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Breccias, Impact and Regolith |
| Listing of Structures |
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Abee, EH4, Cangas de Onis, H5, Bison, LL6 |
| Figure 1. Scale bar 1 cm. |
Abee, EH4, chondrite, impact-melt breccia. |
Regolith breccias are meteorites or rocks formed by the lithification of loose, unconsolidated surface material, known as regolith, on airless planetary bodies like the Moon, Mars, or asteroids. A regolith is a blanket of debris created by the relentless grinding of meteorite impacts, consisting of dust, soil, rock fragments, and glass particles. The transformation of this loose material into a solid breccia occurs when a significant impact compacts and fuses it, often with some degree of localized melting. A meteorite strike delivers a shockwave that compresses the regolith, while heat from the impact may partially melt some components. This process binds the loose fragments into a cohesive rock. As the heat dissipates, the compacted regolith solidifies, locking in its heterogeneous structure. Regolith breccias are texturally complex. They are a mix of rock fragments, including lithic clasts (e.g., basalt or anorthosite on the Moon), shocked minerals (e.g., maskelynite), and possibly projectile fragments from the impactor. They often contain tiny beads of glass formed from molten droplets that cool rapidly after being ejected during an impact. A hallmark of regolith breccias is their enrichment in solar-wind-implanted noble gases-helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe). These gases are implanted into surface grains by the solar wind. This is a a stream of charged particles from the Sun over long periods of exposure. For example: Helium-3: A lightweight isotope used to estimate exposure duration. Neon-20/Neon-22 Ratios: These can indicate solar wind versus cosmic ray contributions. The presence of solar-wind-implanted noble gases ties regolith breccias to the uppermost surface of their parent bodies, where solar wind can penetrate. The importance of regolith breccias is that they are invaluable for studying planetary surface evolution. Their noble gas content reveals exposure ages, while their clasts and matrix preserve records of impact events, space weathering, and regolith mixing. On the Moon, they've helped date the Late Heavy Bombardment (~3.8-4.1 billion years ago). Impact melt breccias differ as they form from high energy impacts when the intense heat and pressure melt portions of the target rock, mixing it with unmelted fragments before solidifying. Unlike regolith breccias, which originate from pre-existing surface debris, impact melt breccias are products of the impact event itself, often involving material from deeper within the parent body. Unlike regolith breccias, impact melt breccias lack significant solar-wind-implanted gases, as their material is typically sourced from beneath the surface or melted during the impact, erasing prior exposure signatures. (1) Together, regolith breccias and impact melt breccias tell complementary stories about planetary surfaces. Regolith breccias chronicle the slow weathering and mixing of the surface, while impact melt breccias capture the dramatic, instantaneous violence of large impacts. (1) A. Rubin, personal communication, 2024. |
| Additional breccias i.e., monomict, polymict, dimict, or genomict, may be presented on this page in supplemental updates. |
| Figure 2. Scale bar 4mm. |
Cangas de Onis, H5 chondrite, regolith breccia |
| Figure 3. Scale bar 1 cm. |
Bison, LL6 chondrite, regolith breccia |
| Breccias, Impact and Regolith |
| Listing of Structures |