They also found that rather than being delivered by comets, the carbon most likely originated deep below the surface, in the form of a now-disrupted and buried ancient graphite-rich crust.
Some of this carbon was later brought to the surface by impact processes after most of Mercury's current crust had formed, said the team led by Patrick Peplowski of the Johns Hopkins University's applied physics laboratory.
The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation.
"We used the spacecraft Messenger's Neutron Spectrometer to spatially resolve the distribution of carbon and found that it is correlated with the darkest material on Mercury, and this material most likely originated deep in the crust," explained Larry Nittler, deputy principal investigator of the Messenger mission.
"Moreover, we used both neutrons and X-rays to confirm that the dark material is not enriched in iron, in contrast to the Moon where iron-rich minerals darken the surface," he added in a paper published in the journal Nature Geoscience.
Messenger obtained the robust data via many orbits on which the spacecraft passed lower than 100 kms above the surface of the planet during its last year of operation.
When Mercury was very young, much of the planet was likely so hot that there was a global "ocean" of molten magma.
From laboratory experiments and modelling, scientists have suggested that as this magma ocean cooled, most minerals that solidified would sink.
A notable exception is graphite, which would have been buoyant and floated to form the original crust of Mercury.
"The finding of abundant carbon on the surface suggests that we may be seeing remnants of Mercury's original ancient crust mixed into the volcanic rocks and impact ejecta that form the surface we see today," Nittler added.
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