Experimental Model of the Formation of Nanophase Metallic Iron in the Lunar Regolith

Doklady Earth Sciences - One of the signs of “space weathering” on the Moon, including the effect of the solar wind and micrometeorite bombardment, is the formation of nanophase...     

Small domes on Venus: Characteristics and origin

Approximately 22,000 small domes have been identified on the 25% of the surface of Venus imaged by Venera 15/16. The word dome is used to imply a broad, lens-shaped, positive topographic feature. The domes: (1) are generally circular in planimetric outline; (2) range in diameter from the effective limit of Venera resolution (2 km) to 20 km; (3) show flank slopes generally 10 ° and possibly 5 °; and (4) occur in association with mottled plains units. Associated features include summit pits, radar bright surfaces, and basal topographic platforms. There are two significant areas of major dome concentrations approximately 180 ° in longitude apart: (1) the largest concentration occurs in the Akkruva Colles area of Niobe Planitia, centered at approximately 45 ° N/120 ° E, just north of the flanks of the Thetis Regio rise; and (2) another concentration occurs in northwestern Guinevere Planitia, centered at approximately 35 ° N/300 ° E, on the north flank of the Beta Regio rise. In addition to these major areas of concentrations, domes occur in smaller concenrations throughout the imaged area of Venus, in association with coronae, arachnoids, intermediate sized hills interpreted to be volcanic constructs, large volcanic centers and calderas. The characteristics and geologic associations of small domes are consistent with an interpretation of their origin as volcanic, and on the basis of their low slopes, individual characteristics, and geologic associations they are interpreted to represent dominantly effusive low shield volcanoes. The large number of small domes implies a large number of multiple centralized eruptions, each one of which represents a discrete, relatively small, volume of material available to build an edifice over a finite time period. Calculated modal volume is 0.73 km³ for individual edifices. Based on the number identified by Venera, the total number of small domes estimated for the entire planet 4.4 × 10⁶ and total edifice volume over the entire planet represents a minimum volume equivalent to a layer approximately 7 m thick over the planet and representing 0.03% of the estimated crustal volume of Venus. In absolute number, size range, and distribution they appear to be similar to terrestrial oceanic seamounts. The global abundance and distribution, size frequency distribution, minimum size, and changes in these characteristics with latitude for the domes will be particularly important in understanding the way in which the domes form and their relationship to global models of tectonism and heat flow on Venus. Increased spatial resolution and coverage from Magellan data will enable a more thorough assessment of these features and associated questions, particularly where radar incidence angles are 15 °.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).