Researchers were immediately suspicious when Mariner 10, after having filmed the dark side of the planet with its television cameras, plunged into the dark mercurial night before re-emerging on the other side. The television cameras were temporarily inactive but the other on-board instruments were working to perfection, and the main purpose of one of these was to measure the solar wind. The scientists did not expect to find traces behind the planet because the flux of particles striking the exposed parts is like being in front of a screen: behind the planet it created a kind of "shadow cone" that extended into space. Instead, Mariner's measurements revealed the presence of particles in places where they should not have been. This meant that the solar wind did not follow the simple trajectory estimated, but behaved as if a magnetic field forced it to follow a different path. But was it really there? Scientists had to wait for the third passage of the probe, at a distance of only 37 kilometers from the planet, to obtain the final confirmation: Mercury does have its own magnetic field, oriented exactly like that of the Earth, although one hundred times less intense. Now, the only thing left to do was to find out how it was generated. The Earth itself is a point of reference, since its magnetic field is produced by the circulation, within the interior of the planet, of massive flows of hot material: just like in an electromagnet, this flux is the source of enough electric current to generate and maintain the field. On one hand, the observations and the astronomical measurements of Mercury have indicated that the planet has a density equal to five and a half times that of water. On the other, Mariner 10 has demonstrated that the surface rocks are similar to those on the Moon or the Earth and these have a density that is only two and a half times that of the water. What does this mean? Let's make another comparison with the Earth. Our planet also has a global density that is higher than that of the surface rock and this is due to the fact that the interior materials are much heavier: the Earth's core is a huge ball of iron and nickel. This is the same situation on Mercury: an enormous "ball" of iron, with a diameter of about 3 600 kilometers (which is slightly larger than the Moon), is hidden under a thin crust of only 640 kilometers. However, the Earth's iron and nickel core is hot and materials are flowing inside at different temperatures: the hottest rise and the coldest sink in a continuous flux that changes our planet day after day. Mercury, instead, seems to be a completely "dead" body from a geological point of view. However, if it is not similar to the Earth, how does its core behave? No one knows for certain. Some think that its magnetism is a type of fossil that has been preserved for billions of years, from when the planet was still "young". Others think instead that this condition was produced by the intense and extensive "bombardment" by the solar wind. Or maybe, Mercury's magnetic field has an origin, that until now, no one has thought of, because we do not know enough about the Earth's history. What is certain is that it would be a real challenge for another probe, even more advanced than the Mariner 10, to travel to the planet and perhaps even land. Maybe we will discover that this small body near the Sun is not an inactive stone, but has a "heart" that is still very much alive!
Researchers were immediately suspicious when Mariner 10, after having filmed the dark side of the planet with its television cameras, plunged into the dark mercurial night before re-emerging on the other side. The television cameras were temporarily inactive but the other on-board instruments were working to perfection, and the main purpose of one of these was to measure the solar wind. The scientists did not expect to find traces behind the planet because the flux of particles striking the exposed parts is like being in front of a screen: behind the planet it created a kind of "shadow cone" that extended into space. Instead, Mariner's measurements revealed the presence of particles in places where they should not have been. This meant that the solar wind did not follow the simple trajectory estimated, but behaved as if a magnetic field forced it to follow a different path. But was it really there? Scientists had to wait for the third passage of the probe, at a distance of only 37 kilometers from the planet, to obtain the final confirmation: Mercury does have its own magnetic field, oriented exactly like that of the Earth, although one hundred times less intense. Now, the only thing left to do was to find out how it was generated. The Earth itself is a point of reference, since its magnetic field is produced by the circulation, within the interior of the planet, of massive flows of hot material: just like in an electromagnet, this flux is the source of enough electric current to generate and maintain the field. On one hand, the observations and the astronomical measurements of Mercury have indicated that the planet has a density equal to five and a half times that of water. On the other, Mariner 10 has demonstrated that the surface rocks are similar to those on the Moon or the Earth and these have a density that is only two and a half times that of the water. What does this mean? Let's make another comparison with the Earth. Our planet also has a global density that is higher than that of the surface rock and this is due to the fact that the interior materials are much heavier: the Earth's core is a huge ball of iron and nickel. This is the same situation on Mercury: an enormous "ball" of iron, with a diameter of about 3 600 kilometers (which is slightly larger than the Moon), is hidden under a thin crust of only 640 kilometers. However, the Earth's iron and nickel core is hot and materials are flowing inside at different temperatures: the hottest rise and the coldest sink in a continuous flux that changes our planet day after day. Mercury, instead, seems to be a completely "dead" body from a geological point of view. However, if it is not similar to the Earth, how does its core behave? No one knows for certain. Some think that its magnetism is a type of fossil that has been preserved for billions of years, from when the planet was still "young". Others think instead that this condition was produced by the intense and extensive "bombardment" by the solar wind. Or maybe, Mercury's magnetic field has an origin, that until now, no one has thought of, because we do not know enough about the Earth's history. What is certain is that it would be a real challenge for another probe, even more advanced than the Mariner 10, to travel to the planet and perhaps even land. Maybe we will discover that this small body near the Sun is not an inactive stone, but has a "heart" that is still very much alive!
