Astronomy college course/Mercury
Mercury is the smallest and closest to the Sun of the eight planets in the Solar System, with an orbital period of about 88 Earth days. Seen from the Earth, it appears to move around its orbit in about 116 days, which is much faster than any other planet. This rapid motion may have led to it being named after the Roman deity Mercury, the fast-flying messenger to the gods.
Mercury's surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.
Mercury does not experience seasons in the same way as most other planets, such as the Earth. It is locked so it rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates exactly three times for every two revolutions it makes around its orbit.
As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years.
Because Mercury's orbit lies within Earth's orbit (as does Venus's), it can appear in Earth's sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to the Earth. Although Mercury can appear as a very bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.
Mercury is even smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm3, only slightly less than Earth's density of 5.515 g/cm3. If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3.
Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.
Geologists estimate that Mercury's core occupies about 42% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core.
Based on data from the Mariner 10 mission and Earth-based observation, Mercury's crust is believed to be 100–300 km thick. One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is believed that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.
Mercury's core: why is it so dense?
Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this.
Impact. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.Early in the Solar System's history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several thousand kilometers across. The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of the Moon.
Rock vapor. Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K. Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the solar wind.
Drag on lighter particles. A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material and not gathered by Mercury.
Each hypothesis predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both will make observations to test them. MESSENGER has found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because potassium and sulfur would have been driven off by the extreme heat of these events. The findings would seem to favor the third hypothesis, however further analysis of the data is needed.
Geology of mercury
Of all the terrestrial planets in the Solar System, the geology of Mercury is the least understood. 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. 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.
Mercury's geological history
After the formation of Mercury along with the rest of the Solar System 4.6 billion years ago, heavy bombardment by asteroids and comets ensued. The last intense bombardment phase, the Late Heavy Bombardment came to an end about 3.8 billion years ago. Some regions or massifs, a prominent one being the one that formed the Caloris Basin, were filled by magma eruptions from within the planet. These created smooth intercrater plains similar to the maria found on the Moon.
Later, as the planet cooled and contracted, its surface began to crack and form ridges; these surface cracks and ridges can be seen on top of other features, such as the craters and smoother plains – a clear indication that they are more recent. Mercury's period of vulcanism ended when the planet's mantle had contracted enough to prevent further lava from breaking through to the surface. This probably occurred at some point during its first 700 or 800 million years of history.
Since then, the main surface processes have been intermittent impacts.
Impact basins and craters
Craters on Mercury range in diameter from small bowl-shaped craters to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed-craters, to highly degraded crater remnants. Mercurian craters differ subtly from Lunar craters — the extent of their ejecta blankets is much smaller, which is a consequence of the 2.5 times stronger surface gravity on Mercury.
The largest known crater is the enormous Caloris Basin, with a diameter of 1550 km, A basin of comparable size, tentatively named Skinakas Basin had been postulated from low resolution Earth-based observations of the non-Mariner-imaged hemisphere, but has not been observed in MESSENGER imagery of the corresponding terrain. The impact which created the Caloris Basin was so powerful that its effects are seen on a global scale. It caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin lies a large region of unusual, hilly and furrowed terrain, sometimes called “Weird Terrain”. The favoured hypothesis for the origin of this geomorphologic unit is that shock waves generated during the impact traveled around the planet, and when they converged at the basin’s antipode (180 degrees away) the high stresses were capable of fracturing the surface. A much less favoured idea was that this terrain formed as a result of the convergence of ejecta at this basin’s antipode. Furthermore, the formation of the Caloris Basin appears to have produced a shallow depression concentric around the basin, which was later filled by the smooth plains (see below).
- Inter-crater plains are the oldest visible surface, predating the heavily cratered terrain. They are gently rolling or hilly and occur in the regions between larger craters. The inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about 30 km in diameter. It is not clear whether they are of volcanic or impact origin. The inter-crater plains are distributed roughly uniformly over the entire surface of the planet.
- Smooth plains are widespread flat areas resembling the lunar maria, which fill depressions of various sizes. Notably, they fill a wide ring surrounding the Caloris Basin. An appreciable difference to the lunar maria is that the smooth plains of Mercury have the same albedo as the older intercrater plains. Despite a lack of unequivocally volcanic features, their localisation and lobate-shaped colour units strongly support a volcanic origin. All the Mercurian smooth plains formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.
The floor of the Caloris Basin is also filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they are volcanic lavas induced by the impact, or a large sheet of impact melt.
One unusual feature of the planet’s surface is the numerous compression folds which crisscross the plains. It is thought that as the planet’s interior cooled, it contracted and its surface began to deform. The folds can be seen on top of other features, such as craters and smoother plains, indicating that they are more recent. Mercury’s surface is also flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17% stronger than the Moon’s on Earth.
Non-crater surface features are given the following names:
- Albedo features — areas of markedly different reflectivity
- Dorsa — ridges (see List of ridges on Mercury)
- Montes — mountains (see List of mountains on Mercury)
- Planitiae — plains (see List of plains on Mercury)
- Rupes — scarps (see List of scarps on Mercury)
- Valles — valleys (see List of valleys on Mercury)