Lunar domes in the Doppelmayer region: Spectrophotometry, morphometry, rheology, and eruption conditions

Abstract

In this study we examine a lunar volcanic region near Doppelmayer, composed of two domical structures previously not studied in detail and a well-known lunar pyroclastic deposit. Dome 1 is situated at selenographic coordinates 41 . 92 ∘ W and 30 . 08 ∘ S , dome 2 at 43 . 42 ∘ W and 30 . 66 ∘ S . We perform a spectrophotometric study of representative locations of the volcanic region based on Clementine UVVIS data. Relying on ground-based high-resolution CCD imagery, we furthermore examine the morphometric characteristics of the two domes, making use of a combined photoclinometry and shape from shading technique. Based on a rheologic model, we examine the physical conditions under which the domes were formed. Dome 1 is spectrally atypically red for mare domes and shows a very weak mafic absorption, implying a low TiO 2 and FeO content. The overall spectral signature corresponds to that of a mixture between mare and highland soils. Dome 1 was formed of lava with a relatively high viscosity value of ∼ 10 7 Pa s , situated between the ranges of values typically observed for mare domes and for the Gruithuisen and Mairan highland domes, respectively, erupting at moderate rates over a long period of time. Dome 1 is larger, steeper, and more voluminous than typical mare domes but has a much lower flank slope than the Gruithuisen and Mairan highland domes. It is an exemplar of a rare type of unusually steep and voluminous mare domes, similar to the well-known mare domes Hortensius 5 and 6 and Herodotus ω . We discuss the relevance of vertical (assimilation of crustal material) vs. lateral (distribution of material across mare-highland boundaries by random impacts) mixing mechanisms as being responsible for the observed spectral appearance of dome 1. The thermal conditions in the lunar interior did not favour the assimilation of crustal wallrock into the ascending magma. Due to the fact that dome 1 is located right on the boundary between hummocky terrain and a mare pond, lateral mixing of mare and highland soils is a much more natural explanation for the observed spectral signature. For dome 2, we find that it is a typical effusive mare dome, given its spectral and morphometric properties and inferred rheologic parameters. An estimation of the dimensions of the feeder dikes of the two domes reveals that the dike which formed dome 1 was five times as broad as the one which formed dome 2, while the dike lengths only differ by about one third. The dike dimensions suggest that their source regions were located below the lunar crust.