Science Summary of Kaguya Mission

Home , Anders (crater), Dirichlet (crater)

Trans. JSASS Aerospace Tech. Japan Vol. 10, No. ists28, pp. Tk_1-Tk_5, 2012

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By Manabu KATO, Susumu SASAKI, Yoshisada TAKIZAWA, and the Kaguya Team

The Institute of Space and Astronautical Science/ Japan Aerospace Exploration Agency, Sagamihara, Japan

(Received June 27th, 2011)

The Kaguya mission completed by collision of the Kaguya spacecraft on targeted place of the Moon on June 11, 2009. The Kaguya science team makes endeavors to archive data for data opening to public, and to study using the Kaguya data. Although data analysis and science study are ongoing, scientific achievements obtained so far are summarized concerning with the origin and evolution of the Moon as a ultimate science target of Kaguya mission: Ubiquitous identification of pure anorthosite in outcrops of central peaks of large craters by Multi-band Imager (MI) and Spectral Profiler (SP), discovery of multi reflectors of radio waves under large mares and ocean in the nearside by Lunar Radar Sounder (LRS), confirmation of free-air gravity anomaly in the whole Moon and identification of farside anomaly different from nearside one of mass concentration by Relay Satellite Transponder (RSAT), confirmation of lunar global topography by Lunar Altimeter (LALT), re-estimation of crustal thickness by Kaguya data of gravity and topography, re-estimation of formation ages of farside mares by crater counting using high resolution images of Terrain Camera (TC), confirmation of magnetic anomalies and mini-magnetosphere by Lunar Magnetometer (LMAG) and Plasma Angle Composition Experiment (PACE), reconfirmation of global distribution of radio-active elements K, U and Th by Gamma-ray Spectrometer (GRS), and discovery of solar wind proton reflection and access into lunar wake by PACE.

Key Words: Kaguya Mission, SELENE Mission, Lunar Science, Lunar Exploration, Remote-Sensing

respectively. MI and SP determined mineralogical 1. Introduction composition of lunar surface by VIS-NIR range spectroscopy. TC, LRS, and LALT measured lunar topography of surface The Kaguya spacecraft was launched on September 14, and subsurface up to about 5 km depth by stereo camera, MHz 2007 from the Tanegashima Space Center TNSC, and inserted radar wave, and laser altimeter. RSAT and VRAD were into lunar polar orbit on October 4. After deployment of two determine precise gravitational fields of whole Moon expandable antenna for radar sounder and mast for magnetic including farside’s never measured so far by relay transponder field measurement, and checking performance of scientific and radio wave source. LMAG measured weak magnetic field instruments, science observations were carried out for twenty distribution of nano Tesla after magnetic cleaning of one months of nominal and extended mission periods. On spacecraft4). UPI primarily observed terrestrial plasmasphere June 11, 2009 the main orbiter impacted on the lunar surface to study aurora, and/or oxygen escape phenomena by of 65.5S/80.4E, crater rim of Gill-B, and its mission was monitoring whole Earth from lunar orbit. HDTV was onboard terminated. One of two sub-satellites, relay satellite Okina to public outreach by broadcasting high definition TV movies collided on lunar farside on February 12, 2009, and ended of Earthrises and lunar surface. gravity measurement by four-way Doppler method, resulting The XRS and the CPS received critical radiation damages in termination of command transmission for another on sensors resulting in failure to get useful data. Other subsatellite Ouna. The Kaguya team continues to archive data instruments were very healthy and succeed to acquire lunar amounting to ten terabytes to release to public and studying data with high quality and quantity never collected so far. the lunar science theme previously allocated in the beginning 1-3) of the mission . Table 1. Science Instruments and Abbreviations. X-ray Spectrometer XRS 2. Science Instruments Gamma-ray Spectrometer GRS Charged Particle Spectrometer CPS Scientific instruments onboard Kaguya and their Multi-band Imager MI abbreviations are summarized in Table 1. Their detailed Spectral Profiler SP specifications are referred in Ref.3. The instruments are Terrain Camera TC categorized into six purposes and plural ones are Lunar Radar Sounder LRS complementally employed using their merits. XRS and GRS Lunar Altimeter LALT determined elemental compositions on lunar surface with Relay Satellite Transponder RSAT different spatial and energy resolutions. CPS and PACE were VLBI Radio Source VRAD Lunar Magnetometer LMAG used to observe plasma particles irradiated on the Moon of Plasma Angle Composition Experiment PACE high energy cosmic-ray and low energy solar wind,

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Radio Science RS and agglutinate15). The study of SPA lithology by MI is also in Upper Atmosphere Imager UPI progress16). High Definition Television System HDTV In order to estimate the size of the lunar core, gravity-field measurements are used to determine the polar moment of 3. Science Achievements of Science of the Moon inertia of the Moon. The data obtained by the Lunar Prospector were used to determine the iron core radius, which 3.1. Science targets was estimated to be 220 to 450 km17). Kaguya measured the Lunar studies have advanced with the integration of gravitational anomaly of the whole Moon, using a four-way scientific data from various categories. The intention, current Doppler technique employing the relay sub-satellite Okina, to results, and perspectives from all categories are described in track the Kaguya spacecraft flown in the lunar farside. The following subsections: gravity free-air anomaly of multi-ring type in the farside can ・Chemical constituents of the Moon be compared with the mass-concentration type anomaly in the ・Interior structure of the Moon nearside18). An error of low-degree spherical harmonic ・Dichotomy of nearside and farside of the Moon coefficients which contribute to the polar moment of inertia ・ Differentiation in the magma ocean and the k2 Love number was also reduced in the Kaguya ・Origin of the lunar magnetic field gravity model SGM100h comparing with the Lunar ・Lunar tectonics. Prospector model LP100K. The differential VLBI technique These science targets will be integrated to study the origin by VRAD can be employed to refine further the coefficients and evolution of the Moon. by precise orbital determination19). 3.2. Chemical constituents of the Moon The results obtained from the Kaguya mission cannot be Determining the chemical constituents of the Moon has used to determine accurately the elemental abundance of the been the first priority in studying the origin of the Moon and entire Moon. This is because the spacecraft did not carry any the chemical distribution in the inner terrestrial zone of the instruments, such as a seismometer in the Apollo missions for primordial solar system. Many model compositions have been in situ measurement of parameters that provide information 5-8) proposed since the Apollo missions . However, these regarding the lunar interior. However, the Kaguya data will compositions were estimated on the basis of geophysical and considerably improve knowledge about the chemical cosmochemical constraints; for example, global rock constituents of the Moon’s lower crust and upper mantle distributions of the lunar surface were not taken into material when they are compared to the Apollo seismological consideration. The ratio of core and mantle/crust could not be investigation results. considered because the size of iron core had not been 3.3. Interior structure of the Moon accurately estimated. As mentioned in the previous subsection, the size of the Two categories of data, namely, elemental abundance of the lunar core can be estimated by the polar moment of inertia of lunar surface (obtained by XRS and GRS) and mineralogical the Moon, which was determined from gravity-field composition (obtained by MI and SP) define the rock types measurements. In the Kaguya mission the shallow interior and and their distribution on the lunar surface. Information about subsurface structures were investigated using LRS. Radar the subsurface constituents in the lunar crust can be acquired soundings using a 5 MHz radio wave revealed the subsurface by investigating the central peaks of the craters that were layer structure, including the density and/or material formed by the rebound of impact shock during crater discontinuities, up to a depth of approximately 5 km. Initial 9) formation . Large basins such as the South Pole-Aitken (SPA) LRS data revealed the subsurface structure of the mare region basin having a diameter of 2500 km and a depth of spreading in the nearside20). approximately 10 km expose interior materials of the lower Gravity and topography data will be used to more precisely crust or extrude the upper mantle of the Moon. It is possible to estimate the thickness of the entire lunar crust. The crust in the estimate the deep crust components by remote analysis of basin areas and mares on the lunar nearside is mostly thin, and central peaks, crater walls, and outcrops in large basin crater the highland on the farside is overlaid on a thick crust. The bored to deep interior. However, small volume of the Moon Kaguya mission was successful in determining the crustal has been only accessed to obtain information about the thickness with greater accuracy21). The maximum thickness of chemical constituents of the whole Moon. lunar crust is found in the Dirichlet-Jackson crater rim of the The MI and SP determined the ubiquitous distribution of farside, where the highest altitude was evidently identified by pure anorthosite in the lower crust layers by showing the the LALT. The minimum thickness is estimated to be under global distribution of it on the central peak outcrops of large Mare Moscoviense of the farside, where an almost 0-m 10, 11) craters . The origin of pure anorthosite is still controversial, thickness of crust was recorded under lava basalt of 600 m because that returned by the Apollo missions, about 92% thickness22). Mantle uplift occurred after formation of the plagioclase and Mg-suite, was thought to have been formed impact basin. in chemical equilibrium by floating in the magma ocean of 3.4. Dichotomy of nearside and farside of the Moon 12-14) early lunar history . Therefore, further study of the The dichotomy of the Moon has been determined from the formation of pure anorthosite is necessary. The SP also topography and rock distributions in the lunar nearside and provided lithology data about the central peaks of craters in farside. Large mares occupy 60% of the lunar nearside. A the SPA basin: they are composed of orthopyroxene, olivine, large altitude difference (more than 19 km) has been observed

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between the rim of the Dirichlet-Jackson crater and the bottom Imbrium- antipodes in the SPA basin and in the of an unnamed small crater in the Antoniadi crater of the SPA Lemonosov-Fleming, the Crisium, the Moscoviense, the basin23). The dichotomy was determined by geological study Apollo basins, isolated anomalies such as the Reiner Gamma, of the material distribution and crustal thickness. The and newly identified eleven isolated anomalies from the 100 formation ages of farside mare areas were re-estimated by km altitude orbit29). The LMAG data, including low-altitude counting craters using high spatial resolution Terrain Camera measurements, are expected to reveal more details regarding images. Mare Moscoviense and the Antoniadi and Apollo anomalies and to help understand the origin of lunar magnetic craters in the SPA basin have been shown to have long- anomalies. lived volcanism to 2.5-2.6Ga compared to a previous 3.7. Lunar tectonics. estimation of 3.5 Ga24). Recently formation ages of nearside The evolution of the Moon is recorded not only in mares are also re-estimated in detailed by crater counting ubiquitous crater formation, but also in various features of the method. They ranged from 3.5 to 3.0Ga and confirm repeated subsurface structure. Volcanic activities to about 3.0 billion eruption of magma25). years after the origin of 4.6 billion ago should be preserved in A large positive gravitational anomaly can be seen in large mares and basins. LRS has provided evidence of nearside mare and basins. A circular, positive, flat 300 to 500 intermittent magma eruption, and detailed tectonic feature of mGal of free-air anomaly reflects spreading basalt mares. stratification in the southern Mare Serenitatis20). Undulating Co-axial multi-ring type anomalies repeating positive and strata shown there also indicate ridge formation by horizontal negative values analyzed by RSAT are preserved in the shortening due to global cooling 2.84 billion years ago formation of multi-ring craters distributed in the farside. 3.8. Origin and evolution of the Moon GRS with high-energy resolution determined quantitatively The ultimate objective of lunar studies is to determine the the distribution of radioactive elements K, U, and Th, which origin and evolution of the Moon. Advanced scientific studies are incompatible elements partitioned into mare materials of such as those described in the previous subsections may lead the nearside26). to the realization of this objective. The Kaguya mission is 3.5. Differentiation in the magma ocean expected to provide new insights for lunar studies.. If the Moon did in fact originate from the formation of the magma ocean, the same evidence must be globally retained on 4. Solar Wind Interaction on the Moon the lunar surface. The rock distribution could thus be used as evidence for the differentiation of the magma ocean. The Interesting results on “Science on the Moon” have been formation of the SPA basin and large mares due to flooded obtained from PACE; these results include evidence that 0.1 magma in the nearside are the main geological events that to 1.0% of the solar wind (SW) protons are reflected back occurred after the formation of the magma ocean 4.6 billion from the lunar surface; He+, C+, O+, Na+, K+, and probable years ago. Therefore, geological recovery or reburying of the Ar+ originating from the Moon are detected by the ion mass basin and mares is necessary to reproduce the magma ocean analyzer IMA30). The perpendicular entry of SW protons into age. A detailed geological study by the Kaguya mission the near-Moon wake is studied; SW protons coming into the clarified the origin of the magma ocean. The magma ocean deepest lunar wake have been studied; IMA has directly model suggests that the Moon was formed after a giant impact. detected ions originating from the Moon in the Earth’s A short period of accretion to the Moon after the giant impact magnetosphere31). led to the heating up of its surface; this induced the formation of molten magma. The ubiquitous occurrence of anorthosite in 5. Terrestrial Plasmasphere from the Moon the central peaks of large craters is attributed to the existence of anorthosite layers under the lunar surface. Detailed UPI instrument also successfully observed plasmasphere of distribution of mafic rocks which coexisted with anorthosite in the Earth from lunar orbit32), which showed first meridian the magma ocean must be globally confirmed in the lunar images to study the dynamics of plasmasphere interacting surface. Spectrometric studies by MI, SP, and the M3 onboard with solar-terrestrial magnetic field. the Indian lunar orbiter Chandrayaan-1 are expected to provide information on the global distribution of minerals and 6. Conclusions rocks. Olivine originated from deep mantle is identified undoubtfully in rim of large crater by SP27). The Kaguya science team has archived the Kaguya data and 3.6. Origin of the lunar magnetic field has made them available to the public33); the team is involved Apollo rock samples contain magnetic minerals that in various studies in which the data are used. Although data indicate magnetization in a weak but ambient magnetic field. analysis and science study are ongoing, the major scientific A previous study28) suggested that the most probable sources achievements to date are summarized as follows: of the ancient lunar magnetic fields were (1) a former core • Identification of ubiquitous pure anorthosite in outcrops of dynamo during a high-field epoch and (2) transient magnetic central peaks of large craters by MI and SP. fields generated by the interaction of impact plasmas with the • Discovery of multiple reflectors of radio waves under large ambient field during brief periods of ejecta emplacement. mares and ocean in the nearside by LRS. Distribution of magnetic anomalies less than 1 nT were • Use of RSAT for confirmation of free-air gravity anomaly definitely confirmed in the Crisium-, the Serenitatis-, and the in the whole Moon and identification of farside anomalies that

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are different from nearside mass concentration anomalies. 8) Ringwood, A.E. and Kessen, S.E.: Basaltic magmatism and the • Confirmation of lunar global topography by LALT. bulk composition of the Moon II. Siderophile and volatile • elements in Moon, Earth and Chondrite: implications for lunar Re-estimation of crustal thickness by Kaguya data of origin, Moon 16, (1977), pp. 425-464. gravity and topography. 9) Cintala, M.J. and Grieve, R.A.F.: Scaling impact melting and • Re-estimation of the formation ages of nearside and farside crater dimensions: Implications for the lunar cratering record, mares by crater counting using high resolution images of TC. Meteorit. Planet. Sci. 33 (1998), pp. 889-912. x Confirmation of magnetic anomalies and mini 10) Ohtake, M., et al.: The global distribution of pure anorthosite on the Moon, Nature 461 (2009), pp. 236-241. magnetosphere by LMAG and PACE. 11) Matsunaga, T., et al.: Discoveries on the lithology of lunar crater • Reconfirmation of global distribution of radio-active central peaks by SELENE spectral profiler. Geophys. Res. Lett.,, elements K, U and Th by GRS. 35, L23201 (2008). • Discovery of SW proton reflection from the lunar surface, 12) Walker, D., et al.: On the chemistry of lunar samples and SW entry into lunar wake, and interaction with the Moon by achondrites. Primarymatter in the lunar highlands: a re-evaluation, in Proc. 8th Lunar Sci. Conf. (1977), pp. 2191–2213. PACE. 13) Warren, P.W.: Lunar anorthosites and the magma ocean • Confirmation of the polar illumination rate by LALT plagioclase-flotation hypothesis: importance of FeO enrichment topographic data. in the parent magma, Am. 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32) Murakami, G. et al.: First sequential images of the plasmasphere from the meridian perspective observed by KAGUYA, Earth Planets Space, 62 (2010), e9–e12. 33) Hoshino, H. et al.: Data processing at KAGUYA operation and analysis center, Space Sci. Rev. 154 (2010), pp. 317-342.

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