Physical characteristics

Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of . Mercury is also 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.

Internal structure

Mercury appears to have a solid silicate crust and mantle overlying a solid, iron sulfide outer core layer, a deeper liquid core layer, and a solid inner core. 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 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 55% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a mantle consisting of silicates. Based on data from the mission and Earth-based observation, Mercury's crust is estimated to be thick. However, this model may be an overestimate and the crust could be thick based on an Airy isostacy model. 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 major 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 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.

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 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 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 there are two space missions set to make observations. 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 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 mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more heterogeneous than either Mars's or the Moon's, both of which contain significant stretches of similar geology, such as maria and plateaus. Albedo features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, and ray systems. Larger albedo features correspond to higher reflectivity plains. Mercury has dorsa (also called "wrinkle-ridges"), Moon-like highlands, montes (mountains), planitiae (plains), rupes (escarpments), and valles (valleys).

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 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 chemically reducing conditions than is found in the 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. 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 fossae are named for works of architecture. Montes are named for the word "hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments 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

Mercury was heavily bombarded by comets and asteroids 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 volcanically active; basins were filled by magma, producing smooth plains similar to the maria found on the Moon. An unusual crater with radiating troughs has been discovered that scientists called "the spider". It was later named Apollodorus.

Craters on Mercury range in diameter from small bowl-shaped cavities 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 in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity. According to International Astronomical Union (IAU) rules, each new crater must be named after an artist that 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 1,550 km. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~2 km 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 are volcanic lava flows induced by the impact or a large sheet of impact melt.

At the 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.

Overall, 46 impact basins have been identified. A notable basin is the 400 km wide, multi-ring Tolstoj Basin that has an ejecta blanket extending up to 500 km from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km 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 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 30 km in diameter.

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to the 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 localisation 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

One unusual feature of Mercury's surface is the numerous compression folds, or rupes, that crisscross the plains. As Mercury's interior cooled, it contracted and its surface began to deform, creating wrinkle ridges and lobate scarps associated with thrust faults. The scarps can reach lengths of 1000 km and heights of 3 km. 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 ~1 to 7 km. 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 km, 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.

The Lunar Reconnaissance Orbiter discovered that similar but smaller thrust faults exist on the Moon.

Volcanism

There is evidence for pyroclastic flows on Mercury from low-profile shield volcanoes. 51 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 8 km in diameter. It is thus a "compound volcano". The vent floors are at least 1 km 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 of the order of a billion years.

Surface conditions and exosphere

The surface temperature of Mercury ranges from at the most extreme places: 0°N, 0°W, or 180°W. 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. The subsolar point reaches about 700 K during perihelion (0°W or 180°W), but only 550 K at aphelion (90° or 270°W).

On the dark side of the planet, temperatures average 110 K.

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 the daylight temperature at the surface of Mercury is 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 where ice can accumulate. Water ice strongly reflects radar, and observations by the 70-meter Goldstone Solar System Radar and the VLA in the early 1990s revealed that there are patches of high radar reflection near the poles. Although ice was not the only possible cause of these reflective regions, astronomers think it was the most likely.

The icy regions are estimated to contain about 1014–1015 kg of ice, and may be covered by a layer of regolith that inhibits sublimation. By comparison, the Antarctic ice sheet on Earth has a mass of about 4 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 or 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 containing hydrogen, helium, oxygen, sodium, calcium, potassium and others at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen atoms and helium atoms probably come from the solar wind, diffusing into Mercury's magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. MESSENGER found high proportions of calcium, helium, hydroxide, magnesium, oxygen, potassium, silicon and sodium. 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 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 1980–1990s, 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.

On November 29, 2012, NASA confirmed that images from MESSENGER had detected that craters at the north pole contained water ice. MESSENGER principal investigator Sean Solomon is quoted in The New York Times estimating the volume of the ice to be large enough to "encase Washington, D.C., in a frozen block two and a half miles deep".

Magnetic field and magnetosphere

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by , it is about 1.1% the strength of Earth's. The magnetic-field strength at Mercury's equator is about . Like that of Earth, Mercury's magnetic field is dipolar. Unlike Earth's, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the 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 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. This contributes to the space weathering of the planet's surface. Observations taken by the 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 wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as flux transfer events, 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 is 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 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 bulges raised by the Sun that are about 17 times stronger than the Moon's on Earth. Combined with a 3:2 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 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.

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 Mercurian day. This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that the Sun's 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 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 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 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 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 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 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 , 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

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 still in Mercury's sky.

The rare 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 point always 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, enforcing 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 chaotically from nearly zero (circular) to more than 0.45 over millions of years due to 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 orbital resonant perihelion interaction with Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the planet will collide with Venus within the next five billion years.

Advance of perihelion

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, but no such planet was ever found.

The perihelion precession of Mercury is 5,600 arcseconds (1.5556°) per century relative to Earth, or 574.10±0.65 arcseconds per century relative to the inertial ICRF. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5,557 arcseconds (1.5436°) per century. 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 42.98 arcseconds per century for Mercury; it therefore requires a little over twelve million orbits for a full excess turn. Similar, but much smaller, effects exist for other Solar System bodies: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.

Habitability

There may be scientific support, based on studies reported in March 2020, for considering that parts of the planet Mercury may have been habitable, and perhaps that life forms, albeit likely primitive microorganisms, may have existed on the planet.

Observation

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.

Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.

Like the Moon and Venus, Mercury exhibits 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 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 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.

Nonetheless, the brightest (full phase) appearance of Mercury is an essentially impossible time for practical observation, because of the extreme proximity of the Sun. 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 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.

Mercury is more often and easily visible from the Southern Hemisphere than from the 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 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).

Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The first of two spacecraft to visit the planet was , which mapped about 45% of its surface from 1974 to 1975. The second is the MESSENGER spacecraft, which after three Mercury flybys between 2008 and 2009, attained orbit around Mercury on March 17, 2011, to study and map the rest of the planet.

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 in a year makes up a seven-year cycle (0.15 × 7 ≈ 1.0), in the seventh year Mercury follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven years before.

Observation history

Ancient astronomers

The earliest known recorded observations of Mercury are from the Mul.Apin tablets. These observations were most likely made by an Assyrian 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 records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu after the messenger to the gods in their mythology.

The ancients 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 Στίλβων Stilbōn, meaning "twinkling", and Ἑρμής Hermēs, for its fleeting motion, a name that is retained in modern Greek (Ερμής Ermis). The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius), which they equated with the Greek Hermes, because it moves across the sky faster than any other planet. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.

The Greco-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 the transits were too infrequent.

In ancient China, Mercury was known as "the Hour Star" (Chen-xing 辰星). It was associated with the direction north and the phase of water in the Five Phases system of metaphysics. Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the "water star" (水星), based on the 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 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.

In medieval Islamic astronomy, the 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 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 Maragha astronomer Qotb al-Din Shirazi in the 13th century. (Note that most such medieval reports of transits were later taken as observations of sunspots.)

In India, the 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 Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. 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 around 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 mean that it has been 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 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 300-meter 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.

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 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, recent technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory 1.5 meter Hale telescope. They provided the first views that resolved surface features on the parts of Mercury that were not imaged in the 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

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 large change in velocity (delta-v) to get to Mercury and then enter orbit, as compared to the delta-v required for, say, 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 the Solar System completely. As a result, only two 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

The first spacecraft to visit Mercury was NASA's (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. 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 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 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, ran out of fuel. Because its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut down. is thought to be still orbiting the Sun, passing close to Mercury every few months.

MESSENGER

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 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.

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 , assorted spectrometers to determine abundances of elements in the crust, and magnetometers 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 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 near 3:26 PM 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.

Comparison

概述

由於水星十分接近太陽，時常被太陽光所籠罩，勘測相當困難，因此我們對水星的所知相當有限，迄今只有兩艘太空船曾大致勘察過水星。第一艘是1974至1975年的水手10號，只描繪了45%的水星表面圖。第二艘是信使號，在2008年1月14日掠過水星，描繪了另外30%的表面。信使號於2011年3月17日再度抵達水星，並且進入環繞軌道，開始對水星表面進行全面的探測。

實際上，水星外觀很像月球，表面有許多的坑穴，沒有天然衛星，也沒有實際的大氣層；它有巨大的鐵核，磁場強度約是地球的1%

。由於水星有著巨大的核，它的密度非常高。水星的表面溫度為90至700K（-180至430 °C），日下點是最熱的地方，靠近地理極的坑穴底部是溫度最低之處。

水星的觀測紀錄可以追溯到西元前3,000年的蘇美爾人，希臘的赫西俄德時代稱之為（拉丁化：「Stilbon'」）（「the gleaming」）和「Hermaon」。今天英文中的名稱來自羅馬，是羅馬神話中眾神的信使墨丘利（），相當於希臘的赫耳墨斯（）和巴比倫的納布。在天文學上的符號是一個古老的星占符號，一個很有風格的版本是帶著有翅膀的頭盔持著眾神手杖（）的「傳信天使」。在西元前5世紀，希臘天文學家認為水星是兩個不同的天體，這是因為它時常交替地出現在太陽的兩側；一顆出現在日落之後，它被叫做墨丘利；另一顆則出現在日出之前，為了紀念太陽神阿波羅，它被稱為阿波羅。畢達哥拉斯後來指出他們實際上是相同的一顆行星。

在印度，水星被稱為「Budha」（बुध），是月亮之神（「Chandra」）的兒子；在希伯來，稱為「Kokhav Hamah」（כוכב חמה），意思是來自太陽的炎熱之星。

在中國，水星是五行之一，又稱為「辰星」。《晉書》內提及：「辰星曰北方冬水，智也，聽也。智虧聽失，逆冬令，傷水氣，罰見辰星。辰星見，則主刑，主廷尉，主燕趙，又為燕、趙、代以北；宰相之象。亦為殺伐之氣，戰鬥之象。又曰，軍於野，辰星為偏將之象，無軍為刑事。和陰陽，應效不效，其時不和。出失其時，寒署失其節，邦當大饑。當出不出，是謂擊卒，兵大起。在於房心間，地動。亦曰，辰星出入躁疾，常主夷狄。又曰，蠻夷之星也，亦主刑法之得失。色黃而小，地大動。光明與月相逮，其國大水。」

內部構造

水星是太陽系內與地球相似的4顆類地行星之一，有著與地球一樣的岩石個體。它在赤道的半徑為2,439.7公里，是太陽系中最小的行星，水星甚至比一些巨大的天然衛星，像是甘尼米德和泰坦，還要小 － 雖然質量較大。水星由大約70%的金屬和30%的矽酸鹽材料組成，水星的密度是每立方公分5.427公克，在太陽系中是第二高的，僅次於地球的每立方公分5.515公克。如果不考慮重力壓縮對物質密度的影響，水星物質的密度將是最高的。未經重力壓縮的水星物質密度是每立方公分5.3公克，相較之下地球物質只有每立方公分4.4。

從水星的密度可以推測其內部結構細節。地球的高密度，特別是核心的高密度，是由重力壓縮所導致的。水星的質量及重力是如此的小，它的內部不會被強力的擠壓，所以它要有如此高的密度，其核心必然是巨大的且含有許多的鐵。

地質學家估計水星的核心佔有體積的55%；地球的核心只佔體積的17%。水星富鐵的核心占據了其總質量的至少60%，它的半徑更是達到了水星半徑的四分之三。最近的研究強烈支持水星有一個熔融的核心，包圍著核心的是500–700公里厚的矽酸鹽地函。太陽系類地行星中，只有水星和地球擁有全球性的磁場。天文學家認為這些磁場是由它們核心外層中的電流所產生。根據水手10號任務和從地球觀察的資料，水星的地殼被認為只有100-300公里的厚度。水星表面的一大特徵是有無數的窄脊，可以延伸到數百公里長，相信都是在水星的地殼凝固後，核心和地函因冷卻而收縮造成的。

水星核心含有的鐵高出太陽系內任何主要的行星，已經有幾種理論被提出來解釋。得到最廣泛支持的理論是水星原本有著類於於常見的球粒隕石金屬-矽酸鹽比率的核心，被認為是太陽系內典型的岩石物質，質量大約是目前質量的2.25倍。在太陽系早期的歷史中，水星可能遭受到一顆直徑數百公里，質量約為其1/6的微行星撞擊。這次撞擊剝離了大量原始的地殼和地函，留下的核心就相對的成為組成中較大的部分。這一假說得到了信使號分光儀對水星表面元素豐度觀測的支持。一個類似的假說，稱為巨大撞擊假說，被用來解釋地球的衛星，月球的形成。

另一假說為，水星在太陽輸出的能量穩定下來之前就已經在太陽星雲中形成。這顆行星原本的質量是目前的兩倍，但在原行星的收縮過程中。當時水星的溫度可能在2,500-3,500K，並且可能高達10,000K，水星表面許多的岩石成份在如此的高溫下可能都汽化，成為大氣層中的"岩石蒸汽"，然後被太陽風帶走了。

第三種假說認為，太陽星雲造成水星吸積的物質被拖曳，這意味著水星表面較輕的物質會從吸積的材料中丟失。每種假說預測的水星表面有不同的成分，信使號和即將執行的貝皮可倫坡號任務都試圖經由觀測來測試上述的學說。信使號已經發現表面的鉀和硫的含量在預測水準之上，巨大撞擊假說的地殼和地函的汽化未曾發生，因為鉀和硫都會在這些事件的高溫下被驅離。此一發現似乎傾向於較輕的行星材料受到拖曳而離開，造成較重的金屬材料被濃縮。

信使號的分光儀已經測量水星的組成，科學家發現水星的岩石所含的鎂比起地球或月球表面要多得多，而鋁則少得多。

表面地質

水星的表面與月球很相似，呈現出像海的廣大平原和大量的撞擊坑，顯示它數十億年來都處於非地質活動狀態。我們對水星地質的認識建立在1975年飛越水星的水手10號和地面的觀測，它是我們了解最少的類地行星。當信使號最近飛越水星的資料被處理過後，這方面的知識將會有所增進。例如，科學家們已經發現一個不尋常的火山口輻射槽，稱之為「蜘蛛」。稍後，被重新命名為阿波羅多羅斯。

在水星表面特徵的命名有著不同的來源，取自已經過世的人名。坑穴使用藝術家、音樂家、書畫家和作家，他們都在各自的領域中有著傑出或基礎的貢獻。山脊或皺脊，以對水星的研究有貢獻的科學家命名；窪地或地溝以建築師來命名。山脈以各種不同語言中熱門的單詞來命名；平原或平原低地以各種不同語言的水星之神名稱來命名。懸崖或峭壁以科學探險船命名；山谷或谷地則使用電波望遠鏡命名。

反照率特徵指使用不同領域的望遠鏡觀測，明顯的有不同反照率的地點。水星擁有山脊 (有時也稱為皺脊)，像月球的高地、山脈 (山)、平原或平原低地 (Planitia)、懸崖 (Rupes) 和谷地 (山谷)。

水星在46億年前形成時，曾經經歷過彗星和小行星短暫的輪番轟擊，在38億年前結束，可能是獨立發生的後期重轟炸期。在這些劇烈形成隕石坑的期間，由於缺乏大氣層來減緩撞擊，行星表面整個都被隕石坑覆蓋著。在這個期間，行星有著火山的活動，像是卡洛里盆地等盆地都被來自行星內部的岩漿覆蓋著，形成如同在月球上發現的海一樣的平原。

信使號於2008年10月28日飛越水星，讓研究人員獲得更多鑑別水星表面渾沌地形的資料。水星的表面比火星和月球更為複雜 (詭異)，它包含了大量在兩者上都值得注意的類似地質，像是海和平原等。

撞擊盆地和坑穴

水星坑穴的範圍，在直徑上從小型的碗型腔到跨越數百公里的多環撞擊坑。從相對新鮮亮麗到高度退化火山口的殘餘物，展示了所有退化階段的現象。水星的撞擊坑與月球的有著微妙的差異，它們的噴發物覆蓋的區域小得多，這顯示水星有較強的表面重力。

已知最大的隕石坑之一是，直徑為1,550公里。撞擊並創建卡洛里盆地的影響是如此的強大，它造成的火山熔岩噴發，留下高度在2公里以上的同心圓環圍繞著隕石坑。在卡洛里盆地的對蹠點是不尋常的、被稱為「怪異地形」的大片丘陵地形區域。這種地形起源的一種假說是：撞擊出卡洛里盆地的激震波環繞著行星，匯聚在盆地的對蹠點（相距180度），結果造成了高應力的裂縫表面；另一種說法則認為是噴出物直接匯聚在卡洛里盆地對蹠點的結果。

整體而言，在已有的水星影像中大約已經發現15個撞擊盆地。一個顯著的盆地是400公里寬、有著多重環的托爾斯泰盆地，它的噴發物覆蓋造成的平原，從山脊和地板延伸達500公里。直徑625公里的貝多芬盆地有著相似規模的噴發覆蓋物。和月球一樣，水星的表面也有遭受太空風化過程的影響，包括太陽風和微隕石撞擊的影響。

平原

水星有兩種地質顯著不同的平原。在坑穴之間，起伏平緩、多丘陵的平原，是水星表面可見最古老的地區，早於猛烈的火山口地形。這些埋藏著隕石坑的平原似乎已湮滅許多較早的隕石坑，並且缺乏直徑在30公里以下，以及更小的隕石坑。還不清楚它們是起源於火山還是撞擊，這些埋藏著隕石坑的平原大致是均勻的分布在整個行星的表面。

平坦的平原是廣泛的平坦區域，布滿了各種大大小小的凹陷，和月球的海非常相似。值得注意的是，它們廣泛的環繞在卡洛里盆地的周圍。不同於月海，水星平坦的平原和埋藏著隕石坑的古老平原有著相同的反照率。儘管缺乏明確的火山特徵，在地化的平台和圓角、分裂的形狀都強烈的支持這些平原起源於火山。值得注意的是，所有水星平坦平原的形成都比卡洛里盆地晚，比較在卡洛里噴發覆蓋物上可察覺的小隕石坑密度可見一斑。卡洛里盆地的地板填滿了獨特的平原地質，破碎的山脊和粗略的多邊形碎裂。不清楚是撞擊誘導火山熔岩，還是撞擊造成大片的融化。

行星表面一個不尋常的特徵是眾多的壓縮皺褶或懸崖，在平原表面交錯著。隨著行星內部的冷卻，它可能會略為收縮，並且表面開始變型，造成了這些特徵。凹陷也在其它地形，像是坑穴和平滑的平原，頂部看見，顯示這些皺褶是在最近才形成的。水星的表面也會被太陽扭曲 － 太陽對水星的潮汐力比月球對地球的強17倍。

信使號在水星北極地區發現了水星上最大的火山平原開闊區之一，覆蓋面積約400萬平方千米，深度幾千米。它幫助確認了火山活動在水星曆史的大多數時間裡對于塑造其地殼起到了關鍵作用。

表面狀態和"大氣層" (外逸層)

由於缺乏大氣的包圍，水星表面的赤道和兩極之間有著陡峭的溫度差，溫度範圍從100K至700K。日下點的溫度在近日點時高達700K，而在遠日點時只有550K；在行星夜晚的那一側，平均溫度是110K。陽光的強度範圍是太陽常數 (1,370 W·m−2) 的4.59和10.61倍。

雖然水星表面的溫度在白天是非常的高，但觀測的結果仍然強烈的支持冰 (凍結的水) 存在於水星。在極區深坑的底部從未被陽光直接照射過，溫度依然維持在102K以下，遠低於全球的平均溫水冰強烈的反射了雷達，金石70米的望遠鏡和VLA在1990年代早期的觀測，透漏了在接近極區有非常高的雷達反射斑點。雖然冰不是造成這些反射區域的唯一可能原因，但天文學家相信冰是最有可能的。

相信冰的區域擁有大約1014–1015公斤的冰，並且可能覆蓋著一層表岩屑，抑制了昇華。相較之下，地球南極的冰層大約有4公斤的冰，火星南極的冰帽大約有1016公斤的冰。水星上冰的來源還不清楚，但有兩種最可能的來源：從行星內部排放出來的，或是彗星撞擊造成的沉積。

2012年11月29日，水星探測衛星信使號團隊發言人表示，科學家在水星北極區域永遠曬不到太陽的陰暗坑洞內發現大量凍冰（重量可能多達1012公噸）。

水星不僅太小，而且太熱，因此它的引力不足以長期留住大氣層；但它確實有一個稀薄的、侷限在表面的外逸層，包含著氫、氦、氧、硫、鈣、鉀和其它元素。這個外逸層並不穩定，原子會不斷的失去和由其它不同的來源獲得補充。氫和氦可能來自太陽風，並在逃逸回太空之前先擴散至水星的磁層。元素的放射性衰變是水星地殼內氦、鈉和鉀的另一個來源。信使號發現鈣、氦、氫氧化物、鎂、氧、鉀、矽和鈉的比例偏高。也有水蒸氣的存在，組合的過程發表如下：彗星撞擊其表面，濺射創造出的水，其中的氫來自太陽風，氧來自岩石，和在極區坑洞內永久陰影下儲存的冰昇華。檢測到許多由水釋出的離子，如O+、OH-、和H2O+則是一個驚喜。由於這些為數可觀的離子是在水星的太空環境中發現的，因此科學家推測是被太陽風從水星表面或外逸層摧毀的分子。

在1980-1990年代，在大氣層中發現鈉、鉀、鈣，相信主要是表面的岩石被微隕石撞擊汽化導致的。在2008年，信使號探測器發現了鎂。研究指出，鈉的排放是區域性的點，對應於這顆行星的磁極。這將顯示出在磁層和行星表面之間的交互作用。

磁場和磁氣層

儘管水星很小和以59天的長週期自轉，水星仍有值得注意的全球性磁場。根據水手10號的測量，他的強度僅有地球的1.1%。在水星赤道的磁場強度大約是300nT。像地球一樣，水星的磁場是雙極的。不同於地球的是，水星的磁極和水星的自轉軸幾乎是一致的。來自水手10號和信使號兩艘太空船的測量，都指出水星磁場的強度和形狀都是穩定的。

這個磁場可能是經由發電機效應形成的，有些類似於地球的磁場。這種發電機效應起因於行星富含鐵的液體核心的循環，特別是行星軌道的高離心率帶來強烈的潮汐作用，使核心保持液態更是發電機效應所必須的。

水星磁場的強度足以偏轉圍繞著該行星的太陽風，創造出磁層。水星的磁層雖然很小，但已足以將地球包含在內，也強到可以將太陽風的電漿拘束在內，對行星表面的太空風化產生貢獻。水手10號太空船的觀測在水星夜半側的磁層內部偵測到低能量的電漿，在磁尾也偵測到高能量的微粒爆炸，這些都顯示了水星磁層的動力學性質。

在2008年10月6日的第二次飛掠水星，信使號發現水星的磁場有甚高頻的「滲漏」。太空船遭遇到磁性的「龍捲風」－ 纏繞扭曲的磁場與行星磁場聯結並深入行星際空間 － 寬度達到800公里，或是行星半徑的1/3。這個龍捲風形成時夾帶著太陽風的磁場聯結到水星的磁場。隨著太陽風刮過水星的磁場，這些聯結的磁場會被攜走和扭曲成類似漩渦狀的結構。這些扭曲的磁通量管，技術上稱為通量傳輸事件，形成行星磁盾中開放的窗口，太陽風可以長驅直入並直接撞擊到水星的表面。

這種聯結行星際和行星磁場的過程稱為磁重聯，在宇宙中是很普遍的。它也發生在地球的磁場，通常也會產生磁場的龍捲風。信使號的觀測顯示重聯結的速率在水星高出了10倍。但依水星和太陽的距離，信使號觀測到的重聯結僅有1/3。

軌道和自轉

水星是所有的行星中離心率最大的；它的離心率是0.21，使它與太陽的距離在4600萬至7000萬公里的範圍之間變動。它以87.969地球日的週期完整地公轉太陽一圈。

右邊的水星軌道圖疊加上有著相同半長軸的圓形軌道，以顯示出軌道離心率造成的影響。以5天為間隔的標示顯示出在近日點時有著較大的距離，清楚的顯示出比較高的軌道速度。球的大小，與它們和太陽的距離成反比，用來說明日心距離的變化。到太陽距離的變化，結合行星繞著自轉軸的自轉軌道共振，造成表面溫度複雜的變化。

這種共振使得一個水星日的長度是水星的兩年，或是大約176個地球日。

水星的軌道平面對地球的軌道平面 (黃道) 有著7度的傾斜，顯示在右圖中。結果是，水星橫越過太陽前方的凌日，只有在水星穿越黃道平面之際，也位於地球和太陽之間時才會發生。平均下來，大約7年才會發生一次。

水星的轉軸傾角幾乎是零，最佳的測量值小於0.027度。這明顯的遠小於木星，它是轉軸傾角第二小的行星，數值為3.1度。這意味著位於水星極點的觀測者，太陽中心點的高度永遠不會高於地平線上2.1弧分。

在水星表面上的某些點，觀測者可以看見太陽上升到半途時，會反轉回去日落，然後再度日出；在所有的點上，這些都發生在同一個水星日。這是因為在近日點前大約4個地球日時，水星軌道的角速度，幾乎與他的自轉速度相同，所以太陽的視運動會停滯；在近日點時，水星軌道的角速度超過水星自轉的角速度。因此，對假設在水星上的觀測者，會明顯的看到太陽逆行。通過近日點4天之後，在這些點上觀測到的太陽視運動又恢復正常了。

水星與地球內合 (最靠近地球) 的周期平均是116地球日，但是由於水星軌道的離心率，這個間隔從105日至129日不等。水星與地球的距離可以近到7730萬公里，但在AD28,622年之前不會接近至8000萬公里以內，最近的接近是在2679年的8210萬公里，然後是4487年的8200萬公里。從地球可以看見它逆行的時間大約是在內合前後的8-15天，所以會有如此大範圍差距變化，完全是因為它有著較大的離心率。

自旋軌道共振

1889年意大利天文學家喬凡尼·斯基亞帕雷利經過多年觀測認為水星自轉時間和公轉時間都是88天。

許多年以來，水星被認為是與太陽同步的潮汐鎖定，在每一次的軌道公轉中都以同一面朝向太陽，就像月球始終以同一面朝向地球。在1965年的雷達觀測，美國天文學家才測量出水星自轉的精確週期是58.646天，證明水星以3:2的自旋軌道共振，每公轉太陽二次時也自轉三次；而水星軌道的高離心率使得此共振穩定 － 在近日點，太陽的潮汐力最強，太陽也平靜 (穩定) 的出現在最靠近水星的天空。

起初，天文學家認為它被同步鎖定的原因是，當水星在適合觀測的位置上時，它幾乎總是在3:2共振的相同位置上，因此呈現相同的面貌。這也是因為水星公轉週期與地球會合週期一半的巧合，由於水星3:2的自旋軌道共振，一太陽日 (太陽兩次過中天的時間間隔) 約176地球日 。一恆星日 (自轉週期)則約59地球日。

模擬的研究顯示水星軌道的離心率是混亂的，在數百萬年的時間內會因為其它行星的攝動從接近0 (圓形) 至超過0.45之間變動。這被認為可以解釋水星的3:2自旋軌道共振 (而非更常見的1:1)，因為這種狀態在高離心率軌道的時期中是可能發生的。數值模擬顯示未來長期軌道共振，與木星的交互作用會造成近日點距離的增加，在未來的50億年內有1%的機率會與金星碰撞。

近日點的前進

1859年，法國數學家和天文學家奧本·勒維耶報告水星環繞太陽的軌道有著牛頓力學和現有已知的行星攝動不能完滿解釋的緩慢進動。他建議用「另一顆行星 (或一系列更微小天體) 位於比水星更靠近太陽的軌道上」來處理這些攝動 (其它的解釋包括太陽略微的扁平)。基於天王星的軌道受到擾動而發現了海王星的成功，使天文學家對這個解釋充滿了信心，並且這個假設的行星被命名為瓦肯，但是始終未能發現這顆行星。

水星相對於地球的近日點進動是每世紀5,600弧秒(1.5556度)，或是相對於慣性ICFR每世紀574.10 ±0.65弧秒；但牛頓力學考慮了來自其它行星所有的影響，預測的進動只有每世紀5,557弧秒 (1.5436度)。在20世紀初期，愛因斯坦的廣義相對論對觀測到的進動提供了解釋。這個效應非常小：水星近日點的相對論進動是每世紀42.98弧秒，剛剛好是之前不足的值；然而，在經歷1,200萬次的公轉之後，它仍有一點點的過剩。其它行星也有非常類似的情形，但是影響小了很多：金星是每世紀8.62弧秒，地球是3.84弧秒，火星是1.35弧秒，伊卡路斯是10.05弧秒。

座標系統

水星的經度是向西增加的，一個被命名為Hun Kal的小坑穴被選定作為經度的參考點，它的中心被定義為西經20°

。

觀測

水星的視星等介於 −2.6等（比最亮的恆星天狼星更亮）和 +5.7等（接近理論上裸眼可見的極限值）之間。這兩個極端值都出現於水星在天空中的視位置接近太陽的時候。由於它很接近太陽，因此觀測上很麻煩，大部分的時間都會迷失在陽光中，只有在日出前或日落後短暫的暮曙光內可以看見。

水星，像其它一些行星和明亮的恆星一樣，可以在日全食的時間被看見。

像月球和金星一樣，從地球上可以觀察到水星的相位。它的「新月」出現在內合，「滿月」出現在在外合。由於它相對的過度貼近太陽，因此從地球上是看不見水星呈現這兩種相位。

水星探索

早期

水星最早被閃族人在（公元前三千年）發現，他們叫它 Ubu-idim-gud-ud。最早的詳細記錄觀察數據的是巴比倫人，他們叫它 gu-ad 或 gu-utu。希臘人給它起了兩個古老的名字，當它出現在早晨時叫阿波羅，當它出現在傍晚叫赫耳墨斯，但是希臘天文學家知道這兩個名字表示的是同一星體。希臘哲學家赫拉克利特甚至已經認為水星和金星（維納斯星）是繞太陽公轉的而不是地球。水星的觀測因為它過於接近太陽而變的非常複雜；在地球可以觀測它的唯一時間是在日出或日落時。

美國國家航空暨太空總署

第一個靠近水星的航天器是水手10號。另一個被美國國家航空暨太空總署批準的計畫，被命名為MESSENGER（「信使號」，是MErcury Surface, Space ENvironment, GEochemistry, and Ranging的字母縮寫，意為 「水星表面，空間環境，地理化學和全向遙測」），信使號已在2004年8月發射，2011年3月18日進入圍繞水星運行的軌道，成為首顆圍繞水星運行的探測器。

水手10號

第一艘探測水星的太空船是NASA的水手10號 (1974-1975年)。這艘太空船使用金星的引力調整它的軌道速度，使它能夠接近水星，並使它成為第一艘使用重力助推效應，和NASA第一次拜訪多顆行星的太空任務。水手10號提供了第一批的水星表面特寫影像，其中立刻顯示出水星有大量環型山的性質，並透漏許多其他類型的地質特徵，像是巨型的陡坡，後來歸因於水星的鐵核冷卻時稍為收縮造成的。不幸的是，由於水星軌道公轉週期的長度，使得水手10號每次接近時觀察的都是水星的同一側。這使得水手10號不可能觀察到完全的水星表面，結果是完成的水星表面地圖少於45% 。

在1974年3月27日，首次飛越水星的兩天前，水手10號的儀器意外的發現水星附近有大量的紫外線輻射，這導致初步認定水星有衛星。不久之後，過量的紫外線被確認是巨爵座31號星的，而水星的衛星就成為天文歷史書上的一個註腳。

這艘太空船三度飛臨水星，最接近時與表面的距離只有327公里。在第一次接近時，儀器偵測到水星有磁場，這使得行星地質學家大為驚訝 － 因為水星的自轉極為緩慢，不致於產生發電機效應。第二次的接近主要是要拍攝影像，但在第三次接近時，獲得了廣泛的磁性資料。這些資料顯示水星的磁場非常類似於地球，使得水星周圍的太陽風產生偏離。水星磁場的起源依然有幾個主要的理論在相互競爭。

在1975年3月24日，就在最後一次接近水星之後8天，水手10號耗盡了燃料。由於不再能精確的控制他的軌道，於是任務控制者關閉了探測器的儀器。水手10號被認為仍然環繞著太陽，每隔幾個月仍會接近水星一次。

信使號

信使號是NASA前往水星的第二艘太空船，於2004年8月3日使用波音戴爾他2型火箭從卡納維拉爾角空軍基地發射。它在2005年8月飛越地球，並在2006年10月和2007年6月掠過金星，將它調整至正確的軌道，以達到能環繞水星的軌道。在2008年1月14日，信使號首度飛越水星，2008年10月6日再度飛越，並於2009年9月29日第三度飛越。在這幾次的飛越中，將水手10號未曾拍攝的半球都拍攝了。探測器在2011年3月18日成功進入繞行水星的橢圓軌道。信使號是在一個大橢圓軌道上以12小時為周期繞水星轉動，距離水星表面最近時距離為200千米，最遠則可達15,193千米。它軌道的最低點位于水星北緯60度的上空，之所以這樣選擇部分是為了能詳細地研究巨大的卡洛里盆地。這個盆地直徑1,550千米，是水星最大的表面特徵。並在2011年3月29日獲得了第一張在軌道上的水星影像。探測器已經完成一年的製圖任務，現在正在進行預定在2013年完成的另一年延伸探測任務。除了繼續觀測水星和繪製地圖之外，信使號也將觀察2012年的太陽極大期。

這項任務要釐清六個關鍵的問題：水星的高密度、地質歷史、磁場的本質、核的結構、兩極是否有冰？以及稀薄的大氣是如何形成的。為了達到這些目的，探測器攜帶了比水手10號的儀器解析度更高許多的影像成像設備，各式光譜儀測量地殼中元素的豐度，和磁強計等設備來測量帶電粒子的速度。詳細測量探測器在軌道速度上的微小變化，用來推斷水星內部構造的詳細資訊。

貝皮可倫坡號

歐洲太空總署計畫和日本合作，以兩艘太空船環繞水星：一艘描繪水星地圖，另一艘研究它的磁氣層，稱為貝皮可倫坡號的探測計畫。在2018年10月20日發射太空船，預期將於2025年前抵達水星。載具將釋放一個磁強計進入環繞水星的橢圓軌道，然後化學火箭將點燃，讓繪製地圖的探測器進入圓軌道。這兩個探測器都將運作一個地球年。繪圖探測器將攜帶類似於信使號的光譜儀，和在許多不同的波長上研究這顆行星，包括紅外線、紫外線、X射線和伽馬射線。

俄國人計畫在2011年-2012年之間用聯盟火箭送出他們的飛船，飛船將在四年後到達水星，將會環繞軌道飛行，繪製地圖並且研究它的磁場。

成為人類殖民地的可能

在水星南北極的環形山是一個很有可能適合成為地球外人類殖民地的地方，因為那裏的溫度常年恆定（大約-200℃）。這是因為水星微弱的軸傾斜以及因為基本沒有大氣，所以有日光照射的部分的熱量很難攜帶至此，即使水星兩極較為淺的環形山底部也總是黑暗的。適當的人類活動將能加熱殖民地以達到一個舒適的溫度，相比週圍大部分區域來說，較低的環境溫度將能使散失的熱量更易處理。

關于水星的科幻

水星是科幻小說作者感興趣的題材。主題主要包括暴露在太陽輻射下的危險；停留在水星緩慢移動的晨昏圈（白天與夜晚之間的界綫）上被過度輻射所傷害的可能和獨裁政府（可能因為水星表面溫度很高的緣故）。

在文化中

在西洋占星學，水星統領的星宮是雙子宮和室女宮。也就是當水星在這些星宮時對這兩個星宮的人影響最大。

在天文學家於最近幾十年創建詳細的水星地圖前，Solitudo Hermae Trismegisti (荒蕪的Hermes Trismegistus)被認為是水星的一大特色，覆蓋了行星1/4的東南象限。

墨丘利，是在古斯塔夫·霍爾斯特的音樂，行星組曲中運動的四棱使者。

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