new observational evidence of global seismic effects of ... · waves accompanying the orientale...

New observational evidence of global seismic effectsof basin-forming impacts on the Moon from LunarReconnaissance Orbiter Lunar Orbiter LaserAltimeter data

M. A. Kreslavsky1 and J. W. Head2

Received 27 September 2011; revised 19 April 2012; accepted 25 April 2012; published 5 June 2012.

[1] New maps of kilometer-scale topographic roughness and concavity of the Moon reveala very distinctive roughness signature of the proximal ejecta deposits of the Orientale basin(the Hevelius Formation). No other lunar impact basin, even the just-preceding Imbriumbasin, is characterized by this type of signature although most have similar types ofejecta units and secondary crater structures. The preservation of this distinctive signature,and its lack in basins formed prior to Orientale, is interpreted to be the result of seismicallyinduced smoothing caused by this latest major basin-forming event. Intense seismicwaves accompanying the Orientale basin-forming event preceded the emplacement of itsejecta in time and operated to shake and smooth steep and rough topography associatedwith earlier basin deposits such as Imbrium. Orientale ejecta emplaced immediatelyfollowing the passage of the seismic waves formed the distinctive roughness signaturethat has been preserved for almost 4 billion years.

Citation: Kreslavsky, M. A., and J. W. Head (2012), New observational evidence of global seismic effects of basin-formingimpacts on the Moon from Lunar Reconnaissance Orbiter Lunar Orbiter Laser Altimeter data, J. Geophys. Res., 117, E00H24,doi:10.1029/2011JE003975.

1. Introduction

[2] Meteoritic impacts have long been understood togenerate seismic waves, which can alter the surface ofplanetary bodies. For example, Houston et al. [1973] arguedthat seismic shaking from impacts contributes to regolithgardening on the Moon. Seismic shaking works moreeffectively on small bodies [e.g., Cintala et al., 1978] andhas been recognized as the primary cause of global resurfa-cing on them [e.g., Asphaug and Melosh, 1993; Richardsonet al., 2004, 2005; Thomas and Robinson, 2005; Asphaug,2008]. Seismic shaking can trigger global downslope massmovement, if the acceleration of the surface due to seismicwaves is comparable to or exceeds gravity. On the basis ofthis criterion, Richardson et al. [2005, equation (7)] obtaineda scaling relationship giving a minimum impactor size forthe global seismic effects. We applied this scaling relation-ship to the Moon, assumed 1 Hz seismic frequency, at whichthe seismic wave attenuation of the Moon is known to below [Nakamura and Koyama, 1982], took a conservative

guess of impact seismic efficiency of 10�5, and obtained a100 km threshold diameter of the projectile. Thus, on theMoon the global seismic effects can be caused only by thelargest impacts in its geological history.[3] The possibility of global seismic effects of large-scale

impact events forming multiring basins on the Moon andother planetary bodies has been recognized long ago [e.g.,Schultz and Gault, 1975; Hughes et al., 1977]. Severalmodeling studies have aimed to understand the concentra-tion of seismic energy in the regions antipodal to impacts[e.g., Hughes et al., 1977; Watts et al., 1991; Boslough andChael, 1994; Lü et al., 2010]. These studies were inspired bythe work by Schultz and Gault [1975], who attributed somespecific morphologies in a region antipodal to Caloris basinon Mercury and possibly in regions antipodal to Imbriumand Orientale basins on the Moon to seismic effects. At thepresent state of knowledge, however, the balance betweenseismic waves and the convergence of basin ejecta [e.g.,Stuart-Alexander, 1978; Wieczorek and Zuber, 2001, andreferences therein] is not clear in terms of the formation ofdifferent morphologies in the antipodal regions. On the otherhand, as noted by Hughes et al. [1977], even outside theantipodal regions, seismic waves can produce stresses highenough to disrupt rocks and accelerations high enough toexceed gravity, and thus there is a possibility that basin-forming impacts have a global effect on morphology notlimited to the antipodal region.[4] Here we present observational evidence that seismic

effects of basing-forming impacts indeed resurface the Moon

1Department of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California, USA.

2Department of Geological Sciences, Brown University, Providence,Rhode Island, USA.

Corresponding author: M. A. Kreslavsky, Department of Earth andPlanetary Sciences, University of California–Santa Cruz, 1156 High St.,Santa Cruz, CA 95064, USA. ([email protected])

Copyright 2012 by the American Geophysical Union.0148-0227/12/2011JE003975

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, E00H24, doi:10.1029/2011JE003975, 2012

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at km scale globally. This evidence results from mappingkilometer-scale topographic roughness and concavity.

2. Mapping Topographic Roughnessand Concavity

[5] We used data from the laser altimeter LOLA [Smithet al., 2010a, 2010b] onboard the Lunar ReconnaissanceOrbiter and mapped the statistical parameters of topographickm-scale roughness and concavity in a manner similar to thatsuccessfully applied to Mars by Kreslavsky and Head [2002].[6] A good statistical measure of roughness should cor-

respond to an intuitive perception of “roughness,” in par-ticular, it should not depend on large-scale tilts of thesurface; it should characterize a typical surface rather than itsprominent features; it should be tolerant of the heavy tails ofstatistical distributions of topographic quantities, typical forplanetary surfaces. For example, the variance of topographyat a given baseline is not a good measure of roughness,because it feels regional slopes. The variance (or standarddeviation) of slope at a given baseline, a popular statistics oftopography, is also not a good measure of roughness,because it, as any distribution moment, is not tolerant toheavy tails of the slope-frequency distributions: rare verysteep slopes cause significant local increase of the variance.If we mapped the variance of slope on the Moon, we wouldsee high values associated with the peculiar steepest topo-graphic features, mostly walls of impact craters; thus, thevariance of slopes is an indicator of peculiar features ratherthan a characteristic of typical, background topographicroughness. For many natural terrains the tails of the slope-frequency distribution are so heavy, that the variance ofslope calculated over an arbitrary smaller subset of data forsome uniform area is systematically lower than the variancecalculated over the whole data set, thus, for such terrains,the variance of slope is not an objective characteristic ofterrain at all.[7] Here we used the interquartile range of profile curva-

ture as a measure of roughness. To map it, we calculated aproxy for the second derivative (“curvature”), c, of along-orbit topographic profiles at a given baseline, l, at the loca-tion of each LOLA laser shot, according to the equation:

c ¼ hþ � h� � 2hð Þ=l2; ð1Þ

where h, h+, and h� are surface elevations at the given lasershot, and shots a half-baseline ahead and a half-baselinebehind, respectively. We used different baselines to calculatecurvature, which allows us to compare roughness at differentspatial scales. For the particular maps presented in thispaper, we used two baselines: l = 0.46 km, (8 consecutiveLOLA shots), and l = 1.8 km (32 consecutive LOLA shots);the signatures of the impact basins that we analyze are bestexpressed at these baselines. For each 1/4� � 1/4� map cell(“pixel”), we selected all data points within the cell andcollected the frequency distribution of the curvature, c, in thegiven map cell. The map cell should be much larger than thebaseline. Typically, each map cell contained 20–200 datapoints. Then, for each map cell, we calculated the median(c1/2) and quartiles (c3/4, c1/4) of the curvature-frequencydistribution in each cell. We use the interquartile range ofthis distribution, c3/4–c1/4 to characterize roughness. Thenumerical values of c3/4–c1/4 are not intuitive and difficult toconceptualize, and we normalized them by a typical valuefor typical highlands. Thus, we defined roughness r as:

r ¼ c3=4 � c1=4� �

=r0; ð2Þ

where r0 = 1.7 � 10�5 m�1 for l = 0.48 km baseline, andr0 = 0.6 � 10�5 m�1 for l = 1.8 km baseline. Figures 1aand 1b show maps of roughness r at the longer and theshorter baselines, respectively. The brightest sites inFigure 1b are saturated: normalized roughness of severalyoung craters significantly exceeds 2, the upper limit of thescale in this image.[8] The interquartile range of curvature possesses the

abovementioned qualities of a good measure of roughness.The use of the second spatial derivative of elevation (cur-vature) made it insensitive to regional slopes. The charac-teristic width of the curvature-frequency distributioncharacterizes sharpness of typical slope breaks and thuscorresponds to intuitive concept of roughness. The use of theinterquartile range, a robust estimator of the scale of fre-quency distributions, made our measure of roughnessinsensitive to the heavy tails. The maps (Figures 1a and 1b)show that typical highlands have rather uniform roughness;old large craters do not come out as individual features. Thisconfirms that our roughness measure does characterize typ-ical background topography rather than features well seen in

Figure 1. Maps of roughness at (a) 1.8 km and (b) 0.46 km baselines and (c) concavity at 1.8 km base-line. The maps are in the Lambert azimuthal equal-area projection centered at the center of the nearsideand cover about 85% of the lunar surface, except a small area near the center of the farside. They are usedas red, green and blue channels for the RGB composite shown in Figure 3, left.

KRESLAVSKY AND HEAD: SEISMIC EFFECTS OF BASIN-FORMING IMPACTS E00H24E00H24

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the images. Another example of a good measure of rough-ness is the median differential slope proposed for Mars byKreslavsky and Head [2000] and applied to LOLA data byRosenburg et al. [2011]. Maps of this measure of roughnessat the same baselines, if properly contrasted, are almostindistinguishable from the maps we used in the present work.[9] Positive values of profile curvature c correspond to

concave segments of topographic profiles, while negativevalues correspond to convex segments. An ideal imaginarysurface made of smooth bowl-shaped craters with sharprims, would be almost entirely concave, except a very smallarea of rim crests. For typical highlands shaped by numerousimpacts and downslope movement of regolith, concavesegments are more frequent than convex segments. Figure 2(middle, bold violet curve) shows the frequency distributionof curvature c/r0 at l = 1.8 km baseline calculated over alarge area of typical highlands (�108 data points). To illus-trate the asymmetry of this distribution its left, negative(convex) branch is duplicated with a thin line flipped to theright. It is seen that the bold line is above the thin line(except for rare high curvature values), which means that thenumber of concave segments exceeds the number of convexsegments; concave segments prevail. The same typicallyoccurs for the curvature-frequency distributions inside themap cells, with the distinction that the total number of datapoints is much smaller and the distributions are much noisier.[10] Concavity is a quantitative measure of prevalence of

concave topographic forms over convex ones. The rationalefor our choice of particular statistical measure of concavity issimilar to that of roughness. For example, skewness of thecurvature-frequency distribution, like any other measurebased on the distribution moments, is not a good measure of

concavity because it is badly affected by the heavy tails ofthe distribution. The median curvature c1/2 does characterizethe misbalance between segments of profiles with the posi-tive and negative values of c and is little affected by the tails.However, if we imaginarily stretched all topography verti-cally, all values of c would proportionally increase, and c1/2would also proportionally increase. To obtain a statistics thatcharacterizes the proportion of concave segments regardlessof the vertical scale, we normalized the median curvature c1/2by the characteristic scale of the curvature variations. Thus,we used the median curvature normalized by its interquartilerange as a measure of concavity v:

v ¼ c1=2= c3=4 � c1=4� �

: ð3Þ

Positive concavity values indicate the prevalence of concavesegments of topographic profiles; negative concavity indi-cates the prevalence of convex topographic forms. For the“Highlands” distribution in Figure 2, equation (3) givesv = 0.033. We calculated concavity v for each map cell andobtained a concavity map. (The median value of concavity incells that belong to the typical highlands is about 0.100,significantly higher than the concavity calculated for thewhole distribution over the same area, 0.033; this occurs dueto the nonlinear nature of the concavity measure we use.)Because concavity has a high level of inherent noise, weapplied additional cosmetic filtering to this map to reduce thenoise in exchange for resolution; Figure 1c shows the result.[11] When we look at the surface images or simulated illu-

minated topography, our eyes catch the most prominent fea-tures; slope breaks associated with these features usuallyoccupy only small proportions of the map cells and hence, dueto our choice of statistics, they have a small influence on theroughness and concavity. Our measures r and v characterize themost typical, dominant topography. In a sense, the roughnessand concavity maps make visible some variations of topo-graphic properties that are not readily apparent in the images.[12] Roughness at both baselines and concavity at the longer

baseline are shown in Figure 3 as an RGB color composite:roughness at longer (Figure 1a) and shorter (Figure 1b) base-lines is coded in the red and green channels, respectively;brighter shades denote rougher surfaces; concavity (Figure 1c)is coded in the blue channel; brighter shades denote higherconcavity. The most obvious feature on this composite map isthe dichotomy between the smooth (dark) maria and the rough(brighter) highlands. In the RGB color composite, typicalhighlands have bluish and pinkish shades, which denotes theprevalence of concave topography (high intensity in the bluechannel). A few of the largest Copernican (the youngest) cra-ters and proximal ejecta of some of them are light green in themap (Figure 3): they are extremely rough, especially at theshorter baseline, and have reduced (in comparison to typicalhighlands) concavity. Several of the largest Eratosthenian andUpper Imbrian craters have distinctive orange or yellowishsurroundings, which indicates excessive roughness of theirproximal ejecta at the longer baseline and reduced concavity.

3. The Roughness and Concavity Signatureof Impact Basins

[13] After the mare - highlands dichotomy, the next moststriking feature is the 930 km diameter Orientale multiring

Figure 2. Bold curves, curvature-frequency distributionscalculated over large areas: “Orientale,” the Hevelius Forma-tion areas marked with arrows in Figure 4a, �2.8 � 107 datapoints; “Highlands,” a large area of typical highlands, younglarge craters excluded, �108 data points; “Imbrium,” the FraMauro Formation area marked with arrows in Figure 4b,�0.7 � 107 data points. Curvature (equation (1)) is calcu-lated at l = 1.8 km baseline and is normalized by its inter-quartile range for highlands. Frequencies are normalizedby their maximum values. Thin curves show left, negative(convex) branches of the distribution flipped to the rightfor comparison with the positive (concave) branches. Thinvertical lines show c1/4 and c3/4 quartiles.


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impact basin (Figure 4a; O in Figure 3) with its prominentzone of continuous ejecta (the Hevelius Formation[McCauley, 1967, 1977; Fassett et al., 2011b]; its typicalexposures are outlined with arrows in Figure 4a) immediatelysurrounding and distal to the topographic rim crest (Cordil-lera Montes ring). The Hevelius Formation is noticeablyrougher than typical highlands (by factors of 1.2 and 1.3 atthe shorter and longer baselines, respectively) and henceappears brighter in Figure 3. The pronounced yellow shade

that characterizes the Hevelius Formation in Figure 3 is dueto its low concavity (�0.04) in comparison to typical high-lands. A set of rough near-radial ray-like features is seenoutside the distal boundary of the Hevelius Formation, andoccur in the position of secondary crater chains radiatingfrom the basin and observed in images.[14] Orientale basin is unique; of the many basins on the

Moon [Wilhelms, 1987], no other impact basin has thisprominent roughness signature in Figure 3. The Imbrium

Figure 3. (left) Color RGB composite of the maps shown in Figure 1. (right) The same configuration, butthe Lambert azimuthal equal-area projection is centered at the center of the farside. I, Mare Imbrium, O,Mare Orientale; arrows show roughness/concavity anomaly in the antipodal region of Orientale.

Figure 4. Parts of the color composite roughness/concavity map in Figure 3 reprojected and centered atthe (a) Orientale and (b) Imbrium basins. Arrows outline typical exposures of continuous ejecta of thesebasins, Hevelius Formation (Figure 4a) and Fra Mauro Formation (Figure 4b).


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basin (Figure 4b; I in Figure 3), which is slightly larger thanOrientale and is slightly older, but very close in age[Wilhelms, 1987; Stöffler and Ryder, 2001], shows no signof such a signature. Its ejecta deposit (the Fra Mauro For-mation [Wilhelms and McCauley, 1971]; its typical expo-sures are outlined with arrows in Figure 4b), where it is notcovered with mare material and not modified by youngcraters, is noticeably smoother than typical highlands (itstypical roughness is 0.6–0.8 at both baselines) and is char-acterized by moderate concavity (0.07).[15] Curvature-frequency distributions calculated over the

entire exposure areas (Figure 2) provide another way tocharacterize the striking difference between the Hevelius andFra Mauro Formations. The distributions show clearly thatthe Hevelius Formation is significantly rougher than typicalhighlands (r = 1.20; orange curve in Figure 2 is wider thanthe violet curve), while the Fra Mauro Formation issmoother (r = 0.53; green curve is narrower). The HeveliusFormation has a small negative concavity (v = �0.008; thethin orange curve is above the bold orange curve inFigure 2), while the Fra Mauro Formation has a small pos-itive concavity (v = +0.012; thin green curve is below thebold one). (The quantitative discrepancy between r and vreported in this paragraph and the median r and v over thecorresponding map areas reported in the beginning of thissection are due to the nonlinear nature of our definitions ofr and v). What is the origin of the unique strong roughnesssignature characterizing the Orientale basin exterior?

4. Interpretation

[16] Orientale is the youngest basin of its size [Wilhelms,1987], and the roughness signature of its ejecta is best pre-served as seen in the images at visible wavelengths [Head,1974; Head et al., 1993; Scott et al., 1977; Spudis, 1993].Ejecta deposits of very old basins have been partly to com-pletely modified by superposed impact craters [Head et al.,2010; Fassett et al., 2011a, 2011b] and by later nearbycrater and basin ejecta (proximity degradation [Head,1975]); at some point in their history they become indistin-guishable from typical highlands. The degradation sequencealone, however, cannot account for the observed uniquesignature of the Orientale basin. The Imbrium basin is onlyslightly older than Orientale: craters larger than 20 kmsuperposed on the Imbrium basin are only factor of 1.5denser than those superposed on the Orientale basin, whileon typical highlands they are a factor of 5 to 13 denser [Headet al., 2010]. Nevertheless, Imbrium ejecta roughness sig-nature is strikingly different; moreover, it is not transitionalbetween the roughness signatures of Orientale ejecta andthat of typical highlands. Schrödinger, a small basin inter-preted to postdate the Imbrium event and predate theOrientale event [Wilhelms, 1987, Fassett et al., 2011b] alsodoes not show a distinctive roughness signature. Thus, somegeologically rapid (because of the small age difference) andpowerful (because of the major change in values) processother than simple gradual accumulation of small impactsmust be responsible for the effective smoothing of all impactbasin ejecta except that surrounding the Orientale basin.[17] We explore the obvious possibility that the basin-

forming impact itself causes such a process. In this scenario,the formation of each new basin could significantly modify

the roughness signature of a preceding basin. Orientale,being the last of the large basins [Wilhelms, 1987; Headet al., 2010], is thus the only well-preserved. What factorsassociated with the Orientale basin-forming event couldaccount for this degradation process? An excellent candidatefor an impact-caused process is intensive global seismicshaking associated with the event. Schultz and Gault [1975]showed that on the Moon the first (the strongest) seismicwaves, both the internal p-waves and the surface-boundRayleigh waves, always travel across the area of basin con-tinuous ejecta before the emplacement of ejecta. The seismicshaking associated with these waves mobilizes the surfacematerial and promotes intensive downslope movement forkilometer-scale and shorter slopes, and tends to form smoothfill in local lows. This process would cause smoothing oftopography, because local highs would get lower, local lowswould get higher, local elevation amplitude would decrease;therefore, typical curvatures would proportionally decrease.Simultaneously, smooth fill of local lows would tend toreplace mixture of concave and convex shapes with smootherpredominantly concave shapes, while convex shapes of localhighs would become sharper and hence cover smaller areas;therefore the downslope movement of material wouldincrease the proportion of area occupied by concave shapesand hence increase concavity.[18] Kilometer-scale roughness of typical heavily cratered

highlands results from a dynamical balance between rough-ening by impacts producing kilometer and larger craters, andsmoothing by small impacts and by seismic shaking bydistal basin-forming impacts. Similarly, kilometer-scaleconcavity of typical highlands results from a dynamicalbalance between an increase due to the formation of (con-cave) craters, regolith gardening, and seismic shaking, and adecrease due to emplacement of ejecta of larger craters.Cratered highlands had experienced many pre-Orientaleshaking events, and thus one more event does not alter thetopography significantly. Proximal ejecta of basins erase theequilibrated topography and replace it with specific ejectatopography. Orientale ejecta are rather pristine (except minormodification by regolith gardening). Imbrium ejecta wereemplaced originally with a topographic signature similar toOrientale ejecta and then were modified (smoothed) by theOrientale seismic episode. They, however, are still far fromthe dynamic equilibrium characteristic of typical highlands:they have not been significantly roughened by kilometer-scale craters. Seismic shaking smoothed the old highlandsless effectively than the fresh Imbrium ejecta, becauseabundant km-size and larger craters on the highlands are noterased by shaking and do contribute to kilometer-scaleroughness, while for the fresh ejecta, large craters are absentand roughness is caused by features of smaller scale.[19] Small, decameter-scale morphologies characteristic of

seismically triggered intensive downwasting (small land-slides, etc.) are likely to have been obliterated and over-printed by subsequent smaller-scale impacts and regolithgardening, but the signature of intensive downwastingwould be preserved at kilometer scales and is revealed by theroughness map (Figure 3). Obliteration of old decameter-scale morphologies is obvious in high-resolution images; itis also evident from the 0.12-km baseline roughness (notshown here [see Rosenburg et al., 2011]): roughness at thissmall scale has a much narrower variability (the highlands


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and maria are similar), except for the youngest, Copernican-aged craters.

5. Discussion

[20] Seismic shaking plays a role as a trigger for down-slope movement of surface material. Seismic energy isexpended to cause mobilization of material; its movement,smooth filling of local lows that causes the decrease ofroughness and increase of concavity, occurs under the forceof gravity. The downslope movement releases gravitationalenergy, which is partly converted to seismic and acousticenergy. In this way the resurfacing process triggered by theintensive seismic disturbance from the impact is self-supportingto some extent. If the specific morphologies in the basinantipodal regions indeed have a seismic origin, the mechanismof influence of seismic waves on topography in those regionsshould be different: concentrated seismic energy is expended toconstruct some topographic features.[21] The region antipodal to Orientale shows anomalous

orange shades in the composite map (arrows in Figure 3): ithas an increased roughness at both scales, especially at thelonger one, and a decreased concavity. This signature israther similar to the signature of the Hevelius Formation. Itmight be a hint that the specific surface morphology here iscaused by concentration of distal Orientale ejecta [Stuart-Alexander, 1978; Wieczorek and Zuber, 2001]. However,the genetic association of this antipodal roughness / con-cavity anomaly with the Orientale-forming impact is notobvious from Figure 3 alone: the anomaly can also becaused by the proximal ejecta of large Upper Imbrian craterHubble. We would like to re-emphasize that our conclusionabout the role of seismic effects is based on comparison ofbasin continuous ejecta, and not on any peculiarities of theantipodal regions.[22] No post-Orientale impact crater on the Moon was

apparently large enough to trigger global resurfacing. How-ever, it is very probable that the seismic effects associatedwith these sub-basin-scale events left local observable tracesin the areas adjacent to the larger impact craters and smallbasins. It is possible that some small-scale morphologiesobserved at high-resolution in association with Copernican-aged craters have a seismic origin.[23] Due to stronger gravity on Mercury, the threshold for

seismic amplitudes required to affect kilometer-scaletopography is higher than on the Moon. The presence of alarge presumably molten core causes the seismic effects oflarge impacts on Mercury to differ significantly from theMoon (see discussion by Lü et al. [2010]). It is interesting tolook for possible topographic indications of seismic effectsassociated with the youngest large impact basin on Mercury[Fassett et al., 2009]. This can be done with data from theMercury Laser Altimeter (MLA) instrument, onboard theMESSENGER spacecraft currently in orbit around Mercury.On Mars, the largest terrestrial planet preserving ancientcrust and impact basins, the roughness / concavity signatureof the basin ejecta is completely overprinted by later modi-fication [Kreslavsky and Head, 2000, 2002] associated withatmospheric and aqueous processes [e.g., Carr and Head,2010].[24] As we already mentioned, seismic shaking has been

recognized as a major mechanism of global resurfacing of

asteroids and other small bodies [e.g., Asphaug and Melosh,1993; Richardson et al., 2004, 2005; Thomas and Robinson,2005]. While on the Moon the global resurfacing affectskilometer-scale topography, but leaves 10-km scale andlarger craters well recognizable, on small bodies the seismiceffects can completely reset the observed geological record(see discussion by Asphaug [2008]).

6. Conclusions

[25] We presented global maps of topographic roughnessand concavity of the lunar surface at the kilometer scale.We compared the roughness / concavity signatures of theHevelius and Fra Mauro Formations with typical highlands.We concluded that seismic shaking by basin-formingimpacts causes significant modification of kilometer-scaletopography of the Moon. During the Nectarian and Imbrian,the epochs of basin-forming impacts, seismic shaking playeda significant role in shaping the surface along with crateringand volcanism.

[26] Acknowledgments. This work was partly funded as part of J.W.H.’s membership on the Lunar Reconnaissance Orbiter Lunar Orbiter LaserAltimeter (LOLA) team, under NASA grant NNX09AM54G to J.W.H.

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