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Contributions of solar wind and micrometeoroids to molecular hydrogen in the lunar exosphere
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Contributions of solar wind and micrometeoroids to molecular hydrogen in the lunar exosphere Dana M. Hurley a,∗, Jason C. Cook b, Kurt D. Retherford c, Thomas Greathouse c, G. Randall Gladstone c, Kathleen Mandt c, Cesare Grava c, David Kaufmann b, Amanda Hendrix d, Paul D. Feldman e, Wayne Pryor f, Angela Stickle a, Rosemary M. Killen g, S. Alan Stern b a
Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA Southwest Research Institute, Boulder, CO 80302, USA c Southwest Research Institute, San Antonio, TX 78228, USA d Planetary Science Institute, USA e Johns Hopkins University, Baltimore, MD 21218, USA f Central Arizona College, Coolidge, AZ 85128, USA g NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA b
a r t i c l e
i n f o
Article history: Received 20 August 2015 Revised 25 March 2016 Accepted 14 April 2016 Available online xxx Keywords: Moon Atmospheres Dynamics Solar wind Impact processes
a b s t r a c t We investigate the density and spatial distribution of the H2 exosphere of the Moon assuming various source mechanisms. Owing to its low mass, escape is non-negligible for H2 . For high-energy source mechanisms, a high percentage of the released molecules escape lunar gravity. Thus, the H2 spatial distribution for high-energy release processes reflects the spatial distribution of the source. For low energy release mechanisms, the escape rate decreases and the H2 redistributes itself predominantly to reflect a thermally accommodated exosphere. However, a small dependence on the spatial distribution of the source is superimposed on the thermally accommodated distribution in model simulations, where density is locally enhanced near regions of higher source rate. For an exosphere accommodated to the local surface temperature, a source rate of 2.2 g s−1 is required to produce a steady state density at high latitude of 1200 cm−3 . Greater source rates are required to produce the same density for more energetic release mechanisms. Physical sputtering by solar wind and direct delivery of H2 through micrometeoroid bombardment can be ruled out as mechanisms for producing and liberating H2 into the lunar exosphere. Chemical sputtering by the solar wind is the most plausible as a source mechanism and would require 10–50% of the solar wind H+ inventory to be converted to H2 to account for the observations. © 2016 Published by Elsevier Inc.
1. Introduction It has been confirmed that water exists in permanently shadowed regions of the lunar poles (Colaprete et al., 2010). However, the source of the water remains an open question. Several candidate sources exist and may each contribute including comets, micrometeoroids, internal water, and solar wind (Arnold, 1979). The solar wind ions comprise primarily protons. Once steady state is achieved, there must be a balance in the influx of protons and the outflow of hydrogen. However, there are multiple pathways for the outflow of hydrogen. Some of those pathways have been observed by spacecraft. In this paper, we quantify the amount converted to
∗
Corresponding author. Tel.: +1 443 778 9126. E-mail address: [email protected] (D.M. Hurley).
H2 . This further constrains the amount of solar wind protons that is available to be converted to water, and potentially migrate to the poles. H2 has been detected spectrally in the lunar exosphere by the Lyman Alpha Mapping Project (LAMP) on NASA’s Lunar Reconnaissance Orbiter (LRO). Initially, it was detected in the vapor plume emanating from the Lunar CRater Observation and Sensing Satellite (LCROSS) impact into the permanently shadowed Cabeus crater (Gladstone et al., 2010). Modeling of the propagation of the vapor plume indicated that 110 kg of H2 was released by the impact (Hurley et al., 2012). The analysis suggested that an energy source such as an exothermic reaction of two hydrogen atoms adsorbed to the surface forming H2 via a grain-catalyzed reaction was consistent with the perceived partition of energy from the impact. Later Stern et al. (2013) detected H2 in the lunar exosphere by integrating observations made at high latitudes on the nightside of
http://dx.doi.org/10.1016/j.icarus.2016.04.019 0019-1035/© 2016 Published by Elsevier Inc.
Please cite this article as: D.M. Hurley et al., Contributions of solar wind and micrometeoroids to molecular hydrogen in the lunar exosphere, Icarus (2016), http://dx.doi.org/10.1016/j.icarus.2016.04.019
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the Moon from the first 3.5 years of LAMP data. They determine a column density of 5.9 × 109 cm−2 , or an equivalent surface density of H2 of 1200 ± 400 cm−3 . This is consistent with the previous upper limit set by Feldman and Morrison (1991) of < 90 0 0 cm−3 based on UV observations from the Apollo 17 orbiter. The Stern et al. (2013) detection of H2 provides an average amount over an extended region of the Moon. Without spatial or temporal resolution, the observation is not able to distinguish between various sources and release mechanisms, but does give a useful upper boundary to the global steady state release of H2 . In addition, H2 has been detected through mass spectrometry in the vicinity of the Moon. The Moon Impact Probe (MIB) released from the Chandrayaan-1 orbiter carried a neutral mass spectrometer, CHandra’s Altitudinal Composition Explorer (CHACE) (Sridharan et al., 2010). The H2 detected along the sub-satellite trajectory on the lunar dayside demonstrated higher surface density near the pole than at low latitudes (Thampi et al., 2015). The estimated surface density at high southern latitude on the dayside was 825 cm−3 , consistent with the Stern et al. (2013) nightside polar averages at a lower surface temperature. More recently, Halekas et al. (2015) analyzed LADEE NMS data using the ion mode to infer the density of the H2 exosphere. Using the specifics of the geometry of the convection electric field and the viewing geometry of the NMS, the analysis is sensitive to ions produced from exospheric neutrals, and therefore traces exospheric density. However, the observations necessarily convolve the ionization production process and the column density along the electric field direction. Water and H2 share the same potential sources on the Moon. Solar wind implanted hydrogen has been considered the most likely source owing to the steady stream of hydrogen incident on the Moon. However, micrometeoroids bring some volatiles with them that would be released on impact upon the surface of the Moon. The Neutral Mass Spectrometer (NMS) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft detected an increase in exospheric water associated with lunar encounters with meteor storms (Mehdi Benna, personal communication). A solar wind proton that encounters the Moon is implanted into the regolith grain it hits. At energies of 1 keV amu−1 , protons penetrate a few 10 s of nm into the rim of the grain. Although most of the incident ions are neutralized on contact, the mission Kaguya measured that 0.1–1% of SW protons are backscattered immediately from the surface (Saito et al., 2008). These percentages have also been confirmed by NASA’s Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) spacecraft (Halekas et al., 2013). An additional population of energetic neutral hydrogen that has been reflected from the surface of the Moon has been observed by SARA on ISRO’s Chandrayaan-1 (Wieser et al., 2009) and Interstellar Boundary Explorer (IBEX) (McComas et al., 2009; Funsten et al., 2013; Allegrini et al., 2013). IBEX observations are consistent with 10% of solar wind protons being converted to energetic neutral atoms (ENA). SARA’s observations show up to 20%. Thus, only 10.1–21% of the incident flux of solar wind protons has been directly accounted for by observations. This leaves 79–90% of solar wind inventory unobserved. Hodges (2011) predicts that nearly all of the solar wind can be converted to neutral H that immediately diffuses out of the regolith while still being consistent with ENA, far ultraviolet (FUV), and ion observations. In that model, the energy distribution of the H is such that most of the atoms are escaping the Moon, however at energies below what is detectable by ENA instruments. Returned lunar regolith from the Apollo missions contain hydrogen that has built up in the rims of regolith grains (Hintenberger et al., 1970; Leich et al., 1973). This reveals that,
as expected from laboratory experiments (Lord, 1968; Zeller et al., 1966; Wehner et al., 1963), the lunar regolith acts as a sink for solar wind elements as small amounts accumulate in the rims until a steady state saturation level is achieved. For solar wind hydrogen, the saturation limit appears to be ∼150 ppm in equatorial soils (DesMarais et al., 1974). Neutron detectors sense an overall decrease in epi-thermal neutron flux in the lunar polar regions (Feldman et al., 1998; Mitrofanov et al., 2010), which signifies an increase in hydrogen abundance near the poles. As this measurement cannot distinguish between binding states of hydrogen, it is not known whether this indicates an increased presence of implanted hydrogen, adsorbed water, or hydrated minerals at high latitudes. This paper presents models of exospheric H2 produced by various source and release mechanisms. The results are discussed in terms of how the likely sources and processes influence the distribution of exospheric hydrogen. Finally, we discuss how the various source and release mechanisms fit into the greater hydrogen and hydrologic cycle on the Moon. 2. Exosphere model 2.1. Model description The exosphere model is a Monte Carlo model that follows particles as they hop along collisionless, ballistic trajectories in the lunar exosphere by following the equation of motion under gravity (Hurley, 2011; Killen et al., 2012) neglecting radiation pressure. The equation of motion is solved using a fourth order Runge–Kutta algorithm (Press et al. 1992) with 1-s time steps. The particles are originally released with a spatial and energy distribution representative of the assumed source. The source’s distribution in phase space is extremely important, and is discussed in detail below. Being a low-mass species, much H2 escapes beyond the Hill sphere of the Moon, i.e., beyond the region where lunar gravity dominates the motion of the particles. In the model, when a particle reaches the Hill sphere at 35 Rmoon , the model counts the particle as “escaping” and no longer traces the particle. Although these particles enter Cis-Lunar space and may return to the Moon, they are not expected to represent a significant portion of the H2 at very low altitude, where measurements occur (<200 km). The model also calculates the loss due to photodissociation for any particle that is in sunlight for each time step. Using the Huebner et al. (1992) photodissociation rate for quiet Sun conditions, photodissociated particles are removed from the simulation. Particles that reencounter the surface of the Moon interact with the regolith. Possible interactions include dissociative adsorption, thermal accommodation, or elastic collisions. We assume that H2 does not adsorb to the surface, thus set loss from dissociative adsorption to zero. Also, this means that the particle is re-released in the model immediately on the next time step. The model parameterizes the energy exchange between the H2 molecule and the surface every time a particle encounters the surface using an accommodation coefficient. The actual thermal accommodation of H2 interacting with lunar regolith is poorly constrained. For the runs presented here, we assume 100% thermal accommodation, meaning the particles are released with no memory of their incident velocity and have a distribution representative of a Maxwell– Boltzmann distribution at the local surface temperature. The local surface temperature is implemented in the model using an empirical function (Hurley et al., 2015). The model neglects energy exchange between translational, vibrational, and rotational degrees of freedom for the H2 molecules. By allocating all of the energy as translational, the model slightly overestimates the velocity of the molecules. The net result would be an overestimate of escape, and therefore a lower density.
Please cite this article as: D.M. Hurley et al., Contributions of solar wind and micrometeoroids to molecular hydrogen in the lunar exosphere, Icarus (2016), http://dx.doi.org/10.1016/j.icarus.2016.04.019
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Fig. 1. Local time profile of simulated H2 surface density for solar wind source normalized to a source rate of 1 g s−1 . The left panel shows H2 released on the dayside with a thermal profile at the local surface temperature. The right panel shows H2 released with higher energy, representing physical sputtering by solar wind ions. Model output from equatorial latitude (|λ| < 30°) is shown in the dashed line; mid latitude (30° < |λ| < 60°) in the dotted line; and high latitude (60° < |λ| < 85°) in the solid line. The surface density is defined as the average model density between the surface and 2 km altitude. Table 1 Model output for different source mechanisms for comparison with LAMP observations.
Source process Thermal, local surface temperature Impact, thermal 10 0 0 K 30 0 0 K Sputtering Thermal source Tstick = 80 K Tstick = 200 K
Spatial location of source
Density for 1 g s−1 source rate (cm−3 )
Source rate to produce 1200 cm−3 (g s−1 )
Source rate to produce 90 0 0 cm−3 (g s−1 )
Global (Fig. 2)
940
1.3
9.6
Dayside (Fig. 1)
540
2.2
17
Dawn + Global (Fig. 3) Dawn + Global (Fig. 3)
160 50
7.6 25
57 180
Dayside (Fig. 1)
0.94
1300
9600
Global (Fig. 4) Global (Fig. 4)
370 50
3.2 24
24 180
However, the correction is expected to be negligible (Moore and Pearson, 1981). 2.2. Spatial distribution We present several model runs and inspect the basic properties of the resulting exosphere. We run a variety of release mechanisms in position and energy. The initial positions include global isotropic, dayside-centered, apex-centered, and combinations of those. The initial energies simulated are mostly thermal velocity distributions with temperatures coming from the local surface temperature, 10 0 0 K or 30 0 0 K. A Sigmund–Thompson distribution for physical sputtering is another case simulated. The results from these model runs are summarized in Table 1. Specific cases are also shown in figures and discussed in the text below. Crider and Vondrak (2002) propose that H2 is produced from the solar wind interaction through chemical sputtering. The spatial distribution of the source is approximately centered at the subsolar point, although incorporating an offset to account for the motion of the Earth–Moon system about the Sun would be more accurate. The concentration of source particles decreases as the cosine of the solar zenith angle moving away from the subsolar point to the terminator. There is no source of particles on the nightside for sputtering release. We simulate both chemical sputtering, where the particles are released with a lower energy, and physical sputtering, where particles are released with higher energy. Because the released product is molecular, chemical sputtering is more appro-
priate (Behrisch and Wittmack, 1991). The results from the model are shown in Fig. 1, where the surface density of H2 is averaged over 30° latitude bins (combining north and south) latitudes and 1 h of local time. The model output from the altitude < 10 km is averaged to approximate the surface density. An assumed source rate of 1 g s−1 is used for the plot. The expected density scales linearly with the source rate. The low energy chemical sputtering gives higher density than high energy physical sputtering because both the scale heights and the escaping fraction are higher for physical sputtering. The distribution of H2 in the physical sputtering model is centered on the subsolar point and reflects the distribution of the solar wind as a source. The high escape rate limits redistribution through the lunar exosphere. In contrast, the chemically sputtered H2 has a local maximum centered around noon, reflecting the source region. However, a smaller escape fraction allows the H2 to remain in the exosphere over several hops. Therefore the larger peak on the nightside demonstrates the thermallycontrolled, semi-bound H2 exosphere. In a thermally equilibrated H2 exosphere, the surface density should be inversely related to surface temperature. A thermally accommodated H2 exosphere produced using an isotropic source is shown in Fig. 2 using the same technique to produce surface density. The thermally accommodated exosphere is similar to the chemical sputtering exosphere with the exception of the density on the nightside being about double in Fig. 2 than in Fig. 1 (left). Because the initial release is isotropic in Fig. 2, half of the particles
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Fig. 2. Modeled local time variation of a thermally accommodated H2 exosphere with an isotropic source released with a velocity distribution at the local surface temperature. As in Fig. 1, the density is normalized to a source rate of 1 g s−1 . The three lines represent the latitude ranges described in Fig. 1.
Fig. 3. Modeled local time variation of the H2 exosphere of the Moon presented in the same manner as Figs. 1 and 2, although here each trace is from the high latitude (60 < |λ| < 85°) band. The density is normalized to the 1 g s−1 source rate. All runs use an isotropic release at the local surface temperature. These runs implemented a finite residence time based on the local surface temperature. The temperature where the probability of release reaches 1 is called the sticking temperature, which is set to 80 K for the dashed line and 200 K for the solid line. The thermal profile with no sticking is shown for reference by the dotted line.
are initially released on the nightside at lower temperatures, and are less likely to be lost on the first hop. More work could be done to model an anisotropic distribution to represent varying diffusion times as a function of local surface temperature (Starukhina, 2006; Farrell et al., 2015). These sputter-produced exospheres are relatively symmetric with respect to the eastern and western hemispheres. A thermally accommodated exosphere is also largely symmetric with respect to the eastern/western hemispheres. There are two mechanisms that could produce an asymmetric H2 exosphere: a non-thermal source that is asymmetric with respect to dawn and dusk or an additional process that is more influential than the thermal control of the exosphere. A mechanism for producing a dawn–dusk asymmetry is a finite residence time on the surface that is modulated by temperature. Although argon demonstrates this effect, helium does not, owing to its high vapor pressure at pre-dawn temperatures (Hodges, 1975). H2 is expected to behave more like helium because it is not expected to interact strongly with the surface. However, a simulation with a long residence time in low temperature regions is shown in Fig. 3. Here, an adsorptive capacity that has a long-duration residence time for T < 80 K is shown in the solid line. The gradual adsorption of the gas across the cold nightside is evident as the density decreases from the dusk terminator, through midnight
and continuing on to reach the minimum at noon. The dashed line shows the H2 distribution when the model implements a higher sticking temperature of 200 K. The entire nightside is colder than this, so this is effectively long term sticking on the nightside. In this case, the overall density of the H2 exosphere is reduced because of the long times that particles reside on the surface and not in the exosphere. There is an overall reduction of a factor of 4–5 on the dayside compared with the 80 K sticking temperature case. The dayside density exceeds the nightside density for a sticking temperature of 200 K, similar to the high energy source mechanism case shown in Fig. 1. Another potential mechanism for producing a dawn/dusk asymmetry is to use a source that is higher at dawn than at dusk. Micrometeoroids are one such source. Owing to the motion of the Earth–Moon system around the Sun, there is a greater flux of micrometeoroids on the dawn hemisphere than on the dusk hemisphere (Fechtig et al., 1974). The Lunar Dust Experiment (LDEX) on LADEE measured the peak in dust production from impacts at 7 am local time (Horanyi et al., 2015). We use a simplified model of impact production that superposes a cosine drop off from the apex direction and an isotropic background with equal magnitude as the dawn-centered population. The model results using the same spatial averaging as in Fig. 1 are shown in Fig. 4. Using impact vaporization, the release energy is high. We assume a Maxwell– Boltzmann distribution with a temperature of 30 0 0 K (Eichhorn, 1976). The resulting exospheric distribution peaks in density at 45 am local time, which is offset 1–2 h from the peak in the source rate in the direction of the lowest temperature. The minimum density occurs at 1-2 pm local time, which is offset 1–2 h in the direction away from the peak of the source rate. Thus a combination in the distribution of the source and the thermalized exosphere modulates the distribution of H2 in the lunar exosphere and produces a dawn/dusk asymmetry. Another model run using a lower temperature of 10 0 0 K for the released particles is also shown in Fig. 4. This run was done to investigate the relative loss rate as a function of release temperature. Furthermore, the Gibson et al. (1972) pyrolysis experiments of returned lunar soil samples released H2 at 573 K < T < 973 K. While the local surface temperature is not expected to exceed 400 K from solar radiation, very localized thermal spikes from meteoroid impacts may play a role in releasing implanted H2 as the impacts heat, melt, or vaporize the local lunar material. 2.3. Source rate Once the exospheric density is measured, one can use the model to determine the source rate of molecules by scaling the density, n, in the model and the source rate, S, used in the model to the observed density:
Sactual = Smodel nactual /nmodel Table 1 provides the source rate inferred for each process modeled using the polar nightside observation from LAMP as the actual density. The energy and the spatial distribution of the source appear in the first and second columns, respectively. The necessary source rate in g s−1 appears in the next column. For reference, the solar wind delivers 31.5 g s−1 of H to the Moon assuming a circular cross section of radius 1738 km and a solar wind flux of 2 × 108 p+ cm−2 s−1 . For thermal source mechanisms, the source rates required to reproduce the LAMP polar measurement (Stern et al., 2013) are ∼1 g s−1 . Impacts, in contrast, are a more energetic source and lead to a higher escape rate. The source rates needed to reproduce the LAMP measurement are on the order of ∼10 g s−1 . Physical sputtering, the most energetic of the processes, requires a source rate of 1300 g s−1 to reproduce the LAMP observation.
Please cite this article as: D.M. Hurley et al., Contributions of solar wind and micrometeoroids to molecular hydrogen in the lunar exosphere, Icarus (2016), http://dx.doi.org/10.1016/j.icarus.2016.04.019
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Fig. 4. Simulated H2 density as a function of local time using micrometeoroids as the release mechanism assuming T = 30 0 0 K (left) and T = 10 0 0 K (right). Both panels are presented in the same manner as Figs. 1–3.
3. Discussion Cremonese et al. (2013) calculate the vapor production from micrometeoroids infall on the Moon. Using a mean mass flux of 6.688 × 10−16 g cm−2 s−1 at the lunar orbit, the rate of mass delivery to the Moon is about 60 g s−1 . Bruck Syal et al. (2015) determine a similar value for delivery as a function of lunar surface area, 1.54 × 10−16 g cm−2 s−1 , where the factor of 4 reduction is from the conversion from cross-sectional area to surface area. Micrometeoroids are not expected to carry large amounts of hydrogen with them. In fact, the relative abundance of hydrogen will span a range of values depending on the source of the micrometeoroids. A reasonable expectation is on the order of a few % in the form of water (Hanner and Zolensky, 2010). Using 10% by weight as a high estimate (e.g., for the case of entirely comet tail dust streams), then micrometeoroids would deliver about 6 g s−1 of water to the Moon. If this water dissociates in the impact vapor, ∼6% would be expected to photodissociate into H2 + O (Huebner et al., 1992). Thus 0.36 g s−1 of H2 + O is an upper limit to the amount of H2 + O coming from the micrometeoroid inventory itself. Because the H2 only comprises 1/9 of the mass of those products, the upper limit to micrometeoroid-delivered H2 is 0.04 g s−1 . This upper limit is quite small in comparison to the source rate needed to reproduce the observations using an impact release mechanism of 25 g s−1 . Even micrometeoroids comprised purely of water ice would not suffice. Direct delivery of H2 via micrometeoroids is not likely the source of the observed H2 . However, in addition to vaporizing themselves, micrometeoroids also vaporize lunar regolith on impact. Killen and Hahn (2015) have linked the density of calcium in the exosphere of Mercury and its dawn/dusk asymmetry to liberation by micrometeoroids. Observations of H2 released by an impact on the Moon have already been made by LAMP during the Lunar CRater Observation and Sensing Satellite (LCROSS) impact (Gladstone et al., 2010). A major difference in the two observations is that LCROSS impacted a volatile-rich region compared to the low volatile concentration in the equatorial regions. The LCROSS impact vaporized 117 kg of H2 , which represented 3.7% by weight of affected regolith (Hurley et al., 2012). The analysis of the timing and the relative abundance of the H2 suggests that the H2 released by the LCROSS impact was formed promptly on impact and was not a product of photodissociation of H2 O. The proposed mechanism explained both the relatively high proportion of H2 in the vapor compared to its abundance in the regolith and the apparently large partition of energy in the vapor component of the ejecta materials because the reaction is exothermic, which provides the translational energy of the
H2 . Therefore, LCROSS demonstrated that H2 can be released from the regolith by an impact. Although the volatile content of Cabeus is much greater than the equatorial regions, it is plausible that impacts in the equatorial region are capable of releasing the small amounts of H that are present there. Cremonese et al. (2013) report a vapor production rate of 1.767 × 10−15 g cm−2 s−1 , or 170 g s−1 integrated over the entire Moon. If the vaporization of material is stoichiometric, using a mature regolith value of 100 ppm by weight of H gives a production rate of 0.017 g s−1 of H-bearing material. However, given the highly volatile nature of H, one should expect it to be volatilized more efficiently than other constituents of the regolith (Bruno et al., 2007). In fact, laboratory temperature-programmed desorption experiments on returned Apollo samples released H2 in the temperature range of 573–973 K, which is considerably lower than the > 1400 K temperature required to initiate melting of lunar regolith (Gibson and Moore, 1972). Cintala (1992) computes that the ratio of impact melt to impact vapor on the Moon is 2.3. The volume of lunar regolith heated to > 600 K is considerably higher. Therefore micrometeoroids can potentially release < 0.17 g s−1 of H2 if the implanted H atoms in regolith with T > 600 K are liberated as H2 . If it is released at a lower temperature than the assumed 30 0 0 K from the impact, then the source rate required to reproduce the observation is reduced. Therefore, we use the T = 10 0 0 K simulation to estimate the distribution of H2 that is released from the regolith from micrometeoroids. The estimate of 7.6 g s−1 is 1.7 orders of magnitude higher than the predicted upper limit available via this process. Therefore micrometeoroid release of implanted H as H2 is not likely the primary source of H2 in the Moon’s exosphere. Solar wind production and diffusion of H2 remains a plausible source mechanism for the lunar exospheric H2 . The source rate of H+ to the Moon is 31.5 g s−1 . It is largely in steady state with the lunar surface where there is an outgoing H is some form for every incoming proton. The exit flux for backscattered protons and energetic neutral H has already been measured, eliminating > 11% of the available inventory. That leaves < 28 g s−1 available for conversion to H2 . Crider and Vondrak (2002) estimate that 60% of the solar wind is converted to H2 , or 19 g s−1 . Assuming that the implanted H diffuses from inside a grain of regolith to reach the surface of the grain and finds another H, it can recombinatively desorb as H2 . Thermal desorption with a dayside source is the appropriate model for this case. In order to produce the upper limit of H2 from observations, about 54% of the solar wind would need to be converted to H2 . To reproduce the Stern et al. (2013) observation, only 7% of the solar wind would need to be converted to H2 and released on the dayside with a thermal distribution at the
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D.M. Hurley et al. / Icarus 000 (2016) 1–7 Table 2 Comparison of mass fluxes from various sources.
Micrometeoroid delivery Solar wind delivery Micrometeoroid liberation
Flux (g cm− 2 s−1 )
Mass rate (g s−1 )
Expected source rate of H2 (g s−1 )
Efficiency that would produce 1200 cm−3 H2
Upper limit using 90 0 0 cm−3 H2
6.67e−16
58
<0.09
Insufficient source
Insufficient source
3.34e−16 1.77e−15
31.5 170
<28 < 0.17
7% Insufficient source
54% Insufficient source
local surface temperature to reproduce the observations. This is the most plausible source mechanism. Because some recent work suggests that Stern et al. (2013) may be an order of magnitude low (Cook et al., 2016), we consider the range from 1200 to 90 0 0 cm−3 as the observed range. The values relative to a 90 0 0 cm−3 density of H2 are reported in Tables 1 and 2 for reference if the LAMP observations are revised. 4. Conclusions Several pathways for incident solar wind protons exist upon interaction with the lunar regolith. One pathway of particular interest is the conversion of solar wind protons into water through interactions with the lunar regolith. This conversion has the potential to contribute to a migrating flux of water to lunar cold traps, that would remain frozen in lunar permanently shadowed regions for extremely long times. This process has not yet been measured in situ on the Moon and can only be accounted for through the unaccounted inventory of solar wind hydrogen. Kaguya has measured an ionized backscatter efficiency of 0.1– 1% (Saito et al., 2008). Chandrayaan-1 and IBEX have measured an energetic backscatter efficiency of 10–20% (e.g., Wieser et al., 2009). The modeling is consistent with H2 being a significant pathway for solar wind protons to leave the Moon. Here, we constrain the inventory that can exit as molecular hydrogen (H2 ) to 7–54%, and thus further constrain the potential inventory available as water to 25–75%. Therefore solar wind remains a potential source of water to lunar permanently shadowed regions. It is important to verify the LAMP observation to narrow this range. Acknowledgments This work was supported by NASA, the Lunar Reconnaissance Orbiter, and the Southwest Research Institute. D.M.H. thanks Nancy Chabot and Joshua Cahill for helpful discussions and inputs that improved the paper. References Allegrini, F., Dayeh, M.A., Desai, M.I., et al., 2013. Lunar energetic neutral atom (ENA) spectra measured by the interstellar boundary explorer (IBEX). Planet. Space Sci. 85, 232–242. Arnold, J.R., 1979. Ice in the lunar polar regions. J. Geophys. Res. 84 (B10), 5659–5668. Behrisch, R., Wittmaack, K., 1991. Introduction. In: Behrisch, R., Wittmaack, K. (Eds.), Sputtering by Particle Bombardment III. Springer-Verlag, New York, pp. 1–13. Bruck Syal, M., Schultz, P.H., Riner, M.A., 2015. Darkening of Mercury’s surface by cometary carbon. Nat. Geosci. 8, 352–356. Bruno, M., Cremonese, G., Marchi, S., 2007. Neutral sodium atoms release from the surface of the Moon and Mercury induced by meteoroid impacts. Planet. Space Sci. 55, 1494–1501. Cintala, M.J., 1992. Impact-induced thermal effects in the lunar and mercurian regoliths,. J. Geophys. Res. 97, 947–973. Colaprete, A., et al., 2010. Detection of water in the LCROSS ejecta plume. Science 330, 463–468. Cook, J.C., Hurley, D.M., Retherford, K.D., et al., 2016. Searching for variations in H2 abundance with local time, magnetotail crossings and meteor showers. In: 47th Lun. Planet. Sci. Conf., p. 2611. Cremonese, G., Borin, P., Lucchetti, A., et al., 2013. Micrometeoroids flux on the Moon. Astron. Astrophys. 551, A27. doi:10.1051/0 0 04-6361/201220541.
Crider, D.H., Vondrak, R.R., 2002. Hydrogen migration to the lunar poles by solar wind bombardment of the Moon. Adv. Space Res. 30 (8), 1869–1874. DesMarais, D.J., Hayes, J.M., Meinschein, W.G., 1974. The distribution in lunar soil of hydrogen released by pyrolysis. Geochim. Cosmochim. Acta 5 (2), 1811–1822. Eichhorm, G., 1976. Analysis of the hypervelocity impact process from impact flash experiments. Planet. Space Sci. 24, 771–781. Farrell, W.M., Hurley, D.M., Zimmerman, M.I., 2015. Solar wind implantation into lunar regolith: Hydrogen retention in a surface with defects. Icarus 255, 116–126. Fechtig, H., Hartung, J.B., Nagel, K., et al., 1974. Lunar microcrater studies, derived meteoroid fluxes, and comparison with satellite-borne experiments. Geochim. Cosmochim. Acta 3, 2463–2474. Feldman, P.D., Morrison, D., 1991. The Apollo 17 Ultraviolet Spectrometer—Lunar atmosphere measurements revisited. Geophys. Res. Lett. 18, 2105–2108. Feldman, W.C, et al., 1998. Fluxes of fast and epithermal neutrons from Lunar Prospector: Evidence for water ice at the lunar poles. Science 281, 1486. Funsten, H.O., Allegrini, F., Bochsler, P.A., et al., 2013. Reflection of solar wind hydrogen from the lunar surface. J. Geophys. Res. 118, 292 http://dx.doi.org/10.1002/ jgre.20055 . Gibson, E.K., Moore, G.W., 1972. Inorganic gas release and thermal analysis study of Apollo 14 and 15 soils. Geochim. Cosmochim. Acta 2, 2029–2040. Gladstone, G.R., et al., 2010. LRO-LAMP observations of the LCROSS impact plume. Science 330, 472–476. Halekas, J.S., Poppe, A.R., McFadden, J.P., et al., 2013. The effects of reflected protons on the plasma environment of the moon for parallel interplanetary magnetic fields. Geophys. Res. Lett. 40, 4544–4548. Halekas, J.S., Benna, M., Mahaffy, P.R., et al., 2015. Detections of lunar exospheric ions by the LADEE Neutral Mass Spectrometer. Geophys. Res. Lett. 42. doi:10. 1002/2015GL064746. Hanner, M.S., Zolensky, M.E., 2010. The mineralogy of cometary dust. In: Henning, T. (Ed.), Astromineralogy. Vol. 815: Lecture Notes in Physics, pp. 203–232. Hintenberger, H., Weber, H.W., Voshage, H., et al., 1970. Rare gases, hydrogen, and nitrogen: Concentrations and isotopic composition in lunar material. Science 167, 543–545. Hodges, R.R., 1975. Formation of the lunar atmosphere. Moon 14, 139–157. Hodges, R.R., 2011. Resolution of the lunar hydrogen enigma. Geophys. Res. Lett. 38 (6), L06201. Horanyi, M., Szalay, J.R., Kempf, S., et al., 2015. A permanent, asymmetric dust cloud around the Moon. Nature 522, 324–326. Huebner, W.F., Keady, J.J., Lyon, S.P., 1992. Solar photo rates for planetary atmospheres and atmospheric pollutants. Astrophys. Space Sci. 195, 1–294. Hurley, D.M., 2011. Modeling of the vapor release from the LCROSS impact: I. Parametric dependencies. J. Geophys. Res. 116, E10 0 07. doi:10.1029/ 2010JE003793. Hurley, D.M., et al., 2012. Modeling of the vapor release from the LCROSS impact: 2. Observations from LAMP. J. Geophys. Res. 117, E00H07. doi:10.1029/ 2011JE003841. Hurley, D.M., et al., 2015. An analytic function for lunar surface temperature for exospheric modeling. Icarus 255, 159–163. Killen, R.M., Hurley, D.M., Farrell, W.M., 2012. The effect on the lunar exosphere of a coronal mass ejection passage. J. Geophys. Res. 117, E00K02. Killen, R.M., Hahn, J.M., 2015. Impact vaporization as a possible source of Mercury’s calcium exosphere. Icarus 250, 230–237. Leich, D.A., Tombrello, T.A., Burnett, D.S., 1973. The depth distribution of hydrogen and fluorine in lunar samples. Geochim. Cosmochim. Acta 2, 1597–1612. Lord, H.C., 1968. Hydrogen and helium ion implantation into olivine and enstatite: Retention coefficients, saturation concentrations, and temperature-release profiles. J. Geophys. Res. 73, 5271–5280. McComas, D.J., et al., 2009. Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon. Geophys. Res. Lett. 36, L12104. Mitrofanov, I., et al., 2010. Hydrogen mapping of the lunar south pole using the LRO neutron detector experiment LEND. Science 330, 483–486. Moore, J.W., Pearson, R.G., 1981. Kinetics and Mechanism: A Study of Homogeneous Chemical Reactions 473. Press, W.C., Teukolsky, S.A., Vetterling, W.T., et al., 1992. Numerical Recipes in C. Cambridge University Press, p. 994. Saito, Y., Yokota, S., Tanaka, T., et al., 2008. Solar wind proton reflection at the lunar surface: Low energy ion measurements by MAP-PACE onboard SELENE (KAGUYA). Geophys. Res. Lett. 35 (24), L24205.
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Contributions of solar wind and micrometeoroids to molecular hydrogen in the lunar exosphere Dana M. Hurley a,∗, Jason C. Cook b, Kurt D. Retherford c, Thomas Greathouse c, G. Randall Gladstone c, Kathleen Mandt c, Cesare Grava c, David Kaufmann b, Amanda Hendrix d, Paul D. Feldman e, Wayne Pryor f, Angela Stickle a, Rosemary M. Killen g, S. Alan Stern b a
Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA Southwest Research Institute, Boulder, CO 80302, USA c Southwest Research Institute, San Antonio, TX 78228, USA d Planetary Science Institute, USA e Johns Hopkins University, Baltimore, MD 21218, USA f Central Arizona College, Coolidge, AZ 85128, USA g NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA b
a r t i c l e
i n f o
Article history: Received 20 August 2015 Revised 25 March 2016 Accepted 14 April 2016 Available online xxx Keywords: Moon Atmospheres Dynamics Solar wind Impact processes
a b s t r a c t We investigate the density and spatial distribution of the H2 exosphere of the Moon assuming various source mechanisms. Owing to its low mass, escape is non-negligible for H2 . For high-energy source mechanisms, a high percentage of the released molecules escape lunar gravity. Thus, the H2 spatial distribution for high-energy release processes reflects the spatial distribution of the source. For low energy release mechanisms, the escape rate decreases and the H2 redistributes itself predominantly to reflect a thermally accommodated exosphere. However, a small dependence on the spatial distribution of the source is superimposed on the thermally accommodated distribution in model simulations, where density is locally enhanced near regions of higher source rate. For an exosphere accommodated to the local surface temperature, a source rate of 2.2 g s−1 is required to produce a steady state density at high latitude of 1200 cm−3 . Greater source rates are required to produce the same density for more energetic release mechanisms. Physical sputtering by solar wind and direct delivery of H2 through micrometeoroid bombardment can be ruled out as mechanisms for producing and liberating H2 into the lunar exosphere. Chemical sputtering by the solar wind is the most plausible as a source mechanism and would require 10–50% of the solar wind H+ inventory to be converted to H2 to account for the observations. © 2016 Published by Elsevier Inc.
1. Introduction It has been confirmed that water exists in permanently shadowed regions of the lunar poles (Colaprete et al., 2010). However, the source of the water remains an open question. Several candidate sources exist and may each contribute including comets, micrometeoroids, internal water, and solar wind (Arnold, 1979). The solar wind ions comprise primarily protons. Once steady state is achieved, there must be a balance in the influx of protons and the outflow of hydrogen. However, there are multiple pathways for the outflow of hydrogen. Some of those pathways have been observed by spacecraft. In this paper, we quantify the amount converted to
∗
Corresponding author. Tel.: +1 443 778 9126. E-mail address: [email protected] (D.M. Hurley).
H2 . This further constrains the amount of solar wind protons that is available to be converted to water, and potentially migrate to the poles. H2 has been detected spectrally in the lunar exosphere by the Lyman Alpha Mapping Project (LAMP) on NASA’s Lunar Reconnaissance Orbiter (LRO). Initially, it was detected in the vapor plume emanating from the Lunar CRater Observation and Sensing Satellite (LCROSS) impact into the permanently shadowed Cabeus crater (Gladstone et al., 2010). Modeling of the propagation of the vapor plume indicated that 110 kg of H2 was released by the impact (Hurley et al., 2012). The analysis suggested that an energy source such as an exothermic reaction of two hydrogen atoms adsorbed to the surface forming H2 via a grain-catalyzed reaction was consistent with the perceived partition of energy from the impact. Later Stern et al. (2013) detected H2 in the lunar exosphere by integrating observations made at high latitudes on the nightside of
http://dx.doi.org/10.1016/j.icarus.2016.04.019 0019-1035/© 2016 Published by Elsevier Inc.
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the Moon from the first 3.5 years of LAMP data. They determine a column density of 5.9 × 109 cm−2 , or an equivalent surface density of H2 of 1200 ± 400 cm−3 . This is consistent with the previous upper limit set by Feldman and Morrison (1991) of < 90 0 0 cm−3 based on UV observations from the Apollo 17 orbiter. The Stern et al. (2013) detection of H2 provides an average amount over an extended region of the Moon. Without spatial or temporal resolution, the observation is not able to distinguish between various sources and release mechanisms, but does give a useful upper boundary to the global steady state release of H2 . In addition, H2 has been detected through mass spectrometry in the vicinity of the Moon. The Moon Impact Probe (MIB) released from the Chandrayaan-1 orbiter carried a neutral mass spectrometer, CHandra’s Altitudinal Composition Explorer (CHACE) (Sridharan et al., 2010). The H2 detected along the sub-satellite trajectory on the lunar dayside demonstrated higher surface density near the pole than at low latitudes (Thampi et al., 2015). The estimated surface density at high southern latitude on the dayside was 825 cm−3 , consistent with the Stern et al. (2013) nightside polar averages at a lower surface temperature. More recently, Halekas et al. (2015) analyzed LADEE NMS data using the ion mode to infer the density of the H2 exosphere. Using the specifics of the geometry of the convection electric field and the viewing geometry of the NMS, the analysis is sensitive to ions produced from exospheric neutrals, and therefore traces exospheric density. However, the observations necessarily convolve the ionization production process and the column density along the electric field direction. Water and H2 share the same potential sources on the Moon. Solar wind implanted hydrogen has been considered the most likely source owing to the steady stream of hydrogen incident on the Moon. However, micrometeoroids bring some volatiles with them that would be released on impact upon the surface of the Moon. The Neutral Mass Spectrometer (NMS) onboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft detected an increase in exospheric water associated with lunar encounters with meteor storms (Mehdi Benna, personal communication). A solar wind proton that encounters the Moon is implanted into the regolith grain it hits. At energies of 1 keV amu−1 , protons penetrate a few 10 s of nm into the rim of the grain. Although most of the incident ions are neutralized on contact, the mission Kaguya measured that 0.1–1% of SW protons are backscattered immediately from the surface (Saito et al., 2008). These percentages have also been confirmed by NASA’s Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) spacecraft (Halekas et al., 2013). An additional population of energetic neutral hydrogen that has been reflected from the surface of the Moon has been observed by SARA on ISRO’s Chandrayaan-1 (Wieser et al., 2009) and Interstellar Boundary Explorer (IBEX) (McComas et al., 2009; Funsten et al., 2013; Allegrini et al., 2013). IBEX observations are consistent with 10% of solar wind protons being converted to energetic neutral atoms (ENA). SARA’s observations show up to 20%. Thus, only 10.1–21% of the incident flux of solar wind protons has been directly accounted for by observations. This leaves 79–90% of solar wind inventory unobserved. Hodges (2011) predicts that nearly all of the solar wind can be converted to neutral H that immediately diffuses out of the regolith while still being consistent with ENA, far ultraviolet (FUV), and ion observations. In that model, the energy distribution of the H is such that most of the atoms are escaping the Moon, however at energies below what is detectable by ENA instruments. Returned lunar regolith from the Apollo missions contain hydrogen that has built up in the rims of regolith grains (Hintenberger et al., 1970; Leich et al., 1973). This reveals that,
as expected from laboratory experiments (Lord, 1968; Zeller et al., 1966; Wehner et al., 1963), the lunar regolith acts as a sink for solar wind elements as small amounts accumulate in the rims until a steady state saturation level is achieved. For solar wind hydrogen, the saturation limit appears to be ∼150 ppm in equatorial soils (DesMarais et al., 1974). Neutron detectors sense an overall decrease in epi-thermal neutron flux in the lunar polar regions (Feldman et al., 1998; Mitrofanov et al., 2010), which signifies an increase in hydrogen abundance near the poles. As this measurement cannot distinguish between binding states of hydrogen, it is not known whether this indicates an increased presence of implanted hydrogen, adsorbed water, or hydrated minerals at high latitudes. This paper presents models of exospheric H2 produced by various source and release mechanisms. The results are discussed in terms of how the likely sources and processes influence the distribution of exospheric hydrogen. Finally, we discuss how the various source and release mechanisms fit into the greater hydrogen and hydrologic cycle on the Moon. 2. Exosphere model 2.1. Model description The exosphere model is a Monte Carlo model that follows particles as they hop along collisionless, ballistic trajectories in the lunar exosphere by following the equation of motion under gravity (Hurley, 2011; Killen et al., 2012) neglecting radiation pressure. The equation of motion is solved using a fourth order Runge–Kutta algorithm (Press et al. 1992) with 1-s time steps. The particles are originally released with a spatial and energy distribution representative of the assumed source. The source’s distribution in phase space is extremely important, and is discussed in detail below. Being a low-mass species, much H2 escapes beyond the Hill sphere of the Moon, i.e., beyond the region where lunar gravity dominates the motion of the particles. In the model, when a particle reaches the Hill sphere at 35 Rmoon , the model counts the particle as “escaping” and no longer traces the particle. Although these particles enter Cis-Lunar space and may return to the Moon, they are not expected to represent a significant portion of the H2 at very low altitude, where measurements occur (<200 km). The model also calculates the loss due to photodissociation for any particle that is in sunlight for each time step. Using the Huebner et al. (1992) photodissociation rate for quiet Sun conditions, photodissociated particles are removed from the simulation. Particles that reencounter the surface of the Moon interact with the regolith. Possible interactions include dissociative adsorption, thermal accommodation, or elastic collisions. We assume that H2 does not adsorb to the surface, thus set loss from dissociative adsorption to zero. Also, this means that the particle is re-released in the model immediately on the next time step. The model parameterizes the energy exchange between the H2 molecule and the surface every time a particle encounters the surface using an accommodation coefficient. The actual thermal accommodation of H2 interacting with lunar regolith is poorly constrained. For the runs presented here, we assume 100% thermal accommodation, meaning the particles are released with no memory of their incident velocity and have a distribution representative of a Maxwell– Boltzmann distribution at the local surface temperature. The local surface temperature is implemented in the model using an empirical function (Hurley et al., 2015). The model neglects energy exchange between translational, vibrational, and rotational degrees of freedom for the H2 molecules. By allocating all of the energy as translational, the model slightly overestimates the velocity of the molecules. The net result would be an overestimate of escape, and therefore a lower density.
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Fig. 1. Local time profile of simulated H2 surface density for solar wind source normalized to a source rate of 1 g s−1 . The left panel shows H2 released on the dayside with a thermal profile at the local surface temperature. The right panel shows H2 released with higher energy, representing physical sputtering by solar wind ions. Model output from equatorial latitude (|λ| < 30°) is shown in the dashed line; mid latitude (30° < |λ| < 60°) in the dotted line; and high latitude (60° < |λ| < 85°) in the solid line. The surface density is defined as the average model density between the surface and 2 km altitude. Table 1 Model output for different source mechanisms for comparison with LAMP observations.
Source process Thermal, local surface temperature Impact, thermal 10 0 0 K 30 0 0 K Sputtering Thermal source Tstick = 80 K Tstick = 200 K
Spatial location of source
Density for 1 g s−1 source rate (cm−3 )
Source rate to produce 1200 cm−3 (g s−1 )
Source rate to produce 90 0 0 cm−3 (g s−1 )
Global (Fig. 2)
940
1.3
9.6
Dayside (Fig. 1)
540
2.2
17
Dawn + Global (Fig. 3) Dawn + Global (Fig. 3)
160 50
7.6 25
57 180
Dayside (Fig. 1)
0.94
1300
9600
Global (Fig. 4) Global (Fig. 4)
370 50
3.2 24
24 180
However, the correction is expected to be negligible (Moore and Pearson, 1981). 2.2. Spatial distribution We present several model runs and inspect the basic properties of the resulting exosphere. We run a variety of release mechanisms in position and energy. The initial positions include global isotropic, dayside-centered, apex-centered, and combinations of those. The initial energies simulated are mostly thermal velocity distributions with temperatures coming from the local surface temperature, 10 0 0 K or 30 0 0 K. A Sigmund–Thompson distribution for physical sputtering is another case simulated. The results from these model runs are summarized in Table 1. Specific cases are also shown in figures and discussed in the text below. Crider and Vondrak (2002) propose that H2 is produced from the solar wind interaction through chemical sputtering. The spatial distribution of the source is approximately centered at the subsolar point, although incorporating an offset to account for the motion of the Earth–Moon system about the Sun would be more accurate. The concentration of source particles decreases as the cosine of the solar zenith angle moving away from the subsolar point to the terminator. There is no source of particles on the nightside for sputtering release. We simulate both chemical sputtering, where the particles are released with a lower energy, and physical sputtering, where particles are released with higher energy. Because the released product is molecular, chemical sputtering is more appro-
priate (Behrisch and Wittmack, 1991). The results from the model are shown in Fig. 1, where the surface density of H2 is averaged over 30° latitude bins (combining north and south) latitudes and 1 h of local time. The model output from the altitude < 10 km is averaged to approximate the surface density. An assumed source rate of 1 g s−1 is used for the plot. The expected density scales linearly with the source rate. The low energy chemical sputtering gives higher density than high energy physical sputtering because both the scale heights and the escaping fraction are higher for physical sputtering. The distribution of H2 in the physical sputtering model is centered on the subsolar point and reflects the distribution of the solar wind as a source. The high escape rate limits redistribution through the lunar exosphere. In contrast, the chemically sputtered H2 has a local maximum centered around noon, reflecting the source region. However, a smaller escape fraction allows the H2 to remain in the exosphere over several hops. Therefore the larger peak on the nightside demonstrates the thermallycontrolled, semi-bound H2 exosphere. In a thermally equilibrated H2 exosphere, the surface density should be inversely related to surface temperature. A thermally accommodated H2 exosphere produced using an isotropic source is shown in Fig. 2 using the same technique to produce surface density. The thermally accommodated exosphere is similar to the chemical sputtering exosphere with the exception of the density on the nightside being about double in Fig. 2 than in Fig. 1 (left). Because the initial release is isotropic in Fig. 2, half of the particles
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Fig. 2. Modeled local time variation of a thermally accommodated H2 exosphere with an isotropic source released with a velocity distribution at the local surface temperature. As in Fig. 1, the density is normalized to a source rate of 1 g s−1 . The three lines represent the latitude ranges described in Fig. 1.
Fig. 3. Modeled local time variation of the H2 exosphere of the Moon presented in the same manner as Figs. 1 and 2, although here each trace is from the high latitude (60 < |λ| < 85°) band. The density is normalized to the 1 g s−1 source rate. All runs use an isotropic release at the local surface temperature. These runs implemented a finite residence time based on the local surface temperature. The temperature where the probability of release reaches 1 is called the sticking temperature, which is set to 80 K for the dashed line and 200 K for the solid line. The thermal profile with no sticking is shown for reference by the dotted line.
are initially released on the nightside at lower temperatures, and are less likely to be lost on the first hop. More work could be done to model an anisotropic distribution to represent varying diffusion times as a function of local surface temperature (Starukhina, 2006; Farrell et al., 2015). These sputter-produced exospheres are relatively symmetric with respect to the eastern and western hemispheres. A thermally accommodated exosphere is also largely symmetric with respect to the eastern/western hemispheres. There are two mechanisms that could produce an asymmetric H2 exosphere: a non-thermal source that is asymmetric with respect to dawn and dusk or an additional process that is more influential than the thermal control of the exosphere. A mechanism for producing a dawn–dusk asymmetry is a finite residence time on the surface that is modulated by temperature. Although argon demonstrates this effect, helium does not, owing to its high vapor pressure at pre-dawn temperatures (Hodges, 1975). H2 is expected to behave more like helium because it is not expected to interact strongly with the surface. However, a simulation with a long residence time in low temperature regions is shown in Fig. 3. Here, an adsorptive capacity that has a long-duration residence time for T < 80 K is shown in the solid line. The gradual adsorption of the gas across the cold nightside is evident as the density decreases from the dusk terminator, through midnight
and continuing on to reach the minimum at noon. The dashed line shows the H2 distribution when the model implements a higher sticking temperature of 200 K. The entire nightside is colder than this, so this is effectively long term sticking on the nightside. In this case, the overall density of the H2 exosphere is reduced because of the long times that particles reside on the surface and not in the exosphere. There is an overall reduction of a factor of 4–5 on the dayside compared with the 80 K sticking temperature case. The dayside density exceeds the nightside density for a sticking temperature of 200 K, similar to the high energy source mechanism case shown in Fig. 1. Another potential mechanism for producing a dawn/dusk asymmetry is to use a source that is higher at dawn than at dusk. Micrometeoroids are one such source. Owing to the motion of the Earth–Moon system around the Sun, there is a greater flux of micrometeoroids on the dawn hemisphere than on the dusk hemisphere (Fechtig et al., 1974). The Lunar Dust Experiment (LDEX) on LADEE measured the peak in dust production from impacts at 7 am local time (Horanyi et al., 2015). We use a simplified model of impact production that superposes a cosine drop off from the apex direction and an isotropic background with equal magnitude as the dawn-centered population. The model results using the same spatial averaging as in Fig. 1 are shown in Fig. 4. Using impact vaporization, the release energy is high. We assume a Maxwell– Boltzmann distribution with a temperature of 30 0 0 K (Eichhorn, 1976). The resulting exospheric distribution peaks in density at 45 am local time, which is offset 1–2 h from the peak in the source rate in the direction of the lowest temperature. The minimum density occurs at 1-2 pm local time, which is offset 1–2 h in the direction away from the peak of the source rate. Thus a combination in the distribution of the source and the thermalized exosphere modulates the distribution of H2 in the lunar exosphere and produces a dawn/dusk asymmetry. Another model run using a lower temperature of 10 0 0 K for the released particles is also shown in Fig. 4. This run was done to investigate the relative loss rate as a function of release temperature. Furthermore, the Gibson et al. (1972) pyrolysis experiments of returned lunar soil samples released H2 at 573 K < T < 973 K. While the local surface temperature is not expected to exceed 400 K from solar radiation, very localized thermal spikes from meteoroid impacts may play a role in releasing implanted H2 as the impacts heat, melt, or vaporize the local lunar material. 2.3. Source rate Once the exospheric density is measured, one can use the model to determine the source rate of molecules by scaling the density, n, in the model and the source rate, S, used in the model to the observed density:
Sactual = Smodel nactual /nmodel Table 1 provides the source rate inferred for each process modeled using the polar nightside observation from LAMP as the actual density. The energy and the spatial distribution of the source appear in the first and second columns, respectively. The necessary source rate in g s−1 appears in the next column. For reference, the solar wind delivers 31.5 g s−1 of H to the Moon assuming a circular cross section of radius 1738 km and a solar wind flux of 2 × 108 p+ cm−2 s−1 . For thermal source mechanisms, the source rates required to reproduce the LAMP polar measurement (Stern et al., 2013) are ∼1 g s−1 . Impacts, in contrast, are a more energetic source and lead to a higher escape rate. The source rates needed to reproduce the LAMP measurement are on the order of ∼10 g s−1 . Physical sputtering, the most energetic of the processes, requires a source rate of 1300 g s−1 to reproduce the LAMP observation.
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Fig. 4. Simulated H2 density as a function of local time using micrometeoroids as the release mechanism assuming T = 30 0 0 K (left) and T = 10 0 0 K (right). Both panels are presented in the same manner as Figs. 1–3.
3. Discussion Cremonese et al. (2013) calculate the vapor production from micrometeoroids infall on the Moon. Using a mean mass flux of 6.688 × 10−16 g cm−2 s−1 at the lunar orbit, the rate of mass delivery to the Moon is about 60 g s−1 . Bruck Syal et al. (2015) determine a similar value for delivery as a function of lunar surface area, 1.54 × 10−16 g cm−2 s−1 , where the factor of 4 reduction is from the conversion from cross-sectional area to surface area. Micrometeoroids are not expected to carry large amounts of hydrogen with them. In fact, the relative abundance of hydrogen will span a range of values depending on the source of the micrometeoroids. A reasonable expectation is on the order of a few % in the form of water (Hanner and Zolensky, 2010). Using 10% by weight as a high estimate (e.g., for the case of entirely comet tail dust streams), then micrometeoroids would deliver about 6 g s−1 of water to the Moon. If this water dissociates in the impact vapor, ∼6% would be expected to photodissociate into H2 + O (Huebner et al., 1992). Thus 0.36 g s−1 of H2 + O is an upper limit to the amount of H2 + O coming from the micrometeoroid inventory itself. Because the H2 only comprises 1/9 of the mass of those products, the upper limit to micrometeoroid-delivered H2 is 0.04 g s−1 . This upper limit is quite small in comparison to the source rate needed to reproduce the observations using an impact release mechanism of 25 g s−1 . Even micrometeoroids comprised purely of water ice would not suffice. Direct delivery of H2 via micrometeoroids is not likely the source of the observed H2 . However, in addition to vaporizing themselves, micrometeoroids also vaporize lunar regolith on impact. Killen and Hahn (2015) have linked the density of calcium in the exosphere of Mercury and its dawn/dusk asymmetry to liberation by micrometeoroids. Observations of H2 released by an impact on the Moon have already been made by LAMP during the Lunar CRater Observation and Sensing Satellite (LCROSS) impact (Gladstone et al., 2010). A major difference in the two observations is that LCROSS impacted a volatile-rich region compared to the low volatile concentration in the equatorial regions. The LCROSS impact vaporized 117 kg of H2 , which represented 3.7% by weight of affected regolith (Hurley et al., 2012). The analysis of the timing and the relative abundance of the H2 suggests that the H2 released by the LCROSS impact was formed promptly on impact and was not a product of photodissociation of H2 O. The proposed mechanism explained both the relatively high proportion of H2 in the vapor compared to its abundance in the regolith and the apparently large partition of energy in the vapor component of the ejecta materials because the reaction is exothermic, which provides the translational energy of the
H2 . Therefore, LCROSS demonstrated that H2 can be released from the regolith by an impact. Although the volatile content of Cabeus is much greater than the equatorial regions, it is plausible that impacts in the equatorial region are capable of releasing the small amounts of H that are present there. Cremonese et al. (2013) report a vapor production rate of 1.767 × 10−15 g cm−2 s−1 , or 170 g s−1 integrated over the entire Moon. If the vaporization of material is stoichiometric, using a mature regolith value of 100 ppm by weight of H gives a production rate of 0.017 g s−1 of H-bearing material. However, given the highly volatile nature of H, one should expect it to be volatilized more efficiently than other constituents of the regolith (Bruno et al., 2007). In fact, laboratory temperature-programmed desorption experiments on returned Apollo samples released H2 in the temperature range of 573–973 K, which is considerably lower than the > 1400 K temperature required to initiate melting of lunar regolith (Gibson and Moore, 1972). Cintala (1992) computes that the ratio of impact melt to impact vapor on the Moon is 2.3. The volume of lunar regolith heated to > 600 K is considerably higher. Therefore micrometeoroids can potentially release < 0.17 g s−1 of H2 if the implanted H atoms in regolith with T > 600 K are liberated as H2 . If it is released at a lower temperature than the assumed 30 0 0 K from the impact, then the source rate required to reproduce the observation is reduced. Therefore, we use the T = 10 0 0 K simulation to estimate the distribution of H2 that is released from the regolith from micrometeoroids. The estimate of 7.6 g s−1 is 1.7 orders of magnitude higher than the predicted upper limit available via this process. Therefore micrometeoroid release of implanted H as H2 is not likely the primary source of H2 in the Moon’s exosphere. Solar wind production and diffusion of H2 remains a plausible source mechanism for the lunar exospheric H2 . The source rate of H+ to the Moon is 31.5 g s−1 . It is largely in steady state with the lunar surface where there is an outgoing H is some form for every incoming proton. The exit flux for backscattered protons and energetic neutral H has already been measured, eliminating > 11% of the available inventory. That leaves < 28 g s−1 available for conversion to H2 . Crider and Vondrak (2002) estimate that 60% of the solar wind is converted to H2 , or 19 g s−1 . Assuming that the implanted H diffuses from inside a grain of regolith to reach the surface of the grain and finds another H, it can recombinatively desorb as H2 . Thermal desorption with a dayside source is the appropriate model for this case. In order to produce the upper limit of H2 from observations, about 54% of the solar wind would need to be converted to H2 . To reproduce the Stern et al. (2013) observation, only 7% of the solar wind would need to be converted to H2 and released on the dayside with a thermal distribution at the
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D.M. Hurley et al. / Icarus 000 (2016) 1–7 Table 2 Comparison of mass fluxes from various sources.
Micrometeoroid delivery Solar wind delivery Micrometeoroid liberation
Flux (g cm− 2 s−1 )
Mass rate (g s−1 )
Expected source rate of H2 (g s−1 )
Efficiency that would produce 1200 cm−3 H2
Upper limit using 90 0 0 cm−3 H2
6.67e−16
58
<0.09
Insufficient source
Insufficient source
3.34e−16 1.77e−15
31.5 170
<28 < 0.17
7% Insufficient source
54% Insufficient source
local surface temperature to reproduce the observations. This is the most plausible source mechanism. Because some recent work suggests that Stern et al. (2013) may be an order of magnitude low (Cook et al., 2016), we consider the range from 1200 to 90 0 0 cm−3 as the observed range. The values relative to a 90 0 0 cm−3 density of H2 are reported in Tables 1 and 2 for reference if the LAMP observations are revised. 4. Conclusions Several pathways for incident solar wind protons exist upon interaction with the lunar regolith. One pathway of particular interest is the conversion of solar wind protons into water through interactions with the lunar regolith. This conversion has the potential to contribute to a migrating flux of water to lunar cold traps, that would remain frozen in lunar permanently shadowed regions for extremely long times. This process has not yet been measured in situ on the Moon and can only be accounted for through the unaccounted inventory of solar wind hydrogen. Kaguya has measured an ionized backscatter efficiency of 0.1– 1% (Saito et al., 2008). Chandrayaan-1 and IBEX have measured an energetic backscatter efficiency of 10–20% (e.g., Wieser et al., 2009). The modeling is consistent with H2 being a significant pathway for solar wind protons to leave the Moon. Here, we constrain the inventory that can exit as molecular hydrogen (H2 ) to 7–54%, and thus further constrain the potential inventory available as water to 25–75%. Therefore solar wind remains a potential source of water to lunar permanently shadowed regions. It is important to verify the LAMP observation to narrow this range. Acknowledgments This work was supported by NASA, the Lunar Reconnaissance Orbiter, and the Southwest Research Institute. D.M.H. thanks Nancy Chabot and Joshua Cahill for helpful discussions and inputs that improved the paper. References Allegrini, F., Dayeh, M.A., Desai, M.I., et al., 2013. Lunar energetic neutral atom (ENA) spectra measured by the interstellar boundary explorer (IBEX). Planet. Space Sci. 85, 232–242. Arnold, J.R., 1979. Ice in the lunar polar regions. J. Geophys. Res. 84 (B10), 5659–5668. Behrisch, R., Wittmaack, K., 1991. Introduction. In: Behrisch, R., Wittmaack, K. (Eds.), Sputtering by Particle Bombardment III. Springer-Verlag, New York, pp. 1–13. Bruck Syal, M., Schultz, P.H., Riner, M.A., 2015. Darkening of Mercury’s surface by cometary carbon. Nat. Geosci. 8, 352–356. Bruno, M., Cremonese, G., Marchi, S., 2007. Neutral sodium atoms release from the surface of the Moon and Mercury induced by meteoroid impacts. Planet. Space Sci. 55, 1494–1501. Cintala, M.J., 1992. Impact-induced thermal effects in the lunar and mercurian regoliths,. J. Geophys. Res. 97, 947–973. Colaprete, A., et al., 2010. Detection of water in the LCROSS ejecta plume. Science 330, 463–468. Cook, J.C., Hurley, D.M., Retherford, K.D., et al., 2016. Searching for variations in H2 abundance with local time, magnetotail crossings and meteor showers. In: 47th Lun. Planet. Sci. Conf., p. 2611. Cremonese, G., Borin, P., Lucchetti, A., et al., 2013. Micrometeoroids flux on the Moon. Astron. Astrophys. 551, A27. doi:10.1051/0 0 04-6361/201220541.
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