Looking like claw marks from some giant space bird, peculiar parallel tracks on Saturn鈥檚 moons Dione and Rhea have researchers baffled.
鈥淚 feel like I鈥檓 going crazy trying to come up with an explanation,鈥� says , a planetary scientist at the National Air and Space Museum in Washington D.C., who presented her team鈥檚 work at a meeting of the 2017 American Geophysical Union in New Orleans, Louisiana, on 11 December.
Previous observers have spotted a few of these streaks before but none had ever systematically charted them. While using data from NASA鈥檚 Cassini spacecraft to map fractured terrain on the two moons, Martin鈥檚 intern kept running into the lengthy features, which couldn鈥檛 be chalked up to known processes that alter the surfaces of moons.
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Their brightness and the fact that they lay atop other parts of the landscape suggested they were relatively young, perhaps not more than a billion years old. After cataloguing them, the team gave the skinny stripes a name: linear virgae, from the Latin word for streak or shaft.
Not cracks or boulder lines
The researchers first thought that the lines were an unknown type of fault or crack. But the virgae neither disrupt the underlying terrain nor cast a shadow, almost as if they were painted on rather than cut into the surface.
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A second suggestion came from rolling boulders, which have been seen producing long linear grooves on Mars鈥� moon Phobos, though these tend to be short, and are known to cast shadows.
Comets and asteroids raining down on the surface offer another explanation. The virgae bear striking resemblance to crater rays, which radiate from an impact in long straight lines.
But the parallel streaks fail to converge on any craters. On Dione, the majority appear to be in almost the exact same orientation, running east to west above and below the moon鈥檚 equator.
Ring rash?
That has Martin reaching for ever more exotic ideas, such as the possibility that the little moons once had ring systems that crashed to their surfaces. But collapsing ring particles should all land near the equator, whereas the streaks on Dione stretch far north and south of it.
On Rhea, whose surface is poorly mapped, the known linear virgae seem to cluster together in a single eastern region near the equator.
鈥淚鈥檓 not sure what to make of it,鈥� says planetary scientist at the Planetary Science Institute in Tucson, Arizona. Perhaps a strange combination of processes both external and internal to the moons could account for the streaks, she says.
Read more: Uranus might have two dark moons we鈥檝e never seen before
