Lunar and Planetary Science XXIX 1458.pdf

ON THE SPECTRAL SIMILARITIES OF 9 AND SOME LUNAR MARE AREAS. V.V. Busarev, Lunar and Planetary Department, Sternberg State Astronomical Institute, Moscow Univer- sity, Moscow 119899, Russia. E-mail: [email protected]

Visible-range reflectance spectra of the S-type [5]. As was demonstrated, there are weak ab- 9 Metis and telescopic reflectance spec- sorption bands at .5, .6, and .7 mm in the telescopic tra of some 13-15 km (diameter) mare areas on spectra of some lunar mare plots under 10-20 km the lunar surface show similar weak (3-6%) ab- spatial resolution which may be explained by an sorption features at .49-.50 mm, .62-.63 mm, and .7 increased content of titanium in ilmenite, pyrox- mm. The lunar mare areas are found in enes, and their products in lunar mare regolith [6, Procellarum, in the basalt unit with medium (2- 7]. The reflectance spectra of lunar mare plots 3%) TiO2 content [1]. with 13-15 km-diameter in Oceanus Procellarum The observations of 9 Metis were made in (curves 1 and 2, Fig. 2) contain weak absorption September 1993 at asteroid phase angle of about bands at .5 mm (ab. 2%, .03 width), .63 mm (ab. 3- 5° with a scanning spectrophotometer mounted on 4%, .03-.04 mm-width), and .7 mm (ab. 3-6%, .03- the 1.25 m telescope in the Crimea. Observations .06 mm-width). Due to the similarity of the ab- of three mare areas in Oceanus Procellarum (east sorption features in parameters on the reflectance of the crater Hermann: j = -1.5°, l = - 51.0°; spectra of 9 Metis and the lunar mare areas they north-east of the crater Flamsteed P: j = -1.7°, l may be created by the same processes. The ab- = -42.1°; north-east of the center Flamsteed P, sorption band at .49-.50 mm could arise under 2 2 2 2 3+ inside: j = 2.5°, l = -43.0°) with a spatial resolu- electron transitions T2( D) ® E( D) in Ti (M1) tion of 13-15 km on the lunar surface were carried [8]. The peculiarity of the band in the reflectance out with the same spectrophotometer mounted on spectra of lunar mare areas is that it is often the .6 m telescope in the same observatory. at a overlaped by a wider one found in ilmenite and lunar phase angle of about 11°. As standards, the titanium pyroxenes and produced by Fe2+® Ti4+ F5-type star, HD334 (according to [2]) for 9 electron transitions [9]. This is better seen on Metis, and the A0-type star, aLyr ([3]) for the curve 3 in Fig. 2 which corresponds to the inner , were used for the observations. For calcu- part of the Flamsteed P region with increased lations the reflectance spectra solar data by Ma- TiO2 content (ab. 7 wt % [1]). The absorption karova et al. [4] were also taken. The spectra feature at .62-.63 mm could be created by Ti3+® 4+ having a resolution of .0048 mm were smoothed Ti electron transitions [10], although in the case 2+ with a .015 mm running box average. The reflec- of 9 Metis the band might arise due to Fe (M2) 3+ tance spectra of 9 Metis (Fig. 1) have a final ac- ® Fe (T) transitions [9], too. Possibly, it is con- curacy of about 2-3% within .41-.68 mm, which firmed by a slightly short-wavelength position (.62 increases to 18% at the blue and 12% at the red mm) of the band in the spectra of 9 Metis (see Fig. ends of the spectra. The spectral curves of the 1 and 2). Finally, the absorption feature at .7 mm 0 4+ 2 3+ mare areas (Fig. 2) have a final accuracy of about could be created by d (Ti ) T2(Ti ) ® .5-1% within .38-.69 mm, which increases to 4% d0(Ti4+)2E(Ti3+) electron transitions [10]. The re- at the blue and 3% at the red ends of the spectra. flectance spectra 1 and 2 (Fig. 2) represent the The reflectance spectra of 9 Metis have weak basaltic formation with medium TiO2 content, absorption bands slightly changing with asteroid about 3% [1]. If we compare them with the aster- rotation at .49 mm (ab. 2-3%, .02 mm-width), .62 oid spectra and assume that the relationship be- mm (ab. 2%, .03 mm), and .7 mm (ab. 5-6%, .04- tween wt % TiO2 in lunar mare soil and the .05 mm-width) (Fig. 1). It seems the spectral fea- .40/.50 mm reflectance ratio for telescopic spectra tures were registered on a spectrum of the aster- (after [9]) is true for the regolith of 9Metis we oid obtained earlier at a lower spectral resolution could estimate the supposed TiO2 content on the Lunar and Planetary Science XXIX 1458.pdf

SPECTRAL SIMILARITY OF 9 METIS: V.V. Busarev

asteroid. The values would be about 3 wt % from the curve 1 and 1.8 wt % from curve 2 (Fig. 1). However, the values may be considered as very approximate. The relationship by Charett et al. [11] works only for mature Ti-rich soils containing abundant opaque agglutinates (up to 60-70%). It should be cleared up at first as far as the regolith on 9 Metis meets the requirements. It would be interesting to discuss how Ti- content materials e.g., such as titanium pyroxenes, could arise on an asteroid in the main belt. This is a problem for the moon, too. Nobody yet knows exactly why Ti-rich rocks are widespread on the moon, while, at the same time, they are relatively Figure 2. Normalized (to unity at 0.56 mm) and rare on the earth’s surface. 9 Metis might have scaled reflectance spectra of lunar mare areas been a part of a large parent planetesimal, which with spatial resolution 13-15 km in Oceanus Pro- was heated up to a high and then, cellarum: 1 – east of crater Hermann (j = -1.5°, after cooling and solidifying, split off after some l = - 51.0°); 2 - north-east of the crater Flam- collisions. Probably, such a scenario is confirmed steed P ( j = -1.7°, l = -42.1); 3 - north-east of by the discover of a small satellite near the aster- the center of crater Flamsteed P, inside: j = 2.5°, oid [12] which may be a remnant of the planete- l = -43.0°) simal. Obviously, new investigations of the aster- oid should be made for checking of the conclu- REFERENCES sions. [1] Pieters, C.M. (1978), Proc. Lunar . Sci. Conf. 9th, 2825-2849. [2] Biryukov, V.V. et al. (1998), Astron. & Astrophys. Transactions (UK), in press. [3] Hayes, D.S. (1985), Calibra- tion of Fundamental Stelar Quantities, IAU Symp. No. 111 (Eds D.S. Hayes et al.), Dor- drecht: Reidel, 225-252. [4] Makarova, E.A. et al. (1991), Flux of Solar Radiation, Moscow, Nauka Press (in Russian). [5] Chapman, C.R., and M.J. Gaffey (1979), (eds T. Ge- hrels, and M.S. Mattews), Tucson, Univ. of Ari- zona Press, 655-687. [6] Busarev, V.V. (1994), Astron. Vestn. 28, No.2, 47-58 (in Russian). [7] Ksanfomality, L.V. et al. (1994), Astron. Vestn. Figure 1. Normalized (to unity at 0.56 mm) and 28, No.6, 49-69 (in Russian). [8] Burns, R.G. et scaled reflectance spectra of 9 Metis. The rec- rd ords corresponding to spectra 1 and 2 were ob- al. (1972), Proc. 3 Lunar Sci. Conf., 533-543. tained at about 1.5 hour-interval, which spread [9] Khomenko, V.M., and A.N. Platonov (1987), over 1/3 of the asteroid’s rotational period. Rock Forming Pyroxenes: Optical Spectra, Colouring, and Pleochroism, Kiev, Naukova Dumka Press (in Russian). [10] Strens R.G.J. et al. (1982), Adv. Phys. Geochem. 2, 327-346. [11] Charette, M.P. et al. (1974), JGR 79, 1605-1613. [12] Sichao, W. et al. (1981), Icarus 46, 285.