This 47 cents postage stamp was issued by America on 31 May 2016 (= Scott Catalogue # 5069). It shows a false color photograph of Planet Mercury that was taken by the MESSENGER spacecraft, the only one to ever orbit Mercury.
The planets of the Solar System are categorized into four types: terrestrial planets, gas giant planets, ice giant planets, and dwarf planets. The terrestrial planets are Mercury, Venus, Earth, and Mars. The gas giant planets are Jupiter and Saturn. The ice giant planets are Uranus and Neptune. Dwarf planets include named and unnamed objects - for example, Ceres, Pluto, Quaoar, Sedna, Eris, Varuna, Makemake, and many others. Only two dwarf planets have been visited and imaged (Ceres and Pluto).
In many ways, Planet Mercury is an "End Member Planet" - it is the # 1 closest planet to the Sun, it is the # 1 highest-density planet (after Earth), it has the # 1 most intense solar radiation and tidal forces, has the # 1 highest range in surface temperatures, and is the # 1 most geochemically altered planet from its primordial state.
Mercury has been visited three times - twice in the form of spacecraft fly-bys (Mariner 10 in the 1970s and BepiColombo in the 2020s) and once by an orbiter (MESSENGER, from 2011 to 2015). BepiColombo is expected to start orbiting Mercury in 2026.
Planet Mercury orbits the Sun at a distance of ~0.31 to 0.47 astronomical units ("A.U."; one astronomical unit is the distance between the center of the Sun and the center of Earth). The orbit of Mercury is prograde, as are all the other planets. "Prograde" means an orbit in a counterclockwise direction when viewed above the North Pole of the Sun. Mercury's rotation is also prograde.
Of the four terrestrial planets, Mercury is the smallest, with a radius of about 2440 kilometers (= distance from the center of the core to the top of the crust). It's about 38% the size of Planet Earth.
The density of Planet Mercury is about 5.4 grams per cubic centimeter, which is the # 2 highest density world (Earth is # 1) - Mercury is very heavy for its size.
Mercury rotates very slowly - for every 2 orbits around the Sun, it rotates 3 times. Compare that with Earth - as Earth orbits once around the Sun, it rotates 365 times. With respect to the background stars, one Mercury day is about 59 days long. One solar day on Mercury (= sunrise to sunrise) is about 176 days - one solar day on Mercury is actually longer than one Mercury year (!), which is about 88 days.
Unlike Earth, Mercury has almost no axial tilt - it spins almost straight up-and-down. As a result, Mercury has no seasons (no fall, no winter, no spring, no summer).
Mercury has no moons / satellites. Only two of the four terrestrial planets have moons (Earth and Mars).
Unlike the other three terrestrial planets, Mercury has no air - no atmosphere. It does have an "exosphere", which refers to chemicals that are so sparsely-scattered around Mercury that the atoms and molecules don't bounce off each other - they don't behave as gases. The exospheric pressure of Mercury is about a quadrillion times less than Earth's atmospheric pressure. The chemicals in the Mercurian exosphere are derived from its own crust and from solar wind. These include atomic hydrogen (H), molecular hydrogen (H2), helium (He), atomic oxygen (O), molecular oxygen (O2), water (H2O), carbon dioxide (CO2), neon (Ne), sodium (Na), magnesium (Mg), silicon (Si), sulfur (S), argon (Ar), potassium (K), calcium (Ca), iron (Fe), and others.
The term "albedo" refers to the amount of sunlight reflected back into space. Mercury has a fairly dark-colored surface - this is called "low albedo". Mercury's value is 8.8%. A light-colored body has a high albedo, with a high percentage of reflected sunlight. Mercury's dark surface is due to space weathering and the presence of opaque minerals in the surface rocks.
Mercury has a weak magnetic field - some worlds have no magnetic field, while others have strong fields. In the case of Mercury, its magnetic field is about 1% as strong as Earth's.
Surprisingly, Mercury's surface temperatures range from very hot to very cold: about +427º Celsius to -193º Celsius (= ~+800º Fahrenheit to -315º Fahrenheit). Being so close to the Sun, one would expect the world to be quite hot all the time. Its slow rotation rate and lack of an atmosphere means the "dark side" of Mercury loses heat quickly and becomes quite cold. Mercury has the # 1 greatest range in surface temperatures in the entire Solar System.
All four terrestrial planets, including Mercury, are differentiated - they all have a central iron core surrounded by a rocky mantle and a rocky crust. Mercury's core is relatively large for the size of the planet, which is strange. The core makes up about 58% of the planet's volume and about 75 to 85% of the planet's radius. The Mercurian core is thought to have two parts, like Earth's core - a solid inner core and a liquid outer core. The top of the outer core may be solid as well. A 2019 study concluded that Mercury's inner core is about half the size of the entire core. The solid inner core is about 2000 kilometers across (= ~1260 miles in diameter). Compositionally, the Mercurian inner core is thought to be a solid iron-nickel alloy (Fe-Ni) or a solid iron-sulfur-silicon alloy (Fe-S-Si). The liquid outer core is possibly composed of molten iron and molten iron sulfide ("fool's gold"), but there could be other elements. The liquid nature of the outer core is demonstrable based on variations in the planet's rotation / spin.
Why is the Mercurian core relatively large? Several hypotheses have been proposed. Example: a giant impact stripped away most of Mercury's mantle and crust. Example: Mercury formed by the accretion of enstatite chondrite-rich planetesimals, which are iron-rich. Example: Mercury formed at the Iron Line in early Solar System history - this region was so hot, only iron condensed from the nebula at this site ~4.5 billion years ago. The Iron Line for the Solar System is at 0.4 A.U., which is exactly where Mercury is located.
Mercury's mantle and crust are composed of silicate minerals, as are the mantle and crust of Earth. These include the minerals olivine, pyroxene, and plagioclase feldspar. The mantle is relatively thin for the size of the world, at about 500 to 700 kilometers thick (one estimate has the mantle at less than 500 kilometers thick). Overall, the crust is thin (~20 to 40 kilometers thick in the North Polar region), but thickens in equatorial areas (~50 to 80 kilometers thick). Thinner crust occurs below some impact basins. Very dark-colored rings occur around some impact craters (e.g., Atget Crater, Basho Crater, Tolstoj Crater, Rachmaninoff Crater). The material was excavated from Mercury's subsurface by impact events and has been interpreted as graphite, one of the polymorphs of carbon (C). Graphite is probably just one component of Mercury's crust, but this is uncertain.
Mercury lacks the traditional weathering styles seen on Earth (mechanical weathering and chemical weathering), but it does have intense space weathering, which does not occur on Earth. Because there is no air and only a weak magnetic field, the Mercurian surface gets nearly the full force of solar wind and solar radiation.
Like the Moon, Mercury has an old, heavily cratered surface. Simple impact craters, complex impact craters, secondary craters, and large impact basins are present. Craters are not uniformly distributed on the planet's surface, however. Some craters have been buried by lava flows. If a crater's outline is still evident, it is nicknamed a "ghost crater".
The # 1 largest impact structure on Mercury is the ~3.8 to 3.9 billion year old Caloris Basin (= light-colored, ~rounded feature in the upper right part of the photo on the stamp). It is the largest geographic feature on the entire planet. It's a huge impact basin with an estimated 1540 kilometer diameter. Concentric rings occur beyond the basin itself. Ringing the basin are the Caloris Mountains - they are about 2 kilometers tall. Impact debris from the Caloris Impact reach about 1000 kilometers beyond the edges of the basin. The original impacting body was probably over 100 kilometers in size. Caloris Basin is floored by lava flows, as is much of the rest of Mercury's surface.
Antipodal to Caloris (= on the opposite side of the planet) is a topographically rough area nicknamed "Weird Terrain" or "Chaotic Terrain". The area is though to have formed when Caloris Impact shock waves traveled through the planet and converged there, or by ejected Caloris impact debris converging there.
"Hollows" are odd features unique to Planet Mercury. Hollows are shallow, irregular, rimless depressions associated with impact craters. There are about 50 heavily hollowed craters on Mercury - most are at mid-latitudes, with none in the Northern Volcanic Field. Hollows are usually developed atop ~flat, thick sheets of impact melt rocks. Apparently, they formed by slow erosion (via sublimation) of a massive sulfide deposit. The sublimating mineral may be oldhamite ((Ca,Mg)S), a calcium magnesium sulfide mineral that is rare on Earth. The oldhamite identification is based on spectroscopic information, consideration of Mercury surface conditions, and lab experiments.
Many worlds have volcanoes, volcanic activity, and volcanic deposits, but some are no longer active. Mercury has extensive volcanic deposits, but there are no actual volcanic mountains or buildups on the planet. Venus, Earth, Mars, and Io do have volcanic buildups. The Mercurian surface is dominated by lava flows greater than 1 kilometer thick that formed by flood volcanism, which refers to huge outpourings of lava from one or more large fractures in the crust. The flood lavas buried many impact craters and volcanic vent areas. Example: the Northern Volcanic Plains cover about 4 to 5 million square kilometers; they are ~3.7 to 3.8 billion years old (another estimate puts the age at ~2.5 billion years).
Mercury's lava is not quite basalt, which is a common mafic, extrusive igneous rock on Earth, Venus, Mars, and the Moon. A common Mercury lava type may be komatiite (or close to that). Komatiite is extremely rare on Earth - it's an extinct, ultramafic, magnesium-rich lava that only erupted during the early Precambrian. Most of Mercury's Northern Volcanic Plains and the Intercrater Plains have Mg/Si and Al/Si ratios consistent with komatiite. Boninite or basaltic komatiites are possible Earth analogues for Mercury's lavas.
Lava flows are called "effusive eruptions" - that was the common volcanic behavior on Mercury. However, some of Mercury's volcanism was explosive in nature. Volcanic vents with explosive volcanic deposits are present (for example, north of Rachmaninoff Crater). Over 39 deposits on Mercury have features consistent with explosive volcanism.
Mercury has an odd, extensive system of cliffs / scarps / escarpments called "rupes" in Latin. They range from a few hundred meters tall to several kilometers tall. Mercurian rupes are though to be fault scarps from thrust faulting - possibly blind thrust faults capped by anticlines (= uparched folds). Thrust faults are low-angle reverse faults - they form by compressional stress. They are common in mountain belts on Earth (for example, the Appalachians in eastern America). Mercury's thrust faulting may be from contraction (shrinking) of the entire planet. That sounds like pseudoscientific nonsense, but it may be analogous to a dried-out, shrunken apple. To get Mercury's system of rupes, the planet's radius may have decreased about seven kilometers. On Earth, thrust faults form by plate tectonic activity, not planetary contraction. Why did Mercury shrink? The core cooled and partially solidified - most solids take up less volume than their liquid counterparts.
Despite being close to the Sun and despite the surface being frequently very hot, there is evidence for the presence of water ice (solid H2O) in impact craters at Mercury's poles. This happens because polar craters are permanently shadowed and are always very cold. Nearly all greater-than-ten kilometer sized impact craters close to the North Pole have inferred water ice deposits. They were first identified in 1991 by Puerto Rico's Arecibo Radio Telescope and later confirmed by the MESSENGER spacecraft.
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