Looking more closely at the spin and gravity of Mercury, we discover the inner solid core of the planet

How to explore the interior of a planet without touching it? Start by observing the rotation of the planet, then measure how your spaceship bypasses it – very, very carefully. That's exactly what NASA's planetary scientists have done, using data from the agency's former mission for Mercury.

It has long been known that Mercury and Earth have metal nuclei. Like the Earth, Mercury's outer core is made of liquid metal, but there have been only hints that the innermost core of Mercury is solid. In a new study, scientists at NASA's Goddard Space Flight Center in the Greenbelt, Maryland, found evidence that Mercury's inner core is actually solid and almost the same size as the inner core of the Earth.

Some scientists compare Mercury to a cannonball because its metal core occupies nearly 85% of the planet's volume. This large nucleus – huge compared to other rocky planets in our solar system – has long been one of the most intriguing mysteries of Mercury. Scientists also wondered if mercury could have a strong inner core.

The discoveries of Mercury's solid inner core, described in Geophysical Research Letters, certainly contributes to a better understanding of Mercury, but its ramifications are wider. The similarity and difference of the hearts of the planets can give us clues about the formation of the solar system and the evolution of rocky planets over time.

"The interior of Mercury is still active, because of the molten nucleus that powers the weak magnetic field of the planet, compared to that of the Earth," said Antonio Genova, an assistant professor at the University. Sapienza of Rome, who led the research at Goddard of NASA. "The interior of Mercury has cooled faster than that of our planet, Mercury could help us predict how the earth's magnetic field will change as the nucleus cools."

To understand what is the core of Mercury, Genova and his colleagues had to get closer figuratively. The team used several observations from the MESSENGER mission (Mercury Surface, Spatial Environment, GEochemistry and Telemetry) to probe the interior of mercury. The researchers mainly examined the rotation and gravity of the planet.

The MESSENGER Space Shuttle entered orbit around Mercury in March 2011 and spent four years observing the planet closest to our Sun until it was deliberately brought to the surface of the planet. April 2015.

MESSENGER's radio observations were used to determine gravitational anomalies (zones of local increase or decrease in mass) and the location of its rotation pole, which allowed scientists to understand the orientation of the planet.

Each planet turns on an axis, also called pole. Mercury runs much slower than Earth, and its day lasts about 58 days. Scientists often use very small variations in the way an object rotates to reveal clues about its internal structure. In 2007, radar observations from the Earth revealed slight changes in Mercury's rotation, called librations, which proved that the core of Mercury must be made of molten metal. But observations of the rotational speed alone were not sufficient to give a clear measure of the nature of the inner core. Could there be a solid core hidden beneath, wondered scientists?

Gravity can help answer this question. "Gravity is a powerful tool for looking deep inside a planet as it depends on the density structure of the planet," said Sander Goossens, a Goddard researcher who collaborated on this study with Genova.

While MESSENGER gravitated around Mercury during his mission and was getting closer and closer to the surface, scientists recorded the acceleration of the spacecraft under the influence of the gravity of the planet. The density structure of a planet can create subtle changes in the orbit of a spacecraft. In the later parts of the mission, MESSENGER flew over 120 miles above the surface and less than 65 miles during his last year. The low-level final orbits provided the best data and allowed Genoa and his team to take the most accurate measurements possible about the internal structure of Mercury.

Genova and his team put the MESSENGER data into a sophisticated computer program allowing them to adjust the parameters and determine the Mercury interior composition to match the rotation and acceleration of the spacecraft. The results showed that for the best match, Mercury must have a large and solid core. They estimated that the solid iron core is about 2,000 kilometers wide and about half of Mercury's total core (about 2,440 kilometers, or nearly 4,000 kilometers wide). In contrast, the Earth's solid core is about 1,400 miles (2,400 kilometers), occupying a little over a third of the entire core of the planet.

"We had to gather information in many areas: geodesy, geochemistry, orbital mechanics and gravity to determine the internal structure of Mercury," said Erwan Mazarico, planetary scientist at Goddard, who also helped Genova reveal the core Mercury solid.

The fact that scientists need to get closer to Mercury to learn more about its interior underscores the power of sending spaceships into other worlds. Such accurate measurements of Mercury's spin and gravity were simply not possible from the Earth. In addition, this result has used the data collected by MESSENGER over several years, information that all scientists can use. It is virtually guaranteed that new discoveries on Mercury are waiting in the MESSENGER archive, every discovery on our local planetary neighborhood allowing us to better understand what lies beyond.

"Every new information about our solar system helps us understand the larger universe," said Genova.