The massive impact, 4.5 billion years ago, threw out so much material that Earth was splattered with the debris.
Mercury, 3,032 miles wide, is the closest planet to the sun and is visible for only a few days each year as an evening or morning star.
A UK scientist working in Switzerland used computers to simulate the impact that created the planet, the National Astronomy Meeting in Leicester were told.
Dr Jonti Horner, of the University of Bern, said: “Mercury is an unusually dense planet which suggests that it contains far more metal than would be expected for a planet of its size.
“We think it was created from a larger parent body that was involved in a catastrophic collision.”
A Mariner 10 image of Mercury, by Nasa, as it appears today is shown above.
© Paul Sutherland. Unauthorised reproduction forbidden.