Schedule

1. Schedule • Today 10/25: Regoliths • Tuesday 10/30: Weathering • Thursday 11/1: Exploration • Tuesday 11/6: Test II • Remember: Papers are due 11/27! • Need to clear topics with me by next Tuesday.

2. Regolith • Regolith: Greek rhegos (blanket) + lithos (stone) the mantle of fragmental and unconsolidated rock material that nearly everywhere overlies bedrock. • This includes the soil of the Earth. • On rocky objects, particularly asteroids and moons, what you see on the surface is regolith.

3. During the early phases of the Apollo Moon landing program, Tommy Gold of Cornell raised a concern that the dust layer at the top of the regolith would be very thick (10’s of meters), low density, and fluffy. • The worry was that the lunar landing module (with the astronauts) would sink beneath the surface. • This caused NASA to fly the a lunar robotic lander program (Surveyor), at a cost ~ $3 billion, before Apollo.

4. The Lunar Regolith • We know the most about the Moon, so lets start there. • Impacts at all scales form and alter the regolith. • Micrometeorites form a fine (<1mm) soil) • Basin-forming impacts excavate a colossal amount of material….this is what forms the surface units of the Moon and other planets. • Take the Imbrian basin • 1200 km in diameter • Ejecta was up to 400 meters thick 600 km from ring edge. • Fractured the crust down to ~25 km • With smaller bodies, a single major impact can resurface the entire object (i.e. Vesta)

5. Terms • Regolith “Soil”: The upper layer of regolith. • Fine particles, very loose, very fluffy, created by micrometeorite bombardment. • About 20 cm deep • Density about 0.9-1.1 g/cm3. Increases with depth to about 1.9 g/cm3. Porosity about 45%. • The regolith becomes progressively more compacted with depth. • Depth of regolith varies with the age of the surface • On the moon Mare has about 4-5 meters, Highlands about 10 meters • Overturn is very slow, 7 cm of overturn can take 109 years. • Megaregolith: Deep shattered layer • The rubble from basin and heavy bombardment ejecta. About 2 km deep in highlands • Structurally disturbed and displaced crust. Between 2~10 km deep. • Bedrock fracturing from the impacts. About 25 km deep.

7. Lunar Soils • Accumulate at a rate of ~1.5 mm/million years • Dominated by <1 mm particles • Mean particle size between 40 to 130 μm • Average particle size of ~65 to 70 μm • Grain density of 3.1 g/cm3 • Bulk density ranges from 1.45 to 1.79 g/cm3, depending on depth

8. Regolith Processes: Comminution • Comminution: breaking of rocks and minerals into smaller particles • Impacts at all scales grind down particle size. • Major impacts produce ejecta blocks • Micrometeorites grind down gravel and blocks to dust (remember they impact with an order of magnitude more force than a bullet)

9. Regolith Processes: Agglutination • Agglutination: welding of mineral and rock fragments together by micrometeorite-impact-produced glass. • High-velocity impacts produce enough heating in Lunar soils to melt material and weld fragments. • This process is limited to the Moon (and probably Mercury) since impact speeds need to be ~10 km/s • Agglutinates are NOT found in meteorites….. Average impact velocities in the asteroid belt are ~ 5 km/s. Too low to produce melting and agglutinates.

10. Regolith Processes: Agglutination • Agglutination works against comminution since it joins small particles to form bigger particles. • This is why Tommy Gold was proved to be wrong…..

11. Impact Gardening • Ejecta is excavated by the impacts and spread over the surface, adding to the regolith. • This process mixes the upper layers of the regolith, depositing fresh material on the surface. • With impact basins, the gardening can be huge…..

12. Regolith Processes: Comminution 6 meters • For asteroids comminution has an additional twist • Low gravity • Low escape velocity • As asteroids get smaller • Low gravity allows progressively larger ejecta debris to escape • Smaller asteroids should have courser regoith soil Eros Itokawa

13. Regolith Processes: Solar Wind Effects • Spallation: formation of elements as a result of cosmic ray impacts that cause protons and neutrons to spall off. • Implantation: See next slide • Vaporization: See next slide • Sputtering: atoms are ejected from a solid target material due to bombardment of the target by energetic particles. The sputtered atoms mostly recondense on grain surfaces. • Charging: Solar ultraviolet and X-ray radiation are energetic enough to knock electrons out of the lunar soil. Positive charges build up until the tiniest particles of lunar dust are repelled and lofted anywhere from m’s to km’s high. Eventually they fall back toward the surface where the process is repeated. On the night side, the dust is negatively charged by electrons in the solar wind.

14. Regolith Processes: Solar Wind Implantation and Vaporization • The elements making up the solar wind are implanted onto the surfaces and shallow interiors of the regolith. • The wind is mostly H and He, so these dominate • The buildup of solar wind H can change the chemistry of the regolith, creating reducing conditions. • When the regolith is briefly heated by impacts, the implanted H drives reduction reactions. • Iron-rich silicates (olivine and pyroxene) are converted to reduced iron and iron-poor ensitite. • This produces particles of submicron Fe which when suspended in agglutinate glass is a powerful reddening agent. • Vaporization: Low-temperature phases can be vaporized during impact and will recondense on surfaces

15. Space Weathering • This term covers the alterations suffered by solid materials when exposed to the space environment. • Crystal damage and spallation from cosmic rays • Irradiation, implantation, and sputtering from solar wind particles • Bombardment and vaporization by different sizes of meteorites and micrometeorites. • Or almost any regolith process….. • The effects of space weathering depend on the chemistry of the target material. For lunar materials and ordinary chondrites, one effect is to darken the material and reddening the spectra.

16. Space Weathering Agent: Solar Wind • Bombardment of helium ions on olivine under vacuum conditions in the lab simulates space weathering of asteroids and other airless bodies. • The observed effects are: • reddening of the spectral slope • slight darkening of the olivine • attenuation of the 1 μm absorption band • formation of metallic iron in the outer layer of the mineral surface in powder and flat slab

17. Space Weathering Agent: Solar Wind • Interpret reflectance spectra of the surfaces of airless planetary bodies • Compare spectra of meteorites to spectra of asteroids, to determine meteorite parent bodies The more we understand the processes and timescales of space weathering, the better we can:

18. Almost all Solid Objects Have Regolith • Asteroids are essentially all regolith or mega-regolith • Most are likely rubble piles • Some may include accreted fragments from impactors (example Almahata Sitta) • Only very rapidly spinning asteroids may be regolith-free • Mars • Moon • Mercury • Earth

19. Asteroid, Meteor, Meteorite In the early morning of October 6, 2008 an asteroid close to Earth was detected by a Catalina Sky Survey (CSS) telescope at Mount Lemmon, Arizona. It entered and broke up in Earth's atmosphere 19 hours later leaving behind a luminous train of clouds in the sky and meteorites on the desert floor. Train station sign in the Nubian Desert CSS telescope discovery images of asteroid TC3 http://www.psrd.hawaii.edu/April10/AlmahataSitta.html

20. Asteroid, Meteor, Meteorite AlmahataSitta is an anomalous polymictureilite: A spectacular mixture of lithologies giving new clues to the mineralogy, density, thermal history, magnetism, and geologic evolution of its parent body. 280 meteorite fragments (weighing 3.95 kilograms) have been found. http://www.psrd.hawaii.edu/April10/AlmahataSitta.html

21. Asteroid, Meteor, Meteorite http://www.psrd.hawaii.edu/April10/AlmahataSitta.html