NASA’s Dawn mission has returned images from the giant asteroid Vesta that scientists say include long, narrow gullies along the walls of relatively young craters.

The discovery has revealed a mystery that scientists are now trying to solve. The images from Dawn show two different types of gullies. Some are straight chutes, while others wind about and end in what NASA is calling “lobe-shaped” deposits.

“The straight gullies we see on Vesta are textbook examples of flows of dry material, like sand, that we’ve seen on Earth’s moon and we expected to see on Vesta,” said Jennifer Scully, a Dawn team member at the University of California, Los Angeles. “But these sinuous gullies are an exciting, unexpected find that we are still trying to understand.”

The “sinuous” gullies are longer, narrower, and curvier than the others. The also begin from V-shaped, collapsed regions researchers have described as “alcoves.” The current hypothesis is that two different processes formed the different gullies.

“On Earth, similar features – seen at places like Meteor Crater in Arizona – are carved by liquid water,” said Christopher Russell, Dawn’s principal investigator at UCLA. “On Mars, there is still a debate about what has caused them. We need to analyze the Vesta gullies very carefully before definitively specifying their source.”

Researchers are looking to Earth and Mars for clues about Vesta’s gullies, but mystery still remains. Scientists have been debating the explanation for Mars’ gullies for years. Some possible causes include liquid water and carbon-dioxide frost that causes fresh flows of Martian sand.

(Image courtesy NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)