Phobos Grooves from Rolling Boulders

Post by Kenneth R. Ramsley, Department of Earth, Environmental and Planetary Sciences, Brown University.

All but one region of Phobos, the largest moon of Mars, is covered by hundreds of valley-like features, usually described as grooves. Most grooves are ~80 to ~200 meters wide and are found in groups of generally parallel members, or families [see Image 1]. Impact craters typically produce slow-moving boulders, and on Phobos there would be little gravity to halt their motions. Did boulders rolling across the surface of Phobos produce the grooves? To answer this question, using a computer model to calculate the fate of rolling boulders, we compare their motions to the geomorphology of the grooves.

Image 1 - Viking Image

Image 1: Dominated by Stickney Crater, a feature nearly half the radius of the moon itself, Phobos is the larger of the two moons of Mars (average diameter, 22 kilometers). Mostly covered in valley-like features, planetary scientists have struggled for more than 40 years to explain the grooves of Phobos (Viking Project, JPL, NASA; Image mosaic by Edwin V. Bell II NSSDC/Raytheon ITSS).

Since their discovery in the late 1970s, numerous hypotheses have tried to explain the grooves. Explanations include fracturing from the comparatively large Stickney impact crater (nine kilometers in diameter). Other ideas point to the close proximity of Mars and suggest that gravitational forces are tearing the moon apart. Still other ideas imagine marks left over from a process that may have captured Phobos as a passing asteroid, or perhaps debris hurled from the surface of Mars 6,000 kilometers away. Including the idea of boulders rolling from Stickney Crater, after 40 years, every idea faces objections.

Video 1 Blog Display Image

Video 1: [] Our computer model is introduced, and in this test we see boulders radiating from Stickney Crater. After circling Phobos, boulders pass tangentially to the east of Stickney in a parallel pattern. Based on this test, we see why grooves in the vicinity of Stickney might not radiate from the crater, and by traveling far enough to encounter previously-emplaced grooves, this also explains how grooves might crosscut other grooves.

One objection to the idea of grooves from rolling boulders assumes that grooves would radiate from the source crater. To assess this and other objections, we model the orbital and rotational motions of Phobos and the gravity of Mars and Phobos. As seen in Video 1, parameters of motion and gravity alter the paths of rolling boulders. After traveling 360° around Phobos, boulders pass by Stickney in a pattern that does not radiate from the crater. In fact, most Stickney boulders travel more than half way around Phobos, and this accounts for a second objection: crosscutting grooves. In geology, we typically expect to see newer features placed atop older features – a.k.a., superposition. However, timescales need not be widely spaced, and in our testing model, we see Stickney boulders traveling dozens of kilometers around Phobos to where they cross the same regions from different directions at different times.

Video 2 Blog Display Image

Video 2: [] Taking flight like a ski-jumper, a test boulder skips past the ‘zone of avoidance’ – showing why this region of Phobos is devoid of grooves. Continuing to observe this test boulder, after traveling 360° around Phobos, the boulder passes through Stickney Crater, showing how Stickney could have produced grooves that subsequently appear inside Stickney.

One region of Phobos has no grooves. If boulders rolled across Phobos, why not here? As we see in Video 2, a test boulder rolls downhill from Stickney Crater, reaches the edge of a persistent reduction in topographical elevation (the groove-free region), and drifts into ballistic flight. Also shown in the video, continuing the theme of geological superposition, the same boulder travels across the floor of Stickney, thereby resolving another objection: the presence of grooves inside Stickney.

Where key objections are explained by our computer model, Stickney boulders appear to offer a plausible explanation for the grooves of Phobos.

Narrated Videos:

Equatorial Tour of Phobos Grooves

Production of Northern Groove Family –

Avoiding the Zone of Avoidance –

Grooves Crosscutting Stickney Crater –

Solitary Grooves Crosscutting Northern Grooves –

 Further Reading:

 Basilevsky, A.T., Lorenz, C.A., Shingareva, T.V., Head, J.W., Ramsley, K.R., Zubarev, A.E. (2014), The surface geology and geomorphology of Phobos. Planetary and Space Science, 102. 95–118.

Hamelin, M. (2011), Motion of blocks on the surface of Phobos: new constraints for the formation of grooves. Icarus, 59, 1293–1307.

Ramsley, K.R. and Head, J.W. (2017), The Stickney Crater ejecta secondary impact crater spike on Phobos: Implications for the age of Stickney and the surface of Phobos. Planetary and Space Science, 138, 7–24.

Ramsley, K.R. and Head, J.W. (2018), Origin of Phobos grooves: Testing the Stickney Crater ejecta model. Planetary and Space Science, [in press].

Wilson, L. and Head, J.W. (2015), Groove formation on Phobos: Testing the Stickney ejecta emplacement model for a subset of the groove population. Planetary and Space Science, 105, 26–42.

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