[1] High-resolution compositional data from Moon Mineralogy Mapper (M3) for the Moscoviense region on the lunar farside reveal three unusual, but distinctive, rock types along the inner basin ring. These are designated "OOS" since they are dominated by high concentrations of orthopyroxene, olivine, and Mg-rich spinel, respectively. The OOS occur as small areas, each a few kilometers in size, that are widely separated within the highly feldspathic setting of the basin rim. Although the abundance of plagioclase is not well constrained within the OOS, the mafic mineral content is exceptionally high, and two of the rock types could approach pyroxenite and harzburgite in composition. The third is a new rock type identified on the Moon that is dominated by Mg-rich spinel with no other mafic minerals detectable (<5% pyroxene, olivine). All OOS surfaces are old and undisturbed since basin formation. They are effectively invisible in image data and are only recognized by their distinctive composition identified spectroscopically. The origin of these unusual lithologies appears to be linked to one or more magmatic intrusions into the lower crust, perhaps near the crust-mantle interface. Processes such as fractional crystallization and gravity settling within such intrusions may provide a mechanism for concentrating the mafic components within zones several kilometers in dimension. The OOS are embedded within highly anorthositic material from the lunar crust; they may thus be near contemporaneous with crustal products from the cooling magma ocean.
We show evidence of very recent (≤25–40 Myr) geologic activity on the eastern flank of Olympus Mons volcano that includes a suite of fluvial (channel networks), volcanic (emplacement of lava flows and dikes), and tectonic (wrinkle ridges and troughs) processes. The combination and youth of these features confirms the importance of geological activity continuing to the present on Mars.
MESSENGER from Mercury The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planet's orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planet's surface. Now, after MESSENGER, only 10% of Mercury's surface remains to be imaged up close. Denevi et al. (p. 613 ) use this near-global data to look at the mechanisms that shaped Mercury's crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercury's surface is dotted with impact craters. Watters et al. (p. 618 ) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercury's contraction as its interior cooled over time. Mercury's exosphere and magnetosphere were also observed (see the Perspective by Glassmeier ). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606 ) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610 ) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercury's exosphere, which may shed light on the processes that create and maintain the exosphere.
Syrtis Major is an old, low relief volcanic plateau near the equatorial regions of Mars. It is a persistent low‐albedo feature on the planet and is thought to contain a high abundance of exposed bedrock and/or locally derived surface material and debris. Spatially resolved variations in surface spectral properties, and therefore composition, are investigated with data from the Imaging Spectrometer for Mars (ISM) instrument. ISM obtained 128 wavelength channel spectra from 0.76 to 3.16 μm for contiguous pixels approximately 22 × 22 km in size across much of the plateau. The value and spatial distribution of four primary spectral variables (albedo, continuum slope, wavelength of the ferric‐ferrous band minimum, area of the ferric‐ferrous absorption) are mapped and coregistered to Viking digital photomosaics. Analysis of these maps shows that although there is a high degree of overall spectral variability on the plateau, the key indicators of mafic mineralogy are relatively homogeneous. Detailed examination of reflectance spectra from representative areas across the plateau indicate the volcanic surface is dominated by augite‐bearing basalts and the pyroxene composition in the basalts is estimated to be 0.275± 0.075 Ca/(Ca+Fe+Mg) and 0.3± 0.1 Fe/(Fe+Ca+Mg). Additional mineral components may include olivine, feldspar, and glass. Most of the spectral variability on the plateau is interpreted to result from mixing of volcanic bedrock and/or locally derived surface material and debris with highly altered dust and soil. In western Syrtis Major the altered material is a transient component on the surface or occurs in large spatially coherent patches (e.g., crater rims). In eastern Syrtis Major it is apparent that the dust components are firmly fixed to the basaltic substrate as a stable oxide rind or coating.
Infrared wavelength observations of Io by the Galileo spacecraft show that at least 12 different vents are erupting lavas that are probably hotter than the highest temperature basaltic eruptions on Earth today. In at least one case, the eruption near Pillan Patera, two independent instruments on Galileo show that the lava temperature must have exceeded 1700 kelvin and may have reached 2000 kelvin. The most likely explanation is that these lavas are ultramafic (magnesium-rich) silicates, and this idea is supported by the tentative identification of magnesium-rich orthopyroxene in lava flows associated with these high-temperature hot spots.
It is commonly assumed that hydrostatic pressure balance arguments can be used to establish a relationship between the maximum height to which a volcanic edifice is able to grow and the depth at which the partial melts providing its magma supply are formed. Such a relationship has been used to infer various aspects of the thermal and stress state of the lithosphere beneath volcanic constructs on Earth, Mars, Io and Venus. We examine the assumptions behind this relationship (which are that: (1) a continuous pressure connection exists between source and summit, (2) the pressure around the magma source is the local hydrostatic pressure dictated by the depth below the geoid, and (3) the melt erupting at the summit has a net positive buoyancy), and show that many of them require geologically unreasonable conditions. We then critically assess the evidence cited in the literature for the relationship and find that there are other factors that may explain the observations. We conclude that volcano heights on the terrestrial planets cannot be related in any simple way to lithospheric thickness or depth to the magma source zone and we review the range of other factors controlling volcano height.
[1] Using the Moon Mineralogy Mapper(M3), we examine the Marius Hills volcanic complex for the first time from 0.46 to 2.97 μm. The integrated band depth at 1 μm separates the mare basalts on the plateau in two units: (1) a strong 1 μm band unit of localized lava flows within the plateau that has similar olivine-rich signatures to those of the nearby Oceanus Procellarum and (2) a weaker 1 μm band unit that characterizes most of the basalts of the plateau, which is interpreted as having a high-calcium pyroxene signature. Domes and cones within the complex belong to the high-calcium pyroxene plateau unit and are associated with the weakest 1 μm band observed on the plateau. This difference could be the result of higher silica content, more opaque minerals, and/or a weaker olivine content of the magma. Finally, the floor of Marius crater has one of the strongest olivine-rich signatures of the entire Marius Hills complex. These compositional differences are indicative of the long and complex volcanic history of the region. The first episode started before the emplacement of the surrounding basalts of the plateau and produced the high-calcium pyroxene flows present on the plateau and their associated domes and cones. The second episode occurred concurrently or slightly after the emplacement of the adjacent Procellarum basalts and produced the olivine-rich basalts seen within the plateau, outside the plateau, and in Marius crater. If the olivine content of the lava flows increases with time, the olivine-rich region on the floor of Marius crater may represent one of the latest episodes of volcanism exposed on the Marius Hills complex.
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