Indicators of a Mercurian Meteorite

representative presently unknown

Mercury is an igneous, differentiated planetary body thought to have undergone initial crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More 3.7–4.4 b.y. ago. Specific features attributed to mercurian meteorites have been proposed by meteoriticists in many published works, from which some pertinent information is presented here. These publications include the following (arranged by date):

• Swapping Rocks: Ejection and Exchange of Surface Material Among the Terrestrial PlanetsRocky planets: Mercury, Venus, Earth, and Mars. These planets have physical characteristics, chemical composition and internal structure similar to the Earth. The terrestrial planets have 0.4% of the total mass of all the planets in the Solar System. Some large satellites of planets are also similar to the characteristics of, J. Melosh and W. Tonks, MeteoriticsScience involved in the study of meteorites and related materials. Meteoritics are closely connected to cosmochemistry, mineralogy and geochemistry. A scientist that specializes in meteoritics is called a meteoriticist. Click on Term to Read More, vol. 28 (1993)
• Mercurian Meteorites: Properties and Probabilities, S. Love and K. Keil, 26th LPSC (1995)
• Recognizing mercurian meteorites, S. Love and K. Keil, Meteoritics, vol. 30 (1995)
• The Delivery of Martian and Lunar MeteoritesAchondrite meteorites from the surface of the Moon. Most were found in the hot deserts of northern Africa and Oman and others were found in the cold desert of Antarctica, although one, a 19-gram specimen, was recovered in 1990 from Calcalong Creek, Australia. These stones are of great importance because, Click on Term to Read More to Earth, B. Gladman and J. Burns, Division for Planetary Sciences, American Astronomical Society (1996)
• EjectaFractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More Transfer Between Terrestrial Planets, B. Gladman, J. Burns, H. Levison, and M. Duncan, 172nd Symposium of the International Astronomical UnionThe International Astronomical Union (IAU) was founded in 1919. Its mission is to promote and safeguard the science of astronomy in all its aspects, including research, communication, education and development, through international cooperation. The IAU also works to promote research, education and public outreach activities in astronomy for the public. Click on Term to Read More (1996)
• Delivery of Planetary Ejecta to Earth, B. Gladman, Doctorate Dissertation, Cornell University (1996)
• Spectra of extremely reducedOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More assemblages: Implications for Mercury, T. Burbine, T. McCoy, L. Nittler, G. Benedix, E. Cloutis, and T. Dickinson, Meteoritics & Planetary Science, vol. 37, #9 (2002)
• Transfer of mercurian ejecta to Earth and implications for mercurian meteorites, B. Gladman, 34th LPSC, #1933 (2003)
• Properties of the Hermean regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles).   Click on Term to Read More: V. New optical reflectance spectra, comparison with lunar anorthosites, and mineralogical modelling, J. Warell and D. Blewett, Icarus, vol. 168 (2004)
• Unique AngriteType of evolved achondrite meteorite that represent some of the earliest stages of asteroidal differentiation and magmatism in our solar system. Angrites are named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil, in early 1869. They are basaltic (mafic) rocks, often containing porous areas, and Click on Term to Read More NWA 2999: The Case For Samples From Mercury, A. Irving, S. Kuehner, D. Rumble III, T. Bunch, J. Wittke, G. Hupé, A. Hupé, American Geophysical Union, FallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More Meeting Abstracts (2005)
• A Fresh PlutonicGeology: Igneous intrusive body that forms when magma is injected into host rocks and solidifies. Plutons occur in the crust of asteroids undergoing differentiation or planets. Named after Pluto, the Roman god of the underworld. Plutonic rocks are the rocks found within a pluton. Astronomy: Category of planet including all Click on Term to Read More Igneous AngriteType of evolved achondrite meteorite that represent some of the earliest stages of asteroidal differentiation and magmatism in our solar system. Angrites are named for the Angra dos Reis meteorite, which fell in Rio de Janeiro, Brazil, in early 1869. They are basaltic (mafic) rocks, often containing porous areas, and Click on Term to Read More Containing Grain Boundary Glass From Tamassint, Northwest Africa, A. Irving, S. Kuehner, and D. Rumble III, American Geophysical Union, Fall Meeting 2006, #P51E-1245
• Coronas and Symplectites in Plutonic Angrite NWA 2999 and Implications for Mercury as the Angrite Parent BodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More, S. Kuehner, A. Irving, T. Bunch, J. Wittke, G. Hupé, and A. Hupé, 37th LPSC, #1344 (2006)
• NWA 2999, A Unique Angrite with a Large Chondritic Component, M. Gellissen, H. Palme, R. Korotev, and A. Irving, 38th LPSC, #1612 (2007)
• Siderophile Elements in MetalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More From Metal-rich Angrite NWA 2999, M. Humayun, A. Irving, and S. Kuehner, 38th LPSC, #1221 (2007)
• The Mn–Cr IsotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: ¹H, ²H (deuterium), and ³H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More Systematics of Bulk Angrites, A. Shukolyukov and G. Lugmair, 38th LPSC, #1423 (2007)
• Grain Boundary Glasses in Plutonic Angrite NWA 4590: Evidence for Rapid Decompressive Partial MeltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting Click on Term to Read More and Cooling on Mercury?, S. Kuehner and A. Irving, 38th LPSC, #1522 (2007)
• The Case Against Mercury As The Angrite Parent Body (APB), M. Hutson, A. Ruzicka, and D. Mittlefehldt, 70th MetSoc, #5238 (2007)
• Plutonic Angrite NWA 4801 and a Model for the Angrite Parent Body Consistent with Petrological and Chronological Constraints, A. Irving, and S. Kuehner, Workshop on Chronology of Meteorites, #4050 (2007)
• Primary Ferric Iron-Bearing Rhönite in Plutonic Igneous Angrite NWA 4590: Implications for RedoxOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More Conditions on the Angrite Parent Body, S. Kuehner and A. Irving, EOS, Trans. AGU 88, #P41A-0219 (2007)
• Mercury: Mg-rich Mineralogy with K-spar and GarnetMineral generally found in terrestrial metamorphic rocks, although igneous examples are not uncommon. Garnet is a significant reservoir of Al in the Earth's upper mantle. The garnet structure consists of isolated SiO4 tetrahedra bound to two cation sites. The A site holds relatively large divalent cations (Ca2+, Mg2+, Fe2+, Mn2+); the Click on Term to Read More, A. Sprague, K. Donaldson Hanna, R. Kozlowski, J. Helbert, A. Maturilli, and N. Izenberg, 39th LPSC, #1320 (2008)
• Mercurian impact ejecta: Meteorites and mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in Click on Term to Read More, B. Gladman and J. Coffey, Meteoritics & Planetary Science, vol. 44, #2 (2009)
• Major-element abundances of surface materials on Mercury: first results from the MESSENGER X-Ray Spectrometer, Weider et al., 74th MetSoc, #5357 (2011)
• Terrene Meteorites: Effects of Planetary Atmosphere on Ejecta Launch, Gladman and Chan, 75th MetSoc, #5147 (2012)
• What sulfides exist on Mercury?, Vaughan et al., 44th LPSC, #2013 (2013)
• Constraining the ferrous iron content of silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the minerals in Mercury’s crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the Click on Term to Read More, Klima et al., 44th LPSC, #1602 (2013)
• The Distribution Of Iron On The Surface Of Mercury From MESSENGER X-ray Spectrometer Measurements, Weider et al., 44th LPSC, #2189 (2013)
• Metal-silicate PartitioningThe tendency of elements to prefer one mineral phase relative to another or to preferentially enter the solid or remain in the liquid during crystallization. Click on Term to Read More Of Si And S In Highly ReducingOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More Conditions: Implications For The Evolutions Of Mercury, Hillgren and Fei, 44th LPSC, #3078 (2013)
• Mapping Major ElementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More Abundances On Mercury’s Surface With MESSENGER X-ray Spectrometer Data, Nittler et al., 44th LPSC, #2458 (2013)
• Correlating Reflectance And X-ray Spectroscopic Data From MESSENGER, Izenberg et al., 44th LPSC, #3018 (2013)
• Variable Sodium On The Surface Of Mercury: Implications For Surface Chemistry And The Exosphere, Evans et al., 44th LPSC, #2033 (2013)
• UngroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More MaficOne of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron (Fe) content. Mafic indicates silicate minerals that are predominantly comprised of Mg and/or Fe.The term is derived from those major constituents: Magnesium + Ferrum (Latin for iron) + ic (having Click on Term to Read More AchondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More Northwest Africa 7325: A Reduced, Iron-poor CumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More OlivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More GabbroWork in progress Coarse-grained igneous rock of basaltic composition that formed at depth and is 90% plagioclase. clinopyroxene, https://www.sandatlas.org/gabbro/ The most important mineral groups that make up this rock type are plagioclase and pyroxene. Plagioclase usually predominates over pyroxene. Plagioclase is sodium-calcium feldspar. It contains more calcium than sodium in gabbro. If there is Click on Term to Read More From A Differentiated Planetary Parent Body, Irving et al., 44th LPSC, #2164 (2013)
• A nonmagnetic differentiated planetary body, Weiss et al., AGU 2013 Fall Meeting, #P51H-04
• Criteria For Identifying Mercurian Meteorites, Vaughan and Head, 45th LPSC, #2013 (2014)
• The origin of boninites on Mercury: An experimental study of the northern volcanicIgneous rock that forms from cooling magma on the surface of a planet or asteroid. plains lavas, Vander Kaaden and McCubbin, Geochimica et Cosmochimica Acta, vol. 173, (15 January 2016)
• Expected Geochemical and Mineralogical Properties of Meteorites From Inferences From MESSENGER Data, McCubbin and McCoy, 79th MetSoc, #6242 (2016)

It is accepted that impact ejecta from the surface of Mercury would be able to escape the planet’s gravityAttractive force between all matter - one of the four fundamental forces. Click on Term to Read More well at a launch speed of 4.2 km/sec, compared to 5.0 km/sec for Mars and 2.4 km/sec for the Moon. However, to reach a heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. Click on Term to Read More orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around Click on Term to Read More and complete a journey to Earth the material has to have an ejection speed about twice the escape velocityVelocity that an object needs to escape the primary gravitational influence of a more massive object: where, m = the object's mass, r = distance from object's center, and G = gravitational constant of the larger object. Click on Term to Read More. After escaping the Sun’s gravity well, the meteoroidSmall rocky or metallic object in orbit around the Sun (or another star). must avoid destruction through collisions with dust-sized particles falling sunward, and overcome orbital collapse due to Poynting-Robertson drag. Accounting for these additional factors, the eventual transfer to an Earth-crossing orbit would require an initial ejection velocity of ~6.2 km/sec (Wetherill, 1984). It has been shown that most Mercurian ejecta would have speeds of 10–30 km/s.

In Gladman (2003), Gladman and Burns (1996), and Gladman et al. (1996), it was determined through numerical simulations that a small flux of Mercurian meteorites to Earth should be possible. They concluded that 30–65% of ejected material would be re-accreted to Mercury in 10 m.y. (75% in 50 m.y.). During that time frame, a portion of the ejected material would make the journey to a Venus-crossing orbit, where 5–10% of that material would be accreted. After many close encounters with Venus, some material would be efficiently delivered to an Earth-crossing orbit. When all is considered, ~0.1% of the material initially ejected from Mercury would accrete to Earth during a time frame of 10 m.y.

In similar studies by Wetherill (1984) and Love and Keil (1995), they calculated that the Earth intercept percentage is lower by a factor of ten; i.e., only 0.01% of the material initially ejected from Mercury would accrete to Earth during a time frame of 2–10 m.y. In contrast, Gladman (1996) determined that 0.5% of Mercurian material would accrete to Earth in a time frame of 50 m.y. Additional results were compiled by Melosh and Tonks (1993), who employed an orbital evolution model to analyze a statistically significant number of particles. Their model demonstrated that almost all of the Mercurian ejecta was either re-accreted to Mercury in a median time frame of ~30 m.y., or that it was collisionally destroyed, with only a very small fraction of the ejecta impacting Venus. Surprisingly, their model also predicted that ~30% of ejecta from Venus should impact Earth in a median time frame of ~12 m.y., which, with other considerations aside, suggests a higher likelihood of finding a venusian meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More than a Mercurian meteorite.

The most recent study of the transfer of Mercurian impact ejecta to Venus and Earth or its re-accretion to Mercury has been conducted by Gladman and Coffey (2009). They utilized numerical calculations of particles to simulate impact-generated ejections from Mercury. In this new study it was determined that previous estimations of impacts onto Mercury were too low, and that higher impact speeds of 20–70 km/s are more realistic. Therefore, according to theory, the ejection velocities used for the simulations were in the range of 4–25 km/s, and the particles were followed over a period of 30 m.y. The transfer efficiency of the ejected meteoroids for various ejection velocities which had sizes of a decimeter or larger were derived. Overall, 50% were re-accreted to Mercury in 30 m.y., and 15% more over the succeeding 30 m.y. The cumulative accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More to Venus was 20%. The cumulative accretion to Earth over the same 30 m.y. for all ejection velocities studied was determined to be 2–5%. The lower ejection velocities (4 km/s) and shorter timeframes (10 m.y.) produce efficiencies that are in agreement with previous studies; the increased velocities (up to ~20 km/s) and extended timeframe (30 m.y.) account for the higher accretion efficiency, about half that of Mars efficiencies.

Indicators of a Mercurian Meteorite

• differentiated igneous rockOne of the three basic types of rock that also include sedimentary and metamorphic. Igneous rocks are formed by the solidification of magma and comprised predominately of silicate minerals. Based on bulk chemical analysis, igneous rocks can be grouped into four major groups based on their SiO2 content: 1. Felsic: Click on Term to Read More/breccia/impact-melt
• moderate refractory oxide enrichment (Ca and Al)
• high abundance of sulfur (up to 4 wt% from MESSENGER observations)
• relatively low amounts of iron (≤4 wt% from MESSENGER observations)
• iron occurring primarily as sulfides, or possibly as meteoritic contamination
• low FeS content, with sulfides occurring primarily as Fe-, Mn-, Ti-, and Cr-sulfides, and minor Mg- or Ca-sulfides; Cl-bearing sulfides or salts are possible
• low alkali element content (MESSENGER observations indicate northern volcanic plains is alkali-rich boninite or Mg-rich trachyandesite with >8 wt% MgO, >52 wt% SiO2, and total alkali content up to 4 wt%)
• MESSENGER observations indicate ~2.6 wt% to ~5 wt% Na across the surface
• XRS surface analysis from MESSENGER reveals 1) Mg/Si, Al/Si, and Ca/Si ratios are like those of terrestrial ultramaficTerm used for silicate minerals with cations predominantly Mg and/or Fe. Mafic minerals are dominated by plagioclase and pyroxene, and also contain smaller amounts of olivine. Click on Term to Read More rocks, indicative of a low plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More abundance and/or a precursor of enstatite chondriteType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More material, and 2) low concentration of Ti (<1 wt%) and Fe (<~2 wt%), high abundance of S (~2–4 wt% in the shallow regolith) as sulfides (FeS and CaS in 1:1 ratio)
• the paleomagnetic signature (natural remanent magnetization) of a meteorite should be consistent with the paleomagnetic intensity of Mercury as measured by MESSENGER—no greater than ~1 microtesla (Weiss et al., 2013, P51H-04)
• possibly a product of partial melting of shallow crust composed primarily of forsteritic olivine, enstatitic pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. Click on Term to Read More, and albitic plagioclase
• probable low abundance of volatile elementsChemical elements that condense (or volatilize) at relatively low temperatures. The opposite of volatile is refractory. Volatile elements can be divided into moderately volatile (Tc = 1230–640 K) and highly volatile (Tc < 640 K). The moderately volatile lithophile elements are: Mn, P, Na, B ,Rb, K, F, Zn. The moderately Click on Term to Read More, e.g., graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More
• major silicates in the form of enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More and calcic and sodic plagioclase (labradorite and albite, respectively)
• minor silicate phases could include MnS and NaCrS2 (caswellsilverite, discovered in two aubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as Click on Term to Read More)
• likely to have O-isotopic compositions close to the TFL
• crystallization age of ~3.7–4.4 b.y.
• probable low content of regolithic solar-wind-implanted gases
• regolithic solar-wind-implanted gases might be fractionated
• high ratio of solar to galactic cosmic ray tracks
• ~5× higher flux of micrometeorites than in lunar breccias
• higher content of microcraters than in lunar breccias
• ~14× higher content of regolithic agglutinates than in lunar breccias
• higher content of impact vapor deposits than in lunar breccias
• higher content of exogenous chondritic material than in lunar breccias
• higher content of regolithic melt-rich material than in lunar breccias
• may contain an exogenous low-Ni metal component
• may exhibit high shock effects from ejection
• may contain evidence of a solar-imparted magnetic field
• possible presence of eclogite with Mg- and Ca-rich mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More compositions
• the pre-atmospheric meteoroid diameter should be ~2 decimeters
• the 4-pi CRE age should be 5–30 m.y.
• the atmospheric entry velocity should be 15–30 km/s with evident ablationGradual removal of the successive surface layers of a material through various processes. • The gradual removal and loss of meteoritic material by heating and vaporization as the meteoroid experiences frictional melting during its passage through the atmosphere. The resulting plasma ablates the meteor and, in cases where a meteor Click on Term to Read More
• falls should generally occur during the morning hours

A mercurian meteorite would presumably be an achondrite and would most likely be mistaken for an aubriteAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8).  They contain large white crystals of enstatite as Click on Term to Read More or an anorthositic lunaite. However, aubrites are inconsistent with a mercurian origin because of their sheer numbers in our collections, and also because of their lack of agglutinates, their high content of solar-wind-implanted gases, and their content of FeNi-metal retaining chondritic trace element abundances. Furthermore, Burbine et al. (2002) estimated that a basaltic crust on Mercury composed of a reduced, aubrite-like mineralogy should have a relatively flat spectra rather than the extremely red spectra actually seen on Mercury. This reddening is thought to be due to space weathering in which micrometeoriteMeteorite so small that it falls to Earth essentially unchanged from how it existed in space. If a meteoroid entering the Earth's atmosphere is sufficiently small (generally less than 10-6 m), it will be slowed by collisions with molecules in the upper atmosphere to a degree where ablation does not Click on Term to Read More impacts and solar windSupersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially particle sputtering produce nanophase iron coatings on surface materials. This high amount of space weathering on Mercury would require reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More of a much higher amount of FeO than that which would be present given an aubritic, enstatite basaltic crust.

While a lunaite should have an FeO content and a crystallization age similar to a mercurian meteorite, it would also have a high content of implanted solar gases. Notably, many lunaites contain clasts composed of high FeO mareBroad low plains surrounded by basin-forming mountains, originally thought to be a sea (pl. maria). This term is applied to the basalt-filled impact basins common on the face of the Moon visible from Earth.   Click on Term to Read More basaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock Click on Term to Read More. Optical and near-infrared spectra of Mercury indicate that the regolith is consistent with a composition having a 3:1 ratio of labradorite and enstatite, commensurate with ~1.2 wt% FeO and 0 wt% TiO2 (Warell and Blewett, 2004).

An intriguing theory was set forth by investigators from the University of Washington in Seattle. They suggested that the angrite group of meteorites might represent material from Mercury that was collisionally stripped from the planetThe term "planet" originally comes from the Greek word for "wanderer" since these objects were seen to move in the sky independently from the background of fixed stars that moved together through the seasons. The IAU last defined the term planet in 2006, however the new definition has remained controversial. Click on Term to Read More, thereby explaining the chemical and mineralogical differences between Mercury and the angrites; e.g., the higher FeO abundance of angrites compared to that on the present surface of Mercury, and the reversal of the Fe/Mn ratios for olivine and pyroxene compared to those values measured for other planetary bodies. Nevertheless, even accepting that collisional-stripping of a hypothetical FeO-rich basaltic (angritic) crust on Mercury occurred, Hutson et al. (2007) findMeteorite not seen to fall, but recovered at some later date. For example, many finds from Antarctica fell 10,000 to 700,000 years ago. Click on Term to Read More it implausible that Mercury initially differentiated under oxidizingOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More conditions to form the angritic crust, and then subsequently differentiated under reducing conditions to form the now-exposed surface that we can observe today. Hutson et al. (2007) have also determined that other mineralogical features identified in angrites, which were attributed to rapid decompression on a planetary-sized body such as Mercury, are more consistent with typical cooling processes during crystallization of a melt. McCubbin and McCoy (2016) suggest that the crystallization age for a mercurian meteorite could likely reflect that for fresh volcanic terranes on Mercury based on data obtained by the MESSENGER spacecraft, estimates for which are 4.1–3.7 b.y.; this is very much younger than the ancient ages calculated for angrites and aubrites.

Ongoing studies of newly discovered angrites by investigators at UWS and other institutions has led to revised models describing the possible connection of angrites to Mercury, such as the occurrence of metasomatic processes to explain the disequilibrium textures observed. Metasomatic processes may also be responsible for the inferred oxidizing conditions under which angrites formed, as evidenced by the association of rhönite and ferric iron in NWA 4590, and by other Fe-metal–oxide associations found to exist in some other angrites. Furthermore, the UWS investigators have modeled the petrogenesis of the various angrite lithologies, the masses of which could now reside in the asteroid beltBelt located between 2.12 and 3.3 AU from the Sun and located between the orbits of Mars and Jupiter containing the vast majority of asteroids. The asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such Click on Term to Read More, or perhaps may still orbit around the original collisionally-stripped parent body (Mercury?).

Other studies that relate to a possible angrite–Mercury connection have been conducted. For example, in a study of siderophile elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on depletions (Ni, Co, and W) and their association to coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More segregation of the angrite parent body, K. Righter (2008) proposed a scenario which would lead to the observed siderophile element depletions observed in angrites. The model is consistent with a small differentiated asteroid having a mantle only a few hundred km in radius surrounding a small metallic core. His model employs the conditions of a reduced peridotite mantle and FeNi-metal core which attained metal–silicate equilibriumTerm used to describe physical or chemical stasis. Physical equilibrium may be divided into two types: static and dynamic. Static equilibrium occurs when the components of forces and torques acting in one direction are balanced by components of forces and torques acting in the opposite direction. A system in static Click on Term to Read More under very low pressures. The oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More state of material derived from this hypothetical body, consistent with that observed in some angrites, could have occurred later, possibly during the eruption phase, through C-assisted redox processes.

In a study of the shock-modified bencubbinite Fountain Hills, and the CB group in general, Weisberg and Ebel (2009) discussed the severe impact-related characteristics of this meteorite and other CB members and demonstrated evidence for a major collision early in the history of the CB parent body. They revealed an abundance of evidence which indicates that Fountain Hills experienced impact shock forces greater than those observed for any other chondritic body, and they contrast this severe impact with the hypothesized collision on Mercury that is considered to have stripped away much of its original mantle. They argue that the general characteristics of bencubbinites, i.e., metal-rich, refractory-rich, and volatile-depleted, are consistent with its formation in the innermost Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids., possibly near the orbit of Mercury. Furthermore, they contend that the bulk composition of bencubbinites shows some similarities to Mercury as well. It may be speculated that bencubbinites formed from a vapor cloud that was produced by a massive collisional impact on Mercury. However, data gathered by the MESSENGER (MErcury Surface, Space ENvironment, GEochemistryStudy of the chemical composition of Earth and other planets, chemical processes and reactions that govern the composition of rocks and soils, and the cycles of matter and energy that transport Earth's chemical components in time and space. Click on Term to Read More, and Ranging spacecraft) orbital spacecraft revealed a source consisting of abundant ilmeniteTi-Fe oxide, TiFeO3, found in achondrites, lunar mare basalts, and shergottites. Ilmenite forms as a primary mineral in mafic igneous rocks. It crystallizes relatively early out of a magma before most of the other minerals, and as a result, the heavier crystals of ilmenite precipitate to the bottom of the magma Click on Term to Read More (up to 22 wt%). Utilizing crystallization models, similar characteristics to Mercury were not obtained from a bencubbinite-like starting composition, but instead, the models are most consistent with an origin from a high-Ti picritic glass similar to that returned by Apollo 15 and 17 (Stockstill-Cahill and McCoy, 2010).

A hydrocode simulation of a hypothetical Mercury-forming hit and run collision was computed by E. Asphaug and A. Reufer (2014) (see below). They theorized that a differentiated chondritic impactor (proto-Mercury) having 25% the mass of Earth could have collided with proto-Venus at a given velocity and angle so that the mantle of the impactor was stripped and dispersed, leaving a largely metallic secondary body which matches the mass (5.5% the mass of Earth) and composition (~70 mass% metallic iron) of Mercury.

Simulation of a hypothetical Mercury-forming hit and run collision from Erik Asphaug. Interestingly, Sanborn and Yin (2014) compared various meteorites utilizing a ε54Cr vs heliocentric distance coupled diagram (see below). The ε54Cr value of –0.55 (±0.08) calculated for NWA 7325 does not plot in the present location expected for Mercury, unless the planet is considered to have formed at a greater heliocentric location before migrating inward in a scenario similar to that presented by Asphaug and Reufer (2014).
Diagram credit: Sanborn and Yin, 45th LPSC, #2018 (2014) A methodical and systematic analysis of the delivery criteria for a mercurian meteorite was presented by W. Vaughan and J. Head (2014), with reference to current improved data obtained by the MESSENGER spacecraft. In lieu of providing a synopsis of their conclusions here, their concise LPSC abstract is best read in its entirety. Although the transfer dynamics of ejecta from Mercury to Earth are more complex than delivery from Mars, the probability of finding a Mercurian meteorite may be higher than once thought. Perhaps several samples are already in the worldwide collections but have not yet been recognized as such. Indeed, it may even be possible to find venerian (Venus) meteorites and terrene/terran (Earth) meteorites, but their recovery is less likely because of other constraining factors such as a high escape velocity, a thick atmosphere, and a short (m.y.) time frame for re-accretion (Gladman and Chan, 2012).

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