Mathematics – Logic
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999nvm..conf...69w&link_type=abstract
Workshop on New Views of the Moon 2: Understanding the Moon Through the Integration of Diverse Datasets, p. 69
Mathematics
Logic
Basalt, Elevation, Lunar Crust, Lunar Mantle, Lunar Maria, Magma, Moon, Topography, Volcanology, Selenology, Clementine Spacecraft, Kreep, Lunar Prospector, Depth
Scientific paper
Although it has been known from the beginnings of the space age that the lunar mare basalts are primarily located on the nearside of the Moon, the definitive cause of this phenomenon has remained a mystery. One popular explanation is that the hydrostatic pressure of the mare source controls the eruption of mare basalts. In this scenario, the depth of the mare source controls the maximum height that magma can rise in the crust. If the maximum depth of the mare source was globally uniform, then mare basalts would only be able to erupt at the surface below a critical elevation. Following the discovery of the Moon's 2-km center-of-mass/center-of-figure offset, many have suggested that the higher elevations of the lunar farside could have prevented farside magmas from reaching the surface due to their lack of the necessary pressure in their source. Recent data obtained from the Clementine and Lunar Prospector missions suggest that this scenario may be a bit simplistic. Using Clementine altimetry data, the full topographic extent of the South Pole Aitken Basin has been determined. Even though the lowest elevations of the Moon were found to occur within this basin's floor, mare flows in this basins are volumetrically insignificant when compared to the nearside basins and Oceanus Procellarum. If elevation was the only factor controlling the eruption of basalts, then this basin should surely have been completely flooded . Gamma-ray data from the Lunar Prospector mission also suggest that elevation is not the only factor that controls the eruption of mare basalts to the surface. Based on the surface distribution of KREEP, as well as results from previous studies, it has recently been argued that the Procellarum and Imbrium region of the Moon is a unique geochemical crustal province enriched in incompatible and heat-producing elements (named the "Procellarum KREEP terrane," or PKT). Wieczorek and Phillips have noted that more than 60% of the Moon's mare basalts reside within this province, and have argued for a genetic relationship between the two. Specifically, by placing a layer of KREEP basalt in this province, their thermal models predict that the lunar mantle should have partially melted only beneath this province. The eruption of mare basalts thus may be primarily controlled by the distribution of heat sources in the Moon. A figure shows the elevation of the lunar mare referenced to the geoid. We reference the mare elevations to the geoid for two reasons. First, magma on the surface of the Moon would flow down the geopotential gradient (i.e., downhill). Secondly, for a given hydrostatic pressure in the mare source, the maximum height a column of magma can extend is a function of elevation referenced to the geoid. We plotted histograms of elevation referenced to the geoid for the entire Moon, the mare within the confines of the Procellarum KREEP terrane, and the remainder of the mare outside this terrane. From this plot it is found that the eruption of mare basalts does indeed appear to be limited by elevation. In particular, no mare flows occur above an elevation of 2.6 km, and the vast majority of flows occur below about 0.5 km. This is true for both mare basalts that erupted inside and outside the confines of the PKT. This result is consistent with the hypothesis that hydrostatic forces in the mare source control the eruption of basaltic magmas, if the maximum depth of the mare source is globally uniform. The maximum elevation of mare basalts can be used to determine the maximum depth of the mare source by performing a simple force balance. For mechanical equilibrium, the pressure at the base of a column of magma must be balanced by the Moon's reference hydrostatic pressure at this depth. We have attempted to maximize the depth of the mare source in this calculation by taking extreme values of the density and bulk modulus of the mare basaltic magmas. These values were computed at the material's liquidus temperature using the technique of Delano and the data of others for the mare basaltic reference suite. The variation in density of this magma with depth was determined using the second-order Birch-Murnagban equation of state, and gravity was computed as a function of depth. If mare basalts only erupt below an elevation of 2.6km, then from the above considerations, a column of magma in hydrostatic equilibrium, at this elevation would extend to a depth of 135 km below the surface. If this column of magma were to extend deeper into the lunar mantle, then it would have been possible for mare basalts to erupt at elevations higher than are observed. It is natural to interpret this depth as the maximum depth of the mare source, and as a working hypothesis we assume this to be the case. We note, however, that this interpretation appears to be inconsistent with the petrologically constrained depth of the mare source. Assuming that the picritic glasses were multiply saturated in olivine and pyroxene in their source, the mare basalts (which are likely derived from these magmas) should have all been derived from depths between 360 and 500 km. In addition, if the magmas parental to the picritic glasses fractionated olivine before erupting, then these depths should be interpreted as a lower limit. In the following sections, we briefly discuss three possible scenarios that may rectify the disparity between the depth of the mare source based on hydrostatic and petrologic arguments. If the density of mare basaltic magmas was less than that of the surrounding crust, then these melts would have been able to rise to the surface based on buoyancy forces alone. Though the upper anorthositic crust of the Moon is certainly less dense than most mare basaltic magmas, many have argued that the lunar crust becomes progressively mafic with increasing depth below the surface. Additional information contained in original.
Phillips James R.
Wieczorek Mark A.
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