The innermost planet in the Solar System, Mercury, is unique in many ways. It is surprisingly dense, reflecting a massive metallic core that has 60% of the mass of the planet (versus less than 31% for Earth, Venus, and Mars). It has a highly eccentric orbit, approaching as close as 0.3236 AU to the Sun and retreating as far as 0.4506 AU on each orbit, covering the entire range in half an orbital period, 44 Earth days. The intensity of sunlight striking Mercury’s orbit drops off with the square of the distance from the Sun, so it varies by nearly a factor of two.
Even odder, the rotation of Mercury is locked onto its orbital period. Most examples of spin-orbit resonances in the Solar System are 1:1 (one rotation per orbit), which means that the smaller body always keeps the same side toward the larger body. Examples include Earth’s Moon and the large Galilean satellites of Jupiter. The most extreme example of a strong lock between rotation and orbit is Pluto and its satellite Charon: they are locked into a 1:1:1 spin-spin-orbit state, in which both Charon and Pluto always keep exactly the face toward each other. Mercury is noteworthy in that it has a 3:2 spin-orbit resonance: Mercury rotates exactly 3 times every 2 Mercury years, or 1 ½ times per year. Thus in consecutive closest approaches to the Sun (perihelion), opposite sides of the planet get baked. This situation is understandable if Mercury is elongated along one of its equatorial axes: the planet then points the ends of this long axis (the so-called tidal bulges) at the Sun alternatingly at each perihelion passage.
Mercury shows no trace having ever had a significant atmosphere or oceans. The faint wisp of gases surrounding Mercury today is in part due to solar wind gases from the Sun temporarily captured by Mercury’s gravity, and in part to atoms, such as sodium, baked out of the surface by the extreme heat and the impact of high-speed solar wind ions. Gases released from Mercury’s interior, such as argon-40 from the radioactive decay of potassium-40 in the crust and mantle, can easily be ionized by ultraviolet radiation from the Sun, entrapped in the magnetic field of the solar wind, and swept away.
How did Mercury get to be like this? The spin-orbit resonance is essentially unavoidable for a planet that orbits so close to a star. The absence of atmosphere and oceans is also unavoidable for a planet with such weak gravity in so hostile an environment. But the high density of Mercury is a continuing puzzle. The smaller bodies that accreted to form Mercury may have collided so violently that brittle silicates were preferentially crushed to dust, while tough grains of metal survived. Dissipation of the finest dust would then leave behind the ingredients of a metal-rich, dense planet. A second possibility is that the material in the zone where Mercury was to accrete was so strongly heated by the early superluminous Sun that the most volatile minerals were evaporated and lost from Mercury’s formation zone, leaving dense, involatile solids behind to form the planet. A third scenario is that Mercury formed normally with a composition similar to that of the other terrestrial planets, but that post-accretion impacts of comets and asteroids eroded away the crust and much of the mantle, blasting them off at high speeds and leaving behind a part of the lower mantle and the well-protected dense metallic core. Each of these three scenarios predicts different surface compositions for present-day Mercury. The goal of the MESSENGER spacecraft, which is due to enter orbit around Mercury in January, is to test these hypotheses by analyzing the crust by means of gamma-ray spectroscopy and probing the interior of the planet by means of measuring its gravitational and magnetic fields and the interaction of Mercury’s core with the magnetic field of the solar wind.
If all goes well, the answers to these puzzles will soon be in our hands.