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OObbsseerrvvaattiioonnss oonn tthhee uupprraannggee eejjeeccttaa ooff oobblliiqquuee aannggllee iimmppaacctt ccrraatteerrssby Barry Fitz­Gerald GLR Group................................................................................................. 4

LLEEAAVVIINNGG NNOO DDOOMMEE UUNNTTUURRNNEEDDReview reprint from LPOD by C. Wood....................................................................................... 19

OOnn tthhee iimmaaggeedd sshhoorrtt aanndd nnaarrrrooww rriillllee nneeaarr RRiimmaa SShheeeeppsshhaannkkssby KC Pau and R. LenaGeologic Lunar Research (GLR) group....................................................................................... 21

UUnnuussuuaall CCrraatteerr EEjjeeccttaa FFeeaattuurreeby Barry Fitz­Gerald GLR Group................................................................................................. 26

TToorrrriicceelllliiby Richard Hill Loudon Observatory............................................................................................ 32

CCOOVVEERR

LLuunnaarr DDoommeess

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OObbsseerrvvaattiioonnss oonn tthhee uupprraannggee eejjeeccttaa ooff oobblliiqquuee aannggllee iimmppaacctt ccrraatteerrss

Barry Fitz­Gerald GLR Group

AAbbssttrraaccttOblique impacts are known to produce a suite of features within the ejecta blanket and on the

planforms of the resulting craters. These changes only become evident at shallow impact angles, withinitially the ejecta becoming sparse uprange, and then with the progressive appearance of a Zone ofAvoidance (ZoA) or Forbidden Zone in the uprange direction, where ejecta appears to be poorlydeveloped or absent. The crater planform remains circular at these impact angles and only departs fromthe axisymmetric at extremely low angles. The edges of theses ZoA's are in many cases defined byconspicuous rays which can, in some cases have a positive relief core, which survives as a topographicfeature long after the bright ray material has become inconspicuous due to space weathering. Thecurrent article discusses this process, and relates it to features observed in the uprange ejecta blanketsof a number of craters of proposed low angle origin. These features form uprange ridges which appearto form in the transition regime between craters that form ZoA,s and craters that exhibit moreasymmetric ejecta such as the well known 'butterfly' pattern of crossrange rays.

The effects of low angle crater forming impactshas been discussed extensively, exploredempirically and observed on the surfaces of theterrestrial planets (Gault & Wedekind,1978 andHerrick & Forsberg­Taylor, 2003). Low angleimpacts can have spectacular consequences onthe planform of the resulting craters, withelongate forms such as Schiller and Messierbeing the most familiar examples. Experimentalwork indicates however that all but the shallowestimpacts (<10°) produce essentially axisymmetriccraters (Gault & Wedekind,1978) with theasymmetry introduced by the impact angle onlybeing evident in the distribution of the ejecta. Thisand later work demonstrated that in obliqueimpacts the ejecta curtain is initially tilteddownrange but becomes more 'upright' as thecratering flow field undergoes the transition fromthat of a moving point source to that of astationary point source later in the crateringprocess (Schultz, et.al, 2000 and Elbeshausen &Wünnemann 2013). This phenomenon will havean effect on the uprange and downrange ejecta,with one aspect being the development of theuprange Zone of Avoidance. Spectacular ZoA'sare seen in the uprange ejecta of Proclus,Petavius B, Jackson, Ohm and Crookes. In thesecraters the ZoA's are defined laterally by brightcardioid (curving or heart shaped) or arachnid

(straighter) rays. These bright rays will however,over time, become less prominent as radiationdarkening and space weathering reduces thecontrast between the rays and the surface onwhich they lie. As the associated craters will beapproximately axisymmetric, evidence for theoblique nature of the impact will thereforedisappear. In a number of cases howeverevidence of the ZoA is preserved in the form of apair of diverging ridges, which originally formedthe rays bounding the ZoA.

Cauchy and Kies A are probable examples of thiscategory of oblique impact, with the divergingridges forming 'wing' like extensions in theuprange directions (Wood 2011 & 2012). The'wings' of Cauchy are composed of a combinationof positive relief ridges proximally, which gradedistally into elongate secondary craters and shortintervening ridges arranged in an en­echelonconfiguration (Fig.1). This would appear to reflectthe transition from the initial high angle, highvelocity ejecta distally, to the lower angle lowervelocity ejecta proximally. In the case of Kies Athe distal crater chain sections are obscure whilstthe proximal ridges are highly conspicuousparticularly under low angle illumination (Fig.2).

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A number of other craters exhibit similarstructures but with the relative development of theridge and crater chain elements in the 'wings'varying. One example is the 10km diameterDemocritus A, which possesses a pair of longfilamentous features which can be traced for over30kms to the east and south­west (Fig.3). Thedistal section of these 'wings' appear to becomposed of bifurcating linear grooves formed bycomplex arrangements of 'en echelon' secondarycraters as noted above.

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FFiigg..11 Cauchy as seen bySELENE in this morningview (a) showing 'wings'to the north­west andsouth­west, and LROCNAC image of yellow boxshowing the en­echelonsecondary craters andridges which form thedistal section of the wing(b). Note sparsity ofejecta within ZoA ascompared to that north ofthe 'wing' .

FFiigg..22 LROC WAC image of Kies A showingpositive relief proximal sections of 'wings' to thewest. Note the cleft in the crater shadow as sunshines through the low point on the cratersuprange rim.

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FFiigg..33 LROC Quickmap image of Democritus A showing 'wings' to the east and south­west (a) and LROCNAC detail of area shown in yellow box showing bifurcating groove morphology of this distal section.

Cauchy, Kies A and Democritus A show their low angle credentials in the form of their rim morphology, withthe uprange rim being depressed relative to the downrange rim (Fig.4), a phenomenon consideredcharacteristic of hypervelocity low angle impacts (Forsberg, et.al, 1998).

FFiigg..44 Surface elevation profiles as shown by LROC GLD100 for Cauchy, Democritus Aand Kies A, showing depressed rim height uprange. Profiles taken along axis anddirection shown by yellow arrows.

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The example of Cauchy demonstrates that these wingsappear to coincide with the edges of the ZoA, withinwhich ejecta is sparse (Fig.1b). These ridges may formas the result of material excavated by secondary craterslining up in an en­echelon manner as seen in the distalsections of the wings in Cauchy (Fig.1) or as a result ofdeposition of material (possibly directly from the ejectacurtain) as demonstrated in the proximal sections of thewings of Kies A (Fig.2). It is possible that bothprocesses occur in sequence during the crater formingprocess, with the formation of the proximal and distalparts of the 'wings' dominated by different processes.This might result in an initial secondary crater formingdominated phase giving rise to the distal sections and adepositional phase the proximal sections. Thisinterpretation is supported by the observation that asimilar configuration of proximal ridge and distal en­echelon crater chains can be seen to form the brightrays bordering the ZoA's of craters such as Proclus andPetavius B.The uprange ejecta of the 12km diameter crater MarkovE may possess another class of ridge like featureproduced in oblique angle impacts. Markov E, whilstbeing more or less circular in outline has an asymmetricejecta blanket where darker material has beenexcavated and deposited onto a lighter mare surface,whilst immediately proximal to the crater rim, brightdeposits may represent the deepest excavated materialof highland composition (Fig.5). The crater itself shows adepressed southern rim and a lack of ejecta in thisdirection, consistent with an impactor approaching fromthe south.

The lack of ejecta in the hypothesised uprange direction(south) is accompanied by the presence of an slightlysinuous ridge immediately uprange of the southern rim.This ridge is of very low relief (~ 50m high) and as notedabove is slightly sinuous, giving an impression of shortchevrons connected end to end.

FFiigg..55 Clementine UVVIS MultispectralMosaic of Markov E showingasymmetric ejecta (a) and Surfaceelevation profiles as shown by LROCGLD100 (b). Profile taken along axis

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FFiigg..66 LROC NAC montage of uprange chevronridge to the south of Markov E. Note the mannerin which the ridge grades into the surroundinguprange ejecta, particularly the decelerationdunes to the west.

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FFiigg..77 Clementine UV­VIS Ratio Map of Markov E showing compositional relationship between thechevron ridge and adjacent near rim ejecta (a) and LROC Quickmap image of ridge and dune field.

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The ridge is approximately 10kms in length andis orientated roughly tangential to the southernrim of Markov E. That this ridge is part of theejecta blanket and not a later tectonic feature(such as a wrinkle ridge) is indicated by themanner in which it grades into the surroundingejecta, particularly a small field of imbricatingdeceleration dunes to the west, which alsoexhibit a subtle chevron configuration (Fig.6).

This relationship is further suggested by theClementine UV­VIS Ratio Map image of MarkovE that shows that the ridge and dune field sharethe same blue spectral signature of highlandanorthosite also evident in the near rim ejecta(Fig.7a). This would indicate a similar originduring crater formation when deeply buriedhighland material was excavated and depositedimmediately outside the rim. These observationsindicate that this chevron ridge originated duringcrater excavation and represents a feature of thecontinuous ejecta blanket.

A similar ridge like feature may be seen withinclose proximity of the north­eastern rim of

FFiigg..88 Selene montage of Milichius showing possibleridge in uprange ejecta to the north­east (a) andSurface elevation profiles as shown by LROCGLD100 showing slightly depressed north­easternrim (b). Note butterfly ejecta pattern in Milichius A (in

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Milichius. As with Markov E, this ridge is oflow elevation and approximately 1km wide,taking the form of gentle curve, tangentialto the crater rim. Indications of obliqueimpact in the crater are however tenuous atbest, with a very slightly depressed rim tothe north­east, but the location of the ridgeadjacent to the rim low point is suggestive(Fig.8).The relationship of these tangentialuprange ejecta features to oblique impactsis demonstrated in a couple of exampleswhere the impacts occurred on inclinedsurfaces, which may in effect mimmic anoblique impact on a horizontal surface.One example, a bright youthful craterapproximately 2kms in diameter, is locatedjust inside the western rim of Galvani B,and appears to represent an impact on theslopes of the inner crater wall. Theglancing nature of the impact appears tohave resulted in an exceedingly shallowcrater (Fig.9), in all probability heavilymodified post impact by downslopemovement of loose debris. In this case theimpactor appears to have approached fromthe north­west and in this direction we seea field of tangentially arranged craterchains exhibiting an en­echelon chevronpattern draped over the western glacis ofGalvani B (Fig.10).

The asymmetry in the uprange anddownrange ejecta configuration is striking,with the downrange (eastern) ejectadominated by long rays without anysuggestion of the tangential arrangementseen to the west. Bright ejecta is

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FFiigg..99 LROC 3D Visualisation tool image of crater onwestern rim of Galvani B (a) and LROC GLD100Surface elevation profiles showing position of impact (b).

a.

concentrated crossrange but predominantlydownrange as can be seen from the ClementineRatio Map image (Fig.10c), with little ejecta in theuprange direction.This might indicate that the impact angle was lowerthan that required to produce a ZoA and approachingthat where crossrange 'butterfly' patterns begin tobecome apparent.

FFiigg..1100 LROC NAC montage (a) detail ofuprange ejecta (b) and Clementine UV­VISRatio Map (b) of crater shown in Fig.9. Notelack of a ZoA.

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The second example is another fresh 2km diametercrater located at the base of the north­westerninner wall of Dante C. In this case the uprangedirection corresponds to the north­western craterwall, and the downrange to the crater floor to thesouth­east. As with the previous example, a zone oftangentially arranged en­echelon crater chainsexists uprange, which in this case 'wrap' slightlyaround the crater towards the downrange direction(Fig.11).

Both of these examples are clearly affected by theterrain on which they are developed, with slopeplaying a significant part in the final form of theejecta and crater itself. They do howeverdemonstrate the clear influence of impact angle onthe ejecta pattern, especially on the production ofthese tangential uprange zones. Neither of theabove has a ridge as observed in Markov E (orpossibly Milichius) but it is possible to propose thatas the impact angle decreases, this zone wouldbecome narrower in extent and eventuallydisappear at very low angles where uprange ejectais altogether absent.

At some point therefore conditions might existwhere the uprange ejecta becomes restricted to avery narrow zone, and is consequentlyconcentrated into the type of ridge like feature wesee in Markov E.

Further examples of the phenomenon proposed inthe above discussion may be provided by thecraters Dawes (17kms) and Sulpicius Gallus

(12kms). Both craters have been discussed byHerrick & Forsberg­Taylor (2003) in relation tooblique lunar impact craters, with Dawes beingdescribed as having a rim asymmetry (ellipticity of< 1.1) and Sulpicius Gallus as being nearlyaxisymmetric despite being considered an obliqueangle crater. Dawes has been the subject ofconsiderable study with particular regard to its rimand floor topography. Rather less comment hasbeen attracted by Rima Dawes, a linear troughtype feature to the north­east. This feature ishowever not restricted to the north­east, butappears to wrap around the craters eastern rim,and is present to the south­east as a troughbetween two ridges. The trough is interrupted halfway along its length by what appears to be animpact melt rich slump off the eastern rim, but therelationship between the northern and southernsections is clear and their mode of origin almostcertainly related. This structure does not appearto be a pre­crater structure, and similarly isunlikely to be a product of post impactmodification. Dawes itself shows clear indications

of an oblique impact origin (Fig.12b) with theejecta clearly divided into two components. Onecomponent is focussed downrange and is visiblein this data as a yellow ray of presumably freshlyfragmented mare material produced in the earlyphase of crater formation when the ejecta curtainwas tilted downrange.

FFiigg..1111 LROC Quickmap of crateron floor of Dante C, with GLD100profile in inset to show position atbase of inner crater wall (a) anddetail of tangentially arrangedejecta uprange (b).

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Further examples of the phenomenon proposed inthe above discussion may be provided by thecraters Dawes (17kms) and Sulpicius Gallus(12kms). Both craters have been discussed byHerrick & Forsberg­Taylor (2003) in relation tooblique lunar impact craters, with Dawes beingdescribed as having a rim asymmetry (ellipticity of< 1.1) and Sulpicius Gallus as being nearlyaxisymmetric despite being considered an obliqueangle crater. Dawes has been the subject ofconsiderable study with particular regard to its rimand floor topography. Rather less comment hasbeen attracted by Rima Dawes, a linear trough typefeature to the north­east. This feature is howevernot restricted to the north­east, but appears to wraparound the craters eastern rim, and is present tothe south­east as a trough between two ridges. Thetrough is interrupted half way along its length bywhat appears to be an impact melt rich slump offthe eastern rim, but the relationship between thenorthern and southern sections is clear and theirmode of origin almost certainly related. Thisstructure does not appear to be a pre­craterstructure, and similarly is unlikely to be a product ofpost impact modification. Dawes itself shows clearindications of an oblique impact origin (Fig.12b) inthe Clementine UVVIS data, with the ejecta clearlydivided into two components. One component isfocussed downrange and is visible in this data as ayellow ray of presumably freshly fragmented marematerial produced in the early phase of craterformation when the ejecta curtain was tilteddownrange.Superimposed on this, and having a moresymmetrical distribution is a deposit showing aslight blue, and representing material excavatedfrom deeper levels, but during a later phase in theprocess when the ejecta curtain had assumed amore 'upright' orientation. This shows that asymmetric and asymmetric ejecta pattern can begenerated in the same impact event.The LROCGLD100 surface elevation plot also shows a slightlydepressed eastern (uprange) rim, which isconsistent with an impactor approaching from theeast (Fig.12b).

Morphologically, Rima Dawes consists ofadjacent, sub­parallel ridges either side of adepressed trough, and not a lunar surface featuresuch as a graben or sinuous rille (Fig.13). Theinner ridge is narrower than the broad outer ridge,

FFiigg..1122 SELENE image of Dawes showingRima Dawes to the north­east (a) andClementine UVVIS Ratio Map and inset LROCGLD100 surface elevation plot showingevidence of oblique impact origin (b).

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b.

with an approximate elevation of a few tens ofmeters at the most for both features. Detailedviews show that the ridges themselves have aveneer of impact melt, which is most apparenton the uprange aspect of the inner ridge whereit can be seen to form a slightly 'stepped'texture, possibly where the edges of individualflow fronts have started to erode. The inner

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ridge also has boulders perched its summit,strongly suggesting an origin during the impactprocess itself. The glacis between the inner ridgeand the eastern rim also has a considerablecovering of impact melt whilst immediately belowthe eastern rim considerable numbers of impactmelt gullies can be seen. It is worth noting that thegreatest development of impact melt appears to bein the north­east quadrant, somewhat at odds withthe oblique impact from the east. This meltdistribution may however have more to do withmelt displacement during the complex wallslumping processes that occurred during the cratermodification stage rather than processes occurringduring the impact event itself. The fact that theseridges have a melt veneer and perched boulderswould suggest they were formed during the impactevent but before the melt was emplaced or theboulders ejected from the crater, presumably at alate stage in the process. The ridges also exhibitthe gentle sinuosity seen in the ridge of Markov E,again suggestive of a common mode of origin(Fig.14). A comparison between Dawes andSulpicius Gallus serves to illustrate the strikingdegree of similarity between these two craters,with features to the east of Sulpicius Gallusmirroring 'Rima Dawes'. The oblique nature of theSulpicius Gallus impact is demonstrated by theClementine UVVIS Multispectral Mosaic (Fig.15c)indicating an impactor from the north­east, andshowing that these ridge like features are again inthe uprange direction. These features are not asconspicuous as in Dawes, indeed they are incomparison quite subtle topographically, but theoverall morphological similarity and comparablelocation in relation to the uprange rims are highlysuggestive of homologous structures in bothcraters (Fig.16).

FFiigg..1133 LROC GLD100 surface elevation plotsradially from crater rim through 'Rima Dawes'.This shows the inner and outer ridges (redarrows) and the relative topography. Note plotsare not equally to scale

The extent of ejecta beyond these uprangeridges in Dawes and Sulpicius Gallus differ inthat in the former, conventional ejecta in theform of the 'herringbone' pattern formed bysecondary cratering can be traced forapproximately 2 crater radii. In the latter casethis ejecta is inconspicuous even at one craterradius distance uprange, indicating an overalllack of uprange ejecta development. Thiswould indicate a difference between the twoimpact regimes, with Sulpicius Gallusrepresenting a shallower impact than Dawes.

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FFiigg..1144 LROCNAC montage ofeastern rim ofDawes (north tothe right) showingthe ridge likenature of theedges of 'RimaDawes, impactmelt gullies in thecrater rim andperched boulderson the ridgecrests.

FFiigg..1155 Sulpicius Gallus as viewed by SELENE (a) Apollo 17 (b) and Clementine UVVISMultispectral Mosaic (c). Note the ridge like structures located to the east of the easternrim and comparable to 'Rima Dawes'.

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FFiigg..1166 Dawes (left ) and Sulpicius Gallus (right) shown from asimilar orientation from the uprange direction. Note similardisposition of ridges uprange of both craters as indicated byarrows. (LROC Quickmap 3D Visualisation tool).

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Two possible larger examples are seen inPlinius and Picard. Plinius (a closeneighbour of Dawes) displays indicationsof an oblique origin in the form of adepressed rim to the east, but thisinterpretation may be complicated by ageneral lowering of the mare surfacetowards the east at this location. A moreconvincing indication of oblique impact isprovided by the Clementine FeO binnedcolour image, which shows impact melt ofa relatively lower lower FeO content(derived from highland material underlyingthe mare) preferentially developed towardsthe west (Fig.17b). If we accept thisevidence as indicative of low angle impactorigin, an uprange direction to the east issuggested. Immediately to the east of theeastern rim the ejecta blanket can be seento be developed into a broad ridge thatappears to 'wrap' around the northern andsouthern rims in a crescent shapedstructure.

The ridge can, in places, be seen to besculpted longitudinally into the now familiarchevron pattern, suggesting an origin in aprocess similar to that described above(Fig.18). As with Dawes, this broad ridgeappears to be developed on the glacis ofthe crater and grades uprange into a moreconventional ejecta blanket with the'herringbone' texture referred to above to adistance of some 2 crater radii.

The evidence for Picard being an obliqueangle impact is subtle, its south easternrim is slightly depressed in relation to thewestern, whilst ejecta, in the form ofshallow elongate secondary craters, canbe traced radially slightly further to thenorth­west but is still conspicuous to thesouth­east. This suggests a degree ofasymmetry with an uprange directiontowards the south­east. As with Plinius, theuprange glacis bears a ridge like rampart,

a. b.

FFiigg..1177 LROC WAC montage of Plinius showing uprangeridge (a) and Clementine binned FeO colour map showingdownrange bias in ejecta (b).

FFiigg..1188 SELENE image of Plinius showing chevron patterningin uprange ridge.

in this example mostly straight, but curving at the endsaround the northern and southern crater rims (Fig.19).Indications of the chevron pattern can be seen at either endof the straight section of the ridge, whilst a corrugatedtexture may indicate a veneer of impact melt.

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FFiigg..1199 Selene image of Picard showing ridge inuprange ejecta to the south­east (a) and Surfaceelevation profiles as shown by LROC GLD100showing depressed rim and ridge on the glacis to thesouth­east.

This ridge is quite a prominent topographicfeature as is shown by the LROC GLD100surface elevation plot (Fig.19b), whilst itslocation on the glacis would argue against itbeing an unrelated pre­impact structure(Fig.20).

Comparing Plinius and Picard using the LROCWAC Colour Shaded Relief Map (Fig.21) servesto illustrate a final important point. Both cratersexhibit indications of enhanced proximal ejectacrossrange, which is a feature of obliqueimpacts which eventually in the shallowest ofimpacts (<5º) gives rise to the 'butterfly' patternof ejecta seen in craters such as Messier(Herrick & Forsberg­Taylor, 2003). This servesto support the interpretation of these craters asbeing the results of oblique impacts. Neitherhowever show any indication of a ZoA, and asnoted both have conventional uprange ejecta inthe form of 'herringbone' secondary cratering.

FFiigg..2200 LROC Quickmap 3D Visualisation tool image ofPicard as viewed from the east (looking downrange).Note the ridge on the glacis of the south­easternrampart.

FFiigg..2211 LROC WAC Colour Shaded Relief Map ofPlinius (a) and Picard (b) showing enhancementof proximal ejecta crossrange as indicated byarrows. In both cases 'uprange' is to the right.

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A final example however provides a possiblelink between craters with ZoA's and theuprange ridges discussed so far. Harding isan irregularly shaped 22km diameter craterthat is not obviously a low angle impactstructure. It has however a prominent butextremely wide ZoA, suggesting that theapproach angle was well below that requiredto form a ZoA but clearly above that requiredto produce a 'butterfly' pattern in the ejecta.The ZoA is not particularly obvious as thesouthern cardioid ray is far more prominentthan the northern one (Fig.22) which appearsless conspicuous possibly as a result ofcompositional differences (possibly with adarker more iron rich lithology contributing tothe northern ray).This gives the crater a somewhat asymmetricappearance, and may contribute to it notbeing widely recognised as a low anglestructure. As with Picard and Plinius a slightcrossrange enhancement of ejecta is furtherindication of an oblique origin. A more detailedexamination of the crater rim area reveals ashort 'V' shaped trough immediately uprangeof the craters north eastern rim, with the apexpointing towards the crater. This feature isextremely subtle (outer ridge ~ 30 to 50meters high and ~ 2km wide), with littletopographic relief and appears to consist oftwo short (~7km) lunate ridges either side of ashallow trough. Its position mid way betweenthe two cardioid rays of the ZoA, and uprangeindicate that it is a feature of the ejecta andnot a pre or post impact structure (Fig.23).In this example it is possible to speculate thatthis 'V' feature represents an incipientexpression of the uprange ridges notedabove. What this observation means in termsof the relationship between ZoA formation andridge formation is a question for furtherconsideration, but it is possible that atdifferent impact angles different featuresdominate, but this is speculation based onlimited observations and the few examplesdiscussed above.

FFiigg..2222 Clementine NIR Multispectral Mosaic ofHarding showing the ZoA to the north­east, withbrighter southern cardioid ray (a) and Selenemontage showing enhanced crossrange ejecta and'V' structure immediately uprange of the crater rim.

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FFiigg..2233LROC Quickmap 3DVisualisation toolimage of Hardingviewed from theuprange direction.Note the 'V' featureuprange.

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DDiissccuussssiioonn

The examples described above are intended to demonstrate the existence of an uprange ejectafeature that develops in oblique impacts. At sufficiently low impacts (~15º) a conspicuous ZoAdevelops where ejecta is sparse uprange. These ZoA's can be bordered by conspicuous brightrays (Proclus, Petavius B), and are also responsible for the diverging uprange ridges ('wings')in older craters, where the bright ray material has been weathered to such an extent that thedifference between it and the surrounding surface is no longer obvious (Cauchy, Kies A). TheZoA results from the dynamics of the ejecta curtain, which is initially tilted downrange during theearly impact but becomes more upright later in the cratering event. As the impact angledecreases the ZoA becomes wider (as seen in Harding) and eventually gives way to an ejectadistribution which is enhanced crossrange and downrange and absent uprange.

In certain circumstances within these low angle regimes a narrow ridge may form in theuprange ejecta which in some circumstances can result in conspicuous topographic features. Itappears that these ridges are produced during the earlier phases of the impact process wherethe ejection and deposition of ejecta are asymmetric due to the tilting of the ejecta curtaindownrange. The ridges may be subject modification, such as impact melt emplacement duringlater stages of the process when the ejection and deposition of ejecta become symmetric.

The relative infrequency of these ridges may be related to the rather narrow range of conditionsduring which their formation is possible. It is probable that this process is the result of thecomplex interplay of a number of factors including impact angle, size and velocity of theimpactor, nature and topography of the target and duration of the impact event. The ridges andtangential ejecta features described above may not be directly comparable, the ridge of MarkovE and those of Picard and Plinius differ in morphology and scale, but the coincidence of theseridges uprange in oblique impacts suggests the existence of a phase in oblique crater formationthat favours enhanced deposition of ejecta material in this location.

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RReeffeerreenncceess

Elbeshausen,D and Wünnemann, K. 2013 Crater Formation in the Transition from Circular to

Elliptical Impact Structures. 44th Lunar and Planetary Science Conference (2013)

Forsberg, N. K., Herrick, R. R., Bussey, B. 1998. The Effects of Impact Angle on the Shape of

Lunar Craters. 29th Annual Lunar and Planetary Science Conference, March 16­20, 1998,

Houston, TX, abstract no. 1691.

Gault, D. E. & Wedekind, J. A, 1978. Experimental Studies of Oblique Impact,

LUNAR AND PLANETARY SCIENCE IX, PP. 374­376.

Herrick, R. R, Forsberg­Taylor N. K. 2003. The shape and appearance of craters formed by oblique

impact on the Moon and Venus. Meteoritics & Planetary Science 38, Nr 11, 1551–1578

Schultz, P.H, Anderson, J.B.L and Hermalyn, B (2009) Origin and significance of uprange ray

patterns. 40th Lunar and Planetary Science Conference (2009)

Schultz, P.H., J.T. Heineck, and J.L.B. Anderson, 2000 Using 3­D PIV in laboratory impact

experiments, in Lunar Planet. Sci. Conf. XXXI, pp. 1902, LPI, Houston, TX

Wood, C, Horns and Wing, LPOD Lunar photo of the Day 18th May 2012

http://lpod.wikispaces.com/May+18%2C+2012

Wood, C, Single Crater Septum, Lunar photo of the Day 19th December 2011

https://lpod.wikispaces.com/December+19,+2011

AAcckknnoowwlleeddggeemmeennttss

LROC images, topographic charts and 3D visualisations reproduced by courtesy of the LROC

Website, School of Earth and Space Exploration,University of Arizona at:

http://lroc.sese.asu.edu/index.html

Clementine Multispectral Images courtesy of the USGS PSD Imaging Node at:

http://www.mapaplanet.org/

Apollo images courtesy Lunar and Planetary Institute web site at:

http://www.lpi.usra.edu

Selene images courtesy of Japan Aerospace Exploration Agency (JAXA) at:

http://l2db.selene.darts.isas.jaxa.jp

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LLEEAAVVIINNGG NNOO DDOOMMEE UUNNTTUURRNNEEDDReview reprint from LPOD by C. Wood

Over the last ten years or so a small international group of professionals in various fields relentlesslyimaged, measured and analysed lunar domes. They started with their own images and traditionaltechniques, and embraced new data and tools as they became available from orbiting spacecraft to refinetheir calculations and increase the types of knowledge gained. Early on simply getting an accuratedetermination of dome height and slope was a significant advance, and now they routinely estimatesurface compositions, eruption conditions and geophysical constraints of dome formation. It is not anexaggeration to say that with the publication of the first book devoted exclusively to domes that Raf,Christian, Jim and Maria Teresa have become the world's leading experts on these small volcanoes. Thefirst third of the book is technical, reflecting its origin in papers the team has published in professionaljournals, The rest is a mare by mare observer's guide to many dozens of domes, each illustrated withspacecraft and amateur images, and derived topographic profiles. The book is not for the casualobserver, but both the professional lunar geologist and the advanced amateur will want it as a compacthandbook of virtually everything we know of some of the Moon's most illusive features.These same four people are the leaders of the Geological Lunar Researches Group that has recentlypublished the 31st issue of Selenology Today, a high level journal that provides opportunities foramateurs to publish new research on the Moon. Congratulations to GLR for ST and Lunar Domes.

19Image from Springer

http://www.springer.com/astronomy/astrophysics+and+astroparticles/boo

CChhaapptteerr 11

This chapter provides an overview of terrestrialand lunar volcanic processes. The dependenceof the shape of a volcanic construct on thephysical and chemical properties of the eruptinglava is described. Furthermore, an introduction tolunar pyroclastic deposits, lunar cones andeffusive lunar domes and their vents as well asan outline of the occurrence of different types ofvolcanic constructs on the Moon and thecorresponding formation mechanisms is given.

CChhaapptteerr 22This chapter describes different methods fordetermining the morphometric properties(diameter, height, flank slope, volume) of lunardomes, a model to estimate the physicalproperties of the dome­forming magma, anddifferent classification schemes for lunar domes.

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CChhaapptteerr 33This chapter gives an outline of methods for describing the lunar surface in terms of specific spectralparameters inferred from multispectral or hyperspectral image data. These allow for the detection ofspecific lunar minerals, an estimation of the abundances of important chemical elements, and a mappingof the lunar surface in terms of its basic petrographic constituents.

CChhaapptteerr 44This chapter describes how the dimensions of the dikes through which the dome­forming magmaascended to the surface can be modelled. Furthermore, a modelling approach to encompass the physicalconditions under which putative intrusive lunar domes were formed is outlined.

CChhaapptteerr 55This chapter describes a classification scheme for lunar domes which relies on the spectral andmorphometric parameters by which they are characterised, the rheologic parameters describing thedome­forming magma, and the physical conditions under which dome formation occurred.

CChhaapptteerr 66This chapter provides an overview of effusive lunar domes bisected by linear rilles. A description of theformation conditions is provided according to the modelling approaches described in the previouschapters.

CChhaapptteerr 77This chapter provides a detailed description of a large number of lunar domes in a variety of regionsaccording to their determined spectral and morphometric properties as well as the inferred rheologicproperties of the dome­forming magma and the corresponding feeder dike dimensions.

CChhaapptteerr 88This chapter provides a discussion of the putative lunar intrusive domes, their morphometric propertiesand the conjunction of the large specimens with linear rilles.

CChhaapptteerr 99This chapter provides a summary of the book and gives a perspective towards future research activitiesregarding lunar domes.

Lunar dome Properties and Formation ProcessesSeries: Springer Praxis BooksSubseries: Astronomy and Planetary SciencesLena, R., Wöhler, C., Phillips, J., Chiocchetta, M.T.

2013, XIII, 174 p. 145 illus., 10 illus. in color.

http://www.springer.com/astronomy/astrophysics+and+astroparticles/book/978­88­470­2636­0

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There are so many rilles existing on the surface ofthe Moon. Some of them are very easy to locate,such as the most renowned Triesnecker complexrilles system and the Hippalus rilles system. Thereare some rilles that need an appropriateillumination condition to identify them, such as theRima Sheepshanks. In this study we examine asmall rille located to the south of the RimaSheepshanks.Possibly it is a rille segment connecting RimaSheepshanks and Rima Gartner. A description ofthe complex rille system, examined in this report, isreported by Wood in a previous LPOD

(http://lpod.wikispaces.com/May+14%2C+2010)

According to Wood (2010) the complex system ofrilles were poorly known and in our knowledge alsoimaged.The narrow rille or rille segment was detected byKC Pau and also imaged by Lena using telescopicCCD images. It is not reported in the Atlas of theMoon by A. Rükl, in the Lunar Quadrant Maps andin the LAC charts.Initially, Pau imaged it on December 29, 2003 witha Toucam camera and a 250 mm Newtoniantelescope (Fig. 1). It is marked by arrows and lies tothe south of the main Rima Sheepshanks.On April 27, 2012, Lena took some AVI clips of theSheepshanks region using a 180 mm Mak­Cassegrain Telescope. The subtle rille or rillesegment was easily detected and the image, madeat 19:21 UT, is shown in Fig. 2 (top and bottom forhigh enlargement).

Another recent image was taken by Pau onFebruary 17, 2013 at 11:56 UT with a 250 mmNewtonian telescope (Fig. 3). Moreover the imagedisplays both the whole length of RimaSheepshanks and the examined feature.However the rille is well detectable in the LunarReconnaissance Orbiter (LRO) WAC imagery,which displays this elusive rilles system (cf. Fig. 4)and in the Clementine imagery (Fig. 5).The narrow rille, located between Gartner C andGalle F, is running to the east­northeast, exactly likeRima Sheepshanks and can be a target for furtherimagery.It is apparently a linear graben, like the RimaSheepshanks. Lunar linear graben probably are thesurface expressions of intruded dikes that fillextensional weak zones radial to basins. Howeverno albedo variations are detectable in this regionand near the rille suggesting the absence ofpyroclastic material, as it is the case for themagmatic origin for Rima Hyginus. The size oftelescope is definitely an important factor indetection of fine rilles or domes. The seeing is themost important factor. If the seeing is steady, theresolution of the image taken with our telescopescan be comparable with larger size telescopes. Thedetection of "subtle" or “news” features on moonneeds patience. Most amateurs nowadays onlydoing casual imaging and observation of the moon.Our aim of imaging is to find something and keeprecords of most features in different illumination.That's the first target and principle of the GLRgroup.

OOnn tthhee iimmaaggeedd sshhoorrtt aanndd nnaarrrrooww rriillllee nneeaarr RRiimmaa SShheeeeppsshhaannkkss

By KC Pau and R. LenaGeologic Lunar Research (GLR) group

AAbbssttrraaccttDuring a survey we detected a small fine rille to the south of the main Rima Sheepshanks. The unnamed rille,likely an extension of the rille segment from Rima Gartner, was initially detected in 2003 with a Toucamcamera. We report our imagery displaying the narrow rille, which is shown in the LROC WAC imagery andClementine imagery.

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Fig. 1

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Fig. 2

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Fig. 4

Fig.5

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26

UUnnuussuuaall CCrraatteerr EEjjeeccttaa FFeeaattuurree..Barry Fitz­Gerald GLR Group

AAbbssttrraacctt

Impact crater ejecta takes a wide variety of forms including rays and secondary craters which are producedby the excavation of target material and their expulsion from the transient cavity during crater formation. Thematerial excavated in most cases ends up distributed more or less symmetrically around the crater, withonly very low angle impacts producing asymmetric distributions. Infrequently an ejecta field will stronglyreflect the azimuthal direction of the impactor, with features such as a Zone of Avoidance or Forbidden Zonein the uprange direction (such as Proclus) or conspicuous downrange rays (such as Messier A) forming.The current article describes an unusual uprange feature which takes the form of a linear gouge extendingaway from a small (~1.2km) very oblique angle crater on the south eastern rim of Vasco da Gamma B.

The crater discussed appears to be the resultof an exceedingly shallow angle impact of asmall body travelling from the south east, withthe rim of Vasco da Gamma B (for brevityreferred to as VdG.B in the following text).The impact appears to have struck the verycrest of the rim of VdG.B, and occurred as asuch a shallow angle that the resulting crater(referred to as crater X) was not excavated toany great depth, and probably no more than afew tens of meters.

As can be seen in Fig.1 VdG.B itself has asomewhat 'teardrop' shaped outline, andabuts a raised massif like area to the southeast. The oblique nature of crater X isindicated by the conspicuous dark Zone ofAvoidance (ZoA) to the south­east and the'butterfly' pattern ejecta laterally, with thesouthern 'wing' developed downslope and thenorthern one along the eastern rim ofVdG.B.

FFiigg..11 LROC image of oblique angleimpact (crater X) on the south eastern rimof Vasco da Gamma B, dashed arrowshows approximate trajectory (a), andLROC Quickmap 3D Visualisation toolimage showing terrain at impact site (b).

FFiigg..22 LROC image showing crater X with prominentdark Zone of Avoidance (ZoA) and gouge like furrow(G) along the inner south­western wall of Vasco daGamma B.

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A more detailed view of crater X as shown inFig.2 reveals that in the downrange direction theinner wall of VdG.B bears a gouge like linearfeature that extends for some 9kms towards thenorth­west and measures approximately 300meters wide. In close up this gouge appears toform, for much of its length, a pair of raggedgrooves cut into the inner crater wall of VdG.B,appearing more prominent proximally, and less sodistally (in this context 'proximal' means proximalto crater X). For the proximal 1/4 of its length thegouge consists only of the lower groove whilst up­slope of this is a crescentic scar which is convexproximally and concave distally. The lower 'horn'of this crescent appears to grade distally into theupper of the two grooves that form the remainderof the gouge like feature (Fig.3).

Each groove appears to be cut into the innercrater wall of VdG.B and to have exposed freshimmature regolith that has formed numeroussmall avalanches and debris slides which inplaces have cascaded down towards the craterfloor of VdG.B. One particularly prominent area ofslumping is marked by high albedo fans of debriswhich are criss­crossed by numerousdiscontinuous boulder trails indicating that the

rocks forming them travelled downslope at someconsiderable velocity and in a saltatory type ofmotion. Numerous other small debris slidesmantle the grooves themselves, some coming torest in lobate fronts on the shelf like lowersurface of the grooves.

These smaller debris slides appear to have alow albedo and may represent slumps of matureregolith from inner crater wall surface higher up­slope (Fig.4). The USGS Map­a Planet RatioMap images give an indication of the amount ofdisruption caused to the inner walls of VdG.B ascan be seen in Fig.5. Here the disrupted craterwall of VdG.B is conspicuous due to the brightblue colouration of the exposed immaturematerial in the gouge and on the slopes aboveand below.

FFiigg..33 LROC image of gouge likefeature showing upper (U) andlower (L) grooves and crescentshaped scar (Cr) that appears tograde into the upper groove distally.

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FFiigg..44 LROC imageshowing upper (U)and lower (L)grooves, dark debrisslides (Dk) and highalbedo debris fanscrossed by bouldertrails (Br). (Note'proximal' is towardsupper left)

FFiigg..55 Clementine UV­VISRation Map image ofcrater X showing brightimmature ejecta includingbutterfly wings. Thegouge like feature anddisruption to the innercrater wall of Vasco daGamma B are also pickedout in bright blue(showing the presence ofimmature regolith).

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Crater X itself is 'D' shaped in planform,with the straight side of the 'D' forming theup­range rim and the convex side thedownrange (Fig.6). The crater floor islittered with both light and dark boulders(possibly indicative if a variable lithology)as well as in­situ exposures of countryrock from which the boulders are derived,but there are no conspicuous areas ofimpact melt. In the ejecta field immediatelydownrange (to the north west) at least twonarrow sinuous features ending in lobatefronts may indicate impact melt channels,these may however be interpreted asdebris slides of unconsolidated fine ejectaand not melt. In this downrange field anumber of fine boulder trails can be seendescending over 1km in height and 2km indistance from crater X towards the floor ofVdG.B. Crater X lacks a raised rim,possibly as a consequence of the very lowangle of impact.

There is a vertical height difference ofsome 1.2kms between the downrangeedge of crater X and the start of the gougefeature which is some 8kms furtherdownrange to the north­west. It is alsoapparent that a line drawn from theproximal end of the gouge south­eastwards through crater X bisects theZoA and therefore lies in line with theinferred trajectory of the original impactor.This would suggest that if this gouge wascaused by ejected material from crater X,that this material travelled from the impactsite on a downwards trajectory of some 9°(Fig.7).In contrast to this, the gouge feature itselfis inclined upwards at an angle ofapproximately 6.5° from its proximal endtowards its termination near the south­western rim of VdG.B. The direction of thegouge however is not a continuation of thisline, but deviates to the north byapproximately 30°,

FFiigg..66 LROC image of crater X showing blocky nature offloor with mass of light and dark boulders possiblyindicating a heterogenous lithology at this location.Impactor approached from lower right.

FFiigg..77 Surface elevation profiles showing downwards trajectoryfrom crater x to proximal end gouge (X­Y) and upwardstrajectory of groove forming objects (Y­Z). Note deflection oftrajectory at Y.

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indicating that the impacting objectswere deflected by the impact with theinner crater wall of VdG.B both inazimuth and altitude. It must bestressed however that this is just aqualitative view and no precisegeometric analysis has beenconducted to support this proposition.The inner crater wall of VdG.Bbetween crater X and the proximalend of the gouge did not escape theravages of ballistic erosion, and linearstriations orientated in anapproximate east to west directioncan be seen incised into its surface,below which are rocky exposures thatappear to have produced a smallcascade of boulders which have lefttrails on the slope towards the craterfloor (Fig.8). It is possible that thesefeatures were produced as aconsequence of the impact thatproduced crater X and thepropagation of high velocity debrisdownrange.

At the very distal end of the gougebelow the south western rim of VdG.Ba small pit may represent a smallcrater produced by the final survivingfragment of the object(s) producingthe grooves, above the pit the rim ofVdG.B appears to have undergone aslight episode of slumping.

FFiigg..88 LROC view of southern inner crater wall of Vasco da GammaB showing dark striated surface (lower edge of frame) and rockyexposures possibly produced by impact of ejecta from crater x. Notethe proximal end of the gouge visible top left.

FFiigg..99 Small pit or crater (circled) at the distal end of the gougefeature just below the south­western rim of Vasco da Gamma B.

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31

DDiissccuussssiioonn

It would be fair to say that the most likely origin for the gouge like feature seen in the above example, is as aresult of high velocity debris travelling on a grazing trajectory along the inner crater wall of VdG.B from anorigin in crater X. The morphology of the gouge would suggest that this material took the form of at least twodiscrete objects (or groups of tightly associate objects) which formed the upper and lower grooves, and atrain of objects of variable size drawn out in a vertical line giving rise to the crescentic scar. The objects thatformed the upper and lower grooves would appear to have impacted and then travelled some 9kms or so,ploughing grooves into the inner crater wall of VdG.B. It is probable that as these objects travelleddownrange they were disrupted into smaller fragments which would account for the shallowing andnarrowing of the grooves to the north­west.

As noted above, the orientation of the gouge feature is deflected northwards by some 30° relative to theproposed impactors azimuth, and is elevated in altitude by some 6.5 relative to its initial origin. Theremarkably straight course of the gouge downrange thereafter suggests that the groove forming object orobjects possessed considerable kinetic energy, possibly greater than that normally partitioned intoconventional cater ejecta. This observation raises the possibility that the groove forming objects were notcrater ejecta (material excavated from crater x and expelled during the crater forming process) but afragment or fragments of the original impactor travelling along their original trajectory, and with a highproportion of their pre­impact velocity. This may be a consequence of impactor decapitation where theimpactor experiences shear failure during the crater X event producing daughter fragments which continuealong the original trajectory to impact further downrange.. Such processes are thought responsible for manyimpact structures (Schultz & Gault 1990) with Messier and Messier A being prime examples, whilst impactordecapitation has also been implicated in the production of basin size structures (Stickle et.al, 2012). In thepresent case sufficient energy was released in the crater X impact event to produce an extensive ejectablanket consistent with an oblique angle impact, as well as excavating a crater, albeit a very shallow one, butpart of the impactor escaped destruction at this point and continued onwards to create the gouges andstriations we see downrange.

If this gouge is a result of impactor decapitation, it might suggest that similar structures exist elsewhere buthave not been identified as impact related structures. The present example is particularly conspicuous dueto the relative youth of crater X and the presence of bright immature ejecta, which if absent would make theidentification of similar structures difficult.

RReeffeerreenncceessSchultz, P. H. & Gault, D. E. 1990 DECAPITATED IMPACTORS IN THE LABORATORY AND ON THEPLANETS; Abstracts of the Lunar and Planetary Science Conference, volume 21, page 1099

Stickle, A.M., Schultz, P.H and Crawford, D.A, 2012. EFFECT OF ASTEROID DECAPITATION ONCRATERS AND BASINS. 43rd Lunar and Planetary Science Conference

AAcckknnoowwlleeddggeemmeennttssLROC images and topographic charts reproduced by courtesy of the LROC Website athttp://lroc.sese.asu.edu/index.html, School of Earth and Space Exploration, University of Arizona.

Clementine UVVIS Clementine UV­VIS Ration Map image courtesy of the USGS PSD Imaging Node athttp://www.mapaplanet.org/

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TToorrrriicceelllliiBy Richard Hill Loudon Observatory

Torricelli along with the rest of Sinus Asperitatis was imaged on May 16, 2013 at 02:19 UT (Fig. 1).

This is a region of irregular craters from Hypatia on the left edge to Isidorus B in the right half of the

image. I would call your attention to Censorinus C just above Isidorus B. Look at the floor

of this crater. On LROC Quick Map it appears to be filled with ejecta from one of the local impacts

(Theophilus?). It is a very complex structure. The finest craters identified in this image were about 1.5

km across. The images were stacks of 300 frame from 1500 frame AVIs using Registax6. The 3

images were then further processed with GIMP and IrfanView and finally assembled with AutoStitch.

Torricelli has been a favourite ofmine for decades since first observing it with my old 2.4" refractor in

the early 1960s. After Triesnecker it may be the most imaged feature I've done on the Moon.

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