HED group meteorites, eucrites, pictures, photos, images

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HED group meteorites
Part 1, eucrites

Pictured above is a 72g individual of the monomict eucrite Millbillillie which fell in October 1960 near Jundee Station, Wiluna district, Western Australia. The image below shows the weathered rind on the opposite surface of the above specimen with the characteristic laterite stain adherent to most Millbillillies. Today scientists agree that the source of the known eucrites is Asteroid 4 Vesta and its smaller companions, the Vestoids. Vesta was discovered on March 29 in 1807 by the German astronomer Heinrich Olbers. After Ceres and Pallas Vesta is the third largest body in the asteroid main belt. With an albedo of 0,423 Vesta is also the brightest asteroid in the night sky. In a dark sky without light pollution it can be viewed with the unaided eye.

Based on Vesta's light spectrum, pyroxene rich mineralogy and what we know about achondrite meteorites, we expect Vesta's surface to be a mixture of three rock types: diogenites, basaltic eucrites, and cumulate eucrites. These rock types crystallized at different times within a cooling magma ocean. Minerals that crystallize early during the cooling process and which are more dense than the surrounding magma sink and form aggregates on the floor of the magma chamber. Such rocks forming from accumulated crystals are named cumulates. Diogenites for example are cumulates that formed at depth, composed mostly of magnesium-rich, calcium-poor orthopyroxene. And there are cumulate eucrites as well. They formed between the bottom and the surface of the magma chamber. Basaltic eucrites have formed much closer to the surface of the Vesta magma oceans. They are are made up of iron rich pyroxene and sodium-poor plagioclase.

The image above shows a 72.90g endcut of NWA 2482, a "polymict breccia of lithic and mineral clasts set into a fine-grained clastic groundmass of plagioclase, Ca-pyroxene, orthopyroxene, and opaque phases" (Meteoritical Bulletin no. 89). Pictured below is a 7.12g part slice of the main group eucrite HaH 286.

Several of the known eucrite meteorites were metamorphosed after their formation by complex processes involving heating and pressure. Regolith forming processes such as impacts and seismic activity contributed further to the alteration of Vesta’s surface. Today vast areas of the asteroid’s surface are covered with polymict eucritic breccias and breccias of diogenites and eucrites. The latter are named howardites, if they contain more then 10 percent diogenitic clasts. Together these igneous rocks form the HED group (Howardites, Eucrites, Diogenites).

Freshly fallen eucrites, like the 35.30g Camel Donga meteorite from Australia (pictured above), are famous for their glass like fusion crust. Eucrites contain a relatively high ammount of calcium-bearing minerals such as plagioclase and augite. When mixed with free metal that has been oxidized to magnetite in the fusion process , these minerals produce a black, glossy fusion rind that often displays striking flow features.

The NASA images above and below show an evolution model (top) and an elevation model (bottom) of Asteroid 4 Vesta acquired by the Hubble space telescope in 1997. The latter clearly shows the 460km wide impact crater on the asteroid's south pole.

Connected to Vesta are a number of smaller but still kilometer-scale bodies that share its mineral composition. These are called the Vestoids. It is believed that they originate from a catastrophic collision Vesta has suffered and whose traces can still be seen in the shape of a large impact crater near Vesta’s south pole. Remarkably it has been found that the Vestoids have a redder continuum across the visible and NIR interval in contrast to a Vesta’s continuum in the same wavelength range.

The reddish spectra of asteroids (not only vestoids) are usually explained by alteration of their surface due to the effects of space weathering. Vestoids appear far more affected by weathering than their parent body Vesta. Recently Shestopalov and Golubeva (2008) suggested that long term seismic activity connected with a 460-km impact basin near the Vesta south pole may be responsible for this phenomenon. By reaching the surface high frequency seismic waves erupting from time to time could trigger regolith forming processes thus overturning and replacing old weathered portions by new, unweathered regoliths. According to Shestopalov and Golubeva another reason for the unweathered surface of Vesta may be large clouds of asteroidal debris trapped in the gravitational sphere of Vesta which shield the asteroid from space weathering. Pictured below is a 58.90g endcut of the eucrite NWA 2126 (provisional). Shifting regolith blankets composed of breccias like these may be responsible for the relatively bright continuum of asteroid 4 Vesta's light spectrum.

Vesta is the first target of the Dawn spaceship which was launched in September 2007 and which is expected to reach the asteroid in 2011. Among other detectors Dawn carries a visible and IR spectrometer and a gamma ray/neutron spectrometer. With the latter it will provide us with a detailed picture of the main and trace element abundances. The NASA image below shows the liftoff of the Delta II rocket carrying the Dawn space probe on the morning of September 27, 2007.

References and further reading: Shestopalov, D. and Golubeva (2008) LPI Contribution No. 1391, p.1116. Martel, L. M. V. (2007) PSRD 11/2007, Sanders, I. S., Scott, E. R. D. (2007) LPI Contribution No. 1338, p.1910. L.McSween H.Y, and Stolper E.M. (1980) Sci. Am. 242, 54-63. Becker R.H. and Pepin R.O (1984) Earth Planet. Sci. Lett. 69, 225-242. Drake M.J. and Righter K. (2002) Nature 416, 39-44. McCord T. et al. (1970) Science 168, 1445-1447. Stolper E.M. (1975) Nature 258, 220-222. Righter K. and Drake M.J. (1997) MAPS 32, 929-944. Consolmagno G. and Drake M.J. (1977) Geochim. Cosmochim. Acta 41, 1271-1282. Binzel R.P. and Xu S. (1993) Science 260, 186-191. Asphaug E. (1997) MAPS 32, 965-980. Thomas et al. (1997) Science 277, 1492-1495. Cruikshank et al. (1991) Icarus 89, 1-13.

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