ULTRA HIGH ENERGY COSMIC RAYS - Somewhat vague term previously applied to cosmic rays with energies ~100 MeV. Currently applied to cosmic rays with energies ≥ 1011 MeV.
UNIVERSE - That which contains and subsumes all the laws of nature, and everything subject to those laws; the sum of all that exists physically, including matter, energy, physical laws, space, and time. Also, a cosmological model of the universe.
UREILITE (URE) - Member of a subgroup of primitive achondrites named for Novo Urei, a village in the Mordova Republic, Russia, where several meteorites fell in late 1886. It has been reported that one stone recovered by local peasants was broken apart and eaten! The report doesn't reveal why this happened - perhaps the freshly fallen meteorite smelled good, or perhaps because it was shaped like a loaf of bread, which some ureilite are. However, not all of the stones were eaten, and Novo Urei became the type specimen of one of the best-represented achondrite groups.
There are two subgroups of ureilite. Main group ureilites are composed mainly of coarse-grained olivine (Fa5-25), pigeonite (En75Wo15) and orthopyroxene (En80-90), set in a dark carbonaceous matrix of graphite and diamond, Ni-Fe metal, and troilite. Polymict ureilites consist of a mixture of different lithologies. Besides clasts from main group ureilites, they contain magmatic inclusions, dark carbonaceous clasts, chondritic fragments of different origins, and various other inclusions. This suggests a surface or regolith origin for the polymict ureilites, an assumption supported by the values for noble gases that have been implanted into the regolith by the solar wind. However, both the origin and the formation history of the ureilites remain enigmatic. Their mineral and oxygen isotopic compositions suggest that they are residues of partial melting, and therefore represent primitive achondrites that probably formed on several parent bodies. On the other hand, REE patterns and other chemical characteristics indicate that ureilites are highly fractionated igneous rocks that formed in different regions of the same parent body; probably a moderately differentiated C-class asteroid that was disrupted by an impact event and then rapidly cooled. An impact history would also explain the occurrence of high-pressure minerals such as diamond and londsdaleite that are formed by intense shock metamorphism. Even this theory isn't without its problems though. Recently, a ureilite from the Libyan Sahara, named DaG 868, was found to contain diamonds, but paradoxically, appears to be nearly unshocked.