Mercury is an igneous, differentiated planetary body thought to have undergone initial crystallization 3.7–4.4 b.y. ago. Specific features attributed to mercurian meteorites have been proposed by meteoriticists in many published works, information from which is presented here; these publications include the following:
Properties of the Hermean regolith: V. New optical reflectance spectra, comparison with lunar anorthosites, and mineralogical modelling, J. Warell and D. Blewett, Icarus, vol. 168, 2004
Transfer of mercurian ejecta to Earth and implications for mercurian meteorites, B. Gladman, LPSC 34, 2003, #1933
The Delivery of Martian and Lunar Meteorites to Earth, B. Gladman and J. Burns, Division for Planetary Sciences, American Astronomical Society, 1996
Ejecta Transfer Between Terrestrial Planets, B. Gladman, J. Burns, H. Levison, and M. Duncan, 172nd Symposium of the International Astronomical Union, 1996
Delivery of Planetary Ejecta to Earth, B. Gladman, Doctorate Dissertation, Cornell University, 1996
Mercurian Meteorites: Properties and Probabilities, S. Love and K. Keil, LPSC 26, 1995
Recognizing mercurian meteorites, S. Love and K. Keil, Meteoritics, vol. 30, 1995
Swapping Rocks: Ejection and Exchange of Surface Material Among the Terrestrial Planets, J. Melosh and W. Tonks, Meteoritics, vol. 28, 1993
Spectra of extremely reduced assemblages: Implications for Mercury, T. Burbine, T. McCoy, L. Nittler, G. Benedix, E. Cloutis, and T. Dickinson, Meteoritics & Planetary Science, vol.37, 2002
Unique Angrite NWA 2999: The Case For Samples From Mercury, A. Irving, S. Kuehner, D. Rumble, T. Bunch, J. Wittke, G. Hupé, A. Hupé, American Geophysical Union, 2005 Fall Meeting, Abstracts
NWA 2999, A Unique Angrite with a Large Chondritic Component, M. Gellissen, H. Palme, R. Korotev, and A. Irving, LPSC 38, 2007, #1612
Siderophile Elements in Metal From Metal-rich Angrite NWA 2999, M. Humayun, A. Irving, and S. Kuehner, LPSC 38, 2007, #1221
The Mn–Cr Isotope Systematics of Bulk Angrites, A. Shukolyukov and G. Lugmair, LPSC 38, 2007, #1423
Grain Boundary Glasses in Plutonic Angrite NWA 4590: Evidence for Rapid Decompressive Partial Melting and Cooling on Mercury?, S. Kuehner and A. Irving, LPSC 38, 2007, #1522
A Fresh Plutonic Igneous Angrite Containing Grain Boundary Glass From Tamassint, Northwest Africa, A. Irving, S. Kuehner, and D. Rumble III, American Geophysical Union, Fall Meeting 2006, #P51E-1245
The Case Against Mercury As The Angrite Parent Body (APB), M. Hutson, A. Ruzicka, and D. Mittlefehldt, 70th MetSoc, 2007, #5238
Coronas and Symplectites in Plutonic Angrite NWA 2999 and Implications for Mercury as the Angrite Parent Body, S. Kuehner, A. Irving, T. Bunch, J. Wittke, G. Hupé, and A. Hupé, 37th LPSC, 2007, #1344
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, 2007, #4050
Primary Ferric Iron-Bearing Rhönite in Plutonic Igneous Angrite NWA 4590: Implications for Redox Conditions on the Angrite Parent Body, S. Kuehner and A. Irving (2007) EOS, Trans. AGU 88, #P41A-0219
Mercury: Mg-rich Mineralogy with K-spar and Garnet, A. Sprague, K. Donaldson Hanna, R. Kozlowski, J. Helbert, A. Maturilli, and N. Izenberg, 39th LPSC, 2008, #1320
It is accepted that impact ejecta from the surface of Mercury would be able to escape the planet's gravity well at a launch speed of 4.2 km/sec, compared to 5.0 km/sec for Mars. However, in order to complete a journey to Earth the material has to escape the Sun's gravity well, 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 ejection velocity of ~6.2 km/sec (Wetherill, 1984).
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 utilized 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., and vice versa, suggesting a higher likelihood of finding a venusian than a mercurian meteorite.
Indicators of a Mercurian Meteorite
differentiated igneous rock/breccia/impact melt
moderate refractory oxide enrichment (Ca and Al)
low FeO content (~5% or less for surface-derived material, commensurate with spectroscopic observations)
low FeS content
low alkali element content
probably has low abundance of volatile elements
probably contains anorthositic plagioclase
likely to have O-isotopic compositions close to the TFL
crystallization age of ~3.7–4.4 b.y.
probably contains low content of regolithic solar-wind-implanted gases
regolithic solar-wind-implanted gases may 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 mineral compositions
A mercurian meteorite would presumably be an achondrite and would most likely be mistaken for an aubrite 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 micrometeorite impacts and solar wind particles produce nanophase iron coatings on surface materials. This high amount of space weathering on Mercury would require reduction 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 mare basalt. 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 has been set forth by investigators from the University of Washington in Seattle. They suggest that the angrite group of meteorites may represent material from Mercury that was collisionally stripped from the planet, 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) find it implausible that Mercury initially differentiated under oxidizing 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 decompresion on a planetary-sized body such as Mercury, are more consistent with typical cooling processes during crystallization of a melt.
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 belt, or perhaps may still orbit around the original collisionally-stripped parent body (Mercury?).
Other studies which relate to a possible angrite–Mercury connection have been conducted. For example, in a study of siderophile element depletions (Ni, Co, and W) and their association to core 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 periotite mantle and FeNi-metal core which attained metal-silicate equilibrium under very low pressures. The oxidation 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.
Although the transfer dynamics of ejecta from Mercury to Earth are much more inefficient than from Mars, the probability of finding a mercurian meteorite does exist, and this fact should be kept in mind during meteorite analyses. Indeed, it may even be possible to find venusian (Venus) meteorites and terrene (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.