A Mercury Meteorite?
NWA 2999, angrite, collected in Morroco, 2004.
Are Angrites From Mercury?:
Angrites are mafic to ultramafic "igneous" or metamorphic rocks that probably derive from a large, differentiated planetary body;
yet, in the absence of any "ground truth" the possibility that they are samples from Mercury rests only on circumstantial arguments:
(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 less than 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?).
A Smithsonian/NASA abstract.
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