Category:Lunar Regolith and Mega-regolith
Regolith is the Greek term for “blanket rock.” Lunar regolith is a layer of loose, unconsolidated rock and rock fragments overlying the bedrock. The Lunar surface is almost entirely regolith, with bedrock only appearing in a few locations. Regolith is not unique to Luna and exists on Earth, Mars, asteroids, and other terrestrial planets and moons. Earth regolith, however, also contains biologically active soil which is lacking in Lunar regolith.
It is estimated that the regolith is 4-5 meters deep in the younger mare areas and 10-15 meters deep in older highland regions (1, pg 286). Our primary source of information about the Lunar regolith comes from the 21 core samples taken by the Apollo missions. Most of these samples penetrated less than 1 meter. Three core samples, however, reached 221 cm, 237 cm, and 292 cm (1, pg 23).
Below the regolith is the mega-regolith. The mega-regolith is thought to consist of large-scale ejecta and impact-fractured bedrock (>1 m) blocks (1, pg. 286). The mega-regolith varies in thickness but is estimated to be up to 10 km thick (1, pg 651). This data is all inferred from orbital data and modeling, as we have not seen or collected any mega-regolith samples (1, pg. 286). We have very little data about the mega-regolith and it is essentially unknown.
Lunar regolith can be divided into three rough categories:
- Pebbles, cobbles, and boulders (≥1 cm) - These are typically individual rock fragments or breccias (rock fragments fused together).
- Fines (<1 cm) - Also informally call Lunar soil but this is misleading and potentially confusing. On Earth, soil refers to the geologic layer that includes organic matter. There is no organic material in Lunar regolith. Technically, Lunar soil should refer to regolith that has had organic material added to it to make it a suitable plant growing medium. Five basic particle types constitute the Lunar regolith fines and dust:(1, pg 288)
- Mineral fragments
- Pristine crystalline rock fragments
- Breccia fragments
- Various glasses
- Lunar dust (<100 µm) - The median particle size of combined Lunar fines and dust is 70 µm, with roughly 10-20% being smaller than 20 µm (1, pg 478). This means over half of the Lunar regolith (minus particles greater than 1 cm) is dust smaller than the human eye can see (1, pg 478).
Lunar Pebbles, Cobbles, and Boulders
(1) Heiken, Vaniman, & French (1991). Lunar Sourcebook: A User's Guide to the Moon. Cambridge: Cambridge University Press. [www.lpi.usra.edu/publications/books/lunar_sourcebook/]
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