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Eclogite
2015-Mar-17, 12:45 PM
There has been speculation that the Angrite meteorites may have originated on Mercury . Only eleven examples of this type are known. They are of basaltic composition, with a variety of textures, including ophitic, porphorhytic, cumulate and plutonic. Dated at 4.55 Ga they are the oldest known volcanic rocks and must have originated on a planetary, i.e. differentiated, body.1

The existence of lunar and especially Martian meteorites in our collections proves that escape from the gravitational well of Mercury (with escape speed of about 4.2 km/sec, less than that of Mars) should be possible....I here present the results of much more extensive simulations of ejecta transfer from Mercury...Given the roughly 20 meteorites of Martian origin in the worldwide collection, the nominal yield calculated here means that we may be near the threshold of expecting a mercurian meteorite to be found. 2

There is a good summary by Irving et al of the circumstantial evidence pointing to an origin on Mercury.3
(1) The virtual lack of Na implies a highly refractory planet (near the Sun?)
(2) Oxygen isotopic compositions are close to and parallel to the TFL (like planetary rocks from Earth, Moon, Mars and Vesta)
(3) Preserved corona textures in NWA 2999 require a parent body capable of km-scale tectonic uplift of lithospheric material (by thrust faulting?)
(4) Each Angrite specimen is texturally different with a unique CRE age
(5) The wide range in CRE ages (55 to <6.1 Ma) suggests that the parent body (APB) is large enough to be struck repeatedly and may still be extant
(6) Very ancient formation ages (>4.555 Ga) imply very rapid core segregation and cooling following APB accretion (consistent with contraction?)
(7) Dynamical calculations predict that ~1% of material ejected from Mercury could reach Earth
(8) The limited shock effects may mean that some angrites, including NWA 2999, were ejected by spallation; others may be impact melts (could vesicles in some quenched specimens be trapped impact rock vapor?).

However, the idea is rejected by most researchers for a variety of reasons, including, mismatch of predicted detailed chemistry with all known meteorite types4 and mismatch of spectral character5.

It remains an interesting problem and it seems likely that one or more specimens from Mercury will turn up in a decade or so, either as fresh discoveries or as misclassified items from the collections. It is likely that information from the Messenger probe, currently in orbit around Mercury, would aid in identifying and confirming potential specimens. Do you think the Angrites may be from Mercury, or can you think of any other candidates, or do you doubt the possibility of any object making it from the surface of Mercury to the surface of the Earth?

1. S. Kuehner et al. LPSC 37: 1344 2006.
2. Gladman, B. "Transfer of Mercurian impact ejecta to Earth and implications for Mercurian meteorites" LPSC 34: 2003.
3. A. Irving et al. Eos, 86(52), abstract no. P51A-0898 2005
4. Love, S.G. & Keil, K. Recognizing Mercurian meteorites Meteoritics 30, 269-278 1995
5. Blewett, D.T & Burbine, T.H. Angrites as Samples of Mercury?: A Spectral Perspective LPSC 38: 2007

Romanus
2015-Mar-17, 02:17 PM
Though I'm no expert on meteorites, I side with those who doubt that we have any Mercurian meteorites, even though they're certainly possible; prospective Mercurian meteorites are too low in sulfur, and much too old. My (unsatisfying) guess is that they're from a long-destroyed protoplanet, perhaps one that had had an impressive magnetic field.

Eclogite
2015-Mar-17, 02:37 PM
My (unsatisfying) guess is that they're from a long-destroyed protoplanet, perhaps one that had had an impressive magnetic field.I considered that too, but the researchers beat us to it. It is possible to determine, quite closely, the transit time for a meteor, from ejection to arrival on the Earth. The Angrites have been in transit for tens of millions of years, not billions. Of course they could be from a remnant of that protoplanet, enjoying a current existence as an asteroid. However, thus far we have no candidates with the right properties to provide a convincing match.

I don't understand your comment on the Angrites being too old. Could you explain why you feel that excludes them?

Romanus
2015-Mar-18, 03:16 AM
^
My understanding is that, like most meteorites, the candidates are about as old as the oldest meteorites, while Mercury apparently had an extended period of volcanism after its formation that should have resulted in younger dates (say, 4 Gya instead of 4.5).