Mercury has no natural satellites and consequently it is not an easy task to determine the planet's mass. By carefully recording the acceleration of the Mariner 10 space probe during its close encounters with planet, however, NASA scientists were able to determine a Mercurian mass equivalent to 1/6,023,600 that of the Sun. This mass, of about 3.3x1023 kg, is some 6% that of the Earth's mass.
Some idea of Mercury's internal structure can be gained from the knowledge of its mass and radius. These two terms indicate that Mercury has a bulk density of 5,430 g/cm3. This density is only slightly smaller than that of Earth, suggesting by analogy that Mercury has a large nickel-iron alloy core, and a thin rocky mantle. The nickel-iron core probably accounts for about 40% Mercury's volume.
Mercury's relatively large nickel-iron core and thin crustal mantle suggests that the planet may have undergone a catastrophic collision during its final stages of formation. A glancing blow from a large planetesimal may have caused most of the planet's initial mantle to be ejected into space, leaving behind a planet with a relatively large core.
Instruments carried on-board Mariner 10 detected a weak Mercurian magnetic field. The magnetic field strength is about 1% that of Earth's. Even though Mercury's magnetic field is very weak, it was a surprise to scientists that it displayed one at all. It is presently believed that planetary magnetic fields are created by the so-called dynamo effect. It is thought that the dynamo effect should operate in those planets that have hot, electrically conducting, liquid inner cores, and that rotate reasonably quickly. Mercury is not thought to satisfy any of the conditions necessary for a planetary dynamo to operate, and consequently the observed magnetic field suggests that the standard picture of Mercury's internal structure needs revising, or that another, presently unknown, mechanism exists through which planets can generate magnetic fields.
Mercury's weak magnetic field is strong enough to force charged particles in the solar wind to flow around the planet. The cavity that consequently exists about the planet is called a magnetosphere, and its existence prevents solar wind material (mainly protons and electrons) impacting directly on the planet's surface.