INDICATORS OF A MERCURIAN METEORITE

representative presently unknown



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, from which some pertinent information is presented here. These publications include the following (arranged by date):

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 and 2.4 km/sec for the Moon. However, to reach a heliocentric orbit and complete a journey to Earth the material has to have an ejection speed about twice the escape velocity. After escaping the Sun's gravity well, the meteoroid 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 meteorite 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 accretion 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

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 particle sputtering 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 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 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 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 belt, 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 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 peridotite 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.

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 System, 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, GEochemistry, and Ranging spacecraft) orbital spacecraft revealed a source consisting of abundant ilmenite (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).

standby for 54Cr vs heliocentric distance diagram
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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