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Mercury
Overview At first glance, Mercury appears to be no different than our moon. It is a dry, nearly airless world that is heavily cratered, and it is only 40% larger than our moon. The planet has not been resurfaced since its formation, and the lack of any erosional processes preserves the craters, much like our moon. Because of its atmospheric absence, the temperatures of the side facing the sun versus that facing away from the sun are very different: The side towards the sun is a sizzling 350 °C (662 °F), while the night side is a cold -180 °C (-292 °F). The reason for this huge difference is that it is the atmosphere (and water in the case of Earth) of planets that moves the heat around, allows night and day temperatures to remain relatively constant. But this is where the resemblance with our moon stops. Mercury has no large mare regions (dark "seas"), it has a significant internal magnetic field, and there is evidence that it has shrunk in size since it formed. Mercury is, in many ways, one of the least-understood planets in our solar system, since it was only visited by one spacecraft in the 1970s. It is hoped that the MESSENGER craft, now en route, will provide many clues to help decipher Mercury's complex history once the craft arrives in 2011. Atmosphere As can be seen in the planetary data table below, Mercury's atmosphere is essentially a vacuum. It exerts a pressure of only approximately 10-15 bars. If you drop a feather on your hand, it exerts a pressure of about 10-3 bars, which is one trillion times more than the atmospheric pressure on Mercury. Its composition is approximately:
The values presented are not well constrained, and the atmosphere may contain trace amounts of argon (Ar), carbon dioxide (CO2), water (H2O), nitrogen (N2), xenon (Xe), krypton (Kr), and neon (Ne). Hopefully, future missions will obtain more accurate measurements. Geology Even though it is a "dead" planet and only 45% of it has been photographed, Mercury has a varied geology. The Mariner 10 images provided a resolution of up to 2-3 km/px (1-2 miles/px). Most of Mercury consists of heavily cratered terrain. However, about 15% of the surface is made of a smooth intercrater plains material. It is unlike mare regions of our moon in that it is the same color and albedo as the rest of Mercury. Partially buried rims of craters within the smooth plains indicates that the plains are probably normal cratered terrain that has been buried by a thin layer of material. There is a debate as to what the origin of this material is - volcanic or impact melt (melt material from crater formation). No volcanic features have been spotted on Mercury, but no good spectroscopy has been done on Mercury, so the composition of the material is unknown. In general, Mercury has less craters on it per unit area than our moon. This could be for several reasons, but one of the most likely is that the intercrater plains cover up many craters that would otherwise be visible. A possible explanation for the marked lack of large basins is that Mercury's giant craters produce so much melt when they form that they fill the crater, making it no longer recognizable. One of the most prominent features of Mercury is the Caloris Basin, which is an impact crater about 1300 km in diameter (about 25% of Mercury's size). It is believed that Caloris Basin formed several billion years ago. Its ejecta has been mapped across a fair portion of the planet, and the basin itself has a varied geology. It has terraced walls, consistent with most large impact basins in the solar system. It has compressional features (wrinkle ridges) within it, and extensional features (graben) radiating away from it. Caloris Basin represents a significant event in Mercury's history. Another important aspect of Mercury's geography is a scarp - a ridge - that lies along most of the photographed equator of Mercury. The scarp's height - on the order of 1 km - indicates that Mercury probably contracted during its history. The contraction would have been a result of the planet cooling as a whole; the height of the scarp indicates Mercury probably shrunk by about 1-2 km in diameter. Time Line Just like Earth's moon, Mercury's history has been divided into five main parts. From oldest to most recent, they are Pre-Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian (a mnemonic is "Time Can't Move Kansas"). The Pre-Tolstojan period is generally defined by the planet's formation and very early history. The planet differentiated into a tiny crust, mantle, and a giant core. This also covers the era of intense bombardment that created multi-ring basins, most of the craters, and some intercrater plains formed. Tolstojan saw continued heavy bombardment, including the creation of the Goya formation. Most smaller basins were created during this time period, and the plains materials continued to form. During the Calorian time, the Caloris Basin and its associated terrain formed. The smooth plains continued to form, as did craters and smaller impact basins. During this time period, the planet also started to contract, and initial thrust faulting occurred. The Mansurian time saw a marked decrease in crater formation; the rest of the rayless craters formed during this time. Contraction continued and stopped, and the lithosphere (region above the mantle) thickened due to the planetary cooling. The current Kuiperian age is characterized by the rayed craters on Mercury. Other Features Mercury is the smallest non-controversial planet in the solar system (Pluto is not considered by most astronomers to be a planet). Therefore, it has very weak gravity. It also has nearly no atmosphere. This is believed to be a result of its weak gravity. However, Mercury might have been hit by an object the size of our moon early in its history. This could have stripped off most of its atmosphere and crust, which could be why it has an enormous core: Mercury's core is 75% of Mercury by volume and 80% of it by mass. This giant iron core is probably the source of Mercury's intrinsic magnetic field. Its field is only about 1% of Earth's but it is still significant that an otherwise geologically dead planet has a magnetic field at all. Its proximity to the Sun has resulted in two interesting characteristics of its orbit. First, it is locked into a 3:2 resonance with the Sun. This means that for every two Mercurian years, it has exactly 3 days, where the day is measured with respect to the stars (sidereal day). Because of its slow rotation rate on its axis, the length of time between sunrise and the next sunrise is nearly 176 Earth days. The other orbital feature is that since it is so close to the Sun, general relativity plays a small role in governing its shape. None of the planets always return to exactly the same location each orbit, as is predicted from Newtonian mechanics, but in Mercury's case, the orbit traces a significant rosette pattern in space. Mercury's perihelion processes at a rate of 574 arcsec per century. In the 1800's, French astronomer Urbain Le Verrier accounted for every classical perturbation (from other solar system bodies tugging on it), but there were still 43 arcsec that were unaccounted for. This extra 43 arcsec is exactly what is predicted with general relativity. Spacecraft that Have Visited or Will Visit
Mythology and Naming Schemes Mercury is named for the ancient Roman god of messengers and merchants who was also the swiftest of the gods. Jupiter gave him a pair of winged sandals, with which he could fly faster than the wind. This is why the planet is named for him: It has the fastest year, circling the Sun in just 88 days, and it also moves the fastest of all the planets at nearly 48 km/s (30 miles per sec). Craters on Mercury are named after dead artists from every culture across the globe in the fields of music, painting, and authors, who have made outstanding or fundamental contributions to their field. Ridges (dorsum singular, dorsa plural) are named after dead scientists who have contributed to the study of Mercury. Plains (planitia) are named after other messenger deities in other cultures, such as Japanese, Norse, and Hindi, or after other names for the planet Mercury in various languages. Scarps (rupes) are names after famous ships of discovery or scientific expeditions. Valleys on Mercury are named after radio telescopes. Data for the Planets
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