# Solar System, technical/Mercury

Mercury is the smallest and least massive of the eight major planets in the Solar System (Pluto is no longer considered a planet). Mercury is the second densest planet in the solar system after Earth. At a mean distance from the Sun of 57.8 million kilometres, it is also the closest. Mercury has a faint atmosphere, mainly helium, and experiences extremes of temperature ranging from over 350°C during the day to -170°C at night. The planet orbits the Sun in 88 Earth-days and rotates on its axis in a period of 58.6 days. The gravity at the surface is 37.7% of what it is on Earth. Mercury's radius is 2439.7 kilometers at the equator, and has a mass 3.302 x 10$^{23}$.

## Some information about the planet
### Orbital characteristics

* Aphelion: 69,816,927 km (0.46669733 AU)
* Perihelion: 46,001,210 km (0.30749909 AU)
* Semi-major axis: 57,909,068 km (0.38709821 AU)
* Eccentricity: 0.205630294
* Orbital period: 87.969 098 d (0.240846264 a)
* Synodic period: 115.88 d
* Avg. orbital speed: 47.362 km/s
* Mean anomaly: 174.795884°
* Inclination: 7.005015818° (3.38° to Sun’s equator)
* Longitude of ascending node: 48.330541°
* Argument of perihelion: 29.124279°
* Satellites: None

### Physical characteristics

* Mean radius: 2439.7 ± 1.0 km (0.3829 Earths)
* Flattening: < 0.0006
* Surface area: 7.48 × 10$^7$ km² (0.108 Earths)
* Volume: 6.083 × 10$^{10}$ km³ (0.054 Earths)
* Mass: 3.3022 × 10$^{23}$ kg (0.055 Earths)
* Mean density: 5.427 g/cm³
* Equatorial surface gravity: 3.7 m/s² (0.38 g)
* Escape velocity: 4.25 km/s
* Sidereal rotation period: 58.646 day (58 d 15.5 h)
* Rotation velocity at equator: 10.892 km/h
* Axial tilt: 0.01°
* Right ascension of North pole: 18 h 44 min 2 s (281.01°)
* Declination of North pole: 61.45°
* Albedo: 0.119 (bond) 0.106 (geom.)
* Surface temp.: min mean max
* Apparent magnitude: up to -1.9
* Angular diameter: 4.5" — 13"
* Adjectives: Mercurian

### Atmosphere

* Surface pressure: trace
* Composition: [citation needed]
* 31.7% Potassium
* 24.9% Sodium
* 9.5% Atomic Oxygen
* 7.0% Argon
* 5.9% Helium
* 5.6% Molecular Oxygen
* 5.2% Nitrogen
* 3.6% Carbon dioxide
* 3.4% Water
* 3.2% Hydrogen

## Cosmic-rays

From the Mariner 10 observations in electron astronomy, it is concluded that "[d]ue to the limited shielding provided by its relatively weak magnetic dipole moment, the surface of Mercury is everywhere subject to bombardment by cosmic rays and solar energetic particles with energies greater than 1 MeV/nucleon."
"Galactic cosmic rays should have very similar fluxes on Mercury and the Moon." "Solar Cosmic Rays which result in the formation of particle tracks also increase by a factor of up to 10 when compared to the Moon. However, surface temperatures reach 700 K, which can result over millions of years in the annealing of irradiation effects."

## Protons

"During the Mercury flyby of Mariner 10, observations of large fluxes of energetic ... protons (0.53 < E < 1.9 MeV) have been reported" but these may be due to "the pileup of low-energy electrons rather than the presence of protons in the vicinity of Mercury."
The Energetic Particles Experiment aboard Mariner 10 "was designed to measure energetic ... protons ... in the interplanetary medium and in the vicinities of Venus and Mercury. The instrumentation consisted of a main telescope and a low-energy telescope. The main telescope consisted of six co-linear sensors (five silicon detectors and one CsI scintillator) surrounded by a plastic scintillator anti-coincidence cup. One pulse height analysis was performed every 0.33 s, and counts accumulated in each coincidence/anti-coincidence mode were measured every 0.6 s. Particles stopping in the first sensor were protons ... in the range 0.62--10.3 MeV/nucleon ... . The aperture half angle for this mode was 47 degrees, and the geometric [factor was] 7.4 sq cm-sr for protons ... . The telescope aperture half-angle decreased to 32 degrees for coincident counts in the first and third sensors. The low-energy telescope, a two-element (plus anti-coincidence) detector with a 38 degree half angle aperture and a 0.49 sq cm sr geometrical factor, was designed to measure 0.53--1.9 MeV and 1.9--8.9 MeV protons without responding to electrons over a wide range of electron energies and intensities."
"The ... solar proton flare on 20 April 1998 at W 90° and S 43° (9:38 UT) was measured by the GOES-9-satellite (Solar Geophysical Data 1998), as well as by other experiments on WIND ... and GEOTAIL. Protons were accelerated up to energies > 110 MeV and are therefore able to hit the surface of Mercury."

## Neutron astronomy

The neutron spectrometer on the MESSENGER spacecraft "[d]etermines the hydrogen mineral composition to a depth of 40 cm by detecting low-energy neutrons that result from the collision of cosmic rays and the minerals."
"During large solar flares, the region near Mercury may be strongly illuminated with solar neutrons."

## Electron astronomy

Mariner 10 has aboard "one backward facing electron spectrometer (BESA). ... An electron spectrum [is] obtained every 6 s, ... within the energy range 13.4-690 eV. ... [B]y taking into account [the angular] distortion [caused by the solar wind passing the spacecraft] and the spacecraft sheath characteristics ... some of the solar wind plasma parameters such as ion bulk speed, electron temperature, and electron density [are derived]."
Mariner 10 had three encounters with Mercury on March 29, 1974, September 21, 1974, and on March 16, 1975.
The BESA measurements "show that the planet interacts with the solar wind to form a bow shock and a permanent magnetosphere. ... The magnetosphere of Mercury appears to be similar in shape to that of the earth but much smaller in relation to the size of the planet. The average distance from the center of Mercury to the subsolar point of the magnetopause is ∼ 1.4 planetary radii. Electron populations similar to those found in the earth’s magnetotail, within the plasma sheet and adjacent regions, were observed at Mercury; both their spatial location and the electron energy spectra within them bear qualitative and quantitative resemblance to corresponding observations at the earth."
"[T]he Mercury encounter (M I) by Mariner 10 on 29 March 1974 occurred during the height of a Jovian electron increase in the interplanetary medium."

## X-rays

"Now, 205 measurements of Mercury's surface composition, made by the X-ray spectrometer onboard Messenger, reveal how much Mercury's surface differs from those of other planets in the solar system."
"The surface is dominated by minerals high in magnesium and enriched in sulfur, making it similar to partially melted versions of an enstatite chondrite, a rare type of meteorite that formed at high temperatures in low-oxygen conditions in the inner solar system."
""The similarity between the constituents of these meteorites and Mercury's surface leads us to believe that either Mercury formed via the accretion of materials somewhat like the enstatite chondrites, or that both enstatite chondrites and the Mercury precursors were built from common ancestors," [Shoshana] Weider [a planetary geologist at the Carnegie Institution of Washington] said."

## Ultraviolets

"[U]ltraviolet observations by Mariner 10 provided evidence for the presence of H and He in the atmosphere (~10$^{11}$ and 10$^{12}$ atoms cm$^{-2}$ respectively$^3$".
Aboard Mariner 10, "[t]he extreme ultraviolet spectrometer consisted of two instruments: an occultation spectrometer that was body-fixed to the spacecraft and an airglow spectrometer that was mounted on the scan platform. When the sun was obscured by the limbs of the planet, the occultation spectrometer measured the extinction properties of the atmosphere. The occultation spectrometer had a plane grating which operated at grazing incidence. The fluxes were measured at 47.0, 74.0, 81.0, and 89.0 nm using channel electron multipliers. Pinholes defined the effective field of view of the instrument which was 0.15 degree full width at half maximum (FWHM). Isolated spectral bands at approximately 75 nm (FWHM) were also measured. The objective grating airglow spectrometer was flown to measure airglow radiation from Venus and Mercury in the spectral range from 20.0--170.0 nm. With a spectral resolution of 2.0 nm, the instrument measured radiation at the following wavelengths: 30.4, 43.0, 58.4, 74.0, 86.9, 104.8, 121.6, 130.4, 148.0, and 165.7 nm. In addition, to provide a check on the total incident extreme UV flux to the spectrometer, two zero-order channels were flown. The effective field of view of the instrument was 0.13 by 3.6 degree. Data also include the interplanetary region."

## Blue astronomy

"MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters. As the spacecraft receded from Mercury after making its closest approach on January 14, 2008, the WAC recorded a 3x3 mosaic covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through the WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here. The human eye is sensitive only across the wavelength range from about 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in black-and-white (single-color) images."
"Color differences on Mercury are subtle, but they reveal important information about the nature of the planet's surface material. A number of bright spots with a bluish tinge are visible in this image. These are relatively recent impact craters. Some of the bright craters have bright streaks (called "rays" by planetary scientists) emanating from them. Bright features such as these are caused by the presence of freshly crushed rock material that was excavated and deposited during the highly energetic collision of a meteoroid with Mercury to form an impact crater. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin. Mariner 10 viewed only the eastern (right) portion of this enormous impact basin, under lighting conditions that emphasized shadows and elevation differences rather than brightness and color differences. MESSENGER has revealed that Caloris is filled with smooth plains that are brighter than the surrounding terrain, hinting at a compositional contrast between these geologic units. The interior of Caloris also harbors several unusual dark-rimmed craters, which are visible in this image. The MESSENGER science team is working with the 11-color images in order to gain a better understanding of what minerals are present in these rocks of Mercury's crust."

## Yellows

"[H]igh-resolution spectral measurements of Mercury show emission in sodium D lines (Potter and Morgan 1985a). This suggests a substantial sodium population in Mercury's atmosphere ... possibly due to photo-sputtering of the planetary surface".

## Radars

"Water ice strongly reflects radar, and observations by the 70 m [Goldstone Deep Space Communications Complex] Goldstone telescope and the [Very Large Array] VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles. While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely."
Radar astronomy of Mercury "[i]mproved [the] value for the distance from the earth [including] [r]otational period, libration, [and] surface mapping, [especially] of [the] polar regions."

## Crater astronomy

"Mercury's surface is heavily cratered and similar in appearance to Earth's Moon ... For example, an unusual crater with radiating troughs has been discovered which scientists called "the spider." ... 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."
"The largest known crater is 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 ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain"."
"Overall, about 15 impact basins have been identified on the imaged part of Mercury. A notable basin is the 400 km wide, multi-ring Tolstoj Basin which 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."
"On November 29, 2012, NASA confirmed that images from the space probe Mercury Messenger had detected that craters at the north pole contained water ice. Sean C. Solomon was quoted in the New York Times as estimating the volume of the ice as large enough to "encase Washington, D.C., in a frozen block two and a half miles deep.""
"A small crater named Hun Kal provides the reference point for measuring longitude. The center of Hun Kal is 20° west longitude."
"Mariner 10 provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature"

## Meteorites

"NWA 7325 is actually a group of 35 meteorite samples discovered in 2012 in Morocco. They are ancient, with Irving and his team dating the rocks to an age of about 4.56 billion years."
"NWA 7325 has a lower magnetic intensity — the magnetism passed from a cosmic body's magnetic field into a rock — than any other rock yet found, Irving said. Data sent back from NASA's Messenger spacecraft currently in orbit around Mercury shows that the planet's low magnetism closely resembles that found in NWA 7325, Irving said."
"NWA 7325 has olivine in it that is insanely magnesium-rich. Iron and magnesium are two elements that are almost always found together in rocks; the ions they make have the same size and charge so they happily occupy the same positions in crystal lattices. It's weird to have a rock that is so dominantly magnesium-rich. Mercury's surface rocks are known (thanks to MESSENGER) to be unusually low in iron."
"NWA 7325's oxygen isotope ratios do not match any known meteorites from any other planet-size body. In fact, they're not particularly similar to much of anything that we've measured oxygen isotope ratios for."
"The ratios of Al/Si (0.224) and Mg/Si (0.332) plus the very low Fe content of NWA 7325 are consistent with the compositions of surface rocks on Mercury [6], but the Ca/Si ratio (0.582) is far too high. However, since NWA 7325 is evidently a plagioclase cumulate (and presumably excavated from depth), it may not match surface rocks on its parent body. The abundance of diopside rather than enstatite might be consistent with some earlier spectral observations of Mercury [7]."
"[I]t's about 23 times harder to get a rock from Mercury to Earth than it is from Mars to Earth. Given that we've got more than 70 known Mars meteorites in our collections, that means we ought to have found 3 (give or take a couple) Mercury meteorites by now."

## Magnetohydrodynamics

"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% as strong as the Earth's. The magnetic field strength at the Mercurian equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar. Unlike Earth, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable."
"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 'tornadoes' form when magnetic fields carried by the solar wind connect to Mercury's magnetic field. As the solar wind blows past Mercury's field, these joined magnetic fields are carried with it and twist up into vortex-like structures. 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."
"The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. The MESSENGER observations show the reconnection rate is ten times higher at Mercury. Mercury's proximity to the Sun only accounts for about a third of the reconnection rate observed by MESSENGER."
There are "observed MHD fluctuations in the magnetosphere" of Mercury.

## Planetary science

"Optical reflectance studies of Mercury provide evidence for Mg silicates."
"Solar heating at ‘‘noontime’’ at Mercury’s equator causes surface temperatures of ~700 K, while the side of Mercury away from the Sun cools to ~100 K (due to radiation losses) during the long night."
"Mercury [has a] tenuous ballistic [atmosphere]."
The MESSENGER X-ray spectrometer (XRS) "[m]aps mineral composition within the top millimeter of the surface on Mercury by detecting X-ray spectral lines from magnesium, aluminum, sulphur, calcium, titanium, and iron, in the 1-10 keV range."

## Classical planets

Wednesday is the day of Mercury and its color varies blue/brown/black.
-5102 b2k, is the historical year assigned to a Hindu table of planets that does include the classical planet Mercury (or Hermes). "Hermès observait en -1660; donc les Indiens observaient en -3102, et ils observaient bien!" "Babylonian astronomy, too, had a four-planet system. In ancient prayers the planets Saturn, Jupiter, Mars, and Mercury are invoked; ... and one speaks of "the four-planet system of the ancient astronomers of Babylonia.""
~ 3300 b2k: “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\u$_4$.Ud ("the jumping planet").”.
~2900 b2k: “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.”.
~2400 b2k: “The ancient Greeks of Hesiod's time knew the planet as (Stilbon), meaning "the gleaming", and (Hermaon). Later Greeks called the planet Apollo when it was visible in the morning sky, and Hermes when visible in the evening. Around the 4th century BC, Greek astronomers came to understand that the two names referred to the same body.”.
“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.”.
"In ancient China, Mercury was known as Chen Xing (辰星), the Hour Star. It was associated with the direction north and the phase of water in the Wu Xing. ... 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 my understanding Mercury was once a satellite of Jupiter, or possibly of Saturn. In the course of the events which followed Saturn’s interaction with Jupiter and its subsequent disruption, Mercury was pushed from its orbit and was directed to the sun by Jupiter. It could, however, have been a comet and the entwined snakes of the caduceus may memorialize the appearance it had when seen by the inhabitants of the Earth."

### Enki

"Enki ... is a god [dingir] in Sumerian mythology ... Beginning around the second millennium BCE [~4000 b2k], he was sometimes referred to in writing by the numeric ideogram for "40," occasionally referred to as his "sacred number." The planet Mercury, associated with Babylonian Nabu (the son of Marduk) was in Sumerian times, identified with Enki."

### Hermes

"Hermes ... Greek : Ἑρμῆς ... [is] son of Zeus and the Pleiade Maia. ... In the Roman adaptation of the Greek pantheon ..., Hermes was identified with the Roman god Mercury, who, [is] inherited from the Etruscans ... The Thracian princes identified him with their god Zalmoxis, considering his ancestor."

### Nabu

"Nabu (in Biblical Hebrew Nebo נבו) is the Assyrian and Babylonian god of wisdom and writing, worshipped by Babylonians as the son of Marduk and his consort, Sarpanitum, and as the grandson of Ea."
"In Chaldean mythology, Nebo was a god whose worship was introduced into Assyria by Pul [Tiglath-pileser III] (Isa. 46:1; Jer. 48:1). The great temple at Birs Nimrud was dedicated to Nebo."

### Ningishzida

"The caduceus was an emblem of the Babylonian deity Ningishzida, and an astronomical tablet from Boghazkoi [Boghaz Keui, in Anatolia] identifies Ningishzida with Nebo-Mercury (Weidner, Handbuch der babylonischen Astronomie, p. 61)."

### Thoth

"Thoth ... [in] the Greeks' interpretation ... was the same as their god Hermes) and ... [Shmounein] in the Coptic rendering. ... The Greeks related Thoth to their god Hermes due to his similar attributes and functions. ... [Egyptian] mythology also credits him with the creation of the 365 day calendar. Originally, according to the myth, the year was only 360 days long and Nut was sterile during these days, unable to bear children. Thoth gambled with Khonsu, the Moon, for 1/72nd of its light (360/72 = 5), or 5 days, and won. During these 5 days, Nut gave birth to Kheru-ur (Horus the Elder, Face of Heaven), Osiris, Set, Isis, and Nepthys."
"In the Ogdoad cosmogony, Thoth gave birth to Ra, Atum, Nefertum, and Khepri by laying an egg while in the form of an ibis, or later as a goose laying a golden egg."