Abstract
Abstract The Ina irregular mare patch, an ∼2 × 3 km summit depression on an ancient ∼22 km diameter shield volcano, displays two very enigmatic units: (1) dozens of dark convex-upward mounds and (2) a very rough, optically immature floor unit with very sharp morphologic contacts between the two. Controversy surrounds the age interpretation of Ina; superposed impact crater size–frequency distributions (CSFDs) suggest an age of ∼33 Ma, consistent with the presence of sharp contacts between the units and indicating that mare volcanism continues to today. Models of the terminal stages of volcano summit pit crater activity suggest an age coincident with the building of the shield, ∼3.5 Ga; these models interpret the CSFD age and sharp contacts to be due to an extremely porous lava lake floor and extrusion and solidification of magmatic foams. We present robotic–human exploration mission concepts designed to resolve this critical issue for lunar thermal evolution.
Highlights
This scenario of waning-stage two-phase lava lake activity and magmatic foam extrusion is supported by a range of observations (Carter et al 2013; Elder et al 2017; Qiao et al 2019, 2020b), including (1) the spatial density disparity of superposed impact craters between the interiors of irregular mare patches (IMPs) and the surrounding mare surfaces, (2) the unconsolidated substrate nature of the Ina mounds derived from Diviner thermophysical measurements, (3) the fine-grained and block-free nature of the small mare-type IMP materials from Arecibo radar mapping data, and (4) the high reflectance and optical immaturity of the IMP deposits
In the late Copernican volcanism scenario (Figure 3), the uneven/hummocky floor terrains are solidified lava lake crust within the shield summit vent being disrupted by lava drainage and deflation, and the smooth mounds are subsequent small lobate lava flows extruded through the cracked floor crusts (Braden 2013; Braden et al 2014)
(5) Nature of ejecta from craters into mound and floor material: Assessment of the morphology and frequency distribution of superposed impact craters of different sizes will help in identifying any distribution that might deviate from standard basaltic morphologies and proportions, or alternatively that might signal the presence of a substrate or regolith of foamy or marco-vesicular nature
Summary
While examining the high-resolution orbital photographs taken by the Apollo 15 crew in August 1971, Whitaker (1972) noted a very unusual and enigmatic depression feature (18.66°N, 5.30°E), ~3 × 2 km in size, with about half of the depression floor covered with blobs of mare materials with an appearance similar to “dirty mercury.” Located in a small mare in the central portion of the nearside Moon, the feature was informally named “D-Caldera”. Post-flow-emplacement impact events were interpreted to cause collapse in the void-rich flows, exposing the rough and coherent interiors of the foamy flows, as observed at many small mare-type This scenario of waning-stage two-phase lava lake activity and magmatic foam extrusion is supported by a range of observations (Carter et al 2013; Elder et al 2017; Qiao et al 2019, 2020b), including (1) the spatial density disparity of superposed impact craters between the interiors of IMPs (both floor and mound units) and the surrounding mare surfaces, (2) the unconsolidated substrate nature of the Ina mounds derived from Diviner thermophysical measurements, (3) the fine-grained and block-free nature of the small mare-type IMP materials from Arecibo radar mapping data (for Cauchy 5), and (4) the high reflectance and optical immaturity of the IMP deposits.
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