Solar Wind Fluence to the Lunar Surface
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44th Lunar and Planetary Science Conference (2013) 2015.pdf SOLAR WIND FLUENCE TO THE LUNAR SURFACE. D. M. Hurley1,3, W. M. Farrell2,3, 1JHU Applied Phys- ics Laboratory ([email protected]), 2NASA Goddard Space Flight Center, 3NASA Lunar Science Institute. Monolayers delivered in one lunation Introduction: The unperturbed solar wind bom- 90 bards the dayside of the Moon with electrons, protons, 60 and heavier ions throughout most of a lunation. Ex- cept when the Moon is in the Earth’s magnetotail for a 30 few days each lunation, the solar wind (shocked solar 0 N. Latitude wind in the magnetosheath, and unshocked solar wind -30 beyond Earth’s bow shock) has access to the dayside -60 surface of the Moon. Investigations of how the solar -90 wind could contribute to the composition and optical 0 90 180 270 360 properties of the lunar surface have a long history (e.g. E. Longitude [1-7]. Yet, it is instructive to revisit this issue and ex- Figure 2. The solar wind proton fluence as a function of amine the solar wind interaction piece by piece. selenographic position is shown in terms of fractions of Delivered Flux: The upper limit on the solar wind an equivalent monolayer of OH. The solid lines neglect thermal effects while the dashed lines include thermal as a potential source of OH can be established by as- effects. suming all of the incident solar wind protons are re- tained in the lunar regolith. The quiescent solar wind is implanted 3He as a resource guide. Fig. 2 shows the variable, but has density, n, of ~5 p+cm-3 and velocity, calculated fluence for one lunation assuming a spheri- v, of ~350 km s-1. The Moon presents an obstacle to cal Moon, no obliquity, and a magnetotail spanning the flow with a cross section of the lunar area, πR2 . moon phase 177°±25° during which the flux is zero. Here, Thus protons are delivered to the Moon at a rate of the fluence is converted to the equivalent number of ~1.7 x 1025 p+ s-1 or ~27 g s-1. Fig. 1 converts this to a monolayers if converted to OH. In addition, [8] find layer depth as a function of time for different resulting for typical solar wind conditions, the solar wind has weight fraction. access to about 20° beyond the terminator owing to the While the Moon traverses the Earth’s magnetotail, thermal velocity of the ions and the pressure gradient it is subjected to a reduced ion flux compared to when across the wake boundary. Flows with a higher mag- it is in the solar wind and magnetosheath. The cumul- netosonic Mach number, i.e., the ratio of flow velocity tive reduction is not uniform across the lunar surface to thermal velocity, take longer to fill in the wake. because the same side of the moon is facing the Sun Thus plasma with low Mach number has access to for every magnetotail crossing. We calculate the flu- more of the lunar surface beyond the terminator. In the ence to each location on the moon over a lunation. [5] magnetosheath, the flow velocity is close to the un- published such a map for the distribution of solar wind shocked solar wind. However, the plasma is hotter, giving it a higher thermal velocity and lower Mach number. For Mach 1, the ions have access to ~40° 100 1 hour 1 day 1 month 1 year beyond the terminator [8]. For any given longitude on the moon, the minimum 10-1 fluence occurs at the poles because the solar wind flows past at a glancing angle. However, at any given -2 10 latitude, the far side of the Moon receives the maxi- mum fluence because it is facing the Sun during during 10-3 periods when the solar wind is unabated by the magne- 10-4 tosphere. The maximum shielding provided by the magnetotail results in an equatorial fluence that is 57% -5 OH concentration (by weight) (by OH concentration 10 of the unabated fluence and occurs close to 0° lng. 10-6 1 Å 1 nm 10 nm 100 nm 1 µm 10 µm 100 µm1 mm 1 cm Reflected Flux: Kaguya has detected solar wind 100 102 104 106 108 1010 ions that are immediately backscattered from the time (s) Moon. The ionized component of the immediate, Figure 1 The globally averaged concentration of OH as high-energy ion backscatter is 0.1-1% [9]. This con- a function of time of OH if all solar wind protons are firms previous estimates based on laboratory studies converted to OH. Separate lines are given for the as- [10]. The neutralized component of the solar wind sumed layer depth. 44th Lunar and Planetary Science Conference (2013) 2015.pdf proton backscatter (i.e., energetic neutral hydrogen) was detected by the Interplanetary Background Ex- plorer (IBEX) with an albedo of 10% of the incident 100 solar wind [11-12]. SARA onboard Chandrayaan-1 DAY NIGHT also detected ENA H reflected from the lunar surface 10-2 up to 20% of incident solar wind protons [13-15]. [16] is able to reproduce the Kaguya protons and Chan- 85° 10-4 drayaan-1 ENAs and calculates an escaping backscat- tered neutral fraction of 98.5% of the incident flux, 75° 60% for E > 25 eV. He finds that collisions within the 10-6 regolith redistribute the energy of the outgoing neutral 60° OH concentration (by weight) (by OH concentration hydrogen out of the range for detection by ENA in- 10-8 struments. 45° 0° Diffusion: One consideration is that solar wind 30° 10-10 protons may diffuse out of the rind on the regolith in a -90 0 90 180 270 temperature-dependent manner. Balancing the inci- longitude (°) dent solar wind flux and the diffusing outward flux Figure 3. Computed OH concentration in the lunar rego- following [17], we calculate the stationary (or instan- lith balancing incident solar wind flux and outgoing dif- taneous) surface concentration as a function of position fusing OH. The concentration is given as a function of on the Moon. Both the incident solar wind flux and longitude in a Sun-State system. the diffusion time are functions of the solar zenith an- activation energy translates into very large differences gle (or insolation angle). To calculate the diurnal OH in computed hydration. concentration, one must convolve the time-varying Conclusions: Each implanted solar wind proton incident solar wind flux and integrated lifetime in the has the potential to react with the regolith to form OH regolith as a function of rotation. as long as diffusion times are long. We find that after At dawn, the solar wind begins to implant solar exposures of > 100 years, the regolith should be satu- wind into the regolith. However, the temperature is rated. Differences in the diffusion time may induce still low enough that the concentration of OH builds up diurnal variations. However, the magnitude of the as the Moon rotates through a portion of the morning. diurnal concentration change appears to be too low to At some time (est. ~8 am local time at equator, later be visible. times at higher latitude), diffusion begins to exceed the implantation rate. Thus OH concentrations will de- References: [1] Zeller E. J. et al. (1966) JGR 71, crease from that point until noon. After noon, the 20. [2] Hapke B. (1965) [3] Gibson E. K. and Moore trend reverses. As the Moon rotates through the after- G. W. (1972) Geochim. Cosmochim. Act. Suppl. 3, noon, diffusion rates continue to slow. Thus OH im- 2029. [4] Pillinger, C. T. (1979) Rep. Prog. In Phys. planted there lasts longer than the instantaneous 42, 897. [5] Johnson J. R. et al. (1999) GRL 26, 385. amount. OH concentrations increase in local time [6] Starukhina L. V. and Shkuratov Y. G. (2000) Ica- throughout the afternoon sector, rapidly near noon. rus 147, 585. [7] Ichimura A. S. et al. (2012) EPSL However, eventually solar wind fluxes attenuate near 345, 90. [8] Futaana Y. et al. (2006) PSS 54, 132. [9] the terminator, slowing the creation of new OH while Saito Y, et al. (2008) GRL 35, L24205. [10] Behrisch the existing, diffusing OH remains implanted in the and Wittmaack (1991) . [11] McComas D. J. et al. grains. At some evening time, the diffusion time will (2009) GRL 36, L12104. [12] Rodriguez M. et al. begin to exceed the time it will take to rotate through (2012) PSS 60, 297. [13] Wieser M. et al. (2010) GRL night. In essence, the solar wind conditions in the pre- 37, L04103. [14] Wieser M. et al. (2011) PSS 59, 798. evening sector are locked in the regolith on the [15] Futaana Y. et al. (2012) JGR 117, E05005. [16] nightside for two weeks while the Moon rotates. The Hodges R. R. (2011) GRL 38, L06201. [17] concentrations calculated here are not high enough to Starukhina, L. (2006) ASR 37, 50. be consistent with the IR observations. Because the diffusion function spans many orders of magnitude for a small range of temperature, a small deviation in the actual surface temperature on the Moon from the smooth function applied in the model would introduce large errors in this calculation. Similarly, the assumed .