Lunar Mega-regolith

Lunar breccia

Wikipedia [Lunaite (lunar breccia) (Northwest Africa 8586 Meteorite) 1 (17383646595).jpg]

The Lunar mega-regolith is a layer of large-scale ejecta that lies beneath the regolith. The mega-regolith is important to Lunar Homesteaders because the plan is to dig beneath the regolith and build as much of the Homestead in the mega-regolith. Unfortunately, we know very little about the mega-regolith.

Here’s how it works. In the past, Luna was hit by a lot of fast-moving rocks. Each one hit the surface and made a crater. The impact broke up the rocks the surface and threw the pieces all over the area. Large and/or faster moving impactors had more kinetic energy which allowed them to create deeper craters and distribute more shattered rocks over a larger area. These impact-broken, distributed rocks are called ejecta. Over time, this constant breaking and moving of rocks created the mega-regolith layer.

Eventually, the number and kinetic energy of impacts decreased. The size of craters got smaller and much of the mega-regolith was left untouched. Only the top layer continued to be “impact gardened”, breaking the rocks into smaller and smaller pieces. This layer is the regolith and it contains the majority of the smallest, dust-sized particles.

Core data

  • No physical samples.
  • Layer below the regolith (4-15 meters thick).
  • The depth of the mega-regolith is uncertain.
    • Average thickness (planet-wide) = 1.85 – 1.95 km
    • Mare thickness (thinner) = several hundred meters thick
    • Highland thickness (thicker) = 2+ km thick
  • Composed of:
    • Chaotically mixed
    • Large scale impact ejecta (>1 meter blocks)
      • polymict breccias (fragmented, shock melted conglomerate of rocks)
        • Mostly local bedrock
    • Regolith
    • Ratio of regolith to ejecta decreases with depth.
  • Particle size
    • Impact ejecta range from 0.1 to 10 meters in size, with an average size of 1 meter.
    • See Lunar regolith for data on regolith particle sizes.
  • Density
    • Bulk density (dry weight per unit volume) (solids and pore space)
      • Average
        • 2350–2600 kg m3 for ejecta
      • Mare
        • 3010 to 3270 kg/m3
      • Highlands
        • 2200–2600 kg/m3
    • Grain (particle) density (dry weight per unit volume) (solids only)
      • Mare
        • High aluminum basalt grain density = 3270 kg/m3 (11, pg 1).
        • High titanium basalt grain density = 3460 kg/m3 (11, pg 1).
        • Rocks that are more mafic than anorthosite are denser (7, pg 672).
        • Lunar basalt densities are significantly higher than terrestrial basalts, reflecting their much higher abundance of FeO (11, pg 3).
      • Highlands
        • Anorthosite average = 2800 to 2900 kg/m3
  • Porosity
    • Porosity decreases exponentially with depth.
    • Average
      • Approximately 20% for ejecta.
    • Mare
      • 7% average (ranges from 2-10%).
    • Highlands
      • 12% on average
      • Varies regionally = 4% to 21%
  • Thermal conductivity
    • Average
      • 0.20 W m−1 K−1
    • Mare
      • ?
    • Highlands
      • ?
  • Chemical parameters
    • The majority of materials is assumed to be from the local bedrock. Basalt for marias and anorthosite for highlands.

Expanded data (all the data)

  • Location
    • Between the regolith and structurally disturbed crust.
      • Mare regolith is about 4-5 meters thick (1, pg 286).
      • Highland regolith is about 10-15 meters thick (1, pg 286).
Lunar surface cross section

Lunar Sourcebook (pg 92)

    • Both highlands and mare have a mega-regolith layer.
    • The modern day megaregolith could have been created by single impacts rather than by multiple large impact events (10, pg 941).
  • Size
    • The depth of the mega-regolith is uncertain and thought to be highly variable depending on the region (1, pg 93).
    • Conservative estimates are that the mega-regolith is at least 2-3 km thick (1, pg 92).
    • The mega-regolith beneath the mare could be several hundred meters thick (2, pg. 379).
    • The mega-regolith is exceptionally thin in mare regions (3, Abstract).
    • Average thickness = 1.85 – 1.95 km (according to one model) (4, pg 2699).
      • About 50% of the simulated area is less than 1 km thick (4, pg 2700).
      • 65-70% of the simulated area is less than 2 km (4, pg 2700).
    • The mega-regolith is 2-3 km thick (9, pg 1).
    • Seismic profiling and measured sound velocities have been used to determine thickness.
      • The Apollo missions left 4 seismometers on the surface.
      • Sound velocity (Vp) within the mega-regolith is 1-2 km/second (1, pg 93).
      • It is difficult to differentiate between rocks and compacted/densified regolith (1, pg 92).
      • P-wave velocities of < 3 km/sec in the upper 3km (fragmental mega-regolith) of the crust vs 3-6 km/sec in the fragmented, but still crystalline substrate (9, pg 1).
    • The average depth to which the Lunar crust is mechanically disturbed is not known (1, pg 92).
  • Composition
    • The mega-regolith is a layer of fragmented, shock-melted, and chaotically mixed impact debris called polymict breccias(1, pg 212).
    • The mega-regolith is a complex zone consisting primarily of large-scale ejecta and impact-fractured, brecciated bedrock (1, pg 286).
    • The mega-regolith is composed of ballistically transported, coarse-grained, polymict ejecta and comminuted melt sheets (1, pg 286).
    • The mega-regolith is composed of breccias, rocks, and soil overlying a basement layer with extremely irregular lower contact (9, pg 1).
    • The lowest section of the regolith may consist of highly fractured bedrock with regolith mixed in and/or with regolith seeped into the fractures (1, pg. 337).
    • The mega-regolith may consist of localized cavities (2, pg 379).
    • Large craters and basins contribute the most material to the mega-regolith (4, pg 2699).
    • Mare Cognitum region the 20 km thick upper layer has high velocity gradients and microcracks may play an important role (5, Abstract).
    • According to one study, the maria mega-regolith acts as a rock layer (basaltic flow units) where craters eject boulder fields. Terrae mega-regolith is relatively pulverized to at least 2 km and craters there eject less rocky rubble (6, Abstract). This makes sense as the thicker terrae mega-regolith absorbs more of the impact energy.
  • Physical parameters
    • May consist of > 1m blocks (1, pg 286).
    • Density
      • Highland crust (not just the mega-regolith) average density is 2550 kg/m3 with lower densities surrounding the Orientale and Moscoviense impact basins. This is lower than the 2800 to 2900 kg/m3 typically used for geophysical models of anorthosite. The lower density could be caused by impact-induced fractures and brecciation (modeled) (7, pg 672).
      • Rocks that are more mafic than anorthosite are denser (7, pg 672).
      • Surface bulk density is near 2400 kg/m3 {model} (8, pg 7).
      • Ejecta associated with impact basins have a bulk density of 2350 to 2600 kg/m3 {model} (8, pg 7).
      • Crustal density may vary by up to 500 kg/m3 from the surface to the deep crust {model} (8, pg 7).
      • High aluminum basalt grain density = 3270 kg/m3 (11, pg 1).
      • High titanium basalt grain density = 3460 kg/m3 (11, pg 1).
      • Lunar basalt densities are significantly higher than terrestrial basalts, reflecting their much higher abundance of FeO (11, pg 3).
      • Feldspathic highland crust has a bulk density of 2200–2600 kg/m3 (11, pg 1).
      • Impact basin ejecta has a bulk density of 2350–2600 kg m3 (11, pg 1).
      • Mare bulk density ranges from 3010 to 3270 kg/m3 (11, pg 3).
      • Upper highland crust bulk density of 2200 to 2600 kg/m3 (11, pg 3).
      • Highland bulk density = 2590 to 2870 kg m3, with a mean value of 2691 kg m3 (12, pg 1).
      • I recall (put can’t haven’t found a source yet) reading that moonquakes and impacts vibrate the regolith and mega-regolith. This causes compaction and increases density. It would be great to find a source or two on this.
Megaregolith density and porosity

Global characteristics of porosity and density stratification within the lunar crust from GRAIL gravity and Lunar Orbiter Laser Altimeter topography data. Shin‐Chan Han, Nicholas Schmerr, Gregory Neumann, Simon Holmes. March 2017.

    • Porosity
      • GRAIL data revealed that the upper few kilometers of the lunar crust has an average crustal porosity of 12%. Porosity decreases with depth with a porosity of ~8% at 10 km and ~3–4% at 20 km (10, pg 942).
      • A thin porous layer at the top 3 km could have a surface porosity of 0.30 – 0.35 {model} (8, pg 7).
      • Ejecta associated with impact basins have a porosity of approximately 20% {model} (8, pg 7).
      • Surface porosity is 0.10 (2600 kg/m3) to 0.17 (2900 kg/m3) {model} (8, pg 7).
      • Porosity decreases exponentially with depth (within the 10-20 km layers) depending on grain density 2600 kg/m3 to 2900 kg/m3 {model} (8, pg 7)
      • Feldspathic highland crust has a porosity of 10–20% (11, pg 1).
      • The average porosity of the top kilometers of Highland regions is about 7.7%±2.8% (12, pg 8).
      • Impact basin ejecta has a porosity of ~20% (11, pg 1).
      • Mare porosity ranges from 2-10%, with an average of 7% (11, pg 3).
      • Shock appears to compact and remove porosity. Even the most highly shocked chondrite meteorite shows a baseline porosity of 5–10%, resulting from microfractures and not macroscopic pores (12, pg 8).
      • The implied porosity of highland crust (not just the mega-regolith) is 12% on average, and varies regionally from 4 to 21%. The porosity inside impact basins is less, due to the reduction in pore space by high impact temperatures. The porosity immediately exterior of many basins is higher than the surrounding area due to the ballistic deposition of ejacta and impact generated shock waves (modeled) (7, 673).
    • Thermal conductivity
      • Roughly 0.20 W m−1 K−1 (3, Abstract)
        • About 13 × lower than the mean conductivity of the lunar lithosphere (3, Abstract)
        • 5–10 × lower than the mean conductivity of the subjacent anorthositic crust (3, Abstract)
      • Heat passes more readily through mare mega-regolith than through highland regions due to its thinness (3, Abstract).
      • Heat flows laterally from the highland toward the mare (3, Abstract).
        • Heat flow is exceptionally high along highland and mare boundaries (3, Abstract).
      • Globally, mega-regolith insulation results in high present‐day mantle temperatures and heat flow (3, Abstract).
      • Large thermal variations in the mega-regolith may be caused by crustal density (porosity and composition) lateral heterogeneity (8, pg 8).
    • Particle size
      • Impact ejecta range from 0.1 to 10 meters in size, with an average size of 1 meter. Higher energy events create larger ejecta {modeled} (10, pg 951).
    • Chemical parameters
      • No physical specimens have been studied.
      • It is assumed that the mega-regolith is mostly made up of rocks that originate from the local bedrock (1, pg 286).

Analysis/Conclusions

  • Most of our knowledge about the mega-regolith is based on a small number of seismic measurements, remote observation analysis, and computer modeling. We have no physical samples to analyze. Effectively, the mega-regolith is a huge unknown.
  • We can assume that the top layers of the mega-regolith consist of larger particles (impact ejecta) mixed in with finer regolith particles. As we go deeper, there will be less regolith and more ejecta. Eventually, we will get deep enough that the impact ejecta will be replaced with in-situ the shattered bedrock.
  • The mega-regolith will probably also have numerous voids, fissures, and cracks. Homesteaders will have to be able to deal with these quickly before the oxygen in the SPORE unit bleeds out.
  • We can assume that Homesteaders won’t know in advance what they will be digging into since the mega-regolith is a chaotic, random mix of ejecta and regolith. SPORE will have to be designed to deal with unknown and constantly changing mega-regolith conditions.

Contributors

  • Ben Smith –  Gathering and analyzing data. Writing this page.
  • Aedan Acheson – Finding papers and publications regarding Lunar mega-regolith.

Sources

  1. Lunar Sourcebook: A user’s guide to the moon. Grant H. Heiken, David T. Vaniman, Bevan M. French. 1991. [www.lpi.usra.edu/publications/books/lunar_sourcebook/].
  2. Regolith Science. Colin Pain and Keith Scott. August 18, 2009.
  3. Megaregolith insulation, internal temperatures, and bulk uranium content of the moon. Paul H. Warren, Kaare L. Rasmussen. April 1987. [Abstract only]
  4. Monte Carlo simulation of lunar megaregolith and implications. Aggarwal, H. R. & Oberbeck, V. R. 1979.
  5. Structure of the Moon. M. Nafi Toksöz, Anton M. Dainty, Sean C. Solomon, and Kenneth R. Anderson. November 1974. [Abstract only]
  6. Blocky craters: Implications about the lunar megaregolith. T. W. ThompsonW. J. RobertsW. K. HartmannR. W. ShorthillS. H. Zisk. November 1979. [Abstract only]
  7. The Crust of the Moon as Seen by GRAIL. Mark A. Wieczorek,1*Gregory A. Neumann, et. al. February 2013.
  8. Global characteristics of porosity and density stratification within the lunar crust from GRAIL gravity and Lunar Orbiter Laser Altimeter topography data. Shin‐Chan Han, Nicholas Schmerr, Gregory Neumann, Simon Holmes. March 2017.
  9. The Significance of Substrate Characteristics in Determining Morphology and Morphometry of Lunar Craters. Head, J. W. Abstracts of the Lunar and Planetary Science Conference, volume 7, page 354, (1976)
  10. Impact Fragmentation and the Development of the Deep Lunar Megaregolith. Sean E. Wiggins, Brandon C. Johnson, et. al. March 2019.
  11. The density and porosity of lunar rocks. Walter S. Kiefer, Robert J. Macke, et. al. April 2012.
  12. Density and porosity of the lunar crust from gravity and topography. Qian Huang and Mark A. Wieczorek. May 2012.

Resources (not used but relevant)

  • NASA Technical Memorandum 84211 – Reports of Planetary Geology Program 1981. Page 129 has the paper PLANETARY MEGAREGOLITHS, which is a good primer on how mega-regolith is formed.
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