The Controversy over the Presence of Water on the Moon

Estelle Asmodelle www.relativecosmos.com Page No: 1 The Controversy over the Presence of Water on the Moon Estelle Asmodelle, University of Central Lancashire, School of Computing, Engineering and Physical Sciences. [email protected] 1.0: Moon Discoveries: The Moon has been our closest neighbour in the solar system for around 4.5 billion years. Early studies of the moon were purely observational and limited to the resolution of the human eye. By the dawn of 1600s science and mathematics were progressing, but it would take a Dutch spyglass invention  later called the telescope  that would allow details of the moon to become more accessible. The first use of the telescope by Galileo, or Thomas Harriot (Grafton, 2009), to view astrophysical objects revealed a wealth of information about the Moon and many other celestial objects. It would take a new technological revolution, the advance of rockets and space missions, to open the secrets that the Moon wanted to reveal. The first mission to the Moon was launched by the Soviets on January 2nd 1959, named Luna 1; this would be the start of 74 missions as of January 2012, not counting flyby missions (Williams, 2011). During orbiting and landing missions, a vast array of experiments were performed to measure many aspects of the Moon, from its gravity, whisper of an atmosphere, regolith constituents, magnetism, structure, temperature, radiation, formation, as well as mapping the Moon’s topology in various wavelengths. However the focus of this report is primarily concerned with the detection of water. The presence of water is crucial for two reasons: it may reveal the presence of life that has remained undetected, and that water is vital for habitation. Future habitation of the Moon will require a supply of water that is accessible, for water can be broken down into hydrogen for use as a rocket propellant, and oxygen for supporting human colonisation. A usable water supply could mean that the possibility of a Moon base could one day become a reality, firstly as a stepping stone to other destinations in our solar system, and secondly as the future location for a telescope site, other research facility, or mining facility (Sanderson, 2006). 2.0: Discussion of Moon: The Moon is slanted ~1.6 from the plane of orbit and ~5.15 to the ecliptic. Hence it does not experience seasons, which also means that there are certain areas of deep craters of the Polar Regions [PR] which are permanently shadowed and never see sunlight with temperatures ~5070 K, being the most likely locations for water ice (Lang, 2003). There are different kinds of water signatures. The first is abundance of hydrogen, H, which implies that H2O could be present. Then there is hydroxyl, HO-, which is simply a single atom of hydrogen and oxygen bound together, while this can be a very good indicator of water embedded in material of some kind, and lastly simply H2O in the form of ice – the best detection. Estelle Asmodelle www.relativecosmos.com 2.1: Physical detection: Prior to any detection most scientists believed that the Moon was devoid of water in any form (Beals et al, 1956). However some scientists posited that water exists on the Moon, in some form, yet it was understood that volatiles, water in this case, would be unstable on the lunar surface because of photodissociation by solar radiation, and gravitational escape. Yet it was realised that water-ice, could be Page No: 2 contained in “cold traps,” (Watson et al, 1961). It was also suggested that photos from NASA’s Lunar Orbiter 4 and 5, revealed physical effects on the surface that could only be explained by water flow (Urey, 1967), while others suggested that any water presence would be < 370 grams per cm2 at a depth of 9m, at most (Anders, 1970). Various detection reports of hydrogen, hydroxyl, water vapour, or water ice, are shown Table 1 below: Table 1 Mission or study Apollo 14 & 12 Clementine Lunar Prospector Cassini Flyby Chandrayaan-1 LRO LCROSS Saal lab study Method / Type of Detection Ion Detector / H2O vapour Bistatic Radar / Polar H2O ice Neutron spectrometer / Hydrogen Visual and Infrared Spectrometer / H2O & HO Imaging Spectrometer / H2O Lyman-Alpha, Neutron & Radar / H2O & HO Lyman-Alpha, Imaging Spectrometer / H & H2O Lunar soil analysis / H & H2O Year 1971 1994 1998 1999 2008 2009 2009 2008 & 2011 Detection level Poor Good Very Good Very Good Excellent* Very Good Very Excellent Very Excellent Table 1: Detections on the Moon related to water signatures. Data compiled from several sources, cited below in the succeeding text. *Denoted the first fairly substantial confirmation. 2.1.1: Apollo: On March 7th 1971 NASA’s Apollo 14 was poised on the Moon as the Suprathermal Ion Detector Experiment [SIDE] was deployed (Freeman et al, 1970). SIDE essentially measured positive ions reaching the lunar surface. A series of intense bursts of ions, measuring ~ 48.6 eV, which was consistent with water vapour ion energy levels, was detected over a 14 hour period. SIDE on Apollo 12, positioned 183 km west of the Apollo 14 site, also confirmed the detection. See Graph 1. observed water vapour ions were likely the result of mission associated water vapour (Freeman & Hills, 1991). Graph 1 Freeman et al conjectured that the observed fluxes were of lunar origin, “the water vapour did not come from man-made sources,” (Freeman et al, 1973), which included contamination of the Moon’s surface by Soviet landers. However there was a retraction in 1991 over the origin of these ions by two of the original authors, which stated that the Total ion detector data from Apollo 14 SIDE - peak at 70 eV corresponds to water vapour ions. The broad spectrum peaking at 500 eV is from the magnetosheath ions. Image courtesy: Freeman. Estelle Asmodelle www.relativecosmos.com Page No: 3 2.1.2: Clementine: There was a 18 year hiatus, between 1972 to 1990, for no missions were deployed to the Moon. The first of which was Hiten in 1990, deployed by the Japanese Space Agency, yet the search for water had all but been abandoned. Diagram 1 Then four years later the Clementine probe, officially referred to as the Deep Space Program Science Experiment [DSPSE], was launched as a joint project between the Strategic Defence Initiative Organization and NASA. One of the objectives of the mission was to make scientific observations of the Moon and an Earth crossing asteroid called 1620 Geographos. Several experiments were carried out but of prime importance are the results of the Bistatic Radar Experiment [BRE]. Clementine used its transmitter to beam 13 cm radio waves into the dark regions of the South Polar Region [SPR], later echoes of these signals were detected on Earth by NASA’s Deep Space Network [DSN]. Computer simulations suggest the area of the North Polar region [NPR] which is in continual shadow ~7,500 km2, with a similar area at the SPR ~6,500 km2 (Martel, 2003). Representation of the orbital geometry of the Clementine BRE, with lunar polar tilt ~1 .6° and lunar tilt toward Earth ~-5°, and the bistatic angle between spacecraft, lunar surface, and Earth receiver are shown. Diagram courtesy S. Nozette et al. Graph 2 As ice is partly transparent to radio energy, so the radio waves penetrate the ice and scatter from internal reflections. Referring to Diagram 1below, the energy peak at =0 is when Clementine and the receiving antenna on Earth are aligned. The magnitude and polarisation of these echoes was consistent with an icy surface. Radio waves were transmitted from the Clementine spacecraft during four orbits, Orbits 301 and 302 passed over the NPR, while orbits 234 and 235 passed over the SPR (Nozette et al, 1996). Graph 2 shows the results: Graph showing results of The Clementine Bistatic Radar Experiment. The graph shows a peak in the power curve of Orbit 234 indicative of water ice. Diagram credit: Nozette et al. In December 1996 the Pentagon announced that the Clementine data, “indicated ice in the bottom of a crater on the South Pole of the Moon” (NRL, 2010). Others have been critical, “we find weak suggestions of enhanced echoes at the time of South Pole backscatter,” (Simpson & Tyler, 1998). Estelle Asmodelle www.relativecosmos.com Image 1 Image 1 shows the SPR, with possible ice deposits marked in blue at orbit 235. Photo credit: Naval Research Laboratory. Interestingly, the BRE had previously been performed with Apollo 14, 15, and 16. Whereby radio waves were transmitted from the Command and Service Modules to the Moon's surface and later the bounced back signals recorded at tracking stations in California (Bahar et al, 1994). Page No: 4 In the permanently shaded craters of both PRs, the NS detected epithermal neutrons which result from collisions of normal fast neutrons with hydrogen atoms. These epithermal fluxes were detected: at the NPR ~ 4.6%, and at the SPR ~ 3.0%. This result was consistent with deposits of hydrogen, in molecular water ice form (Feldman et al, 1998). This data indicated that almost pure water ice could be buried as deep as ~40 cm into the regolith. Initially it was believed that the deposits could cover an area 2 ~10,00050,000 km , but later estimates place this value to be ~1850 km2 at each PR, with a total mass estimate of ice ~ 6 trillion kg (Williams, 2011). Graph 3 Since that time radar studies of the permanently shaded craters at the NPR and SPR of Mercury have shown similar “quasispecular scattering properties,” to the Moon, being consistent with volume scattering from ice deposits (Starukhina & Shkuratov, 1997). 2.1.3: Lunar Prospector: Lunar Prospector, 1998, was primarily designed for a low polar orbit of the Moon. It was fitted with a neutron spectrometer [NS] to search for deposits of hydrogen at SPR and NPR. The NS can detect: lowenergy or thermal neutrons, medium-energy or epithermal neutrons, and high-energy or fast neutrons. NS data graph, showing epithermal neutrons counts with respect lunar latitude, lower values indicate hydrogen abundances. Image credit NASA. Based on the neutron data it was estimated that the total amount of water on the Moon could be anywhere ~10300 million metric tons, with an error of the order of 10 (NASA, 2008). In any case the results confirmed that there was, “abundant H, and hence probably abundant water ice,” (Binder, 1998). Estelle Asmodelle www.relativecosmos.com Page No: 5 Image 2 Image shows the dark blue and purple areas at the PRs indicate neutron emissions consistent with hydrogen deposits covered by desiccated regolith, possible indicators of water in the form of ice or hydrated minerals. Image Credit: Feldman et al / NASA. At the end of the mission the Lunar Prospector was deliberately impacted into a shadowed area of a crater near the SPR in the hope that the impact would spray water vapour from the ice deposits high enough for Earth based telescopes to detect such an emission, but no plume was observed (LPI, 2004). 2.1.5: Cassini Flyby: Although NASA’s Cassini spacecraft only performed a fly-by of the Moon in 1999, it still made an important spectral analysis of the Moon’s albedo. The Visual and Infrared Mapping Spectrometer [VIMS] showed a broad absorption at ~3 m, indicative of water absorption and ~2.8 m, attributed to hydroxyl. The amounts of water present could be ~ 10 to 1000 ppm (Clark, 2009). high-resolution spatial and spectral map of the entire lunar surface, to reveal the composition of the regolith. Subsequently M3 made direct detection of absorption features ~ 2.83.0 μm on the lunar surface, typically indicative of hydroxyl or water containing materials (Pieters, 2009). This was in agreement with previous findings. In the month of October 2009, three peer reviewed papers were published in the journal Science, which indicated the results of such studies were conclusive evidence for water molecules on the Moon (Lucey, 2009). Image 3 2.1.6: Chandrayaan-1: Chandrayaan-1, a 2008 Indian Space Research Organization [ISRO] mission, was designed to orbit the Moon over a two year period. The spacecraft was carrying a NASA contributed instrument: the Moon 3 Mineralogy Mapper [M ], being an imaging spectrometer that has provided the first Two images from the M3 showing water in a very young lunar crater on the far side. Image credit: ISRO/NASA/JPLCaltech/USGS/Brown Univ. Estelle Asmodelle www.relativecosmos.com Image 4 Page No: 6 further examination suggested that larger high-energy neutrons were seen in LEND data, (Lawrence et al, 2011). Image 3 Image taken by the M3. Blue is the water signature, green is the surface as measured by reflected infra-red, and red shows pyroxene, an iron bearing mineral. Image credit: ISRO /NASA /JPL-Caltech/Brown/USGS. It has also been suggested that direct detection of water in situ, chemically created in place, has also been imaged. Measurements carried out by the on-board Chandra’s Altitudinal Composition Explorer [CHACE] detected the presence of water-ice on the surface and at higher lunar latitudes than expected. This could be signatures of water in solid and gaseous phases (Sridharan et al, 2010). 2.1.7: Lunar Orbiter (LRO): Reconnaissance LRO is the first mission of NASA's Robotic Lunar Exploration Program, while the mission objectives also including searching for polar volatiles, especially water ice. The Lunar Exploration Neutron Detector [LEND] detected several regions, that were not permanently shadowed, where the epithermal neutron flux from the surface is lower, indicative of abundant hydrogen. The hydrogen seemed, “consistent with buried water ice from cometary impacts, hydrogen implantation from the solar wind, and/or other as yet unknown sources,” (Mitrofanov et al, 2010). This finding was criticized: as Map of the SPR by LEND, blue represents the low levels of epithermal neutrons, signalling hydrogen rich, while red areas have the highest neutron counts and are relatively hydrogen-poor. Credit: NASA/Goddard/Institute of Space Research. LRO also selected a suitable site where a subsequent mission, LCROSS, would impact its spent rocket stage. As of early 2012, LRO using the Lyman Alpha Mapping Project (LAMP) instrument has fairly substantially shown than farultraviolet [FUV] wavelengths, showing that the PRs are darker by 70%, than the previously estimated 40%. This essentially translates to surface water frost content at the PRs to 1-2% (Gladstone et al., 2012). Image 4 Image of Shackleton crater, a 21-km-diameter permanently shadowed crater adjacent to the lunar South Pole. Credit: NASA/Zuber, M.T. et al., Nature, 2012. Estelle Asmodelle www.relativecosmos.com Page No: 7 Graph 4 Then in June of 2012, a NASA team, using laser light from LRO's laser altimeter, examined the floor of Shackleton crater in the SPR, and found that the crater's floor is brighter than those of other nearby craters, which is consistent with the presence of small amounts of ice (Zuber et al., 2012) as shown in image 4 above. 2.1.8: Lunar Crater Observation and Sensing Satellite (LCROSS): LCROSS was conceived as a low-cost method of determining the nature of hydrogen previously detected at the lunar PRs. On the 9th of October 2009, a spent Centaur rocket impact at the lunar SPR crater Cabeus, ejecting debris, dust, and vapour. During the impact several remote observations detected the presence of water. The plume was observed by LCROSS, which showed a total water ice vapour, within the field of view ~155 ± 12 kg. Of the total excavated regolith that was observable, the concentration of water ice in the regolith at the impact site is ~5.6 ± 2.9% (Colaprete et al, 2010). Near-infrared spectra of LCROSS impact. The blue curve is the calculated value with water vapour and ice added. The blue areas indicate the water absorption bands. Credit: LCROSS/NASA/A.Colprete. The event may have produced: vapour, photodissociated water or subliming H2O, and an OH exosphere created by the release of water ice and vapour into sunlight (Heldman et al, 2011). Diagram 2 Image 5 Diagram of the analysis of impact: with flash, curtain and crater stages shown, and types of hydrogen measured. Image credit: Heldmann et al. (http://www.nasa.gov/mission_pages/LCROSS/searchforwater/LCROSS_impact.html) Furthermore LRO’s LAMP also analysed the plume, and detected molecular hydrogen by way of sunlit fluorescence. The hydrogen content observed was much higher than that produced by dissociation of water alone, which further indicates there would have been trapped hydrogen in addition to water (Retherford et al, 2010). A mid-infrared photo of LCROSS Centaur Impact Flash. Credit: NASA, LCROSS Mission Team. 2.1.9: Moon Samples: In 2008 Saal et al published findings of examination of volcanic glasses returned by Estelle Asmodelle www.relativecosmos.com earlier NASA Apollo missions. This study found considerably more water than thought possible, providing the then best estimate of water in soil ~745 ppm, with a minimum ~260 ppm (Saal et al, 2008). Image 6 Page No: 8 were highly suggestive of water, yet were considered by many to be inconclusive (Simpson & Tyler, 1998). In contrast the Lunar Prospector epithermal fluxes at NPR ~ 4.6%, and at the SPR ~ 3.0%, indicated that deposits could be ~10300 million metric tons (Binder, 1998), other studies suggest a maximum of ~6 trillion kg (Williams, 2011). Although the Cassini finding was interesting, suggesting that amounts of water present could be ~ 10 to 1000 ppm, but suspiciously the results were not published until the Chandrayaan-1 results were published in 2009 (Clark, 2009). This is photo of super-tiny melt inclusions in lunar soil, revealing the magnitude of water inside the moon. Credit: Saal lab/Brown University. Others have criticized this analysis based on isotopes in the samples, concluding the Moon is anhydrous, or waterless (Sharp, 2010). However more recent studies from Hauri et al. found that the lunar magma may contain 100 times higher water content than previous thought. Additionally, the results are inconsistent with the current formation model of the Moon. The lunar melt inclusions seem to contain ~6151410 ppm water, which is similar to primitive terrestrial mid-ocean ridge basalts mantle (Hauri et al, 2011). 3.0: Conclusion: On the Apollo landers, SIDE with its ~ 48.6 eV detection, was a mass analyser and not a mass spectrometer (Cadogan, 1981), and so these detections are tenuous at best. Furthermore the BRE scanning area was not situated around the poles (Bahar et al, 1994) but equatorially. The Clementine probe results of the NPR~7,500 km2, and the SPR ~6,500 km2 A definitive result was obtained by Chandrayaan-1 with direct detection of absorption features ~2.83.0 μm on the lunar surface, indicative of hydroxyl, confirming the Cassini finding. The early LRO’s results were promising but again not definitive. LRO is still an active mission and continues to provide data, and has indirectly provided fairly conclusive evidence via LCROSS. The LCROSS impact data measured the total water ice vapour, ~155 ± 12 kg, being ~5.6 ± 2.9% of water ice in the regolith (Colaprete et al, 2010). These data were confirmed by several observation platforms and so seems the most reliable thus far. Furthermore, the recent results in 2012 with LRO are fairly substantial. Lastly, the Hauri et al. studies have raised estimates of water in soil even higher, from the initial ~260745 ppm, to ~6151410 ppm (Hauri et al, 2011), indicating that parts of the lunar interior may contain as much water as the Earth’s mantle. Clearly however the controversy continues with recent peer reviewed material making claims, as did the scientists of the 1970s, Estelle Asmodelle www.relativecosmos.com that the Moon is rocky and waterless (Laursen, 2010). Any individual study has provided inconclusive results, but taken collectively Page No: 9 there is a conclusive result: there does indeed appear to be water on the Moon. However estimates vary greatly and so more data is required to determine the exact amount of water present. _____________________________________________________________________________________________ AUTHOR: Estelle Asmodelle is a computer scientist and a student in Astronomy with the University of Central Lancashire, in the UK. Estelle also writes articles in astrophysics, is an associate member of the Institute of Physics (UK), and a member of the Australian Society for General Relativity & Gravitation. She also runs a blog on astrophysics at www.relativecosmos.com _____________________________________________________________________________________________ 3.0: Bibliography: Anders, E. (1970). Science, 169, 1309-1310. Bahar et al. (1994). IEEE Transactions on Geoscience and Remote Sensing, 32(1), 72-77. Beals et al. (1956). Canadian Scientists Report, Part 1, 50, 203. Binder, A. B. (1998). Science, 281, 1475-1476. Cadogan, P. H. (1981). The Moon Our Sister Planet. 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