Mercury is the smallest planet of the Solar System and the nearest to the Sun. Like Venus, it follows a path around the Sun in the same orbit as Earth and is tidally locked in a 3:2 spin-orbit resonance, making it far more difficult to observe with a telescope. As a result, NASA launched its MESSENGER probe in 2004 to take a closer look at the surface.
However, it was revealed during Brian Cox’s new BBC programme “The Planets” how the space agency got more than they planned.
The physics professor said last month: “MESSENGER was able to do much more than just take images of Mercury’s surface.
“By tracking radio signals emitted by the spacecraft, we’re able to see very slight changes in the orbital path around Mercury as seen from Earth.
“That allows us to map out Mercury’s gravitational field.
“There are also instruments that allow us to see how the planet wobbles around as it spins on its axis.
“Putting all these measurements together allows us to take a cross section through the planet to see what it’s made of.
“When we do that, we find something very strange – Mercury’s core extends out about 85 percent from the centre of the planet to the surface – it’s as if the rocks of the surface were smashed away.”
However, the find was far from the most remarkable.
Dr Cox also revealed how they made a seemingly “impossible” discovery on the surface.
He added: “There’s more – the tiny probe began detecting chemical elements in concentrations that no one thought possible this close to the Sun.
“The discovery of relatively large concentrations of elements like sulphur and potassium on Mercury’s surface was a huge surprise.
“If you think back to the time when the planets were forming, you don’t expect high concentrations of those elements close to the Sun where Mercury orbits today, because they’re so-called volatile elements and boil away easily.
“So you only find high concentrations further out in the colder reaches of the Solar System.
“So Mercury is an enigma and discoveries like these force us to completely rethink our theories about the formation of the planet.”
During the same show, Dr Cox also revealed how NASA uncovered “flowing rivers” on the surface of Venus.
He said: “At the time when life was just about beginning on the Earth, three-and-a-half to four billion years ago, the Sun was fainter and that means that Venus was cooler.
“In fact, temperatures on Venus at that time would have been like a pleasant spring day here on Earth.
“Within a few million years of its formation, the surface of Venus had cooled and the planet found itself at just the right distance from the faint young Sun for Venus to experience a sight familiar to us here on Earth.
“The heavens opened and great torrents flooded the surface, rivers of water flowed and Venus became an ocean world.
“The planet’s atmosphere allowed it to hold on to the oceans by acting as a blanket, keeping the surface temperate thanks to the greenhouse effect.”