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Planetary image interpretation and mapping Phil Stooke USGS map I-515 Copernicus region The first area mapped using these methods E. M. (Gene) Shoemaker and R. Hackman, 1962. “Stratigraphic Basis for a Lunar Timescale”, available HERE. Image from the Consolidated Lunar Atlas, available online at LPI Names: Many other named features not shown here. Details at the USGS Planetary Nomenclature website Scale: Copernicus is about 100 km across Observations: Craters: produced by asteroid impacts Secondary craters: made by big blocks thrown out of the primary crater Rays: surface material disturbed by ejecta and/or thrown out of primary Mountains: part of the rim of a gigantic crater (basin) Impact is the dominant process here Context Context is crucial to understanding Here we see Copernicus near the bottom. The mountains above it (north) are part of a circular pattern – the rim of a giant crater (called a basin). Without context you don’t know that. Lunar Orbiter 4 image 4-114-M (always identify your images) The image is available from LPI’s Lunar Orbiter Photo Gallery Observations: Smooth plains: lava flows filling low areas inside and outside the giant ‘basin’ Domes: small volcanoes Ridges: deformed surfaces of lava flows Dark hills: in a few cases, volcanic ash deposits Volcanism has also contributed to the landscape Units: Geologic (lithostratigraphic) units are individual bodies of rock or other material formed by a specific event or process. The landscape is a patchwork of these units We can recognize them on Earth by composition, texture, fossils etc. We can try to recognize them on other worlds by morphology and texture (and composition with more recent data) Units: Forgive the very crude outlines! Brown: material of mountains Pink: material of hills Note: I’m describing the rock – the material – not the landform (mountain or hill) I think not just of the surface appearance – this goes down into the crust as a 3D mass of rock. Units: Yellow: material of craters with rays green: material of craters without rays Purple: material of partly filled craters Note: I’m mapping ejecta deposits, impact melt, all materials associated with the crater. They could be subdivided (Smaller features omitted for clarity) Units: Blue: material of smooth plains red: material of domes Note: I have not mapped rays separately, but I could if needed, especially in a detailed map of a small area. (Smaller features omitted for clarity) Units: Here’s the USGS version, much prettier but basically doing the same thing (all such maps available at the LPI website under Resources Lunar Atlases – Lunar Map Catalog USGS map I-515, Geologic Map of the Copernicus Quadrangle of the Moon. Schmitt, H., Trask, N. and Shoemaker, E., 1967. Ages – young or old? Copernicus must be young – its rays lie on top of the smooth plains Plains must be younger than the basin – they fill it and cover most of its ejecta Craters without rays: older, their rays mixed into the local regolith by small impacts Ages – young or old? Lunar Orbiter 4 image 4-126-H2 Crater at left – Copernicus secondaries on its rim – older than Copernicus Crater at right – very few superposed craters and no Copernicus secondaries on its rim or ejecta – younger than Copernicus (always identify your source images) Ages – young or old? One of the ‘filled craters’ Covered with Copernicus secondaries – older than Copernicus But… it lies on the basin rim, so must be younger than the mountains and hills Lunar Orbiter 4 image 4-126-H2 Ages – young or old? Domes – older than Copernicus – secondaries cross the dome at middle left Younger or older than plains? No real evidence here, one way or the other Lunar Orbiter 4 image 4-126-H1 Unit description and interpretation We try to keep these separate. If the interpretation is wrong, the unit mapping may still be useful with a new interpretation Examples: Mountain material: Material of large steep-sided elevated areas. Interpretation: rim materials of large impact basin Hill material: Material of small isolated hills and regions of many hills. Interpretation: ejecta of large impact basin Dome material: Material of smooth round to elongated elevated hills, most with summit pits. Interpretation: volcanic shields and cinder cones, pits are calderas or vents Geologic history We try to put it all together. How did the surface get to be the way it is now? 1. Organize materials by order of formation: Youngest - craters with rays - craters without rays - plains and domes - filled craters - the materials of the large impact basin (Imbrium basin) Oldest 2. Describe as a narrative: A very large impact formed the Imbrium basin, destroying any older features in this map area and producing a mountainous rim and hilly ejecta deposit. Some craters formed on top of those materials. Lava flows flooded low areas inside the basin and on its ejecta, forming Mare Imbrium and Oceanus Procellarum. Some cones and domes formed at about the same time. Numerous craters formed after that. Older craters, including Eratosthenes, had their rays removed by small impacts (gardening of the regolith). Younger craters such as Copernicus still show rays and many secondaries.