Photo: Grooves on Mars' moon Phobos are likely caused by tidal forces – the mutual gravitational pull of the planet and the moon. But some of Phobos' crater chains don't line up. Credit: NASA/JPL-Caltech/University of Arizona Scientists have resolved a long-standing mystery of oddly aligned chains of craters on the Martian moon Phobos.
A new study shows that the peculiar chains of craters likely were created by material that had been boosted into space by a previous impact. After some time in orbit, the material crashed back into Phobos, forming chains of craters that are not aligned with surface features caused by the moon's deadly gravitational embrace with Mars.
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Phobos, a small, potato-shaped moon of Mars, is covered with bulges and parallel linear features that are mostly aligned with tidal stresses as Phobos spirals closer to Mars, planetary scientists Michael Nayak and Erik Asphaug, with the University of California at Santa Cruz, write in a paper published in this week's Nature Communications.
"As the tidal bulge grows, surface stresses increase and cause striations. However, many of Phobos' linear features do not align with any interpretation of tidal stress, giving rise to alternative models," the scientists said.
They modeled the flight paths of material ejected into orbit by a primary impact strike and found that it left a chain of craters on Phobos as the process repeated.
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The model was a match to one chain of craters on Phobos that had been unexplained by previous models.
Nayak and Asphaug conclude that these types of impacts likely created some of Phobos' crater chains, but that the massive tidal forces acting on Phobos also play a large role in shaping its surface features.
The scientists also noted that the lack of a crater chain near Stickney, Phobos' largest impact basin, suggests it formed when the moon was farther away from Mars.
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