ANALYSIS OF EJECTA OF TYCHO USING HIGH-RESOLUTION PHOTOGRAPHS AND MULTI-SPECTRAL BAND IMAGES. N. Hirata, H. Otake, M. Ohtake and J. Haruyama,, Advanced mission researchcenter, National Space Development Agency of Japan (Sengen 2-1-1, Tsukuba, Ibaraki, 305-8505, Japan,naru@hope.tksc.nasda.go.jp).

Introduction: Impact cratering is one of the mostfundamental geologic processes of the solid bodies ofthe solar system. Understanding of its process is animportant field of planetary science [e.g. 1]. The dis-tribution and lithology of crater ejecta provide goodinformation about impact process. Terrestrial cratersare affected by subsequent erosion and other degrada-tion. Lunar craters are good targets, because they arewell-preserved in the moon environment.

In this study, we have investigated ejecta of thecrater Tycho. Since Tycho (D = 85 km) is the largestand youngest crater on the nearside of the moon [1, 2],it is a good case study target. We particularly focusedon the nature of impact melt. Impact melt can be easilyrecognized on remote sensing images, and its genera-tion and transportation occupy a principal position instudy of impact cratering. We use high-resolutionphotographs of Lunar Orbiter and multi-band images ofClementine UVVIS camera. Lunar Orbiter photographsare suitable for photogeologic analyses, and Clementi-ne images provide geochemical and mineralogical in-formation.

Photogeologic analysis: Tycho ejecta field is char-acterized by an inner hummocky zone and an outerradially zone (Fig. 1). The inner hummocky zone ex-tends much to north-south direction. In geologic mapbased on Lunar Orbiter photographs, the inner zone isdivided into two units: blocky rim material (Ccrb), andhummocky rim material (Ccrh) [3]. The innermostblocky rim region is characterized by small angularhummocks about 1/2 km diameter. It develops adjacentto the north rim of Tycho, and its maximum extentfrom the rim is about 20 km. While hummocky rimejecta also has rough and knobby surface, hummocksin this region are larger than those in blocky rim mate-rial zone.

The radially zone material accounts for much ofejecta of Tycho. It has the continuous surface with ropyand braided texture radial from Tycho. In the geologicmap, the materials of radially zone (categorized as ra-dial rim material; Ccrr) extend about one crater di-ameter.

Many massive deposits of impact melt are seenwithin and around Tycho in forms of melt pool, flowand veneer [4]. The major clusters of melt pools are onthe eastern side of Tycho (Fig. 1). The pools in thisarea is about 3 km across. Minor pools can also be seenaround Tycho. High-resolution photographs of Lunar

Orbiter V show detailed structures of melt deposits inthe north region of Tycho. Small (< 1 km) melt poolsare seen both in the inner hummocky zone and in theouter radially zone. Many melt flows and veneers arealso observed in the northern region.

Multi-band image analysis: Large and fresh Co-pernican-age craters in lunar highland including Tychooften have dark rings in their surroundings [5, 6].Multi-spectral images and Earth-based spectroscopicobservations show that dark ring materials have spec-tral properties with low 750 nm albedo, high 750/415nm ratio, steep NIR continuum and weak or no bandminimum near 1000 nm [5-8]. It is argued that the darkring material contains glassy materials. Laboratoryresults showed that the 750/415 nm ratios of fusedglasses of lunar samples are lower than that of theircrystalline parents [9, 10].

Clementine UVVIS multi-band image shows theasymmetric and complex distribution of the dark ringmaterial around Tycho (Fig. 2). Comparing with LunarOrbiter photographs, the distribution of the dark ringcorresponds to that of the radial rim material (Ccrr) andthe hummocky rim material (Ccrh). These ejecta mate-rials contain much glassy materials. The 750/415 ratiois low just on the crater rim, and rise to the maximumzone apart from the rim 5-30 km. The dark ring ex-tends to about 0.8 crater diameter from the rim andfades into the background terrain. In the north andnorthwest region of Tycho, the dark ring is most devel-oped, and the 750/415 nm ratio is highest. At adjacentof the north rim, however, there is a zone with quitelow 750/415 nm ratio corresponding to the field of theblocky rim material.

The large melt pools on the east rim can be recog-nized in the Clementine image as bright patches withlow 750/415 nm ratio. The spectral properties of themassive melt deposits are not different from those ofcrystalline materials within the crater.Ejecta interpretation and cratering process: On thebasis of the analysis of Lunar Orbiter photographs andClementine images, we have reorganized Tycho ejectato three categories:

(1) Glassy ejecta contains a lot of impact melt, so itcan be recognized as the dark ring. Its surface texture ishummocky or radial.

(2) Non-glassy ejecta is seen only in the north rim,corresponding to the blocky rim material.

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ANALYSIS OF TYCHO EJECTA: N. Hirata et al.

(3) Massive melt deposits are made of crystallinemelt.

Two types of ejecta containing impact melt, theglassy ejecta and the massive melt deposits, have manydifferences: material type, the maximum distance fromthe crater, and the asymmetrical distribution. They areprobably ejected and deposited by different processes.A model based on experiments and theoretical calcula-tions had been developed for transportation of impactmelts [11, 12]. Our observations are consistent with themodel.

During the excavation stage of cratering process,the upper 1/3 materials in the transient cavity are exca-vated and ejected, and the rest are displaced downwardinto the floor [1]. The excavated portion of the impactmelt is dispersed ballistically with other less shockedmaterials. They are cooled during their trajectories, andwhen the melt falling back, are mixed with cold localmaterials and quenched. The glassy ejecta correspondto these ballistic ejecta.

As noted by many workers, the massive melt de-posits obviously emplaced upon the ballistic ejecta[e.g. 4, 12]. The melt that remains within the transientcavity forms a lining of the cavity floor. As the modifi-cation stage of cratering process begins, the melt liningis given a lateral component of movement by reboundof the transient cavity and by terrace formation. Themelt is ejected at low velocities and emplaced on thecrater rim [12].

Origin of the non-glassy ejecta is unclear. The glasscontent of ejecta decreases near the rim, which is re-flected in a low 750/415 nm ratio. The ejecta near therim is from the outer area of the transient cavity. Sinceit contains less impact melt, and the crumble of the rimpeak also buries glassy material [6]. It is not enough,however, to explain the wide expanse and the charac-teristic texture of the blocky rim material in the northof Tycho. The key evidence about this problem is thatthe north region is also the area that the dark ring ismost evident. It is reasonable that same processes workon the distribution of these ejecta.

References: [1] Melosh H. J. (1989) Impact Cra-tering: A Geologic Process. [2] Wilhelms D. G. (1987)The geologic history of the moon. [3] Pohn H. A.(1972) Geologic map of Tycho quadrangle of the moon.USGS Map I-713. [4] Howard K. A. and Wilshire H. G.(1975) J. Res. U.S. Geol. Survey 3, 237-251. [5]McEwen A. S. et al. (1993) JGR, 98, 17207-17231. [6]Hirata H. et al. (1999) LPS XXX, #1350. [7] Pieters C.M. et al. (1994) Science, 266, 1844-1848. [8] SmerkarS. and Pieters C. M, (1985) ICARUS, 63, 442-452. [9]Tompkins S. et al. (1997) LPS, XXVIII, 1441-1442.[10] Tompkins S. et al. (1996) LPS, XXVII, 1335-1336. [11] Grieve R. A. F. et al. (1977) in Impact andExplosion Cratering. pp. 791-814. [12] Hawke B. R.and Head J. W. (1977) in Impact and Explosion Cra-tering. pp. 815-841.

Fig. 1. Lunar Orbiter photograph LO V-125M.

Fig. 2. 750/415 nm ratio mosaic of Tycho.

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