Planetologists have long known that the Earth and Mercury have metal cores. The core of Mercury occupies approximately 85% of the total volume of the planet. The planet's outer core is made of liquid metal, but it has now become clear that the inner core is solid.
A team of scientists from the Goddard Space Flight Center, Massachusetts Institute of Technology, Sapienza University, Case University Western Reserve Region and Columbia University have found evidence that the inner core of Mercury is solid and almost the same size as the inner core of the Earth. The research article was published in Geophysical Research Letters.
"The interior of Mercury is still active thanks to the molten core feeding the planet's weak magnetic field compared to Earth's," says Dr. Antonio Genova of Sapienza University. - The interior of Mercury cooled faster than our planet. Mercury can help predict how Earth's magnetic field will change as the core cools."
To figure out what the core of Mercury is made of, Dr. Jenova and his colleagues had to, figuratively speaking, take a closer look at the planet. They used radio observations from the MESSENGER mission to determine gravitational anomalies (areas of local increase or decrease in mass) and the position of the axis of rotation of the first planet in the solar system, which allowed them to understand its orientation.
Graphical representation of the internal structure of Mercury / © NASA's Goddard Space Flight Center
Each planet rotates on an axis, also known as a pole. Mercury rotates much slower than Earth: a day on the planet lasts 58 Earths. Scientists often use small variations in the rotation of an object to understand what its internal structure is.
In 2007, radar observations from Earth showed that there are small changes in the rotation of Mercury - librations - that helped to determine that at least part of the planet's core is liquid. However, observations of the rotational speed alone were not enough to carry out accurate studies of the composition of the inner core. Gravity helped answer the question of whether a liquid core could be hiding there.
"Gravity is a powerful tool for exploring the interior of a planet, as it depends on the density of its structure," says Dr. Sander Goossens of the Goddard Space Flight Center.
The scientists fed the MESSENGER data into a computer program that allowed them to tweak the parameters and figure out what the internal composition of Mercury should be in order to match its rotation and the gravitational maneuvers of spacecraft.
The results showed that Mercury has a large, solid inner core that best fits these data. Scientists have calculated that the solid iron core is about two thousand kilometers across and makes up about half of the entire core of the planet (about four thousand kilometers across). By comparison, the Earth's solid core is roughly 2,400 kilometers across.
“We had to combine information from different fields - geodesy, geochemistry, orbital mechanics and gravity - to figure out what the internal structure of Mercury is,” said Dr. Erwan Mazariko of the Goddard Space Flight Center.