Scientists say they have uncovered firm evidence that Mars's biggest moon, Phobos, is made from rocks blasted off the Martian surface in a catastrophic event.
The origin of Mars's satellites Phobos and Deimos is a long-standing puzzle.
It has been suggested that both moons could be asteroids that formed in the main asteroid belt and were then "captured" by Mars's gravity.
The latest evidence has been presented at a major conference in Rome.
The new work supports other scenarios. Material blasted off Mars's surface by a colliding space rock could have clumped together to form the Phobos moon.
Alternatively, Phobos could have been formed from the remnants of an earlier moon destroyed by Mars's gravitational forces. However, this moon might itself have originated from material thrown into orbit from the Martian surface.
Previous observations of Phobos at visible and near-infrared wavelengths have been interpreted to suggest the possible presence of carbonaceous chondrites, found in meteorites that have crashed to Earth.
This carbon-rich, rocky material, left over from the formation of the Solar System, is thought to originate in asteroids from the so-called "main belt" between Mars and Jupiter.
But, now, data from the European Space Agency's Mars Express spacecraft appear to make the asteroid capture scenario look less likely.
The researchers also used Mars Express to obtain the most precise measurement yet of Phobos' density.
"This number is significantly lower than the density of meteoritic material associated with asteroids. It implies a sponge-like structure with voids making up 25%-45% in Phobos's interior," said Dr Rosenblatt.
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