NASA’s Dawn spacecraft departed the asteroid Vesta three months ago, leaving behind an intriguing mystery: How did gullies form in some of its craters’ walls?

Pictures taken by Dawn show two types of narrow channels on Vesta. The

straight channels likely formed by falling dry material, similar to sand — a phenomenon also seen on the moon.

ANALYSIS: Vesta Peppered with Carbon from ‘Dark’ Asteroids

But the other channels curve and end in lobe-shaped deposits. On Earth,

similar features, seen in places such as Meteor Crater in Arizona, have been carved by flowing water.

Curving gullies also have been found on Mars, though scientists are still debating how those formed.

The discovery of sinuous gullies on Vesta was a surprise, say scientists who unveiled their find at the American Geophysical Union conference in San Francisco this week.

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“We need to analyze the Vesta gullies very carefully before definitively specifying their source,” Dawn lead scientist Christopher Russell, with the University of California, Los Angeles, said in a statement.

“The sinuous gullies are longer, narrower, and curvier than the short, wide, straight gullies. They tend to start from V-shaped, collapsed regions described as ‘alcoves’ and merge with other gullies,” NASA wrote in a

report posted on its website.

“Scientists think different processes formed the two types of gullies and have been looking at images of Earth, Mars and other small bodies for clues,” NASA said.

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Dawn is now en route to the largest object in the asteroid belt, the dwarf planet Ceres.

Image: Dawn observation of the long, narrow, sinuous gullies that scientists have found in the crater walls on the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA