejecta (mostly rocks ejected from Stickney Crater) created PARALLEL (not radial) grooves and chains of pits as those rocks rolled and bounced over and through the rotating surface of Phobos.

Key facts: 1. the escape velocity of Phobos is only ~25 mph, so Phobos would not have to spin very fast; 2. Stickney is a very large crater relative to Phobos, and the primary impactor could have imparted significant angular momentum; 3. statistically, an off-center impact is the most likely scenario; 4. this results in parallel grooves at all latitudes, as we see on Phobos, which is hard to achieve otherwise; and 5. there appear to be a few radial grooves that turned into parallel grooves as they were being formed traveling away from Stickney Crater, this suggests the acceleration Phobos was experiencing.
Maggnus
5 / 5 (7) Sep 02, 2016
"Mark's Groovy Theory" (hypothesis)

After a large, off-center impact formed Stickney Crater, Phobos received enough angular momentum (was spinning fast enough) that the secondary ejecta (mostly rocks ejected from Stickney Crater) created PARALLEL (not radial) grooves and chains of pits as those rocks rolled and bounced over and through the rotating surface of Phobos.
A better theory than one requiring magical electric bolts that come from nowhere and cannot be explained by physics or any other science.
Mark Thomas
2 / 5 (3) Sep 02, 2016
Thanks Maggnus (I think, but not much of victory beating magical electric bolts). The parallel grooves at wide-ranging latitudes is the key. Very difficult to form that unless Phobos itself is spinning.

While any hypothesis may be wrong, there is no doubt Stickney crater is very large compared to the size of Phobos. So the idea that some angular momentum would have been imparted to Phobos seems almost unavoidable. The only question is whether Phobos was left spinning fast enough to cause the parallel grooves. Somewhere in the galaxy this has to happen on occasion, and Phobos may be our closest example.
Maggnus
5 / 5 (5) Sep 02, 2016
While any hypothesis may be wrong, there is no doubt Stickney crater is very large compared to the size of Phobos. So the idea that some angular momentum would have been imparted to Phobos seems almost unavoidable. The only question is whether Phobos was left spinning fast enough to cause the parallel grooves. Somewhere in the galaxy this has to happen on occasion, and Phobos may be our closest example.

This is something you could work out quantitatively. Have you ever tried? You could probably get an idea of the impactor size from the crater it left, and the rest is just (Just lol) plugging in numbers.

I am not sure that an impactor of that size could impart angular momentum to a second object the size of Phobos, but it's thought a similar event is responsible for Venus's rotation. Give it a go!

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