How would our theories of the Moon's history have been affected if astronauts had discovered sedimentary rock on the Moon?

The formation of sedimentary rocks is a process deeply intertwined with the presence of water and an atmosphere.

Sedimentary rocks typically originate when particles settle out of water or air and accumulate in layers, which over time, are compacted and solidified into rock. This is a process that commonly takes place on Earth, where water plays a critical role in weathering and eroding existing rocks. These particles include minerals, organic matter, and fragments of older rocks. Over extremely long periods, these materials are buried, compacted, and eventually cemented together by minerals precipitating from water percolating through the sediments.

• Weathering: Breakdown of rocks through the action of weather, water, or biological activity.
• Erosion: The movement of sediment by wind, water, ice, or gravity.
• Compaction: The process by which sediments are squeezed together by the weight of overlying materials.
• Cementation: The process where dissolved minerals crystallize and glue sediment grains together.

It's critical to understand that the presence of sedimentary rocks would suggest a history of such processes taking place in an environment.

Contrasting with Earth's dynamic and water-rich surface, the lunar surface presents a starkly different environment.

The Moon's surface is primarily composed of basaltic lava flows and anorthosites.

• Basalts: These igneous rocks are the result of volcanic activity and have formed the dark, flat planes known as the lunar maria.
• Anorthosites: A type of intrusive igneous rock, primarily composed of plagioclase feldspar, which makes up the lighter highland regions.

The Moon is also blanketed in a layer of regolith, a fine-grained soil formed by the incessant barrage of meteorites over billions of years. This dusty layer comprises broken and powdery materials without any water to bind them into rocks. The absence of an atmosphere and liquid water means the processes for forming sedimentary rocks, as observed on Earth, do not actively occur on the Moon. Given the Moon's current state – no atmosphere, no liquid water, and a composition made of materials that crystallize directly from magmatic processes – the discovery of sedimentary rocks would indeed be a scientific anomaly.

Sedimentary rocks on the Moon would demand a radical revision of our current understanding of lunar history.

If such rocks were discovered, implications would be profound:

• Presence of Water: It could indicate that there was once standing water on the Moon, suggesting periods when the Moon's environment was dramatically different from what we see today.
• Atmospheric Conditions: The existence of an atmosphere, albeit in the distant past, could be inferred to facilitate the weathering and transportation of sediments.
• New Geological Processes: Sedimentary rocks might hint at previously unknown geological or geochemical processes that might have taken place on the Moon's surface.
• Lunar Exploration: A re-examination of lunar samples from past missions would likely follow, as well as the planning of new missions aimed at understanding this unexpected aspect of lunar geology.

This discovery would stir the scientific community, leading to new hypotheses about the Moon’s formation and its past environmental conditions. As a testament to the dynamic nature of science, finding sedimentary rocks on the Moon would ignite a new era of lunar exploration and study, pushing the boundaries of our knowledge about our nearest celestial neighbor.