Washington: Mercury and other unusually metal-rich objects in the solar system may be relics left behind by hit-and-run collisions in the early solar system, scientists say.

The origin of planet Mercury has been a difficult question in planetary science because its composition is very different from that of the other terrestrial planets and the moon.

This small, innermost planet has more than twice the fraction of metallic iron of any other terrestrial planet. Its iron core makes up about 65 per cent of Mercury's total mass; Earth's core, by comparison, is just 32 per cent of its mass.

To explain the mystery of Mercury's metal-rich composition, Arizona State University professor Erik Asphaug and Andreas Reufer of the University of Bern have developed a new hypothesis involving hit-and-run collisions, where proto-Mercury loses half its mantle in a grazing blow into a larger planet (proto-Venus or proto-Earth).

One or more hit-and-run collisions could have potentially stripped away proto-Mercury's mantle without an intense shock, leaving behind a mostly-iron body and satisfying a number of the major puzzles of planetary formation - including the retention of volatiles - in a process that can also explain the absence of shock features in many of the mantle-stripped meteorites.

Asphaug and Reufer have developed a statistical scenario for how planets merge and grow based on the common notion that Mars and Mercury are the last two relics of an original population of maybe 20 bodies that mostly accreted to form Venus and Earth. These last two planets lucked out.

"How did they luck out? Mars, by missing out on most of the action ? not colliding into any larger body since its formation ? and Mercury, by hitting the larger planets in a glancing blow each time, failing to accrete," said Asphaug.

By and large, dynamical modellers have rejected the notion that hit-and-run survivors can be important because they will eventually be accreted by the same larger body they originally ran into.

Their argument is that it is very unlikely for a hit-and-run relic to survive this final accretion onto the target body.

"The surprising result we have shown is that hit-and-run relics not only can exist in rare cases, but that survivors of repeated hit-and-run incidents can dominate the surviving population," said Asphaug.

"That is, the average unaccreted body will have been subject to more than one hit-and-run collision. We propose one or two of these hit-and-run collisions can explain Mercury's massive metallic core and very thin rocky mantle," said Asphaug.

"The implication of the dynamical scenario explains, at long last, where the 'missing mantle' of Mercury is ? it's on Venus or the Earth, the hit-and-run targets that won the sweep-up," said Asphaug.

The study was published in the journal Nature Geoscience.---- PTI