As NASA's Messenger spacecraft spiralled to its doom on the surface of Mercury, it sent back a final cache of data which may have solved the mystery of the planet's dark hue.
Mercury is one of the darkest major objects in the solar system with a carbon-covered surface - but how the carbon got there in the first place has puzzled astronomers.
After a decade in space and rapidly running out of fuel, Messenger carried out some final tests in 2014 before plummeting into Mercury's surface.
The data collected has now led scientists to believe that Mercury's dark crust was once molten magma that was buried by volcanoes and other impacts.
Then, over time, the dark carbon was exposed as asteroids repeatedly struck the planet, creating Mercury's graphite skin.
The data was gathered using a neutron spectrometer and X-ray devices, before being sent back to Earth.
Then it was analysed by researchers from the Johns Hopkins University's applied physics laboratory, and the Carnegie Institution for Science.
The findings overturn the previously favoured theory that the carbon was delivered by passing comets.
Mercury is so dark that only 12% of the sunlight falling on the planet is reflected - the reason for which was a mystery until now.
Our moon has a similar dark hue, but that is caused by an abundance or iron-rich minerals.
Messenger was launched in 2004 and took seven years to reach Mercury, at which point it settled into orbit.
During that time it mapped the planet's surface, measured its magnetic field, and discovered water ice at its north pole.
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