NEEP602-Lecture 11/Fall97

NEEP602 Course Notes (Fall 1997)
Resources from Space

Lecture #11: What does the Moon tell us about our future?

Title: Evolution of the Moon: The Apollo Model, continued

Visuals

Slides illustrating the phases of lunar evolution

Notes

Cratering on the Moon: A dominating process!

Most impact velocities are between 15 and 20 km/sec, giving associated target pressures at the point of impact of several hundred GPa and extraordinary amounts of heat as conversion of kenetic energy into forward and rearward shock waves takes place almost instantaneously. About 98% of the projectile is melted, vaporized, or ionized and returned to space based on the amount of extralunar material that can be identified in the lunar regolith. The general characteristics of craters as a function of diameter are as follows: <10 m diameter craters (transition up to 100 m diameter) o normally do not penetrate regolith o depth to diameter ratio variable o glass-lined pits in center of fresh craters o deep pits in the center of some craters o inner benches common if target material statified o impact breccias present in and outside larger craters >100m <10 km diameter craters (transition up to 30 km diameter) o normally penetrate regolith o hemispherical hole with raised rim o depth to diameter ratio 1:3 to 1:4 o ejecta blankets extend to about one crater diameter o target strata overturned but preserved in ejecta blanket o inner benches and impact breccias associated with smaller craters >30 km <100 km diameter craters (see Copernicus) o flat floors with central mound or peak o slumped (landslide) benches on walls o indications of pools and flows of impact melt o ejecta blankets extend to about one crater diameter o target strata overturned but preserved in ejecta blanket o secondary impact craters, crater clusters, and crater chains extent many crater diameters >100 km diameter basins (see Orientale) o generally flat, fractured floors with partial of complete inner rings of peaks o slumped (landslide) benches on walls o extensive pools and flows of impact melt inside and outside the primary impact crater rim o ejecta blankets extend many crater diameters with evidence of significant lateral flow (do lithostatic pressures become a factor?) o one to six rings of mountains outside primary impact crater rim (multiring basins generally >400 km in diameter, largest is South Pole-Aitken with a diameter of about 2500 km) Lunar Evolution: The Apollo Model Major Lunar Features: End of the Young Large Basin Stage

Note: Lunar wide, light-colored debris deposits in closed basins, whose deposition appears to post-date the large basins, may have several origins, including gas-charged impact debris and gas-charged precursors to eruptions of mare basalt that entrained light-colored crustal debris.

Stage Six: The Basaltic Maria - 3.8-3.0(?) eons. Mare basalt, including pyroclastic debris from late fire fountains that appear late in each basins history, erupted at the lunar surface and intruded the subsurface.

Note: The Earth continues to evolve toward compositional, thermal, and gravitational equilibrium due to convection, melting, and differentiation (Bowring and Housh, 1995, Monastersky, 1997). The influence of such processes on the Moon was much more limited and largely concluded with the end of the Basaltic Maria period, about 3.0 eons ago, although there is some evidence of continuing local mare basalt eruptions in the Procellarum region.

Major Lunar Features: End of the Basaltic Maria Stage

Full Moon (nearside) albedo variations

	The full moon albedo images of the Moon show the contrast in the distribution of mare basalts (dark gray) between the nearside and the farside
	The map of iron concentration on the Moon reflects the distribution of mare basalt in the albedo images shown above.
	The map of titanium distribution, however, contrasts distinctly with that for iron. Compare, for example, the Serenitatis and Tranquillitatis basins, right of and a little above center in the left hand image.

Map of Nearside Maria (Wilhelms, 1987) [250 kB]

Mare Imbrium surface structures at sunrise from 60 nm.

Taurus-Littrow Mare from 60 nm.

Boulder field around Camelot crater at Taurus-Littrow.

Mare basalt boulder in situ at Taurus-Littrow.

Vesicular basalt from Taurus-Littrow.

Non vesicular basalt from Taurus-Littrow.

Orange soil deposit in rim Shorty crater.

Shorty Crater, about 80m in diameter.

Area of orange soil deposits in the Sulpicious Galles region on the southern rim of Serenitatis.

Major Issue: Why did relatively little basaltic maria erupt in the very deep, old basins of the farside?

Equivalent gravitational potential surfaces still lie beneath or close to the bottom of these basins?
Regional chemical and radioisotopic heterogeneties in the lunar mantle were extreme enough to preclude the genereation of significant volumes of mare basalt? (Spudis, et al, 1994, in Nozette, et al, 1994)
Thicker crust on the farside inhibited large volume eruptions as that crust would counter the pressure built up by a volume change from rock to liquid (Yingst, R.A., and Head, J.W., 1995, as reported in Science News, v 148, 324).
- Does not explain relative absence of maria in farside basins that overlie thin crust (Zuber, M.T., 1995, as reported in Science News, v 148, 324).
Old large basin forming events on the farside, such as South Pole- Aitken removed most of the insulating layer formed during the Cratered Highlands Stage (Lecture 10), preventing the accumlation of sufficient heat to melt large volumes of the mantle.

Stage Seven: The Mature Surface - 3.0(?) to present. Formation of rayed impact craters and the regolith continued in all regions and included the implantation of solar wind gases in the regolith materials.

Changes to Major Lunar Features: Mature Surface Stage

Rayed crater formation and degredation

Stage characterized by continued impact activity, but by the absence of impact events that could form craters more than about 200 km in diameter.
Copernicus,a typical rayed crater about 1 b.y. old

Major Issue: Why does the farside of the Moon have about twice as many rayed craters under 10 km in diameter than predicted by crater counts on the nearside (Moore, J.M., and McEwen, A.S., 1996, as reported in Science News, v 149, 235).

Regolith formation: "...is a terrestrial term, also used for the Moon. It has been defined as 'a general term for the layer or mantle of fragmental and 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 bedrock. It includes rock debris of all kinds, including volcanic ash .. '... lunar regolith consists of particles <1 cm in size although larger cobbles and boulders, some as much as several meters across, are commonly found at the surface....much of the pulverized material is melted and welded together to produce breccias (fragmental rocks) and impact melt rocks, which make up a significant portion of the regolith ..." (Heiken, et al, 1991)

Shoemaker, et al., 1968, is a particularly important reference in regard to characterization of the lunar regolith.

A particularly important part of the lunar regolith consists of aggregates of rock, mineral, and glass fragments, held together by impact melt glass, called agglutinates. Further, the lunar regolith contains adsorbed solar wind gases, meteoritic material, and the products of solar and cosmic radiation.

Mixing of the regolith derived from various bedrock material is a function of the age of the separating contact.

Mixing between the Apollo 17 light mantle material (109 m.y) and older mare is on the order of 10s of meters.

Mixing between mare basalts (3-4 b.y.) and older highlands is on the order of 100s of kilometers (Fischer and Pieters, 1995)

Pit bottomed crater in the regolith.

Normal regolith exposed in shallow trench

Incompletely mixed regolith units.

Mass wasting

Movement of regolith down slope,triggered by impacts, gravitationally mobilized, and possibly lubricated and bouyed by solar wind volatiles released by particle interaction during initial motion.

Taurus-Littrow Mare from 60 nm.

Landslide from the South Massif, Valley of Taurus-Littrow probably caused by the impact of ray material from the Tycho cratering event, ~2000 km to the southwest.

The light colored swirls

Major Issue: What created the light-colored swirls that have formed on, and probably in, the regolith, particularly in regions east of Smythii.

Alterations of the regolith due to the impact of cometary material.

Alterations of the regolith due to out-gassing of the deep interior of the Moon, possibly the relatively undifferentiated lower mantle (below ~500 km).

There are some suggestions that the largest areas of swirls are antipodal to young large basins, such as Orientale for those east of Smythii.

Field of light colored swirls east of Smythii

Overhead view of zoned swirl east of Smythii

The possibility of polar ice.

Major Issue: Has cometary ice been deposited in the permanently shadowed regions of the South Pole during this final stage of lunar evolution?

More than 15,000 sq. km. of area has been shown by Clementine to be in presently in permanent shadow in the present phase of precession of the lunar pole (Shoemaker, et al, 1994, in Nozette, et al, 1994)
Theory predicts that some of any water and other volatiles released on the lunar surface by comentary impacts (or lunar out-gassing) would be precipitated in these very cold shadowed areas (Watson, et al., 1961).
Limited data from a bi-static radar experiment is consistant with but not conclusive of ice being present in this region (Nozette, et al, 1996)
Radar mapping of the South Polar region from Earth suggests that very rough surfaces are responsible for the ice-like returns from the Clementine data (Stacy, et al, 1997)

	Orbital relations in the Earth-Moon system. Note that lunar spin axis is nearly perpendicular to ecliptic, resulting in the sun always being near the horizon at the poles.
	Mosaic of ~1500 Clementine images of the south pole of the Moon. Note relatively large area of darkness near pole. This dark region suggests that permanently shadowed zones may exist here.
	Visual and radar images of the lunar south pole. Radar image is from Arecibo Observatory and shows landscape detail in the non-sunlit areas
	Geometric relations between Earth, Moon and Clementine spacecraft. Using the onboard S-band radio, radio was beamed into the dark areas near the poles. The coherent opposition backscatter effect (CBOE) is caused by radio penetration of ice and is seen when the source (spacecraft) is aligned with the target (Moon) and receiver (Earth). This condition is called beta=0.
	Mosaic of south pole showing permanent shadow (red), and the beta=0 groundtracks for orbits 234 and 235. Note that orbit 234 tracked directly over the dark areas of the pole while orbit 235 was confined to zones containing no permanent shadow.
	Radar results (ratio of RCP/LCP) for all 4 orbits. Orbits 301 and 302 were over the north pole (very small area of permanent darkness) and orbit 235 was over the non-dark portion of the south pole (see figure above). Only orbit 234, directly over the south pole, shows the CBOE effect. The magnitude of 1 dB indicates that roughly 1 km3 of ice may exist here (= 10^9 m^3 = 1 mT).

Tidal interaction with the Earth and Sun.

The investigation of tidal rhythmites on Earth indicate that the Earth's rotation has slowed during at least the last 900 m.y., increasing the length of the day from about 18 hours to the current 24 hours and an increase in the semimajor axis of the Moon's orbit by about 10% or 34,400 km (Sonett, et al, 1996) as angular momentum has been transfered, minus losses due to tidal heating.

Space weathering. (see Clark and Johnson, l996)

Micrometeorites

Erosion of rock surfaces by ejection of impact melt, fractured rock and minerals, and vapor.

Reduction of Fe⁺² to iron in the presence of solar wind hydrogen.

Microfracturing of rock surface minerals beneath "zap" pits.

Rounding of rock edges

Gardening and maintanence of optical uniformity (fairy castle structure) of upper millimeter or so of the regolith surface.

Creation and redistribution of a thin, irregular brownish glass patina on rock surfaces.

Heat and cold (expansion and contraction)

Apparently minor particle movement and fracture working suggested by seismic noise patterns.

Sunlight (largely UV)

Likely cause of the production of sodium atoms observed a part of the lunar "atmosphere" (Medillo, M., and Baumgardner, J., 1995)

Solar wind and solar flare ions

Hydrogen and helium ions cause sputtering of ions from exposed mineral grains while at the same time they are imbedded in the grain at depths of a few tens of Angstroms.

Galactic and extragalactic cosmic rays

Human activities

EVOLUTION OF THE MOON: SUMMARY

Lunar Evolution: The Apollo Model The figure summarizes what is known about the internal structure and properties of the Moon, based on a synthesis of geophysical, petrological, and geochemical data from Apollo. A schematic cross section of the Moon as it probably exists today

Questions

1. What are the arguments, pro and con, for an origin of the Moon through the "great Mars-sized asteroid impact on the Earth theory" (fission)? (Start with Hartmann, 1986 and 1997, and Alfven, H., and Arrhenius, G., 1972, in Lecture #10, but later refinements by both should be sought in the literature.)

2. What might be the implications of the events of Stages Two through Seven of lunar evolution on the evolution of life on Earth? (See the text for Lectures #10 and 11 as a start.)

3. List six major scientific issues yet to be resolved about the evolution of the Moon other than its Beginning.

Text

Evolution of the Moon: The Apollo Model (continued)

Based on material originally published by the author in American Mineralogist, v 76, 773-784.

References (see Lecture #10)

Bowring, S.A., and Housh, Todd, 1995, Earth's Early Evolution. Science, v 269, 1535-1540.

Clark, B.E., and Johnson, R.E., l996, Interplanetary Weathering: Surface Erosion in Outer Space, EOS, v 77, 141-145

Fischer,E.M., and Pieters,C.M., 1995, Lunar surface aluminum and iron concentration from Galileo solid state imaging data, and the mixing of mare and highland materials, Journal of Geophysical Research, v 100, 23,279-23,290.

Hartmann, W. K., 1997, A Brief History of the Moon, The Planetary Report, v 17, no. 5, 5-11.

Heiken, G., et al, 1991, Lunar Source Book, Cambridge University Press, Cambridge.

Flynn, B. and Medillo, M., 1995, Simulations of the lunar sodium atmosphere, Journal of Geophysical Research, v 100, 23,271-23,278.

Monastersky, R. 1997, Global Graveyard, article in Science News, v 152, 46-47.

Shoemaker, E.M., et al, 1968, Television observations from Surveyor , In Surveyor Project Final Report, Part II, JPL Technical Report 32-1265, NASA SP-146, p 21-136.

Sonett, C.P., et al, 1996 Late Proterozoic and Paleozoic Tides, Retreat of the Moon, and Rotation of the Earth, Science, v 273, 100-104.

Stacy, N.J.S., et al, 1997, Arecibo Radar Mapping of the Lunar Poles: a Search for Ice Deposits, Science, v 276, 1527-1530.

Williams, D.A., et al, 1995, Multispectral studies of western limb and farside maria from Galileo Earth-Moon Encounter 1, Journal of Geophysical research, v 100, 23291-23299.

Nozette, S., et al, 1994,The Clementine Mission to the Moon ,Science, v 266, 1835-1862.

Nozette, S., et al, 1996,The Clementine Bistatic Radar Experiment, Science, v 274, 1495-1498.

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Lunar Evolution: The Apollo Model	Major Lunar Features: End of the Young Large Basin Stage