|
The geology of Mercury is the least understood of all the terrestrial planets in the Solar System. This stems largely from Mercury's proximity to the Sun which makes reaching it with spacecraft technically challenging and Earth-based observations difficult. Mercury's surface is dominated by impact craters, basaltic rock and smooth plains, many of them a result of flood volcanism, similar in some respects to the lunar maria,〔〔 and locally by pyroclastic deposits. Other notable features include vents which appear to be the source of magma-carved valleys, often-grouped irregular-shaped depressions termed "hollows" that are believed to be the result of collapsed magma chambers,〔 scarps indicative of thrust faulting and mineral deposits (possibly ice) inside craters at the poles. Long thought to be geologically inactive, new evidence suggests there may still be some level of activity.〔〔 Mercury's density implies a solid iron-rich core that accounts for about 60% of its volume (75% of its radius).〔 Mercury's magnetic equator is shifted nearly 20% of the planet's radius towards the north, the largest ratio of all planets.〔 This shift lends to there being one or more iron-rich molten layers surrounding the core producing a dynamo effect similar to that of Earth. Additionally, the offset magnetic dipole may result in uneven surface weathering by the solar wind, knocking more surface particles up into the southern exosphere and transporting them for deposit in the north. Scientists are gathering telemetry to determine if such is the case.〔 After having completed the first solar day of its mission in September 2011, more than 99% of Mercury's surface had been mapped by NASA's ''MESSENGER'' probe in both color and monochrome with such detail that scientists' understanding of Mercury's geology has eclipsed the level achieved following the ''Mariner 10'' flybys of the 1970s.〔 ==Difficulties in exploration== Reaching Mercury from Earth poses significant technical challenges, because the planet orbits so much closer to the Sun than does the Earth. A Mercury-bound spacecraft launched from Earth must travel 91 million kilometers into the Sun’s gravitational potential well. Starting from the Earth’s orbital speed of 30 km/s, the change in velocity (delta-v) the spacecraft must make to enter into a Hohmann transfer orbit that passes near Mercury is large compared to other planetary missions. The potential energy liberated by moving down the Sun's potential well becomes kinetic energy; requiring another large delta-''v'' to do anything other than rapidly pass by Mercury. In order to land safely or enter a stable orbit the spacecraft must rely entirely on rocket motors because Mercury has negligible atmosphere. A direct trip to Mercury actually requires more rocket fuel than that required to escape the Solar System completely. As a result, only two space probes, ''Mariner 10'' and ''MESSENGER'', both by NASA, have visited Mercury so far. Furthermore, the space environment near Mercury is demanding, posing the double dangers to spacecraft of intense solar radiation and high temperatures. Historically, a second obstacle has been that Mercury's period of rotation is a slow 58 Earth days, so that spacecraft flybys are restricted to viewing only a single illuminated hemisphere. In fact, unfortunately, even though Mariner 10 space probe flew past Mercury three times during 1974 and 1975, it observed the same area during each pass. This was because Mariner 10's orbital period was almost exactly 3 sidereal Mercury days, and the same face of the planet was lit at each of the close approaches. As a result, less than 45% of the planet’s surface was mapped. Earth-based observations are made difficult by Mercury's constant proximity to the Sun. This has several consequences: #Whenever the sky is dark enough for viewing through telescopes, Mercury is always already near the horizon, where viewing conditions are poor anyway due to atmospheric factors. #The Hubble Space Telescope and other space observatories are usually prevented from pointing close to the Sun for safety reasons (Erroneously pointing such sensitive instruments at the Sun is likely to cause permanent damage). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Geology of Mercury」の詳細全文を読む スポンサード リンク
|