1. Schedule Remember: Papers are due November 4

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. 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

4. 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.

5. 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)

6. 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/cm 3. Increases with depth to about 1.9 g/cm 3. 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 10 9 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.

8. 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

9. 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)

10. 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.

11. 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…..

12. 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…..

13. Regolith Processes: Comminution 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 6 meters

14. 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.

15. 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

16. 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.

17. PSRDPSRD http://www.psrd.hawaii.edu/Aug09/solarwind.helium.html presents 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

18. PSRDPSRD http://www.psrd.hawaii.edu/Aug09/solarwind.helium.html presents Space Weathering Agent: Solar Wind The more we understand the processes and timescales of space weathering, the better we can: Interpret reflectance spectra of the surfaces of airless planetary bodies Compare spectra of meteorites to spectra of asteroids, to determine meteorite parent bodies

19. Our “Type Section” for Space Weathering has been the Moon Lunar weathering generates nano-phase Fe (amongst other things) on the grain surfaces and in glassy rims. This is EXTREMELY optically active and produces the characteristic lunar “red slope” in the visible and near-IR spectra. Another effect is the darkening of the reflectance and attenuation of the absorption bands.

20. But it is not just the Moon that is exposed to this environment. Space weathering can be viewed as the response to energetic inputs that drive the surface composition away from equilibrium. –The result are chemical reactions and evolution that can be understood from underlying thermodynamic driving forces. Try a little forward modeling

21. Look at the Chemistry…. By using techniques and insights developed in materials science and physics we can: –Assess the environment of the common asteroidal and planetary materials –Forward model the weathering reactions –Make testable predictions about processes and products. Start with the idea that key weathering reactions are driven by: –The chemical environment of space (hard vacuum, low fO2, solar wind H, sputtering) –Thermal energy supplied by micrometeorite impacts. –Follow the chemical products……

22. Take Olivine Turns out that the thermodynamics of olivine make it very susceptible to weathering. In a reducing environment when you add energy (heat) olivines lose oxygen and metallic cations (producing nano-phase Fe) –The mineral becomes more disordered and less optically active. –At the same time, the weathering product (npFe 0 ) creates a powerful optical component (i.e. lunar red slope).

23. We have done a few experiments Olivine weathering has been simulated a number of ways. –Most commonly with laser zapping. We did this by changing the chemistry and the kinetics (reducing environment and warming things up) The result is lunar-like spectra From T. Kohout et al., Icarus 2014

24. But this is just a start…. Turns out that surfaces with exposed npFe 0 are an ideal environment for catalyzing further reactions. Mineral decompostion (and the production of catalitic materials) can be thought of as the first stage of space weathering. The problem on the Moon is that there is nothing much to catalyze. From T. Kohout et al., Icarus 2014

25. But what if you do have something else to react with? The second stage of weathering depends upon the presence of “feedstock” components that can participate in catalyzed chemical reactions on exposed surfaces. On Volatile-rich Asteroids….. –Reactive surfaces use the volatiles coming out of frost-line small bodies as the feed-stock (CO, H20, NH3) for catalytic reactions (for example: Fischer-Tropsch Type (FTT)).

26. Fischer-Tropsch Reactions? Developed in Germany in the 1920s, this is a catalytic technology to convert hydrogen and CO into liquid hydrocarbons. –The process has widespread industrial applications, particular in the generation of liquid fuels from coal or gas. (2n + 1) H 2 + n CO  C n H (2n+2) + n H 2 O Can use iron, cobalt, and ruthenium as catalysts. Varying pressure and temperature varies the reaction outputs, typically long-chained alkanes. BUT, if NH 3 is in the feedstock FTT can produce amino acids.

27. Weathering on Volatile-rich Carbonaceous Asteroids Fischer-Tropsch Type (FTT) reactions using the heat supplied by micrometeorite bombardment would “weather” volatile-rich surfaces, producing….. –Long-chain hydrocarbons, polyaromatic hydrocarbons, amino acids, various complex organics. –“Maturity” in this case probably relates to the abundance and variety of organics. Space weathering on volatile-rich asteroids produces organics… –Which is what we see in CI and CM carbonaceous chondrites.

28. How does the “weathered” surface survive? This is a surface effect…..how can we end up with this stuff as part of the interior of a meteorite and survive atmospheric entry? Remember that asteroids are mostly (almost all) rubble piles. –They go thru cycles of disruption and reaccretion. –Impact ejecta buries surface material. –The stratigraphy gets inverted all the time. For example, about 12% of OC falls have solar wind implanted gases. They are samples of the regolith.

29. A few examples Tysnes Island (H4 ) Mifflin (L5) Fayetteville (H4~6)

30. Conclusions: A Theory of Space Weathering The basic scheme can be applied to ANY airless body –Thermodynamics and mineral kinetics are the drivers. –Outputs depend on the chemistry of the available inputs. –Decomposition of common rock-forming minerals can create strongly catalytic regoliths. The range of weathering products are not random, but the predictable (and testable) outcome of the source’s mineral kinetics and chemical feedstock. Weathering products do not have to be optically active. –Probably most weathering products are spectrally neutral or even suppress an object’s reflectance spectrum because decomposition makes the minerals more disordered.

31. Conclusions: Volatile-Rich Asteroids For volatile-rich asteroids, major weathering products include a range of carbon-rich compounds –Weathering in the presence of catalysts transforms “feedstock” materials into a range of products including long-chain organics and amino acids. The generation of pre-biotic compounds are probably a routine and predictable by-product of common space weathering processes. –The precursors of life are probably abundant in any space- weathered asteroid belt, in any solar system, and only wait being accreted to a hospitable environment.

32. Asteroid, Meteor, Meteorite http://www.psrd.hawaii.edu/April10/AlmahataSitta.html 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. CSS telescope discovery images of asteroid TC 3 Train station sign in the Nubian Desert

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

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