SR04 Regolith Flashcards
Regolith
Definitions of Regolith
1.„… a general term for the unconsolidated rock material, whether residual or transported and of highly varied character, that
nearly everywhere forms the surface of the land and overlies or covers the bedrock…“
(Bates and Jackson 1980)
2.„… highly variable, usually unconsolidated but sometimes recemented, granular layer at the surface of planetary bodies,
overlying bedrock“
(Clarke 2008)
3.“Superficial layer or blanket of loose particulate rock material found on planet Earth or any other hard celestial object”
(R. W. Fairbridge)
4.In places this covering is made up of material originating through rock-weathering or plant growth in situ. In other instances it is
of fragmental and more or less decomposed matter drifted by wind, water or ice from other sources. This entire mantle of
unconsolidated material, whatever its nature or origin, it is proposed to call the regolith.
(Merrill, 1897)
Earth regolith subdivisions
- Soil (upper part, organics)
- Saprolith (primary rock fabric
preserved) - Pedolith (rock fabric completely
altered) - Alluvium (transported)
- Volcanic ash
- Duricrust (cemented)
- Salts
- Biota (organics)
Moon regolith subdivisions
- Soil (typ. <1 cm)
- Dust (typ. <30 μm)
- Surficial regolith
- Megaregolith
Mars regolith subdivisions
Sand
* Dust (typ. <30 μm)
Asteroid Regolith
Entire astroid can be of regolith „rubble pile“
Earth vs Moon vs Mars Regolith thickness
Earth: up to hundreds of meters
Moon: On average 4-5 m
in mare and 10-15 m
in highland, always
<20 m
Mars:Several meters
Origin of regoliths (Earth, Moon, Mars, Asteroid, Other moons)
Earth: Weathering and erosion (wind/water), biological processes
Moon: Space weathering, impact cratering
Mars: Weathering (wind/water), impact cratering
Asteroid: impact cratering
Other moons: (cryo)volcanism, impact cratering
Earth Pedolith and Saprolith
Regolith is subdivided into:
* Pedolith: zone in which the parent fabric has been destroyed, new
fabrics formed or soil developed
* Saprolith: zone in which the primary rock fabric is preserved
Depending on bedrock type and landscape setting, various parts of this
mature zonation may be absent, eroded or buried
The weathering front marks the depth limit of chemical alteration
Thickness correlates with age of a certain location
Thicker regolith, older terrain
Two regolith accumulation stages
- Thin initial layer of regolith (< few cm), bedrock is excavated by both small and large impacts
è rapid regolith layer buildup - Larger layer of regolith (> 1 m), only rare and large impacts penetrate and bring up new regolith, small
impacts disturb and mix upper layers
è slow regolith layer growth
* There is a natural limit of regolith thickness through self-shielding
* Loss of regolith occurs via ejecta (more relevant for smaller bodies)
Deepest soil drilling was 3 m on Moon (Apollo 17)
- Superficial regolith
Fine-grained, reworked surface deposit - Upper megaregolith/large scale ejecta
ballistically transported - Lower megaregolith/Structurally disturbed crust
- Unfractured bedrock
Mars regolith structure
1) Eroded ejecta deposit
2) Ejecta blocks
3) Brecciated bedrock
4) In-situ bedrock
Classification of regolith particle types on airless bodies
(1) Mineral fragments
(2) Pristine crystalline rock fragments
(3) Breccia fragments
(4) Glasses (impact glasses, ropy glasses, shocked minerals, volcanic glasses)
(5) Agglutinates
Classification of regolith particles based on source of the material on airless bodies
(1) Regolith-derived particles: agglutinates, fragments of regolith breccia, and heterogeneous glasses, all
formed by impacts into the regolith
(2) Bedrock-derived particles: pieces of the bedrock, monomict breccias, and polymict breccias
Space weathering
- Comminution (mechanical breaking through micrometeoroid bombardment)
- Agglutination (mineral/rock welding)
- Solar wind particle implantation (depth <1 μm, highest concentration in smallest grains because of high
surface/volume ratio) - Exposure to solar flares and galactic cosmic rays
Agglutinates
Individual particles, usually <1 mm
* Aggregates of smaller soil particles (mineral grains, glasses, and even older agglutinates)
* Bonded together by vesicular, flow-banded glass
* Make up 60 vol% in mature soil
* Unique on atmosphere-less bodies
* Irregular shape, vesicles, branching/dendritic morphology
* Always contain fine-grained Fe metal droplets
* Always contain solar wind elements (He, H)
Agglutinates Formation
- Soil with solar wind elements (He, H) is hit by impact
- Melting (è glass) and liberation of gases (è vesicles)
- H reduces FeO in the glass è resulting in Fe and H2O (escapes)
- Cooling, trapping of some gases
Therefore:
* Metallic Fe is a good indicator for maturation (surface exposure)
* Intensity of ferromagnetic resonance can be measured (IS)
* Ratio to total iron is one of the best measures for maturity, i.e. „maturity index“ (IS/FeO)
Effects of space weathering
Degradation of absorption VIS/NIR lines
* Reduction of overall albedo (darkening)
* Reddening of the VIS/NIR spectrum
* Bluing of the NUV spectrum
è Critical issues for composition studies from absorption spectroscopy (e.g. asteroid classification)
Maturation
Maturity indices (surface exposure age):
* IS/FeO
* Mean grain size
* Track density (damages by solar flare particles)
* Agglutinate content
* Content of solar wind elements
* Depletion of volatiles
* Presence and degree of isotopic mass fractionation
* Many of these indices tend to saturate and become constant over time (e.g. grain size achieves
equilibrium between comminution and agglutinate formation)
* Different indices relate to different depths (e.g. solar wind at surface and cosmic rays at several meters)
No good correlation between different maturity indices
–>Combination of different indices is recommended
No saturation over time: IS/FeO
–>Index of choice in most cases, although several
indices should be combined
Crater rays – What are they?
Impact ejecta
Fragmental material ejected from primary and secondary craters during impact events
Turnover/Gardening leads to:
- Size sorting
- Horizontal/vertical mixing of regolith layers
Lunar Orange Soil (74220, 74240, 74260, 74001/2)
Volcanic glass, particles <100 �m, originated a lava fountain (pyroclastic deposit)
* Enriched in volatile elements (Zn, Pb, S, Cl etc) and containing up to 50 ppm water
Lunar Green Soil (15425-15427, 15365-15377)
- Volcanic glass, particles <100 �m, originated a lava fountain (pyroclastic deposit)
- Glass beads are only about ~5 % of rock
- Enriched in volatile elements (Zn, Cd, Br, Se, Te, Ge, In, Tl, Bi, Ag and Sb)
Martian “blueberries”
- Hematite (iron oxide, α-Fe2O3) spherules of several millimeters in diameter
- They are grey but look bluish next to the ubiquitous rusty reds on Mars
- Their formation required aqueous chemistry and involved flows of acidic, salty, liquid water
