Chapter 2: Energy at the Lunar Surface - Lunar and Planetary Institute

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SOLAR ENERGY based on the present mass-frequency distribution shows. Since phase 3 of the flux history covers most of post-mare time, which accounts for over 60 per cent of lunar history, and the time during which much of the lunar soil evolved, it is instructive to look at estimates of the current meteoroid energy flux. Iridium and osmium are rare in terrestrial sediments and comparatively more abundant in meteorites such that their concentrations can be used as an index to the amount of meteoritic material included in deep-sea sediment (Barker and Anders, 1968). By accepting the chemical composition of C1 carbonaceous chondrites as being representative of non-volatile cosmic matter Barker and Anders (1968) found the terrestrial accretion rate for meteoritic material to be 1.2 (k0.6) x 10-8 g cm-2 yr-1 over the past 105 to 106 yr. They were also able to set a stringent upper limit on the accretion rate by assuming that all 0s and Ir in the sediments was cosmic. The upper limit is 2.9 x 10-8g cm-2 yr-1 . The flux on the earth is enhanced by gravitational focusing (f = 1.8) by comparison with the moon (f = 1.03) but the difference is well within the uncertainties attached to the above estimates. Similarly trace element enrichment in lunar soil and soil breccia samples from the Apollo 11 site indicated a mass flux of 3.8 x 10-9 grn c-2 yr-1 (Ganapathy et al., 1970a). Subsequently Ganapathy et al. (1970a, b) estimated the mass flux at 4 x 10-9 g cm-2 yr-1 using similar trace element studies and soil samples from the Apollo 12 site. The Apollo 11 and 12 sites are both located on mare areas so that the flux estimates probably represent estimates of Hartmann's (1970) phase 3 of the flux history. Dohnanyi (1971) used the estimates of the modern mass frequency distribution to arrive at an integrated mass estimate of 2 x 10-9 cm-2 yr-1 for the moon. Gault et al. (1972) arrived at a similar figure and later refined the figure to 1.4 x 10-9 g cm-2 yr-1 (Gault et al., 1974). From the available information and assuming a root-mean-square velocity of 20 km s-1 the post-mare energy flux from impacting meteoroids at the lunar surface has been between 2 to 8 x 103 ergs cm-2 yr-1. By comparison with the terrestrial environment this energy flux is extremely small. However, the lack of an atmosphere and the large amount of geologic time available make meteoroid impact the most effective sedimentary process on the lunar surface. The partitioning of the flux energy is complex and is dealt with in some detail in Chapter 3. Solar Energy At the avarage distance of the earth-moon system from the sun the
SOLAR ENERGY 43 radiant energy flux is 4.4 x 1013 ergs cm-2 yr-1 at a normal angle of incidence. On the earth this energy fuels a global heat engine which provides most of the chemical and mechanical energy for producing and transporting sedimentary materials. Re-direction of radiant energy into sedimentary processes on the earth is dependent upon fluid intermediaries - the atmosphere and hydrosphere. Without fluid intermediaries solar energy is almost totally ineffective - such is the case on the moon. The radiant-energy flux contributes an average of 1.1 x 101 3 ergs cm-2 ~r-1 to the lunar surface. That is, at least 10 orders of magnitude larger than the meteoroid flux contribution. Most of this energy is simply reradiated into space during the lunar nights. However, a small, possibly significant, amount of solar energy is effective in the sedimentary environment. Electrostatic Transport Electrostatic transport of lunar surface materials was first suggested by Gold (195 5). The proposed driving mechanism was transient charge separation due to photoelectron ejection from fully sunlit surfaces. However, later theoretical studies suggested that under daytime conditions soil grains would rise only a few millimeters at most above the surface (Gold, 1962, 1966; Singer and Walker, 1962 ; Heffner, 1965). Subsequently, Surveyor spacecraft recorded a bright glow along the western lunar horizon for some time after sunset suggesting that a tenuous cloud of dust particles extends for up to 30 cm above the lunar surface during and after lunar sunset. Criswell (1972) and Rennilson and Criswell (1974) suggest that this was due to the retention of large differences of electrical surface charge across lightldark boundaries due to the highly resistive nature of the lunar surface. The dust motion could result in an annual churing rate of 10-3 g cm-2 at the lunar surface which is about 4 orders of magnitude greater than the rate at which soil accumulates from meteoroid impact erosion (Criswell, 1972). The horizon glow observed by the Surveyor spacecraft appears to be produced by particles with a diameter of 10pm. As discussed in Chapter 6, such particles form about 10 per cent of the soil mass. The concept of electrostatic transport of lunar materials thus appears to be very model-dependent, but is potentially capable of transporting very large volumes of lunar soil. Thermal Effects Thermal extremes in the terrestrial environment, particularly in hot or cold deserts, are important in shattering rock exposures and producing sedimentary materials. However, despite the dryness of these terrestrial
Page 1 and 2: CHAPTER 2 Energy at the Lunar Surfa
Page 3 and 4: DISTRIBUTION OF METEOROIDS Heliocen
Page 5 and 6: MEASURING THE METEOROID FLUX Measur
Page 7 and 8: MEASURING THE METEOROID FLUX suffic
Page 9 and 10: MASS-FREQUENCY DISTRIBUTION observa
Page 11 and 12: MASS-FREQUENCY DISTRIBUTION experim
Page 13 and 14: PHYSICAL PROPERTIES Escape Velociti
Page 15 and 16: PHYSICAL PROPERTIES he ratio a/b is
Page 17 and 18: PHYSICAL PROPERTIES r Meteorites I
Page 19 and 20: PHYSICAL PROPERTIES 37 requires pro
Page 21 and 22: HISTORY OF METEOROID FLUX History o
Page 23: METEOROID ENERGY Solar Flare Track
Page 27 and 28: GRAVITATIONAL ENERGY Fig. 2.16. Enl
Page 29 and 30: GRAVITATIONAL ENERGY Fig. 2.19. Bou
Page 31 and 32: GRAVITATIONAL ENERGY similarly the
Page 33 and 34: GRAVITATIONAL ENERGY Fig. 2.24. Soi
Page 35 and 36: GRAVITATIONAL ENERGY of seismic eve
Page 37 and 38: INTERNAL ENERGY 55 rate proportiona
Page 39 and 40: OTHER ENERGY SOURCES Fig. 2.28. A g
Page 41 and 42: CONCLUDING REMARKS \ Apollo Orbit I
Page 43 and 44: REFERENCES N. H and Ferry, G. W., 1
Page 45 and 46: REFERENCES Mc~onnell, J. A. M., Fla

Chapter 2: Energy at the Lunar Surface - Lunar and Planetary Institute

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