SCIENCE CHINA Physics, Mechanics & Astronomy • Research Paper • December 2010 Vol.53 No.12: 2172–2178 Integrated Lunar Scientific Results Obtained by Chang’E-1 doi: 10.1007/s11433-010-4159-y Research on water ice content in Cabeus crater using the data from the microwave radiometer onboard Chang’e-1 satellite MENG ZhiGuo1, CHEN ShengBo1*, Edward Matthew OSEI Jnr2, WANG ZiJun1 & CUI TengFei1 1 College of Geoexploration Science and Technology, Jilin University, Changchun 130026, China; 2 Department of Geomatic Engineering, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana Received May 20, 2010; accepted July 30, 2010; published online October 21, 2010 The existence, formation and content of water ice in the lunar permanently shaded region is one of the important questions for the current Moon study. On October 9, 2009, the LCROSS mission spacecraft impacted the Moon, and the initial result veri- fied the existence of water on the Moon. But the study on formation and content of water ice is still under debate. The exis- tence of water ice can change the dielectric constants of the lunar regolith, and a microwave radiometer is most sensitive to the dielectric parameters. Based on this, in this paper, the radiation transfer model is improved according to the simulation results in high frequency. Then the mixture dielectric constant models, including Odelevsky model, Wagner and landau-Lifshitz model, Clau-sius model, Gruggeman-Hanai model, etc., are analyzed and compared. The analyzing results indicate that the biggest difference occurs between Lichtenecker model and the improved Dobson model. The values estimated by refractive model are the second biggest in all the models. And the results from Odelevsky model, strong fluctuation model, Wagner and Landau –Lifshitz model, Clausius model and Bruggeman-Hanai model are very near to each other. Thereafter, the relation be- tween volume water ice content and microwave brightness temperature is constructed with Odelevsky mixing dielectric model and the improved radiative transfer simulation, and the volume water ice content in Cabeus crater is retrieved with the data from microwave radiometer onboard Chang’e-1 satellite. The results present that the improved radiative transfer model is proper for the brightness temperature simulation of the one infinite regolith layer in high frequency. The brightness tempera- ture in Cabeus crater is 69.93 K (37 GHz), and the corresponding volume water ice content is about 2.8%. Cabeus crater, CELMS, radiation transfer simulation, mixture dielectric constant model, water ice content PACS: 98.70.Sa, 90.50.sb, 13.85.Tp 1 Introduction these problems in the current lunar research [1]. Special sensors have been equipped in the Chandrayaan-1 orbiter Interest in the Moon started to increase at the beginning of (India) and Lunar Reconnaissance Orbiter (USA) to look for the 21st century as Selene (Japan), Chang’e-1 (China), water ice in the permanently shaded regions. Chandrayaan-1 (India) and Lunar Reconnaissance Orbiter The possible existence of water ice at the lunar poles was (USA) have successfully accomplished their missions. firstly suggested by Watson [2]. Watson and Arnold found However, there are many problems left for us to solve ur- that temperatures within permanently shaded regions were gently. The existence, the formation and the content of wa- sufficiently low that water delivered to the Moon by mete- ter ice in the lunar pole area is an important one among oroids and comets could be preserved as the water ice for the lifetime of the Moon [2,3]. The Clementine bistatic ra- dar data indicated that the water ice could be presented at *Corresponding author (email: [email protected]) least at the lunar south pole [4]. Subsequent studies sup- © Science China Press and Springer-Verlag Berlin Heidelberg 2010 phys.scichina.com www.springerlink.com MENG ZhiGuo, et al. Sci China Phys Mech Astron December (2010) Vol. 53 No. 12 2173 ported this conclusion and concluded the formation and the model (two-layer model), which is also employed in this range of the water ice [5–7]. In April of 1994, the 2.273 study. The study results by Shkuratov [25], Jin [26], Meng GHz bistatic radar onboard Clementine satellite collected [27] and Wang [23] indicate that the complete lunar regolith the data that showed apparently strong ice signal, and this layer model should consider the temperature profile, dielec- provided the direct evidence of the water ice in the pole tric constant distribution, effective surface reflectivity and regions for the first time [4]. But a reanalysis of the slope of the regolith-rock interface (Figure 1). Clementine data have raised questions regarding this con- Based on the complete regolith layer model and two- clusion and thought that only the radar data was not enough stream approximation principle [26,28], the total microwave to extract the information about water ice. [8] A later analy- brightness temperature TB, detected by the microwave sen- sis of Lunar Prospector (LP) neutron data, γ spectrometer sor with the observation angle θ0, includes two parts. One data and radar data found abundant hydrogen existed at the part, TB1, is the radiation from the regolith layer, and the permanently shadowed regions in both poles of the Moon, other, TB2, is from the rock layer. TB1 also comprises its up- the content was about 0.3%–1% and preserved as the pure welling radiation, T1up, and the downwelling one, T1dn. Then ice in the lunar regolith [9–10]. However, Nozettel and the radiation transfer equation can be write as: Hodges thought that the signature is not from the water ice TTT= +=++ T T T. (1) but from H based on the theoretical modeling[11,12]. BB1211 B updnup 2 Campbell also concluded that there was no water ice at the Here lunar south pole using earth-based radar data [13]. Crider z and Vondrak presented that the hydrogen atoms also has the d ′′ −∫ kza11()secdθ z 1− rp1 same radar features as the water ice by simulation [14]. TkzTzz= () ()seceθ 0 d, (2a) th 111up∫ 1− L a On 9 October 2009, the spacecraft of the LCROSS mis- 0 sion impacted the Moon, which verified the existence of water ice on the Moon [15,16]. But the study on the forma- d (1− rr ) Tk= pp12 ()zT ()seczθ tion and the content is still in debate. The existence of water 111dn ∫ 1− L a ice can change the dielectric constant of the lunar regolith 0 dd [17–20]. Based on this, Meng [17] evaluates the water ice ′′′′ −++(∫∫kzaxa11 ( )sec(θθ )d z kz 11 ( )secθ d z ) content in the lunar pole regions with the Clementine UVVIS × edz 0 z,(2b) data and the improved Dobson model. On the other hand, the passive microwave can penetrate several meters in dry z (1−−rr )(1 ) Tk= pp12()zT ()seczθ medium and it is the best tool to get the dielectric informa- 222up ∫ 1− L a tion of the medium [21–23]. Onboard Chinese Chang’e-1 d zd lunar exploration orbiter, the microwave radiometer has ′′ ′′ −−∫∫kzaa22()secdθθ z kz11 ()secd z successfully collected the lunar microwave brightness tem- ×⋅ede,d z 0 (2c) perature [24]. The initial results indicate that there are sev- eral abnormal regions in the lunar pole region, which are where d is the thickness of the lunar regolith. The oblique of corresponding to the permanently shaded regions [25]. the lower interface is considered. And θ1 and θ2 are the in- The previous researches indicate that the extraction of cident angles of the regolith and the rock, respectively. p = the water ice information using microwave radiometer data h, v, which denotes the horizontal polarization and the ver- is feasible. Therefore, in this paper, the passive microwave tical one. ka1(z) is the absorption coefficient of the regolith radiation transfer equation is employed and improved based in the depth z and ka2(z) is the rock’s, which are the func- on the numerical simulation in high frequency. The regolith tions of the dielectric constants of the regolith and the rock. in Cabeus crater is seen as the mixture of regolith and ice rp1 is the effective reflectivity of the free space – regolith and the mixture dielectric constant models are compared to interface and rp2 is that of the regolith-rock interface establish the relation between the volume water ice content and the effective dielectric constant. Finally, the volume water ice content is retrieved based on the improved radia- tion transfer simulation, the selected mixture dielectric con- stant model and the brightness temperature data from mi- crowave radiometer onboard Chang’e-1 satellite. 2 Improved radiation transfer simulation 2.1 Radiation transfer simulation Figure 1 Passive microwave radiation transfer simulation in the lunar The lunar regolith model usually used is regolith-rock mod- regolith. 2174 MENG ZhiGuo, et al. Sci China Phys Mech Astron December (2010) Vol. 53 No. 12 They are the functions of the dielectric constants of the free the surface temperature and the deep temperature are 71.28 d and 134 K, respectively. Figure 2 shows that the difference −2()secd∫ kza11θ z between the two types of regolith-rock models is great if the space, the regolith and the rock. L = e 0 , and lunar regolith thickness is small. But this difference is up to 1/(1−L) is multi-reflection coefficient. T(z) is the tempera- zero if the regolith thickness is more than 3.6 m. The aver- ture profile in the regolith. age thickness of the regolith layer is about 4–5 m for maria and about 10–15 m for highlands [33]. Almost all perma- 2.2 Improved radiation transfer simulation nently shaded regions are located in highlands.