Shakespeare quadrangle
Quadrangle on Mercury
The Shakespeare quadrangle is a region of Mercury running from 90 to 180° longitude and 20 to 70° latitude. It is also called Caduceata.
The Borealis quadrangle is north of Shakespeare quadrangle. To the west is Raditladi quadrangle, and to the east is Victoria quadrangle. To the southwest is Tolstoj quadrangle, and to the southeast is Beethoven quadrangle.
Mariner 10 imaging.
Prior to the images taken by MESSENGER, the only spacecraft images of Mercury were those taken by the Mariner 10 spacecraft, which made three passes of the planet in 1974–75 (Murray and others, 1974a,b; Strom and others, 1975a). Most images used in mapping the geology of the Shakespeare quadrangle were taken during the near-equatorial first pass, with close encounter or the dark side of the planet. The second, south-polar pass did not image the Shakespeare quadrangle at high resolution. High-resolution images of small areas within the quadrangle were also obtained during the third pass, when the spacecraft was on a near-encounter north-polar trajectory. Because the spacecraft viewed the same areas from different positions during the first and second passes, stereoscopic pictures are available for certain areas of the southern hemisphere; however, such pictures are not available for the Shakespeare quadrangle. All of the Mariner 10 passes occurred under similar lighting conditions. Across the Shakespeare quadrangle, these conditions varied from low light at the terminator near the west boundary to higher sun at the east boundary. Consequently, lighting conditions were favorable for determining fine-scale relief in the west, but progressively less so toward the east. Conversely, albedo features such as bright crater rays, which are conspicuous in the eastern part, become increasingly difficult to recognize westward toward the terminator. This range of lighting conditions across the quadrangle results in inconsistent geologic mapping, because topography, albedo, and surface texture are critical for characterizing individual materials units. The average resolution of the pictures used from the first pass is just over 1 km.
"MESSENGER" images.
The "MESSENGER" spacecraft mapped all of Mercury's quadrangles at much greater detail than that of "Mariner 10". For example, the fresh crater Degas was imaged at very high resolution, and the possible volcanic vents within To Ngoc Van, Scarlatti, and Gibran craters were imaged.
Regional setting.
The dominant feature in the Shakespeare quadrangle is the Caloris Basin, 1,300 km in diameter. This impact basin is the largest and best preserved on the hemisphere of Mercury observed by Mariner 10. Almost the entire eastern half of the basin is in the Raditladi quadrangle; the west half was in the nightside hemisphere of Mercury during all the Mariner 10 passes, and part of the southern half lies in the adjacent Tolstoj quadrangle (Schaber and McCauley, 1980). Surrounding Caloris is a discontinuous annulus of its ejecta deposits, called the Caloris Group. Caloris ejecta are embayed and partly covered by a plains unit that lies mostly in large, roughly circular depressions, some of which may be ancient degraded basins. This plains material also occurs in the floors of old craters and in small irregular topographic lows.
The eastern part of the Shakespeare quadrangle consists mainly of cratered terrain and intercrater plains. Over the whole of the mapped area are scattered fresh craters superposed on other units; in the eastern part the large fresh craters show well- developed bright rays.
Stratigraphy.
Pre-Caloris materials.
The oldest recognizable unit in the quadrangle is the intercrater plains material. These plains were originally described by Trask and Guest as intercrater plains. The unit has a surface expression of rolling to hummocky plains in the areas between large craters and is exposed mainly in the eastern part of the mapped area. The surface of the unit is pockmarked with craters, many of which are small (about 5 to 10 km in diameter), elliptical, and shallow; they are inferred from their shape to be secondary craters associated with larger craters and basins. Trask and Guest concluded that the surface of these plains represents a primordial surface of Mercury on which craters have been superposed. The large extent of this surface compared to its counterpart on the Moon was thought to reflect the restricted distribution of ejecta around each individual crater caused by the relatively high gravity on Mercury. Because of this high gravity, considerable areas were unaffected by crater and basin ejecta. However, Malin and Guest and O'Donnell (1977) have shown that in some areas the intercrater plains overlie highly degraded craters, a relation suggesting either that the intercrater plains were formed during a specific time in Mercury's history and that cratering occurred both before and after their emplacement, or, alternatively, that the intercrater plains were formed by a continuous process throughout cratering history.
In several parts of the quadrangle, especially on the margins of large expanses of smooth plains materials, is a unit of smoother and less rolling plains that have a lower crater density. Following Schaber and McCauley (1980), this unit is called intermediate plains material. It is difficult to map with precision because it grades into both the intercrater plains and the smooth plains. Also, its recognition depends on lighting conditions that vary across the mapped areas, especially east of longitude 120°. The presence of this unit suggests that the plains-forming process spanned much of the early geologic history of Mercury and continued long after the peak of cratering. In the southern part of Sobkou Planitia, intermediate plains have a lower albedo than the adjacent plains. In some places, they may simply represent areas of intercrater plains that have been partly flooded by the younger smooth plains material.
Lineated plains material was recognized by Trask and Guest as forming terrain consisting of lines of hills and valleys, some of which are as much as 300 km long. This unit modified older large craters and intercrater plains. Its features are similar to those of the lunar Imbrium sculpture (Gilbert 1893) and to the hills and valleys radial to the Nectaris Basin on the Moon (Stuart-Alexander, 1971). The lineations were probably formed in a similar way to those of the Imbrium sculpture, which resulted from excavation by projectiles ejected at low angles from the Imbrium Basin; however, some of the mercurian valleys may be the result of faulting. Most of the lineated material in the Shakespeare quadrangle appears to be subradial to an ancient basin lying between Odin Planitia and Budh Planitia centered at latitude 28° N., longitude 158° W. However, except for its northernmost exposure, the surface of this unit is mantled by a facies of the Odin Formation.
Hilly plains material consists of low, rounded, closely spaced hills with relatively few superposed craters. The hills range in size from 1 to 2 km across and were estimated to have heights of 100 to 200 m by Trask and Guest, who first recognized this unit and named it hilly terrain. The main tracts of hilly material occur in a roughly concentric band outside the Caloris ejecta. It is possible that this unit is associated with Caloris, although apart from geographical distribution, there is no supporting evidence. In some places, contact relations suggest that the hilly plains material may be older than intermediate plains material. Also, patches of the hilly material may be associated with intercrater plains materials in the eastern part of the quadrangle, where lighting conditions do not allow its recognition.
Caloris group.
Rock units associated with the Caloris Basin are particularly important for the stratigraphy of Mercury. It has been demonstrated that the history of the Moon was punctuated by a series of major impacts that have emplaced ejecta over widespread areas; the rock units associated with these impact basins were used to divide the lunar stratigraphic column into a series of well-defined time units (Shoemaker and Hackman, 1962; McCauley, 1967; Wilhelms, 1972). These relations are particularly clear for the Imbrium Basin (Wilhelms and McCauley, 1971) and Orientale Basin (Scott and others, 1977).
Recognizable ejecta units extend outward from the Caloris Basin as far as one basin diameter; these units can be used to divide the mercurian stratigraphic column in much the same way as basin ejecta were used on the Moon. A stratigraphic and structural comparison between the Caloris and Orientale Basins has been made by McCauley (1977).
In the Shakespeare quadrangle, only a lineated facies of the Van Eyck Formation is recognized, whereas in the Tolstoj quadrangle to the south, both it and a secondary crater facies are mapped (Schaber and McCauley, 1980; McCauley and others, 1981).
Post-Caloris materials.
The plains material that forms the floor of the Caloris Basin has not been included in the Caloris Group and is mapped separately from the smooth plains. In many way, the Caloris-floor plains are similar to the smooth plains, except that they have been buckled and fractured into numerous ridges and grooves that intersect to form a grossly polygonal pattern. The dominant trends of these features are concentric and radial to the center of Caloris. On the basis of photometric evidence, Hapke and others (1975) suggested that the central part of the basin floor may be 7±3 km lower than the outer edge. Strom and others argued that the ridges were formed by compressive stress generated by subsidence of the floor, and the fractures by subsequent uplift of the center of the basin to produce crustal lengthening and the observed fracture pattern. The origin of the material itself is doubtful. It may consist of sheets of volcanic material emplaced shortly after the basin was formed or it may be material formed by the Caloris event either as melt or as the upper part of a plug of plastic material that rose in the crater floor as part of the impact process. Whatever the origin of this material, it seems clear that it covers the original floor of the excavated crater.
The smooth plains material forms essentially level tracts, flooring depressions in the mercurian surface. The most extensive of such areas in this quadrangle are Sobkou and Budh Planitiae. The surface of the smooth plains material is relatively sparsely cratered, and overlap relations indicate that these plains units are younger than the intercrater plains and intermediate plains. Smooth plains also embay units of the Caloris Group. Smaller patches of smooth plains occur in depressions and old crater floors. In many areas, especially those closer to the Caloris Basin, they exhibit mare ridges like those on the Moon and thus have a rolling appearance. The boundary between smooth plains and the Odin Formation is not everywhere clear, except at high resolution. Smooth plains are mapped in the Shakespeare quadrangle only where there is no clear evidence of small hills characteristic of the Odin Formation.
Interpretation of the origin of the smooth plains is difficult but significant, because it bears directly on the internal constitution and thermal history of Mercury. Like the lunar maria, the smooth plains occur on the floors of large craters and basins, and the broad swath of plains around Caloris finds an analogy with Oceanus Procellarum around Imbrium on the Moon. However, the Caloris plains differ from the maria in having no observed positive-relief volcanic features such as those sparsely scattered on the lunar maria. The absence of sharp albedo differences between the smooth plains and the older terrain (Hapke and others, 1975), compared with the distinct difference in albedo between the lunar maria and highlands, may be more indicative of composition than of origin of the rocks. On the basis of distribution and volume, Strom and others argued that in most areas the smooth plains consist of extensive sheets of basic lava similar to the lunar maria. Schultz (1977), studying modified impact craters, also argued in favor of volcanism. On the other hand, Wilhelms pointed out that the lunar light plains could also serve as an analog of the mercurian smooth plains: Apollo 16 samples indicate that lunar light plains consist of cataclastic breccia and impact melt, interpreted as being emplaced by large impact events (James, 1977). Wilhelms, therefore, proposed that the smooth plains on Mercury may be related to the Caloris impact directly, as breccias and impact melts, rather than as lavas. However, the light plains on the Moon are nowhere so well developed or extensive as the plains around Caloris, and if Wilhelms' explanation is correct, considerable differences must exist between large impact events on the Moon and Mercury. Most likely, large parts of the smooth plains are of volcanic origin, although in some areas they may be of impact-melt origin.
The very smooth plains on Mercury were included in the smooth plains unit by Trask and Guest. Here the geologic units are mapped separately, because very smooth plains material is clearly younger than smooth plains material. The very smooth plains unit, which is featureless and has no resolvable superposed craters, is possibly ejecta fallback on the floors of craters. However not all craters contain this material; some are floored by material with a rugged surface mapped as crater floor material, because it is analogous to the floor material in younger lunar craters such as Copernicus or Aristarchus. One other possibility is that the very smooth plains are volcanic.
Crater materials.
The craters on Mercury show various states of preservation, ranging from crisp-featured craters with bright rays to those that are almost totally obliterated and consist only of a subdued ring of heavily cratered hills. As on the Moon, the principal eroding process is likely impact; thus, a fresh crater will be degraded systematically over time. Craters of similar size that show similar states of preservation are therefore considered to be of about the same age. Craters are mapped according to a fivefold classification on the basis of their states of degradation (McCauley and others, 1981). The components used to define the crater ages are rays, secondary craters, ejecta facies, central peaks and rings, rim form, and inner terraces. As a crater ages, the number of superposed craters increases and each of the morphologic elements becomes more subdued. Volcanic activity also may bury or destroy certain crater components, but the crater may still be dated by the preservation of the remaining rim. On the basis of mapping in this quadrangle and in the adjacent Tolstoj quadrangle (Schaber and McCauley, 1980), the Caloris impact is considered to have occurred in late c3 time (McCauley and others, 1981).
One problem with the above-mentioned crater-dating technique on Mercury is that secondary craters occur closer to the principal crater and are thus more clustered than on the Moon, where they are relatively widespread. In consequence, an older crater adjacent to a fresh one becomes strongly degraded as a result of heavy bombardment by secondary craters from the younger crater and appears much older than it is.
Ghost craters are unusual forms that occur in the Suisei Planitia. They are buried and rounded in profile, with only their rim crests rising above the surrounding smooth plains. Therefore, these craters cannot be assigned a specific age; they may be of any age from late c1 to late c3.
Structure.
The most conspicuous structural elements in the quadrangle are the radial and concentric ridges and cracks inside the Caloris Basin and the ridges developed in the Odin Formation and smooth plains unit immediately outside Caloris. O'Donnell and Thomas (personal communication, 1979) have suggested, on the basis of orientation of features outside Caloris, that these ridges and scarps largely follow preexisting radial and concentric fracture patterns in the mercurian lithosphere initiated by the Caloris impact, similar in character to those around Imbrium on the Moon (Mason and others, 1976). Caloris itself consists of a single mountain ring and a weak outer scarp. A few sinuous scarps also occur in this quadrangle, including the Heemskerck Rupes which cuts the older intercrater plains. Scarps of this type are considered by Strom and others to be compressive thrust faults resulting from overall shortening of the mercurian crust early in its history.
Geologic history.
The history of the Shakespeare quadrangle as evidenced by materials exposed at the surface begins with the formation of intercrater plains material and of impact craters both older and younger than these plains. Some c1 and c2 craters were superposed on the intercrater plains. The intermediate plains material and lineated plains unit were emplaced over the intercrater plains, as were most craters of c3 age. Then followed the major asteroidal impact that produced the Caloris Basin and the emplacement of rocks of the Caloris Group around the basin. Comparison of crater populations on surfaces older and younger than Caloris suggests that at the time of the Caloris impact, the population of craters smaller than 30 km in diameter was eradicated from the pre-Caloris terrain (Guest and Gault, 1976). Gault and others (1976) suggested that the smaller craters were destroyed by the Caloris event and by other basin-forming events elsewhere on the planet at about the same time.
The smooth plains material was then emplaced. Some c3 craters were formed after the Caloris event and after some of the smooth plains were formed. Superposed on the smooth plains unit and on all older deposits were craters of c4 age, inside which was emplaced the very smooth plains material (unit pvs). Analogy with the Moon suggests that most of the recorded events in the history of Mercury occurred during the first 1.5 b.y. of the planet's life; the oldest major rock units in this quadrangle are probably at least 2 to 3 b.y. old. The geologic history of Mercury has been summarized by Guest and O'Donnell (1977), Davies and others, and Strom.
References.
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Mercury (planet)
First planet from the Sun
Mercury is the first planet from the Sun and the smallest in the Solar System. In English, it is named after the ancient Roman god (Mercury), god of commerce and communication, and the messenger of the gods. Mercury is classified as a terrestrial planet, with roughly the same surface gravity as Mars. The surface of Mercury is heavily cratered, as a result of countless impact events that have accumulated over billions of years. Its largest crater, Caloris Planitia, has a diameter of , which is about one-third the diameter of the planet (). Similarly to the Earth's Moon, Mercury's surface displays an expansive rupes system generated from thrust faults and bright ray systems formed by impact event remnants.
Mercury's sidereal year (88.0 Earth days) and sidereal day (58.65 Earth days) are in a 3:2 ratio. This relationship is called spin–orbit resonance, and "sidereal" here means "relative to the stars". Consequently, one solar day (sunrise to sunrise) on Mercury lasts for around 176 Earth days: twice the planet's sidereal year. This means that one side of Mercury will remain in sunlight for one Mercurian year of 88 Earth days; while during the next orbit, that side will be in darkness all the time until the next sunrise after another 88 Earth days.
Combined with its high orbital eccentricity, the planet's surface has widely varying sunlight intensity and temperature, with the equatorial regions ranging from at night to during sunlight. Due to the very small axial tilt, the planet's poles are permanently shadowed. This strongly suggests that water ice could be present in the craters. Above the planet's surface is an extremely tenuous exosphere and a faint magnetic field that is strong enough to deflect solar winds. Mercury has no natural satellites.
As of the early 2020s, many broad details of Mercury's geological history are still under investigation or pending data from space probes. Like other planets in the Solar System, Mercury was formed approximately 4.5 billion years ago. Its mantle is highly homogeneous, which suggests that Mercury had a magma ocean early in its history, like the Moon. According to current models, Mercury may have a solid silicate crust and mantle overlying a solid outer core, a deeper liquid core layer, and a solid inner core. There are many competing hypotheses about Mercury's origins and development, some of which incorporate collision with planetesimals and rock vaporization.
Nomenclature.
Historically, humans knew Mercury by different names depending on whether it was an evening star or a morning star. By about 350 BC, the ancient Greeks had realized the two stars were one. They knew the planet as , meaning "twinkling", and , for its fleeting motion, a name that is retained in modern [[Greek language|Greek]] ( ). The Romans named the planet after the swift-footed Roman messenger god, [[Mercury (mythology)|Mercury]] (Latin ), whom they equated with the Greek Hermes, because it moves across the sky faster than any other planet, though some associated the planet with [[Apollo]] instead, as detailed by [[Pliny the Elder]]. The [[astronomical symbol]] for Mercury is a stylized version of Hermes' [[caduceus]]; a [[Christian cross]] was added in the 16th century:[[File:Mercury symbol (fixed width).svg|16px|☿]].
Physical characteristics.
[[File:Terrestrial planet size comparisons-2.jpg|thumb|upright=1.4|Mercury to scale among the [[Inner Solar System]] [[planetary-mass object]]s beside the Sun, arranged by the order of their orbits outward from the Sun (from left: Mercury, [[Venus]],
[[Earth]], the [[Moon]], [[Mars]] and [[Ceres (dwarf planet)|Ceres]])]]
Mercury is one of four [[terrestrial planet]]s in the [[Solar System]], which means it is a rocky body like Earth. It is the smallest planet in the Solar System, with an [[equator]]ial [[radius]] of . Mercury is also [[list of Solar System objects by radius|smaller]]—albeit more massive—than the largest [[natural satellite]]s in the Solar System, [[Ganymede (moon)|Ganymede]] and [[Titan (moon)|Titan]]. Mercury consists of approximately 70% metallic and 30% [[silicate]] material.
Internal structure.
[[File:Mercury with magnetic field.svg|left|thumb|upright=1.6|Mercury's internal structure and magnetic field]]
Mercury appears to have a solid silicate [[Crust (geology)|crust]] and mantle overlying a solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core. The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements. The planet'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 from both planets, the materials of which Mercury is made would be denser than those of Earth, 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 [[planetary core|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.
The radius of Mercury's core is estimated to be , based on interior models constrained to be consistent with a [[moment of inertia factor]] of . Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core. The mantle-crust layer is in total thick. Projections differ as to the size of the crust specifically; data from the "Mariner 10" and "MESSENGER" probes suggests a thickness of , whereas an [[Airy isostacy]] model suggests a thickness of . One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought 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 has a higher iron content than that of any other planet in the Solar System, and several theories have been proposed to explain this. 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 <templatestyles src="Fraction/styles.css" />1⁄6 Mercury's 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 Earth's Moon.
Alternatively, Mercury may have formed from the [[solar nebula]] before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the [[protostar|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]]. A third hypothesis proposes that the solar nebula caused [[drag (physics)|drag]] on the particles from which Mercury was [[Accretion (astrophysics)|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 space missions have been tasked with making observations of this composition. The first "[[MESSENGER]]", which ended in 2015, 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 said potassium and sulfur would have been driven off by the extreme heat of these events. "[[BepiColombo]]", which will arrive at Mercury in 2025, will make observations to test these hypotheses. The findings so far would seem to favor the third hypothesis; however, further analysis of the data is needed.
Surface geology.
Mercury's surface is similar in appearance to that of the Moon, showing extensive [[Lunar mare|mare]]-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more [[heterogeneous]] than the surface of [[Mars]] or the Moon, both of which contain significant stretches of similar geology, such as [[Lunar mare|maria]] and plateaus. [[Albedo]] features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, and [[ray system]]s. Larger albedo features correspond to higher reflectivity plains. Mercury has "[[wrinkle-ridge]]s" (dorsa), Moon-like [[highland]]s, mountains (montes), plains (planitiae), escarpments (rupes), and valleys ([[Vallis (planetary geology)|valles]]).
[[File:Unmasking the Secrets of Mercury.jpg|thumb|left|[[MESSENGER#Scientific instruments|MASCS]] spectrum scan of Mercury's surface by "MESSENGER"]]
The planet's mantle is chemically heterogeneous, suggesting the planet went through a [[magma ocean]] phase early in its history. Crystallization of minerals and convective overturn resulted in a layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early [[Chemical reduction|chemically reducing]] conditions than is found on other terrestrial planets. The surface is dominated by iron-poor [[pyroxene]] and [[olivine]], as represented by [[enstatite]] and [[forsterite]], respectively, along with sodium-rich [[plagioclase]] and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of the crust are high in carbon, most likely in the form of graphite.
Names for features on Mercury come from a variety of sources and are set according to the [[IAU]] [[planetary nomenclature]] system. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or [[fossa (geology)|fossae]] are named for works of architecture. Montes are named for the word "hot" in a variety of languages. [[Plain]]s or planitiae are named for [[Mercury (god)|Mercury]] in various languages. [[Escarpment]]s or [[rupēs]] are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity.
Impact basins and craters.
[[File:PIA19421-Mercury-Craters-MunchSanderPoe-20150416.jpg|thumb|left|Enhanced-color image of craters [[Munch (crater)|Munch]] (left), [[Sander (crater)|Sander]] (center), and [[Poe (crater)|Poe]] (right) amid volcanic plains (orange) near [[Caloris Basin]]]]
Mercury was heavily bombarded by comets and [[asteroid]]s during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the [[Late Heavy Bombardment]] that ended 3.8 billion years ago. Mercury received impacts over its entire surface during this period of intense crater formation, facilitated by the lack of any [[atmosphere]] to slow impactors down. During this time Mercury was [[volcano|volcanically]] active; basins were filled by [[magma]], producing smooth plains similar to the maria found on the Moon. One of the most unusual craters is [[Apollodorus (crater)|Apollodorus]], or "the Spider", which hosts a series of radiating troughs extending outwards from its impact site.
[[Craters on Mercury]] range in diameter from small bowl-shaped cavities to [[multi-ringed impact basin]]s 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 in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity. According to [[International Astronomical Union]] rules, each new crater must be named after an artist who was famous for more than fifty years, and dead for more than three years, before the date the crater is named.
The largest known crater is [[Caloris Planitia]], or Caloris Basin, with a diameter of . The impact that created the Caloris Basin was so powerful that it caused [[lava]] eruptions and left a concentric mountainous ring ~ tall surrounding the [[impact crater]]. The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they were volcanic lava flows induced by the impact or a large sheet of impact melt.
At the [[antipodes|antipode]] of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury, converging at the basin's antipode (180 degrees away). The resulting high stresses fractured the surface. Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.
[[File:EW1027346412Gnomap.png|thumb|Tolstoj basin is along the bottom of this image of Mercury's limb]]
Overall, 46 impact basins have been identified. A notable basin is the -wide, multi-ring [[Tolstoj Basin]] that has an ejecta blanket extending up to from its rim and a floor that has been filled by smooth plains materials. [[Beethoven Basin]] has a similar-sized ejecta blanket and a -diameter rim. Like the Moon, the surface of Mercury has likely incurred the effects of [[space weathering]] processes, including solar wind and [[micrometeorite]] impacts.
Plains.
There are two geologically distinct plains regions on Mercury. Gently rolling, hilly [[Inter-crater plains on Mercury|plains in the regions between craters]] are Mercury's oldest visible surfaces, predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about in diameter.
Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to lunar maria. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localization and rounded, lobate shape of these plains strongly support volcanic origins. All the smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.
Compressional features.
An unusual feature of Mercury's surface is the numerous compression folds, or [[rupes]], that crisscross the plains. These exist on the Moon, but are much more prominent on Mercury. As Mercury's interior cooled, it contracted and its surface began to deform, creating [[wrinkle ridge]]s and [[lobate scarp]]s associated with [[thrust fault]]s. The scarps can reach lengths of and heights of . These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent. Mapping of the features has suggested a total shrinkage of Mercury's radius in the range of ~. Most activity along the major thrust systems probably ended about 3.6–3.7 billion years ago. Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few kilometers, that appear to be less than 50 million years old, indicating that compression of the interior and consequent surface geological activity continue to the present.
Volcanism.
[[File:Picasso crater.png|thumb|[[Picasso (crater)|Picasso crater]]—the large arc-shaped pit located on the eastern side of its floor is postulated to have formed when subsurface magma subsided or drained, causing the surface to collapse into the resulting void.]]
There is evidence for [[pyroclastic flow]]s on Mercury from low-profile [[shield volcano]]es. Fifty-one pyroclastic deposits have been identified, where 90% of them are found within impact craters. A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval.
A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to in diameter. It is thus a "[[compound volcano]]". The vent floors are at least below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into a conduit. Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years.
Surface conditions and exosphere.
[[File:North pole of Mercury -- NASA.jpg|thumb|Composite of the north pole of Mercury, where NASA confirmed the discovery of a large volume of water ice, in permanently dark craters that are found there.]]
The surface temperature of Mercury ranges from . It never rises above 180 K at the poles, due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. At [[perihelion]], the equatorial [[subsolar point]] is located at latitude 0°W or 180°W, and it climbs to a temperature of about . During [[aphelion]], this occurs at 90° or 270°W and reaches only . On the dark side of the planet, temperatures average . The intensity of [[sunlight]] on Mercury's surface ranges between 4.59 and 10.61 times the [[solar constant]] (1,370 W·m−2).
Although daylight temperatures at the surface of Mercury are generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K, far lower than the global average. This creates a [[Cold trap (astronomy)|cold trap]] where ice can accumulate. Water ice strongly reflects [[radar]], and observations by the 70-meter [[Goldstone Solar System Radar]] and the [[Very Large Array|VLA]] in the early 1990s revealed that there are patches of high radar [[Reflection (physics)|reflection]] near the poles. Although ice was not the only possible cause of these reflective regions, astronomers thought it to be the most likely explanation. The presence of [[ice|water ice]] was confirmed using "MESSENGER" images of craters at the north pole.
The icy crater regions are estimated to contain about 1014–1015 kg of ice, and may be covered by a layer of [[regolith]] that inhibits [[Sublimation (phase transition)|sublimation]]. By comparison, the [[Antarctica|Antarctic]] ice sheet on Earth has a mass of about 4×1018 kg, and Mars's south polar cap contains about 1016 kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from [[outgassing]] of water from the planet's interior and deposition by impacts of comets.
Mercury is too small and hot for its [[gravity]] to retain any significant [[atmosphere]] over long periods of time; it does have a tenuous surface-bounded [[exosphere]] at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). It includes [[hydrogen]], [[helium]], [[oxygen]], [[sodium]], [[calcium]], [[potassium]], [[magnesium]], [[silicon]], and [[hydroxide]], among others. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. [[Hydrogen atom]]s and [[helium atom]]s probably come from the solar wind, [[diffusion|diffusing]] into Mercury's [[magnetosphere]] before later escaping back into space. The [[radioactive decay]] of elements within Mercury's crust is another source of helium, as well as sodium and potassium. Water vapor is present, released by a combination of processes such as comets striking its surface, [[sputtering]] creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like O+, OH−, and [[hydronium|H3O+]] was a surprise. Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.
Sodium, potassium, and calcium were discovered in the atmosphere during the 1980s–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts including presently from [[Comet Encke]]. In 2008, magnesium was discovered by "MESSENGER". Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.
According to NASA, Mercury is not a suitable planet for Earth-like life. It has a [[exosphere|surface boundary exosphere]] instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions. Some parts of the subsurface of Mercury may have been [[Planetary habitability|habitable]], and perhaps [[life form]]s, albeit likely primitive [[microorganism]]s, may have existed on the planet.
Magnetic field and magnetosphere.
[[File:Mercury Magnetic Field NASA.jpg|thumb|Graph showing relative strength of Mercury's magnetic field]]
Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, [[magnetic field]]. According to measurements taken by "Mariner 10", it is about 1.1% the strength of [[Earth's magnetic field|Earth's]]. The magnetic-field strength at Mercury's equator is about 300 [[Tesla (unit)|nT]]. Like that of Earth, Mercury's magnetic field is [[dipolar]] and nearly aligned with the planet's spin axis (10° dipolar tilt, compared to 11° for Earth). Measurements from both the "Mariner 10" and "MESSENGER" space probes have indicated that the strength and shape of the magnetic field are stable.
It is likely that this magnetic field is generated by a [[Dynamo theory|dynamo]] effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong [[tidal heating]] effects caused by the planet's high orbital eccentricity would serve to keep part of the core in the liquid state necessary for this dynamo effect.
Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth, is strong enough to trap solar wind [[plasma (physics)|plasma]]. This contributes to the space weathering of the planet's surface. Observations taken by the "Mariner 10" spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere.
During its second flyby of the planet on October 6, 2008, "MESSENGER" discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes"—twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space—that were up to 800 km wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as [[flux transfer event]]s, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via [[magnetic reconnection]]. This also occurs in Earth's magnetic field. The "MESSENGER" observations showed the reconnection rate was ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed by "MESSENGER".
Orbit, rotation, and longitude.
Mercury has the most [[Orbital eccentricity|eccentric]] orbit of all the planets in the Solar System; its eccentricity is 0.21 with its distance from the Sun ranging from . It takes 87.969 Earth days to complete an orbit. The diagram illustrates the effects of the eccentricity, showing Mercury's orbit overlaid with a circular orbit having the same [[semi-major axis]]. Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. In the diagram, the varying distance of Mercury to the Sun is represented by the size of the planet, which is inversely proportional to Mercury's distance from the Sun.
This varying distance to the Sun leads to Mercury's surface being flexed by [[tidal bulge]]s raised by the [[Sun]] that are about 17 times stronger than the Moon's on Earth. Combined with a 3:2 [[#Spin-orbit resonance|spin–orbit resonance]] of the planet's rotation around its axis, it also results in complex variations of the surface temperature. The resonance makes a single [[solar day]] (the length between two [[meridian (astronomy)|meridian]] transits of the Sun) on Mercury last exactly two Mercury years, or about 176 Earth days.
Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the [[ecliptic]]), the largest of all eight known solar planets. As a result, [[transits of Mercury]] across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between Earth and the Sun, which is in May or November. This occurs about every seven years on average.
Mercury's [[axial tilt]] is almost zero, with the best measured value as low as 0.027 degrees. This is significantly smaller than that of [[Jupiter]], which has the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 [[arcminutes]] above the horizon. By comparison, the [[angular size]] of the Sun as seen from Mercury ranges from <templatestyles src="Fraction/styles.css" />1+1⁄4 to 2 degrees across.
At certain points on Mercury's surface, an observer would be able to see the Sun peek up a little more than two-thirds of the way over the horizon, then reverse and set before rising again, all within the same [[Extraterrestrial skies#Mercury|Mercurian day]]. This is because approximately four Earth days before perihelion, Mercury's angular [[orbital speed|orbital velocity]] equals its angular [[rotational velocity]] so that the Sun's [[improper motion|apparent motion]] ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a [[apparent retrograde motion|retrograde]] direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes. A similar effect would have occurred if Mercury had been in synchronous rotation: the alternating gain and loss of rotation over a revolution would have caused a libration of 23.65° in longitude.
For the same reason, there are two points on Mercury's equator, 180 degrees apart in [[longitude]], at either of which, around perihelion in alternate Mercurian years (once a Mercurian day), the Sun passes overhead, then reverses its apparent motion and passes overhead again, then reverses a second time and passes overhead a third time, taking a total of about 16 Earth-days for this entire process. In the other alternate Mercurian years, the same thing happens at the other of these two points. The amplitude of the retrograde motion is small, so the overall effect is that, for two or three weeks, the Sun is almost stationary overhead, and is at its most brilliant because Mercury is at perihelion, its closest to the Sun. This prolonged exposure to the Sun at its brightest makes these two points the hottest places on Mercury. Maximum temperature occurs when the Sun is at an angle of about 25 degrees past noon due to [[Diurnal temperature variation#Temperature lag|diurnal temperature lag]], at 0.4 Mercury days and 0.8 Mercury years past sunrise. Conversely, there are two other points on the equator, 90 degrees of longitude apart from the first ones, where the Sun passes overhead only when the planet is at aphelion in alternate years, when the apparent motion of the Sun in Mercury's sky is relatively rapid. These points, which are the ones on the equator where the apparent retrograde motion of the Sun happens when it is crossing the horizon as described in the preceding paragraph, receive much less solar heat than the first ones described above.
Mercury attains an inferior conjunction (nearest approach to Earth) every 116 Earth days on average, but this interval can range from 105 days to 129 days due to the planet's eccentric orbit. Mercury can come as near as to Earth, and that is slowly declining: The next approach to within is in 2679, and to within in 4487, but it will not be closer to Earth than until 28,622. Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of an inferior conjunction. This large range arises from the planet's high orbital eccentricity. Essentially, because Mercury is closest to the Sun, when taking an average over time, Mercury is most often the closest planet to the Earth, and—in that measure—it is the closest planet to each of the other planets in the Solar System.
Longitude convention.
The longitude convention for Mercury puts the zero of longitude at one of the two hottest points on the surface, as described above. However, when this area was first visited, by "Mariner 10", this zero meridian was in darkness, so it was impossible to select a feature on the surface to define the exact position of the meridian. Therefore, a small crater further west was chosen, called [[Hun Kal]], which provides the exact reference point for measuring longitude. The center of Hun Kal defines the 20° west meridian. A 1970 International Astronomical Union resolution suggests that longitudes be measured positively in the westerly direction on Mercury. The two hottest places on the equator are therefore at longitudes 0° W and 180° W, and the coolest points on the equator are at longitudes 90° W and 270° W. However, the "MESSENGER" project uses an east-positive convention.
Spin-orbit resonance.
[[File:Mercury's orbital resonance.svg|thumb|After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.]]
For many years it was thought that Mercury was synchronously [[tidally locked]] with the Sun, [[rotating]] once for each orbit and always keeping the same face directed towards the Sun, in the same way that the same side of the Moon always faces Earth. Radar observations in 1965 proved that the planet has a 3:2 spin-orbit resonance, rotating three times for every two revolutions around the Sun. The eccentricity of Mercury's orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly stationary in Mercury's sky.
The 3:2 resonant tidal locking is stabilized by the variance of the tidal force along Mercury's eccentric orbit, acting on a permanent dipole component of Mercury's mass distribution. In a circular orbit there is no such variance, so the only resonance stabilized in such an orbit is at 1:1 (e.g., Earth–Moon), when the tidal force, stretching a body along the "center-body" line, exerts a torque that aligns the body's axis of least inertia (the "longest" axis, and the axis of the aforementioned dipole) to always point at the center. However, with noticeable eccentricity, like that of Mercury's orbit, the tidal force has a maximum at perihelion and therefore stabilizes resonances, like 3:2, ensuring that the planet points its axis of least inertia roughly at the Sun when passing through perihelion.
The original reason astronomers thought it was synchronously locked was that, whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury's 3:2 spin-orbit resonance, a solar day lasts about 176 Earth days. A [[sidereal day]] (the period of rotation) lasts about 58.7 Earth days.
Simulations indicate that the orbital eccentricity of Mercury varies [[chaos theory|chaotically]] from nearly zero (circular) to more than 0.45 over millions of years due to [[Perturbation (astronomy)|perturbations]] from the other planets. This was thought to explain Mercury's 3:2 spin-orbit resonance (rather than the more usual 1:1), because this state is more likely to arise during a period of high eccentricity. However, accurate modeling based on a realistic model of tidal response has demonstrated that Mercury was captured into the 3:2 spin-orbit state at a very early stage of its history, within 20 (more likely, 10) million years after its formation.
Numerical simulations show that a future [[Secular resonance|secular]] [[Orbital resonance|orbital resonant]] interaction with the perihelion of Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the orbit will be destabilized in the next five billion years. If this happens, Mercury may fall into the Sun, collide with Venus, be ejected from the Solar System, or even disrupt the rest of the inner Solar System.
Advance of perihelion.
[[File:Drehung der Apsidenlinie light.svg|right|thumb|[[Apsidal precession]] of Mercury's orbit]]
In 1859, the French mathematician and astronomer [[Urbain Le Verrier]] reported that the slow [[precession]] of Mercury's orbit around the Sun could not be completely explained by [[Newtonian mechanics]] and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller "corpuscules") might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation. Other explanations considered included a slight oblateness of the Sun. The success of the search for [[Neptune]] based on its perturbations of the orbit of [[Uranus]] led astronomers to place faith in this possible explanation, and the hypothetical planet was named [[Vulcan (hypothetical planet)|Vulcan]], but no such planet was ever found.
The observed [[perihelion precession]] of Mercury is 5,600 [[arcseconds]] (1.5556°) per century relative to Earth, or per century relative to the inertial [[International Celestial Reference Frame|ICRF]]. Newtonian mechanics, taking into account all the effects from the other planets and including 0.0254 arcseconds per century due to the oblateness of the Sun, predicts a precession of 5,557 arcseconds (1.5436°) per century relative to Earth, or per century relative to ICRF. In the early 20th century, [[Albert Einstein]]'s [[general theory of relativity]] provided the explanation for the observed precession, by formalizing gravitation as being mediated by the curvature of spacetime. The effect is small: just per century (or 0.43 arcsecond per year, or 0.1035 arcsecond per orbital period) for Mercury; it therefore requires a little over 12.5 million orbits, or 3 million years, for a full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.6247 arcseconds per century for Venus, 3.8387 for Earth, 1.351 for Mars, and 10.05 for [[1566 Icarus]].
Observation.
[[File:Mercury.jpg|thumb|Image mosaic by "Mariner 10", 1974]]
Mercury's [[apparent magnitude]] is calculated to vary between −2.48 (brighter than [[Sirius]]) around [[superior conjunction]] and +7.25 (below the limit of naked-eye visibility) around [[inferior conjunction]]. The mean apparent magnitude is 0.23 while the standard deviation of 1.78 is the largest of any planet. The mean apparent magnitude at superior conjunction is −1.89 while that at inferior conjunction is +5.93. Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun's glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight.
Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The [[Hubble Space Telescope]] cannot observe Mercury at all, due to safety procedures that prevent its pointing too close to the Sun. Because the shift of 0.15 revolutions of Earth in a Mercurian year makes up a seven-Mercurian-year cycle (0.15 × 7 ≈ 1.0), in the seventh Mercurian year, Mercury follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven Mercurian years before.
Like the Moon and Venus, Mercury exhibits [[Lunar phase|phases]] as seen from Earth. It is "new" at [[inferior conjunction]] and "full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of its being obscured by the Sun, except at its new phase during a transit. Mercury is technically brightest as seen from Earth when it is at a full phase. Although Mercury is farthest from Earth when it is full, the greater illuminated area that is visible and the [[opposition effect|opposition brightness surge]] more than compensates for the distance. The opposite is true for Venus, which appears brightest when it is a [[crescent]], because it is much closer to Earth than when [[gibbous]].
[[File:PIA19247-Mercury-NPolarRegion-Messenger20150316.jpg|thumb|left|False-color map showing the maximum temperatures of the north polar region]]
[[File:Mercury Venus Moon over San Jose 08 Jan 2024.png|thumb|right|Mercury (lower left) as seen from [[San Jose, California]] with Venus and the Moon.]]
Mercury is best observed at the first and last quarter, although they are phases of lesser brightness. The first and last quarter phases occur at greatest [[elongation (astronomy)|elongation]] east and west of the Sun, respectively. At both of these times, Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion. At greatest "western" elongation, Mercury rises at its earliest before sunrise, and at greatest "eastern" elongation, it sets at its latest after sunset.
[[File:PIA19422-Mercury-CarnegieRupes-MDIS-MLA-20150416.jpg|thumb|right|False-color image of [[Carnegie Rupes]], a tectonic landform—high terrain (red); low (blue).]]
Mercury is more often and easily visible from the [[Southern Hemisphere]] than from the [[Northern Hemisphere|Northern]]. This is because Mercury's maximum western elongation occurs only during early autumn in the Southern Hemisphere, whereas its greatest eastern elongation happens only during late winter in the Southern Hemisphere. In both of these cases, the angle at which the planet's orbit intersects the horizon is maximized, allowing it to rise several hours before sunrise in the former instance and not set until several hours after sundown in the latter from southern mid-latitudes, such as Argentina and South Africa.
An alternate method for viewing Mercury involves observing the planet with a [[telescope]] during daylight hours when conditions are clear, ideally when it is at its greatest elongation. This allows the planet to be found easily, even when using telescopes with apertures. However, great care must be taken to obstruct the Sun from sight because of the extreme risk for eye damage. This method bypasses the limitation of twilight observing when the ecliptic is located at a low elevation (e.g. on autumn evenings). The planet is higher in the sky and less atmospheric effects affect the view of the planet. Mercury can be viewed as close as 4° to the Sun near superior conjunction when it is almost at its brightest.
Mercury can, like several other planets and the brightest stars, be seen during a total [[solar eclipse]].
Observation history.
Ancient astronomers.
[[File:Mercury-bonatti.png|thumb|Mercury, from "Liber astronomiae", 1550]]
The earliest known recorded observations of Mercury are from the [[MUL.APIN]] tablets. These observations were most likely made by an [[Assyria]]n astronomer around the 14th century BC. The [[cuneiform]] name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU\U4.UD ("the jumping planet"). [[Babylonian astronomy|Babylonian records]] of Mercury date back to the 1st millennium BC. The [[Babylonia]]ns called the planet [[Nabu]] after the messenger to the gods in [[Babylonian mythology|their mythology]].
The [[Greeks in Egypt|Greco]]-[[Ancient Egypt|Egyptian]] astronomer [[Ptolemy]] wrote about the possibility of planetary transits across the face of the Sun in his work "Planetary Hypotheses". He suggested that no transits had been observed either because planets such as Mercury were too small to see, or because transits were too infrequent.
[[File:Shatir500.jpg|thumb|left|[[Ibn al-Shatir]]'s model for the appearances of Mercury, showing the multiplication of [[epicycles]] using the [[Tusi couple]], thus eliminating the Ptolemaic eccentrics and [[equant]].]]
In [[ancient China]], Mercury was known as "the Hour Star" ("Chen-xing" ). It was associated with the direction north and the phase of water in the [[Five Phases]] system of metaphysics. Modern [[Chinese culture|Chinese]], [[Korean culture|Korean]], [[Japanese culture|Japanese]] and [[Vietnamese culture|Vietnamese]] cultures refer to the planet literally as the "water star" (), based on the [[Five elements (Chinese philosophy)|Five elements]]. [[Hindu mythology]] used the name [[Budha]] for Mercury, and this god was thought to preside over Wednesday. The god [[Odin]] (or Woden) of [[Germanic paganism]] was associated with the planet Mercury and Wednesday. The [[Maya civilization|Maya]] may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the [[underworld]]. Mercury was sometimes known as [[Stilbon (mythology)|Stilbon]] ([[Greek language|Greek]]: Στίλβων) meaning 'the shining, glittering'.
In [[medieval Islamic astronomy]], the [[Al-Andalus|Andalusian]] astronomer [[Abū Ishāq Ibrāhīm al-Zarqālī]] in the 11th century described the [[deferent]] of Mercury's [[geocentric orbit]] as being oval, like an egg or a [[Pine nut|pignon]], although this insight did not influence his astronomical theory or his astronomical calculations. In the 12th century, [[Ibn Bajjah]] observed "two planets as black spots on the face of the Sun", which was later suggested as the transit of Mercury and/or Venus by the [[Maragheh observatory|Maragha]] astronomer [[Qotb al-Din Shirazi]] in the 13th century. Most such medieval reports of transits were later taken as observations of [[sunspot]]s.
In India, the [[Kerala school of astronomy and mathematics|Kerala school]] astronomer [[Nilakantha Somayaji]] in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the [[Tychonic system]] later proposed by [[Tycho Brahe]] in the late 16th century.
Ground-based telescopic research.
The first telescopic observations of Mercury were made by [[Thomas Harriot]] and [[Galileo]] from 1610. In 1612, [[Simon Marius]] observed the brightness of Mercury varied with the planet's orbital position and concluded it had phases "in the same way as Venus and the Moon". In 1631, [[Pierre Gassendi]] made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by [[Johannes Kepler]]. In 1639, [[Giovanni Zupi]] used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited the Sun.
A rare event in astronomy is the passage of one planet in front of another ([[occultation]]), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737, is the only one historically observed, having been seen by [[John Bevis]] at the [[Royal Greenwich Observatory]]. The next occultation of Mercury by Venus will be on December 3, 2133.
The difficulties inherent in observing Mercury meant that it was far less studied than the other planets. In 1800, [[Johann Schröter]] made observations of surface features, claiming to have observed mountains. [[Friedrich Bessel]] used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°. In the 1880s, [[Giovanni Schiaparelli]] mapped the planet more accurately, and suggested that Mercury's rotational period was 88 days, the same as its orbital period due to tidal locking. This phenomenon is known as [[synchronous rotation]]. The effort to map the surface of Mercury was continued by [[Eugenios Antoniadi]], who published a book in 1934 that included both maps and his own observations. Many of the planet's surface features, particularly the [[List of albedo features on Mercury|albedo features]], take their names from Antoniadi's map.
In June 1962, Soviet scientists at the [[Institute of Radio-engineering and Electronics]] of the [[USSR Academy of Sciences]], led by [[Vladimir Kotelnikov]], became the first to bounce a radar signal off Mercury and receive it, starting radar observations of the planet. Three years later, radar observations by Americans [[Gordon H. Pettengill]] and Rolf B. Dyce, using the [[Arecibo radio telescope]] in [[Puerto Rico]], showed conclusively that the planet's rotational period was about 59 days. The theory that Mercury's rotation was synchronous had become widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.
In 1965, Italian astronomer [[Giuseppe Colombo]] noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance. Data from "Mariner 10" subsequently confirmed this view. This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every "second" orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.
Ground-based optical observations did not shed much further light on Mercury, but radio astronomers using interferometry at microwave wavelengths, a technique that enables removal of the solar radiation, were able to discern physical and chemical characteristics of the subsurface layers to a depth of several meters. Not until the first space probe flew past Mercury did many of its most fundamental morphological properties become known. Moreover, technological advances have led to improved ground-based observations. In 2000, high-resolution [[lucky imaging]] observations were conducted by the [[Mount Wilson Observatory]] Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the "Mariner 10" mission. Most of the planet has been mapped by the Arecibo radar telescope, with resolution, including polar deposits in shadowed craters of what may be water ice.
Research with space probes.
[[File:MESSENGER Assembly.jpg|thumb|left|"MESSENGER" being prepared for launch]]
[[File:PIA18389-MarsCuriosityRover-MercuryTransitsSun-20140603.gif|thumb|Mercury transiting the [[Sun]] as viewed by the Mars rover [[Curiosity (rover)|"Curiosity"]] (June 3, 2014).]]
Reaching Mercury from Earth poses significant technical challenges, because it orbits so much closer to the Sun than Earth. A Mercury-bound spacecraft launched from Earth must travel over into the Sun's gravitational [[potential well]]. Mercury has an [[orbital speed]] of , whereas Earth's orbital speed is . Therefore, the spacecraft must make a larger change in [[velocity]] ([[delta-v]]) to get to Mercury and then enter orbit, as compared to the delta-v required for, say, [[Exploration of Mars|Mars planetary missions]].
The [[potential energy]] liberated by moving down the Sun's potential well becomes [[kinetic energy]], requiring a delta-v change to do anything other than pass by Mercury. Some portion of this [[delta-v budget]] can be provided from a [[gravity assist]] during one or more fly-bys of Venus. To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. [[Aerobraking]] is ruled out because Mercury has a negligible atmosphere. A trip to Mercury requires more rocket fuel than that required to [[escape velocity|escape]] the Solar System completely. As a result, only three space probes have visited it so far. A proposed alternative approach would use a [[solar sail]] to attain a Mercury-synchronous orbit around the Sun.
"Mariner 10".
[[File:Mariner 10.jpg|thumb|"Mariner 10", the first probe to visit Mercury]]
The first spacecraft to visit Mercury was NASA's "Mariner 10" (1974–1975). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational "slingshot" effect and the first NASA mission to visit multiple planets. "Mariner 10" provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps that were later ascribed to the effect of the planet shrinking slightly as its iron core cools. Unfortunately, the same face of the planet was lit at each of "Mariner 10"'s close approaches. This made close observation of both sides of the planet impossible, and resulted in the mapping of less than 45% of the planet's surface.
The spacecraft made three close approaches to Mercury, the closest of which took it to within of the surface. At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury's rotation was expected to be much too slow to generate a significant [[dynamo]] effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet's magnetic field is much like Earth's, which deflects the solar wind around the planet. For many years after the "Mariner 10" encounters, the origin of Mercury's magnetic field remained the subject of several competing theories.
On March 24, 1975, just eight days after its final close approach, "Mariner 10" ran out of fuel. Because its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down. "Mariner 10" is thought to be still orbiting the Sun, passing close to Mercury every few months.
"MESSENGER".
[[File:Details of MESSENGER's Impact Location.jpg|thumb|Estimated details of the impact of "MESSENGER" on April 30, 2015]]
A second NASA mission to Mercury, named "MESSENGER" (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. It made a fly-by of Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury. A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008, and a third on September 29, 2009. Most of the hemisphere not imaged by "Mariner 10" was mapped during these fly-bys. The probe successfully entered an elliptical orbit around the planet on March 18, 2011. The first orbital image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission, and then entered a one-year extended mission into 2013. In addition to continued observations and mapping of Mercury, "MESSENGER" observed the 2012 [[solar maximum]].
[[File:MercuryTopo.png|thumb|right|Topography of Mercury based on MDIS (Mercury Dual Imaging System) data]] The mission was designed to clear up six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe carried imaging devices that gathered much-higher-resolution images of much more of Mercury than "Mariner 10", assorted [[spectrometer]]s to determine the abundances of elements in the crust, and [[magnetometer]]s and devices to measure velocities of charged particles. Measurements of changes in the probe's orbital velocity were expected to be used to infer details of the planet's interior structure. "MESSENGER"'s final maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015. The spacecraft's impact with Mercury occurred at 3:26:01 p.m. [[Eastern Time Zone|EDT]] on April 30, 2015, leaving a crater estimated to be in diameter.
"BepiColombo".
The [[European Space Agency]] and the [[Japanese Space Agency]] developed and launched a joint mission called "BepiColombo", which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere. Launched on October 20, 2018, "BepiColombo" is expected to reach Mercury in 2025. It will release a magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for one terrestrial year. The mapper probe carries an array of spectrometers similar to those on "MESSENGER", and will study the planet at many different wavelengths including [[infrared]], [[ultraviolet]], [[X-ray]] and [[gamma ray]]. "BepiColombo" conducted the first of its six planned Mercury flybys on October 1, 2021, and the sixth was completed on January 9, 2025. The spacecraft will enter the planet's orbit in 2026.
"Perseverance" rover.
On March 5, 2024, NASA released images of transits of the moon [[Deimos (moon)|Deimos]], the moon [[Phobos (moon)|Phobos]] and the planet Mercury as viewed by the "Perseverance" rover on the planet Mars.
See also.
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Notes.
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References.
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External links.
[[Category:Mercury (planet)| ]]
[[Category:Planets of the Solar System]]
[[Category:Terrestrial planets]]
[[Category:Astronomical objects known since antiquity]]
[[Category:Solar System]]