User:Marshallsumter/Keynote lectures (draft)/Mercury

"A large part of Mercury could be covered in dried lava. The planet's northern plains appear smooth because lava may have poured over the surface, smoothing it out as it moved. Although scientists don't see volcanic activity on Mercury's surface now, many think it could be a good explanation for the way the planet looks today. The two crater rings in this image might have been smoothed by volcanic material, according to NASA."

Intercrater plains
"Mercury's oldest surface is its intercrater plains [1,3], which are present (but much less extensive) on the Moon. The intercrater plains are level to gently rolling terrain that occur between and around large craters. The plains predate the heavily cratered terrain, and have obliterated many of the early craters and basins of Mercury [1,4]; they probably formed by widespread volcanism early in mercurian history."

In the image on the right are abundant shallow elongate craters and crater chains are present on the plains. This image, taken during the first mission of Mariner 10, shows a large tract of inter-crater plains centered at 3° N, 20° W. The scarp running down the middle of the image, Santa Maria Rupes cuts through both the plains and large craters. The scene is 200 km across; north is the top.

Caloris basins


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".

"Scientists think that a huge asteroid slammed into Mercury about 4 billion years ago, creating a giant crater about 960 miles (1,545 km) across. Called the Caloris Basin, the crater could fit the whole state of Texas inside it. Researchers have calculated that the asteroid that created the basin had to have been about 60 miles (100 km) wide."

"Analysis of radio tracking data have enabled maps of the gravity field of Mercury to be derived. In this image [second down on the right], overlain on a mosaic obtained by MESSENGER's Mercury Dual Imaging System and illuminated with a shape model determined from stereo-photoclinometry, Mercury's gravity anomalies are depicted in colors. Red tones indicate mass concentrations, centered on the Caloris basin (center) and the Sobkou region (right limb). Such large-scale gravitational anomalies are signatures of subsurface structure and evolution. The north pole is near the top of the sunlit area in this view."

"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 [on the left] 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."

Planetary astronomy
On the right is a computer program result for the orbit of Mercury. View is from above the ecliptic (North Pole). Mercury is in yellow. A circular orbit with the same semi-major axis is in grey for reference. The orbit is plotted in brighter colours above the ecliptic and darker below. Major axis is drawn showing perihelion (q) and aphelion (Q). Positions show every 5 days before and after the perihelion on May 20, 2006. For illustration the size of the sphere is inversely proportional to the distance from the Sun. The Sun is in the center. Yellow segment points toward the vernal point. Data for the plot is from the Jet Propulsion Laboratory.

The second image on the right down from the top is an animation of the revolutions of Mercury, Venus and Earth around the Sun. Mercury takes 88 days to complete an orbit, thus the animation shows it revolving around the Sun approximately 4.14 times (yellow trail) compared to Earth's 365 days (blue trail).

The third lower diagram on the right shows how Mercury's orbital period and rotational period are locked in a 3:2 resonance.

After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated.

Minerals
"The [Mercury Atmosphere and Surface Composition Spectrometer] MASCS instrument was designed to study both the exosphere and surface of Mercury. To learn more about the minerals and surface processes on Mercury, the Visual and Infrared Spectrometer (VIRS) [VIRS Color Composite Wavelengths: 575 nm as red, 415 nm/750 nm as green, 310 nm/390 nm as blue] portion of MASCS has been diligently collecting single tracks of spectral surface measurements since MESSENGER entered orbit. The track coverage is now extensive enough that the spectral properties of both broad terrains and small, distinct features such as pyroclastic vents and fresh craters can be studied. To accentuate the geological context of the spectral measurements, the MASCS data have been overlain on the MDIS monochrome mosaic."

Astrognosy
A theory for the internal structure of Mercury is shown on the right, where
 * 1) Crust - 100-200 km thick,
 * 2) Mantle - 600 km thick, and
 * 3) Nucleus - 1,800 km radius.

"Mercury's iron core takes up about 75 percent of the planet's radius. The huge core has more iron in it than any other planet's in the solar system."

Astrogony
"Scientists aren't exactly sure how Mercury's giant iron core formed, but researchers think it has something to do with its formation. If the planet formed quickly, it could have left a thin shell of crust over the relatively large core."

Hominins “lived without town or laws, speaking one tongue under the rule of Jove. But after Mercury explained the languages of men (whence he is called hermeneutes, ‘interpreter,’ for Mercury in Greek is called Hermes; he, too, divided the nations) then discord arose among mortals.”

“The meaning is clearly that Hermes invented one language for one people, another for another. The whole account reminds one of the Biblical Tower of Babel.”

"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."

Electromagnetics
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.

"The Mariner 10 spacecraft had revealed that Mercury possessed a magnetic field similar to Earth's, albeit one that is about 100 times weaker."

"This means Mercury's core has to be at least partially liquid. This was a surprise at first, because Mercury is very small, so you would expect it to cool quickly after it formed and be completely solid. [But,] if there was a little bit of nonmetallic stuff in Mercury's core, that'd lower its freezing point and make it hard to be completely solid."

In "the fall of 2014 and 2015, when the spacecraft flew incredibly close to the planet's surface, at altitudes as low as 9 miles (15 kilometers)."

"The signals we detected are really small, and very, very hard to measure. We'd never have been able to measure them if not for these really risky low-altitude observations in the last few months of the MESSENGER mission."

"The scientists detected magnetized rocks in a part of Mercury's crust that, due to the presence of many craters from cosmic impacts, appears to be quite ancient. [Shown in the second image down on the right.] The researchers suggest the rocks were once magnetized by the planet's magnetic field, and based on the age and amount of the magnetized rocks, as well as how strongly they were magnetized, the investigators deduced that Mercury's magnetic field has persisted for 3.8 billion years."

"The strength of Mercury's magnetic field may have ranged anywhere from its strength today to something about 100 times stronger, comparable to the strength of Earth's magnetic field at Earth's surface today."

"Being able to pin down how long Mercury has had a magnetic field helps us narrow down scenarios for the early history of Mercury and how it has changed over time. This in turn helps us understand more about planetary evolution in general."

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."

Neutrons
The neutron spectrometer on the MESSENGER spacecraft determines 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."

"The Messenger spacecraft, which swung into orbit around Mercury in March 2011 and has completed its primary mission, took a closer look by counting particles known as neutrons that are flying off the planet. High-energy cosmic rays break apart atoms, and the debris includes neutrons."

"But when a speeding neutron hits a hydrogen atom, which is almost the same weight, it comes to almost a complete stop, just as the cue ball in billiards transfers its momentum when it hits another ball. Water molecules contain two hydrogen atoms, and thus when Messenger passed over ice-rich areas, the number of neutrons dropped."

“Water ice is the only candidate we’ve got that fits all those observations.”

"The ice is almost pure water, which indicates that it arrived within the last few tens of millions of years, possibly from a comet that smacked into Mercury."

"Several young craters on the surface of Mercury could be candidates for such an impact."

"Not all of the icy regions were bright. In slightly warmer regions, where temperatures exceed minus 280, the ice was covered by a dark layer about half a foot thick."

"Between the scorched equator and the frozen poles, temperatures on Mercury can be temperate, especially a few feet below the surface, where the soil insulates against the temperature swings between day and night — an ideal location to build a colony."

Electrons
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
The MESSENGER X-ray spectrometer (XRS) maps 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.

"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."

Opticals
"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 Tolstoj basin (355 km in diameter) can be seen at the bottom edge of the frame [on the right], its center filled with smooth plains and surrounded by a large region of low-reflectance ejecta. The fresh, bright-rayed crater Nureyev is visible near the limb."

Ultraviolets
"[U]ltraviolet observations by Mariner 10 provided evidence for the presence of H and He in the atmosphere (~1011 and 1012 atoms cm-2 respectively3".

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."

Visuals
"Mercury, the closest planet to the Sun, possesses a lot of ice — 100 billion to one trillion tons [...] near Mercury’s poles, deep within craters where the Sun never shines, temperatures dip to as cold as minus 370."

Blues
"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."

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".

Reds
"A higher-reflectance, relatively red material occurs as a distinct class of smooth plains that were likely emplaced volcanically; a lower-reflectance material with a lesser spectral slope may represent a distinct crustal component enriched in opaque minerals, possibly more common at depth."

"The distinctively red smooth plains (HRP) appear to be large-scale volcanic deposits stratigraphically equivalent to the lunar maria (20), and their spectral properties (steeper spectral slope) are consistent with magma depleted in opaque materials. The large areal extent (>106 km2) of the Caloris HRP is inconsistent with the hypothesis that volcanism was probably shallow and local (10); rather, such volcanism was likely a product of extensive partial melting of the upper mantle."

"Despite the dearth of ferrous iron in silicates, Mercury's surface nonetheless darkens and reddens with time like that of the Moon. This darkening and reddening has been interpreted to be the result of production of nanophase iron (e.g., Pieters et al., 2000; Hapke, 2001), which could be derived from an opaque phase in the crustal material or from delivery by micrometeorite impacts (Noble and Pieters, 2003). On the Moon, deposits that are brighter and redder than the average Moon spectrum appear to be lower in iron (e.g., highland material); deposits that are darker and redder than average are higher in iron (e.g., low-Ti mare material) (Lucey et al., 1995)."

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 improved the value for the distance from the earth including rotational period, libration, and surface mapping, especially of the polar regions.

Craters
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.

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.

"This spectacular view of the crater Degas [lower image on the right] was obtained as a high-resolution targeted observation (90 m/pixel). Impact melt coats its floor, and as the melt cooled and shrank, it formed the cracks observed across the crater. For context, Mariner 10’s view of Degas is shown at left. Degas is 52 km in diameter and is centered at 37.1° N, 232.8° E."

The Scarlatti crater on Mercury has its center located at 40.71°N latitude, 101.09°W longitude.

Scarlatti crater is a pit crater.

"Pit craters are rimless steep-sided depressions that are inferred to have formed by non-impact processes."

Pit "craters on Mercury formed through collapse into an underlying drained magma chamber. Pit-floor craters thus provide evidence for near-surface magmatic activity on Mercury and extend the range of evidence for magmatism beyond such surface expressions as smooth plains and pyroclastic deposits."

Atmospheres
"As the MESSENGER spacecraft approached Mercury, the UVVS field of view was scanned across the planet's exospheric "tail," which is produced by the solar wind pushing Mercury's exosphere (the planet's extremely thin atmosphere) outward. This figure [on the right] shows a map of the distribution of sodium atoms as they stream away from the planet (see PIA10396); red and yellow colors represent a higher abundance of sodium than darker shades of blue and purple, as shown in the colored scale bar, which gives the brightness intensity in units of kiloRayleighs. The escaping atoms eventually form a comet-like tail that extends in the direction opposite that of the Sun for many planetary radii. The small squares outlined in black correspond to individual measurements that were used to create the full map. These measurements are the highest-spatial-resolution observations ever made of Mercury's tail. In less than six weeks, on October 6, 2008, similar measurements will be made during MESSENGER's second flyby of Mercury. Comparing the measurements from the two flybys will provide an unprecedented look at how Mercury's dynamic exosphere and tail vary with time."

"These figures [on the lower right] show observations of calcium and magnesium in Mercury's neutral tail during the third MESSENGER Mercury flyby. The distribution of neutral calcium in the tail appears to be centered near the equatorial plane and the emission rapidly decreases to the north and south as well as in the anti-sunward direction. In contrast, the distribution of magnesium in the tail exhibits several strong peaks in emission and a slower decrease in the north, south, and anti-sunward directions. These distributions are similar to those seen during the second flyby, but the densities were higher during the third flyby, a different "seasonal" variation than for sodium. Studying the changes of the "seasons" for a range of species during MESSENGER's orbital mission phase will be key to quantifying the processes that generate and maintain the exosphere and transport volatile material within the Mercury environment."

Meteorites
"NWA 7325 is actually a group of 35 meteorite samples discovered in 2012 in Morocco. They are ancient, [...] 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."

"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."

"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."

Mercury
"There is also the shrinking issue. As the planet’s core cooled, it contracted, which led to the crust crumpling and fracturing into a rugged landscape. Detailed mapping of the cracks at the surface could help determine how quickly the shrinking is happening and reveal more about the underlying processes driving it."

"The [BepiColombo] spacecraft, scheduled to launch in October 2018, will investigate the existence of water ice at Mercury’s poles and its volcanoes, and attempt to explain the surprising discovery that the solar system’s smallest planet appears to be shrinking."

Astrogeology
"The crater in the lower right-hand corner of this image [on the right] has a patch of very dark material located near its center. The region of this image has been seen only with the Sun high overhead in the sky. Such lighting conditions are good for recognizing color differences of rocks but not well suited for ascertaining the topography of surface features from shadows. The shape of the surface in this area is difficult to resolve given the lighting angle, but the dark patch is not in shadow. Dark surfaces have also been seen on other regions of Mercury, including this dark halo imaged during the second Mercury flyby (PIA11357) and near such named craters as Nawahi, Atget, and Basho seen during MESSENGER's first Mercury encounter. The example here is particularly striking, however, and from this NAC image the material may appear even darker than in other example areas. The dark color is likely due to rocks that have a different mineralogical composition from that of the surrounding surface. Understanding why these patches of dark rocks are found on Mercury's surface is a question of interest to the MESSENGER Science Team. The right edge of the image here aligns with this previously released NAC image (see PIA11763), where other dark surface material, as well as patches of light-colored rocks, can be seen."

The second lower image on the right from the top shows "a double-ring impact basin, with another large impact crater on its south-south-western side. Double-ring basins are formed naturally when a large meteoroid strikes the surface of a rocky planet. Smaller, more recent impacts also formed comparatively fresh craters across the entire surface visible in this image. The floor within the inner or peak ring appears to be smoother than the floor between the peak ring and the outer rim, possibly the result of lava flows that partially flooded the basin some time after impact."

The lowest image from the top on the right is a closeup of the weird terrain of Mercury.

"Weird terrain best describes this hilly, lineated region of Mercury. Scientists note that this area is at the antipodal point to the large Caloris basin. The shock wave produced by the Caloris impact may have been reflected and focused to the antipodal point, thus jumbling the crust and breaking it into a series of complex blocks. The area covered is about 800 km (497 mi) on a side."

Sun-Mercury system
In the image on the right sunspot #923, which is just below the equator at the left-hand side, is much bigger than Mercury. Two more sunspots are on the right-hand side at the equator. Mercury is a small black dot in the lower middle of the solar disk. The picture was taken with a white filter.

The diagram in the second image from the top on the right shows that because "Mercury and Venus orbit the Sun within Earth's orbit, they stay close to the Sun in the sky as seen from Earth. At their greatest angular distance from the Sun they are said to be at elongation. Here the two planets are shown at eastern elongation; they set after the Sun and appear as evening stars."

The third down image on the right shows "five versions of observations that NASA's Curiosity made about one hour apart while Mercury was passing in front of the sun on June 3, 2014. Two sunspots, each about the diameter of Earth, also appear, moving much less than Mercury during the hour."

"This is the first transit of the sun by a planet observed from any planet other than Earth, and also the first imaging of Mercury from Mars. Mercury fills only about one-sixth of one pixel as seen from such great distance, so the darkening does not have a distinct shape, but its position follows Mercury's expected path based on orbital calculations."

"This is a nod to the relevance of planetary transits to the history of astronomy on Earth."

"Observations of Venus transits were used to measure the size of the solar system, and Mercury transits were used to measure the size of the sun."

"The observations were made on June 3, 2014, from Curiosity's position inside Gale Crater on Mars. In addition to showing the Mercury transit, the same Mastcam frames show two sunspots approximately the size of Earth. The sunspots move only at the pace of the sun's rotation, much slower than the movement of Mercury."

Astrography
The geography of the surface of Mercury (or Hermes) could be called Hermography or maybe Hermeography.

The image on the right is part of the first-ever complete map of Mercury's surface.

The image on the left is a topographic map of part of Mercury's surface.

"Measurements from MESSENGER's MLA instrument during the spacecraft's greater than four-year orbital mission have mapped the topography of Mercury's northern hemisphere in great detail. The view shown here is an interpolated shaded relief map of these data. The lowest regions are shown in purple, and the highest regions are shown in red. The difference in elevation between the lowest and highest regions shown here is roughly 10 kilometers! Among the prominent features visible here are the smooth northern volcanic plains and the enigmatic northern rise. The low-lying craters near the north pole host radar-bright materials, thought to be water ice. Linear artifacts can be seen in some areas of this map. These are due to individual MLA tracks that need minor adjustments in order to fit the rest of the data. Crossover analysis and better knowledge of the spacecraft position can be used to adjust these tracks and improve the map."

Classical planets
In antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun (and closest to the Earth); all easily visible without a telescope. They are Mercury, Venus, Mars, Jupiter, and Saturn.

Wednesday is the day of Mercury and its color varies blue/brown/black.

Ancient history
The ancient history period dates from around 8,000 to 3,000 b2k.

~ 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\u4.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.

Technology
Mariner 10, sections above, made a true color image of Mercury that shows its reddish color.

The image on the right is a mosaic of Mercury taken by the Mariner 10 spacecraft during its approach on 29 March 1974. The mosaic consists of 18 images taken at 42 s intervals during a 13 minute period when the spacecraft was 200,000 km (about 6 hours prior to closest approach) from the planet.

"The MESSENGER spacecraft [an artist's impression on the lowwer right] is the first ever to orbit the planet Mercury, and the spacecraft's seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System's innermost planet. During the first two years of orbital operations, MESSENGER acquired over 150,000 images and extensive other data sets. MESSENGER is capable of continuing orbital operations until early 2015."

Hypotheses

 * 1) The current orbit and location of Mercury in the Solar System may have been arrived at within hominin recorded history.