PROPOSAL for research project “Lunar Volatiles” in the International Space Science Institute for the call of 2011 1. Scientific rationale, goals and timeliness of the project 1.1. Rationale of the project We propose the research project “Lunar Volatiles” to address to the recent findings and current development of a new field of lunar science, concerning the origin, transport and accumulation of volatiles at lunar poles. The first suggestion of water ice in polar craters was due to observations of the bistatic radar on board Clementine [1], the first mission of the new period of lunar exploration began in the 90’s and continuing now. However, this result was not supported by subsequent high-resolution Earth-based radar measurements [2]. Evidence for the presence of water ice in polar regolith was provided by the Lunar Prospector Neutron Spectrometer (LPNS), in the form of extended suppression of neutron emissions around both lunar poles [3]. The first direct detection of polar enhancement of H2O and/or OH in the regolith was performed by means of the M3 hyper-spectral IR mapping spectrometer onboard the ISRO Chandrayaan-1 mission [4]; however such IR data characterize only the uppermost few micrometers of the surface. The latest proof for the presence of localized areas with a relatively high content of water and other volatiles at the lunar poles has been recently provided by the remote sensing measurements of NASA’s LRO and LCROSS missions: the orbital neutron spectrometer telescope LEND identified the polar crater Cabeus, as the most promising LCROSS impact site with an anomalously high content of hydrogen [5], and instruments onboard LRO and LCROSS measured direct signatures of water, H2 and another volatiles in the plume material ejected by the LCROSS artificial impact event [6, 7]. The average quantity of water in the regolith of Cabeus was estimated to be about 0.5 – 4.0 % by weight [5], substantially higher than previously measured amounts of < 0.1 % in regolith at moderate latitudes. These discoveries highlight the exploration of a New Moon at the poles, which is very much different from the Moon studied by Apollo and Lunas. 1.2. Goals of proposed project 1.2.1. GOAL I: to study the origin of Neutron Suppression Regions (NSRs), as local spots with high content of hydrogen/water. Until now, comets were considered as the main source of volatiles on the Moon. Permanently shadowed regions (PSRs) at polar craters were thought to be cold traps for condensation of water molecules in transient atmospheres resulting from cometary impacts. Currently available LEND neutron data has allowed identification of several local areas around both lunar poles which most plausibly display high Hydrogen content of several % of WEH (Water-Equivalent Hydrogen) within a 1 meter thick layer of the regolith. They are defined [5] as the local Neutron Suppression Regions, or NSRs. Among them, the strongest suppression effect of epithermal neutrons is associated with the NSR centered on Cabeus [5]. This NSR has a total area of about 700 km2. The average enhancement of Hydrogen in this NSR is about 360 ppm in comparison with the local vicinity. The northern part of this NSR, with an area of about 300 km2, lies in the PSR within Cabeus; the surface of this region is never illuminated by direct sunlight, and its temperature is always below 100 K [8]. However, the southern part of the Cabeus NSR, with an area of about 400 km2, is sporadically illuminated during the lunar polar day, when the surface temperatures increases well above 100 K, suggesting that any water ice should sublime from the upper most layer of the regolith. 2 Another well-observed NSR with an area about 1500 km2 was detected within another polar crater known as Shoemaker. Its boundary coincides well with the outer contour of the PSR in the bottom of this crater [9]. The average enhancement of Hydrogen within the NSR is about 190 ppm in comparison with the vicinity. The third example of a well-defined NSR is located at the northern polar crater Rozhdestvesnky [9]. The LEND-based NSR (with a total area of about 240 km2) also coincides with the PSR within this crater. The average enhancement of Hydrogen in this PSR is about 330 ppm. The surfaces of the NSRs in Shoemaker and Rozhdestvensky are permanently cold, and offering nearly ideal conditions for permanent storage of frozen water in the regolith. On the other hand, there are many similar other craters at both the south and north poles, which also have associated PSRs, but do not exhibit any effect of stronger neutron suppression in comparison with illuminated surfaces at the same latitude. GOAL I is to understand the origin of Neutron Suppression Regions at lunar poles with local enhancement of hydrogen in the shallow subsurface, and their relationship to coincident PSRs and daily illuminated regions. 1.2.2. GOAL II: to study physics of volatiles at lunar poles. There is no simple correspondence between NSRs and PSRs. Moreover, the majority of PSRs do not manifest any stronger suppression of neutron emission in comparison with daily illuminated surface at the same latitude [5, 9]. Therefore, the simple picture of water cold-trapping at PSRs is insufficient to explain the observations. Another mechanism is required to understand the origin, transport and accumulation of volatiles at particular polar regions, presently detected as NSRs. In addition to comets, another process should be considered for delivery of hydrogen from space to the Moon – the solar wind. It is known that protons, as ions of hydrogen, are continuously implanted in the regolith. This protons can combine with oxygen to produce OH [10]. This process is thought to take place everywhere on the Moon, when the surface is exposed to the solar wind. The protons implantation rate is much larger at equatorial and moderate latitudes than at the poles due to geometry. However, volatile molecules must travel a large distance over the Moon, if they were produced at moderate latitudes and accumulated at the poles. Large fraction of them would escape back to space from the exosphere, and in the case of light molecules of H2 one should not expect any substantial accumulation from moderate latitudes to the poles. We seek to study the mechanisms of local production of water molecules, which take place at polar slopes with high incident flux of solar irradiation – water molecules from such natural chemical plants could be locally accumulated in the nearest vicinity. The physics of diffusion of water molecules in the regolith [11] should be considered in order to understand the origin of NSRs. Models [12] and laboratory data [13] for water ice condensation, migration and sublimation from the Ice-Lab Facility at the California Institute of Technology will be used to implement this Goal, along with analysis of space data from Chandrayaan-1, Lunar Prospector, LRO and LCROSS. GOAL II is to consider the physics of newly discovered NSRs, as local lunar environments with concentration of hydrogen-bearing volatiles, and to study the mechanisms of their production, migration and accumulation. 1.2.3. GOAL III: to perform comparable studies of volatiles at the Moon, Mercury, Phobos and comets. The Moon is not the only place in the inner solar system, where volatiles may be produced and accumulated at the surface. Another example is the Mercury. Radio data for this planet show anomalous reflections from polar craters, which are likely associated with layers of water ice in the subsurface [14]. This planet also collides with comets and interacts with the solar wind, both processes on Mercury could be much stronger than on the Moon. Therefore, we seek to perform a study comparing the physical mechanisms on Mercury and the Moon that are responsible for production, migration and accumulation of volatiles at local regions around poles. The analysis of expected data from MESSENGER [15] will constitute an important part of the proposed study for understanding the difference and/or similarity between the physics at poles of the Mercury and at the Moon. 3 Phobos is the small moon of Mars, whose regolith is also exposed to solar wind. The data from the Phobos Sample Return mission (PhSR) [16] will allow comparison of the physics of volatiles on that body with similar processes at lunar poles. The difference in the physical conditions of the two moons will help discriminate among major processes of production, transport and leakage of volatiles in space-looking regolith. Finally, the surface of a comet represents another environment, where processes of interaction between space and regolith could be similar, but much more active than on the Moon and Mercury. At far distances from the Sun, the cold surface of a comet may accumulate volatiles from space irradiation and/or due to slow diffusion from the interior. At small solar distances, where the surface of a comet is irradiated and heated, volatiles should be actively evaporated from the subsurface regolith. Data from ESA’a mission Rosetta [17] will probe in situ the physics of volatiles at the space-exposed surface of the comet, which is the natural traveler from the outer to inner parts of the solar system. If comets are indeed the source of volatiles of the Moon (and of Mercury), their composition may be obtained from the Rosetta data and compared with composition of volatiles at polar deposits near the lunar poles. GOAL III is to perform comparable studies of volatiles on Moon, Mercury, Phobos and comets to understand the origin and evolution of volatiles on the airless celestial bodies in the inner solar system. 1.3. Timeliness of proposed project and strategy of implementation For each of the three goals enumerated, the joint work will focus on open questions of the current state of the field, on problems of data analysis and interpretation, and on the potential directions of further development.