IMPACT MELT DISTRIBUTION AND EMPLACEMENT ON TYCHO: A NEW LOOK AT AN OLDQUESTION Aisha R. Morris1, James W. Head1, Jean-Luc Margot2, Donald B. Campbell3, 1Department ofGeological Sciences, Brown University, Providence, RI, 2National Astronomy and Ionosphere Center, AreciboObservatory, Arecibo, PR, 3Department of Astronomy, Cornell University, Ithaca, NY (aisha_morris@brown.edu).

Introduction: A new digital elevation model (DEM)(Fig. 1) of the lunar impact crater Tycho derivedfrom Earth-based radar observations [1] can be usedto address questions about Tycho’s formation andevolution. Specifically, the distribution of impactmelt on Tycho’s rim is known to be asymmetric [2](Fig. 2b) and the crater has a very asymmetric ejectadeposit, interpreted to mean that it impactedobliquely. Oblique impact, rim topography, and wallslumping have been documented as factors importantin explaining the asymmetric distribution of impactmelt on crater rims [3]. Here we discuss thetopography of the crater, including terrain thatexisted prior to its formation, and use thisinformation to address the relative roles of obliqueimpact, pre-existing topography and slumping in thedistribution and emplacement of impact melt.

Setting: Tycho is a Copernican age (50-100 m.a.) [4]crater on the nearside of the Moon in the southernhighlands (43˚S, 349˚E) and it is an excellentexample of a complex crater (flat floor, central peaks,terraced walls). It is 85 km in diameter with anaverage depth of 3970 m below the 1738 km radiuslunar sphere [1]. The main central peak rises 2400 mabove the crater floor. The average rim crestelevation is +730 m, with a mean rim crest to floordepth of 4700 m [1]. The depth-diameter ratio ofTycho is about 0.05, which is typical for freshcomplex craters measured by Pike [5]. It is evident in a full-moon image that there is anasymmetry of the crater ejecta. There are raysdistributed asymmetrically around the crater, with anejecta exclusion zone to the SW. There are muchlonger rays trending to the NE than in any otherdirection. Based on previous work on the secondarycrater field [4], asymmetry of the rays of Tycho [2],and work on terrestrial craters [6], the crater appearsto be the result of an oblique impact from the WSW. Prior to the impact event, the topography of theregion was dominated by craters in various stages ofdegradation and infilling (Fig. 2a). To the NE aretwo unnamed craters that intersect the rim of Tychoand beyond them is Pictet E, a shallow 60 km craterwith a depth of about 1.4 km. To the E is thebroadest region of the rim of Tycho and the 50 kmcrater Pictet, with a depth of about 2.6 km. A roughtopographic low lies between the crater Pictet A, tothe S of Pictet, and the SE rim of Tycho. Severalunnamed craters lie to the SE and S and their edgesintersect the rim of Tycho, creating local highs. Therough terrain to the SW and W lacks any significantolder craters, and is relatively topographically high.

To the W, beyond the rim of Tycho, there are a fewsmall (<30 km) pre-impact craters in a broaddepression that does not significantly affect thetopography of Tycho’s rim. The topography of the crater Tycho shows anasymmetry that is present in all of the components ofthe crater. The interior of Tycho is generallyasymmetrical, with a deeper floor and narrower wallon the E and NE sides. The floor is about 200 mdeeper on the E and NE sides and is morphologicallysmoother in the deeper region. The walls of thecrater are terraced, composed of cuspate, slumpedblocks around the entire perimeter of the crater [7].The width of the wall is greater in the WSW region,implying more extensive slumping in that area. Thetoes of the WSW area of the wall do not terminate ina relatively straight line, as on the ENE side, but theydissipate in mounds and hummocks that almost reachthe central peak. The central peak of Tycho is notone central peak, but a cluster of peaks, with onelarge, main peak. The smaller peaks are on the N/NEside of the main peak, and the main peak isapparently less steep on the E side than on the Wside. According to estimates by Cintala and Grieve,the central peak of Tycho should have originated at adepth of ~15 km, making that the depth of maximumimpact melting [8]. The rim of Tycho is morphologically distinct due tothe relatively young age of the crater. The rim crestelevation profile of Margot et al. shows asignificantly higher elevation on the NE region, thehighest the rim crest reaches [1]. The lowest rimcrest elevations are in the SSW, with an elevation ofabout 200 m, and in the NNW, with an elevation ofabout 150 m. In general, the elevation of the rimcrest varies between 200 and 1200 m, with a meanvalue of 730 m [1].

Role of topography and angle of incidence:Previous work has suggested that pre-existingtopography may play a major role in the distributionand emplacement of impact melt [6]. It is evidentthat the craters that intersect Tycho’s rim exert astrong control on the resulting topography of Tycho.As seen in previous studies of crater rim topography[2, 3], where Tycho’s rim intersects a topographichigh, (e.g. the rim of an older crater such as theunnamed craters on the SE side of Tycho’s rim), thatportion of the rim is topographically higher. Wherethe rim of Tycho encounters a topographic low, suchas the center of an unnamed older crater to the S ofTycho, that portion of Tycho’s rim is topographicallylower.

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IMPACT MELT ON TYCHO: A. R. Morris et al.

The amount and location of wall slumping, whichis partially controlled by the topography of the pre-existing terrain and the angle of incidence, providesanother method to analyze the evolution of the crater[7]. Tycho appears to have slumped most extensivelyon the WSW portion of the wall, and it appears thatprior to wall failure, this area was topographicallyhigher than much of the rest of the crater. Alongwith the projectile’s angle of incidence [4], this mayhave contributed to the failure of the wall and theextensive slumping seen in this area.

Impact melt: The impact melt on Tycho is locatedon the floor of the crater as a large pond, on theterraced walls as pools and flows, and on the rim andsurrounding areas outside the crater as a veneer,flows and ponds. The melt on the floor of the crateris composed of two distinct provinces. The ENE halfof the floor is smoother and topographically lowerthan the WSW, which has abundant hummocks andhills in the melt. The walls of the crater aredominated by melt ponds collected in the back-tiltedwall slump blocks. There is also evidence on thewalls of the crater of channels flowing downslopeand emptying onto the floor of the crater. Due toshadowing, the E and NE walls are not visible on theOrbiter V image. The largest melt pools on the wallsof the crater are on the S walls, and there are a fewsmall ponds on the NW, but very few on the W andSW walls. Evidence for flows is present on all of thevisible walls. The most abundant impact melt ponds on the rim ofthe crater are on the NE and SE segments, withscattered deposits on the N rim and only rare visibledeposits on the W rim. The deposits on the NE andespecially the SE rim are composed of large ponds,which extend farther from the rim on the NE area (~1crater radius) than on the SE (~1/2 c.r.). Thesefigures are in agreement with figures determined byHoward and Wilshire for the expected distances ofimpact melt away from the crater rim [2]. The N rimdeposits have much smaller ponds that are widely

scattered, and they are slightly less than 1 c.r. awayfrom the rim. There may also be a hard rock veneer,with flows extending away from the crater rim [3].Although mapping is made difficult due toshadowing, the few W rim deposits visible are widelyscattered small ponds with very few extending pastthe rim of the crater. But if these deposits were ofcomparable size to the E deposits, they should bevisible, despite the shadowing.

Discussion: The distribution of the impact melt inrelation to the topography of Tycho suggests thatthere were three factors affecting impact meltemplacement. The first was the oblique impact;which preferentially emplaced melt in the NE regionof the crater rim. A second factor was the pre-existing interior topography, which caused the wallsto slump preferentially on the SW, disrupting themelt that was ponded on the floor and giving it acomponent of motion over the rim on the E side ofthe crater. The third factor was exterior topography.Melt on the rim followed the rough pre-existingtopography of the region, flowing into thetopographic low between Tycho and Pictet A andponding in that region, and flowing down toward theunnamed crater U. Pre-existing topography clearlyexerts controls on the ultimate radial extent of impactmelt ponds on the eastern crater rim.

References: [1] Margot, J., et. al., J. Geophys. Res.,104, No. E5, 11,875-11,882, 1999. [2] Howard, K.A., H. G. Wilshire, Lun. Sci. IV, 389-390, 1973. [3]Hawke, B. R., J. W. Head, in Impact and ExplosionCratering, Pergamon Press, 815-841, 1977. [4]Luchitta, B., Icarus, 30, 80-96, 1977. [5] Pike, R. J.,Proc. Lun. Sci. Conf. 8th, 3427-3436, 1977. [6]Schultz, P. H., R. R. Anderson, in The MansonImpact Structure, Iowa: Anatomy of an ImpactCrater, Geol. Soc. Of Am., 397-417, 1996. [7] Settle,M., J. W. Head, J. Geophys. Res., 84, No B6, 3081-3096, 1979. [8] Cintala, M. J., R. A. F. Grieve, Met.& Planet. Sci., 33, 889-912, 1998.

Figure 2a. Lunar Orbiter V image ofTycho. Crater is 85 km wide. E-Pictet E;U-Unnamed; P-Pictet; A-Pictet A.

Figure 2b. Sketch map detailinglocation of impact melt deposits andapproximate angle of impact.

Figure 1. DEM of Tycho, with referenceto the 1738 km radius lunar sphere. AfterMargot et al. 1999. North is up.

Lunar and Planetary Science XXXI 1828.pdf