Giant gullies found on the asteroid Vesta look as if they were created by liquid water, Nasa scientists have said.
The evidence has left researchers baffled, and has been described as "unexpected" and "exciting".
Vesta is one of the largest asteroids in the Solar System, and is about 326 miles across with the same Surface area three times that of the United Kingdom.
It alone comprises about 9% of the asteroid belt's mass and is the brightest asteroid visible from Earth.
The Dawn spacecraft reached Vesta in July 2011 and left its orbit in September 2012.
The gullies in question have been found on the walls of geologically young craters on the asteroid.
Nasa said: "Some that look like straight chutes and others that carve more sinuous trails and end in lobe-shaped deposits."
"The mystery, however, is what is creating them?"
Above: This image shows examples of long, narrow, sinuous gullies that scientists on NASA's Dawn mission have found on the giant asteroid Vesta.
Presenting the evidence at the American Geophysical Union conference in San Francisco, Nasa said while the straight gullies on the asteroid are "textbook examples' of dry material like Sand, the sinuous features are different.
They are longer, narrower and have more curves than the dry kind, and look very similar to those created by liquid water on Earth. They have also been found on Mars, though what made them is still debated.
While it sounds unlikely that water would exist on an asteroid, it has been found before. The asteroid 24 Themis was confirmed to have ice on its surface in 2010, while a 2005 report claimed the largest asteroid in the Solar System (Ceres) might contain "more fresh water than Earth" at its core.