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To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. [81] Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map. [14] This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. 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. [153], 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. [119], Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.[120]. [152], The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. However, using this time-average definition of 'closeness' - as noted above - it turns out that Mercury is the closest planet to. [72] By comparison, the 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. [178] Mariner 10 is thought to be still orbiting the Sun, passing close to Mercury every few months. Smallest and innermost planet from the Sun in the Solar System, The so-called "Weird Terrain" formed at the point, Animation of Mercury's and Earth's revolution around the Sun, Size comparison with other Solar System objects. the semi-major axis, and [1], Mercury's atmosphere consists of different types of elements. The observation demonstrated conclusively that Mercury orbited around the Sun. [71], 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[73] containing hydrogen, helium, oxygen, sodium, calcium, potassium and others at a surface pressure of less than approximately 0.5 nPa (0.005 picobars). [93] 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. [24] 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.[94]. 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. c its orbit appears vertical) and is at maximum elongation from the Sun (28°) and also when the Sun is 18° below the horizon, so the sky is just completely dark. It is important to be clear about the meaning of 'closeness'. [154] This phenomenon is known as synchronous rotation. [24], A rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. The hydrogen and helium in Mercury's atmosphere are believed to come from the solar wind. Mercury's higher velocity when it is near perihelion is clear from the greater distance it covers in each 5-day interval. 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. [80] Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. It was discovered in 1985 by Drew Potter and Tom Morgan. It was discovered in 1986 by Drew Potter and Tom Morgan. [151] The next occultation of Mercury by Venus will be on December 3, 2133. Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Observable universe → UniverseEach arrow (→) may be read as "within" or "part of". [69] Although ice was not the only possible cause of these reflective regions, astronomers think it was the most likely. It has a combined pressure level of about 10−14 bar (1 nPa). 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. [24] Essentially because Mercury is closest to the Sun, when taking an average over time, Mercury is the closest planet to the Earth,[97] and - in that measure - it is the closest planet to each of the other planets in the Solar System. 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. [139] Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the "water star" (水星), based on the Five elements. 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. 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. ϵ [129] The cuneiform name used to designate Mercury on the Mul.Apin tablets is transcribed as Udu.Idim.Gu\u4.Ud ("the jumping planet"). is the orbital eccentricity, This large range arises from the planet's high orbital eccentricity. Mercury can come as near as 82.2 gigametres (0.549 astronomical units; 51.1 million miles) to Earth, and that is slowly declining: The next approach to within 82.1 Gm (51.0 million miles) is in 2679, and to within 82.0 Gm (51.0 million miles) in 4487, but it will not be closer to Earth than 80 Gm (50 million miles) until 28,622. [161], 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. 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[87] This also occurs in Earth's magnetic field. At both of these times Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion. [162] Data from Mariner 10 subsequently confirmed this view. the orbital period. [113] 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. Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking meteoroids. , where {\displaystyle a} [148] (Note that most such medieval reports of transits were later taken as observations of sunspots. The planet is rendered invisible from Earth on both of these occasions because of its being obscured by the Sun,[119] except its new phase during a transit. 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. 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. 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. 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. Radioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. 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. Like the Moon and Venus, Mercury exhibits phases as seen from Earth. {\displaystyle e} [2] The first elements discovered were atomic hydrogen, helium and atomic oxygen. Oxygen is believed to come from Mercury's crust. For example, essentially because Mercury is closer to the Sun than Venus, Mercury spends more time in proximity to Earth; it could, therefore, be said that Mercury is the planet that is 'closest to Earth when averaged over time'. However, this does not mean that the two planets are closest over time. 2 [132] They knew the planet as Στίλβων Stilbōn, meaning "twinkling", and Ἑρμής Hermēs, for its fleeting motion,[133] a name that is retained in modern Greek (Ερμής Ermis). [182] Most of the hemisphere not imaged by Mariner 10 was mapped during these fly-bys. [81] Unlike Earth's, Mercury's poles are nearly aligned with the planet's spin axis. Viewed from low latitudes and at the right times of year, the ecliptic intersects the horizon at a steep angle. [68] 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. [96] Its period of retrograde motion as seen from Earth can vary from 8 to 15 days on either side of inferior conjunction. Due to Mercury's 3:2 spin-orbit resonance, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days. 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