Phobos

On The Origin Of Phobos’ Groovy Mystery

Article Updated: 26 Sep , 2016
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Mars’ natural satellites – Phobos and Deimos – have been a mystery since they were first discovered. While it is widely believed that they are former asteroids that were captured by Mars’ gravity, this remains unproven. And while some of Phobos’ surface features are known to be the result of Mars’ gravity, the origin of its linear grooves and crater chains (catenae) have remained unknown.

But thanks to a new study by Erik Asphaug of Arizona State University and Michael Nayak from the University of California, we may be closer to understanding how Phobos’ got its “groovy” surface. In short, they believe that reaccretion is the answer, where all the material that was ejected when meteors impacted the moon eventually returned to strike the surface again.

Naturally, Phobos’ mysteries extend beyond its origin and surface features. For instance, despite being much more massive than its counterpart Deimos, it orbits Mars at a much closer distance (9,300 km compared to over 23,000 km). It’s density measurements have also indicated that the moon is not composed of solid rock, and it is known to be significantly porous.

(a) Spacecraft image of Phobos (photo credit: ESA/Mars Express) showing the observed catena of interest (red arrows); (b) reimpact map for a primary impact at Grildrig, azimuth ?? [0: ) rendered in three dimensions. Relative sizes and orientations between a and b are similar and may be correlated from Drunlo, Clustril, Grildrig, Gulliver and Roche craters, respectively. From the correlation, the highlighted catena likely originates from sesquinary ejecta from Grildrig.

Image of Phobos showing the observed catena of interest (left) and reimpact map for a primary impact at Grildrig (right). Credit: ESA/Mars Express

Because of this proximity, it is subject to a great deal of tidal forces exerted by Mars. This causes its interior, a large portion of which is believed to consist of ice, to flex and stretch. This action, it has been theorized, is what is responsible for the stress fields that have been observed on the moon’s surface.

However, this action cannot account for another common feature on Phobos, which are the striation patterns (aka. grooves) that run perpendicular to the stress fields. These patterns are essentially chains of craters that typically measure 2o km (12 mi) in length, 100 – 200 meters (330 – 660 ft) in width, and usually 30 m (98 ft) in depth.

In the past, it was assumed that these craters were the result of the same impact that created Stickney, the largest impact crater on Phobos. However, analysis from the Mars Express mission revealed that the grooves are not related to Stickney. Instead, they are centered on Phobos’ leading edge and fade away the closer one gets to its trailing edge.

For the sake of their study, which was recently published in Nature Communications, Asphaug and Nayak used computer modelling to simulate how other meteoric impacts could have created these crater patterns, which they theorized were formed when the resulting ejecta circled back and impacted the surface in other locations.

Credit: ESA/DLR/FU Berlin-Neukum

Image showing the Stickney crater (left) and how ejecta from an impact can form patterns (right) and crater chains (catenae). Credit: ESA/DLR/FU Berlin-Neukum

As Dr. Asphaug told Universe Today via email, their work was the result of a meeting of minds that spawned an interesting theory:

“Dr. Nayak had been studying with Prof. Francis Nimmo (of UCSC), the idea that ejecta could swap between the Martian moons. So Mikey and I met up to talk about that, and the possibility that Phobos could sweep up its own ejecta. Originally I had been thinking that seismic events (triggered by impacts) might cause Phobos to shed material tidally, since it’s inside the Roche limit, and that this material would thin out into rings that would be reaccreted by Phobos. That still might happen, but for the prominent catenae the answer turned out to be much simpler (after a lot of painstaking computations) – that crater ejecta is faster than Phobos’ escape velocity, but much slower than Mars orbital velocity, and much of it gets swept up after several co-orbits about Mars, forming these patterns.”

Basically, they theorized that if a meteorite stuck Phobos in just the right place, the resulting debris could have been thrown off into space and swept up later as Phobos swung back around mars. Thought Phobos does not have sufficient gravity to reaccrete ejecta on its own, Mars’ gravitational pull ensures that anything thrown off by the moon will be pulled into orbit around it.

Once this debris is pulled into orbit around Mars, it will circle the planet a few times until it eventually falls into Phobos’ orbital path. When that happens, Phobos will collide with it, triggering another impact that throws off more ejecta, thus causing the whole process to repeat itself.

The streaked and stained surface of Phobos. (Image: NASA)

The streaked and stained surface of Phobos, with the Stickney crater shown in the center. Credit: NASA/JPL/Mars Express

In the end, Asphaug and Nayak concluded that if an impact hit Phobos at a certain point, the subsequent collisions with the resulting debris would form a chain of craters in discernible patterns – possibly within days. Testing this theory required some computer modelling on an actual crater.

Using Grildrig (a 2.6 km crater near Phobos’ north pole) as an reference point, their model showed that the resulting string of craters was consistent with the chains that have been observed on Phobos’ surface. And while this remains a theory, this initial confirmation does provide a basis for further testing.

“The initial main test of the theory is that the patterns match up, ejecta from Grildrig for example.” said Asphaug. “But it’s still a theory. It has some testable implications that we’re now working on.”

In addition to offering a plausible explanation of Phobos’ surface features, their study is also significant in that it is the first time that sesquinary craters (i.e. craters caused by ejecta that went into orbit around the central planet) were traced back to their primary impacts.

The many faces of Mars inner moon, Phobos (Credit: NASA)

Mosaic of space images showing the many “faces” of Mars inner moon, Phobos. Credit: NASA

In the future, this kind of process could prove to be a novel way to assess the surface characteristics of planets and other bodies – such as the heavily cratered moons of Jupiter and Saturn. These findings will also help us to learn more about Phobos history, which in turn will help shed light on the history of Mars.

“[It] expands our ability to make cross-cutting relationships on Phobos that will reveal the sequence of geologic history,” Asphaug added. “Since Phobos’ geologic history is slaved to the tidal dissipation of Mars, in learning the timescale of Phobos geology we learn about the interior structure of Mars”

And all of this information is likely to come in handy when it comes time for NASA to mount crewed missions to the Red Planet. One of the key steps in the proposed “Journey to Mars” is a mission to Phobos, where the crew, a Mars habitat, and the mission’s vehicles will all be deployed in advance of a mission to the Martian surface.

Learning more about the interior structure of Mars is a goal shared by many of NASA’s future missions to the planet, which includes NASA’s InSight Lander (schedules for launch in 2018). Shedding light on Mars geology is expected to go a long way towards explaining how the planet lost its magnetosphere, and hence its atmosphere and surface water, billions of years ago.

Further Reading: Nature Communications

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8 Responses

  1. clandau says:

    I think the grooves are Trough Cross Beds from sedimentary layers of a piece of Mars as these grooves go straight through both young and old craters. The catenae or chains of craters would be due to tidal heating not chain impacts. These mini calderas are in chains as in a fault zone.
    I really am tired of seeing every scientist see every hole in the ground as an impact. If I live long enough, I will soon be told that Yellowstone Caldera is an impact crater. Maybe a great many of these mini craters are like mini Vesicular lava bubble craters from internal heating, not impacts. In any case this is probably not lava or basalt, but a sedimentary Trough Cross Bedded chunk of Mars formed from wind or water deposits. I think astronomers remain fixated on the 1994 chain of meteor impacts they saw happen on Jupiter and see catenae only formed in this manner. There is another means of creating long chains via stresses caused by dykes and faults and in this case by tidal heating.

    Chris Landau (geologist)

  2. Aqua4U says:

    I’m going to stick with an impact creating antipodal shock waves as the root cause. If you’ll recall, we’ve seen similarly appearing ‘grooves’ at Vesta.

    • danR2 says:

      “These patterns are essentially chains of craters that typically measure 20 km (12 mi) in length, 100 – 200 meters (330 – 660 ft) in width, and usually 30 m (98 ft) in depth.” [emph added]
      A parsimonious theory needs to explain all the salient phenomena with the fewest moving parts.

  3. clandau says:

    Look again. I disagree completely with that hypothesis. The simplest solution is always best and as far as I am concerned the accretion theory is a huge stretch.(and not in a good way). Look how the layers run unalteted through new and old craters. That in itself tells you the accretion theory does not make sense.
    Look how the layers curve up in classic trough cross bed sets.
    I stand by my hypothesis. I am a field geologist. What geology did these people who came up with this theory ever do?
    Chris Landau
    September 1, 2016

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