NASA's Messenger spacecraft orbited Mercury for four years, and in that time it collected a wealth of data and images as it mapped the planet's gravitational field. Now scientists have announced that Mercury's magnetic field is four million years old.

Mercury's core is massive, making up 85 percent of its radius. The planet's crust is thinner at the poles and thicker at low latitudes, which suggests that the planet's outer core is liquid. These facts leads the scientists to conclude that just below the planet's surface there is a layer of liquid iron sulphide.

Other factors of the planet's makeup provide the data for various interesting conclusions. Elevation changes on Mercury's surface are far less extreme than those on Mars or our moon. In a major volcanic plain in the northern latitudes of Mercury there is an observable uplift caused by the formation of the plain.

Messenger ultimately crashed into the surface of Mercury. Before the crash the spacecraft maintained a distance of 124 to 310 miles per hour. The high-speed crash-at about 8,700 miles per hour-allowed for careful observations at close ranges of less than 10 miles before the impact. The spacecraft detected magnetic signals on Mercury, and the crash itself revealed the magnetic field which is millions of years old.

The study's lead author, Catherine Johnson, is a planetary scientist at the University of Columbia. Johnson states that Mercury's magnetic signals are too low to be analyzed and detected under normal circumstances.

Johnson said: "If we didn't have these recent observations, we would never have known how Mercury's magnetic field evolved over time."

Johnson and her team also indicate that the magnetic field of Mercury is likely as strong as Earth's, albeit one that has decreased considerably. The source of the magnetic field is the core's liquid metal. Johnson also explained that Mercury's maintenance of its heat despite its small size also contributes to its magnetic field.