Mercury May Once Have Been As Large As Earth, Says Scientist

As the closest planet to the Sun, Mercury is a celestial marvel with a bounty of unique attributes waiting to be explored. The planet experiences extreme temperature fluctuations due to its proximity to the Sun. Daytime temperatures can soar up to a blistering 800 degree Fahrenheit (427 degrees Celsius), while night-time temperatures plummet to a frigid -290 degrees Fahrenheit (-180 degrees Celsius). Orbiting the Sun at a breakneck pace, Mercury completes a full orbit in just 88 Earth days, making it the shortest year of any planet in our solar system. But studying the planet is very difficult due to its position and the impact of the Sun's gravity.

This is why scientists have resorted to finding clues on Earth that could help them understand the composition of Mercury. One such scientists is Nicola Mari, a planetary geologist at the University of Pavia in Italy. He studies the ways that our neighbours in the solar system formed and evolved.

And during a recent visit to Cyprus, Mr Mari made an astonishing discovery - that Mercury was once as big as the Earth.

Why Cyprus?

In Cyprus, the geologist was looking for "boninite", a rock that is thought to bear an uncanny similarity to the rocks found on Mercury - a supposition which, if right, could be a clue to the planet's unique origins.

Cyprus was formed under the Tethys Ocean more than 90 million years ago and was thrust towards the surface by colliding tectonic plates. "In certain areas of Cyprus's mountains, it's like you are still walking over an ancient ocean bed," Mr Mari told the BBC.

He was astonished to find the similarity between the composition of rocks found on Mercury and boninites. "They weren't just similar; they were identical," said Mr Mari. The mix of elements such as magnesium, aluminium and iron was the same as that seen on the mysterious planet with the huge core. The only difference was that the rocks from Cyprus had been oxidised - which is inevitable given Earth's oxygen-rich atmosphere.

Mr Mari said further study of these rocks might help reveal some clues about the geological activity in Mercury's past.

The theories around origin of Mercury

Mercury is a planet of extremes. Only two space missions have been able to study our neighbour - Mariner 10 and Messenger. They flew close enough to map the surface of Mercury and revealed some major surprises about its structure.

They revealed that Mercury's core was unexpectedly big and the crust, surprisingly thin. Further, spectrometry of Mercury's surface revealed that Mercury has a much high concentration of thorium than its nearest neighbours.

Thorium should have evaporated in the extreme heat of the early Solar System. Instead, its thorium content is closer to that of Mars - three planets away - which would have formed at cooler temperatures due to its distance from the Sun.

The data led scientists to believe that Mercury was formed near to Mars - and that it started out with a much bigger mass, around the size of the Earth, that would befit its large core. They hypothesised that during its early years, Mercury collided with another planetary body that sent it spinning towards the Sun giving it the current structure.