SECONDARY CRATERING ON MARS: IMPLICATIONS FOR AGE DATING AND SURFACEPROPERTIES. A.S. McEwen1. 1LPL, University of Arizona, Tucson, AZ 85721.

Introduction: Are small (< 1 km diameter) craterson Mars dominated by primary impacts or by the sec-ondary impacts of much larger primary craters? Manycraters are obvious secondaries, closely associated withthe primary crater and with distinctive morphologiessuch as irregular shapes and occurrence in chains andclusters with herringbone patterns. However, there hasbeen a longstanding controversy about the relativeabundances of small primaries versus distant seconda-ries on the Moon. Distant secondaries are expected tobe more circular and isolated (except when concen-trated in rays) than the obvious secondaries, difficult todistinguish from degraded (shallow) primaries.

The answer to this question has implications for ageconstraints on young surfaces and implied climatechange, the physical properties of impact-generatedregolith, provenance of surface rocks accessible to sur-face exploration or sample return, engineering consid-erations (landing safety and rover trafficability), andthe origin of Martian meteorites.

Background. The size-frequency distribution(SFD) of craters are commonly described by a powerlaw of the form N(D) = kD-b, where N is the incre-mental number of craters in a logarithmic diameterinterval, D is crater diameter, k is a constant dependingon crater density, and b is a power-law exponent. Pri-mary craters on the Moon and Mars with diametersfrom about 1 to 100 km have b ~2, whereas secondarycraters have a “steeper” SFD with b ~4. The SFD ofthe lunar maria and other plains, excluding obvioussecondaries, show a steeper slope for craters smallerthan ~1 km, which Shoemaker (1965) [ref. 1] inter-preted as the crossover point from the two distribu-tions: primaries dominate for craters larger than ~1 kmand distant (or “background”) secondaries dominate atsmaller sizes. Shoemaker noted that this crossoverdiameter should vary as a function of proximity to cra-ter rays. Away from known crater rays, Shoemakerestimated that distant secondaries dominate at craterdiameters smaller than ~200 m.

Shoemaker’s interpretation was accepted by manyworkers [e.g., Wilhelms et al. 1978, ref. 2] and appliedto Mars [Soderblom et al. 1974, ref. 3], whereas othersbelieved that small circular craters are chiefly primary[e.g., Neukum et al. 1975, ref. 4]. There has not beenany convincing way to distinguish small primariesfrom background secondaries on the Moon. The ob-servation of a steep SFD (b ~4) for small craters (0.1 to1 km) on Gaspra, where secondaries must be extremelyrare, has been considered conclusive evidence for asteep primary SFD for small craters (Neukum andIvanov 1994 [5], Chapman et al. 1996 [6]). The smallcraters on Gaspra may be produced by impact ejecta

from other asteroids, but still constitutes part of what isconsidered the primary flux onto the Moon or Mars.

Many workers are now convinced that small craterson the terrestrial planets are dominated by primaries;secondaries are not even mentioned in recent reviewpapers [e.g., 7, 8]. However, a steep (b ~4) SFD forboth secondaries and small primaries does not auto-matically mean that primaries dominate. There hasbeen no convincing refutation of Shoemaker's 1965arguments about expected abundances of secondarycraters, including background secondaries.

Recent Results. Several recent results bolster thecase made by Shoemaker that very large numbers ofsecondary craters are possible:

1. Delivery of Martian Meteorites. The discoveryof Martian meteorites requires the existence of manydistant secondary craters on Mars. Head et al. [2002,ref. 9] estimated that the probability of finding on Eartha rock ejected from Mars is 10-6 to 10-7, so an impactevent delivering a discovered meteorite to Earth musteject at least 106 rocks larger than 3 cm diameter atgreater than Mars escape velocity (5 km/s). Their hy-drocode modeling indicates that a 150-m impactor(producing a 3-km diameter crater) into basaltic plainswith negligible regolith will eject >107 fragments largerthan 3 cm. Many more and larger fragments must fallback onto Mars. An event producing a 7 km diametercrater in basaltic lavas on Mars will (in the hydrocodemodels) produce > 106 secondary craters larger than 10m diameter from fragments ejected from 4 to 5 km/s [J.N. Head, personal communication, 2003]. Such high-velocity fragments can land anywhere on Mars [cf.. 10]and will not be concentrated in identifiable rays.

2. Cratering of Europa. Bierhaus et al. [2001, ref.11] studied secondary craters produced by the 25-kmdiameter crater Pwyll on Europa, which has bright raysextending for over 1000 km. Their results (-4.2 power-law exponent) and a largest secondary crater of 1.25km diameter (5% of primary; Melosh 1989 [12]) sug-gests that Pwyll produced ~106 secondary craters largerthan 50 m diameter. They counted a total of over29,500 craters on 95 images with resolutions betterthan 100 m/pixel (covering just 0.01% of Europa), andargue that the majority of small craters on Europa aresecondaries based on cluster analyses. The young ageof Europa's surface, sparse primary cratering, and per-haps a relative paucity of small cometary bodies, to-gether make the surface a "clean slate" for identifica-tion of secondary crater characteristics, unlike the bat-tered lunar surface. Mars also has some very youngsurfaces and much better imaging data is available.

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3. Discovery of a Large Rayed Crater on Mars.McEwen et al. [2003, ref. 13] reported the discovery ofa 10-km diameter crater in the young volcanic plains ofCerberus (SE Elysium Planitia) with rays of secondarycraters (apparent in THEMIS IR mosaics) extendingmore than 800 km from the primary crater. The raysare associated with ~105 to 107 secondary craters rang-ing from 15 to 100 m in diameter. About 75% of thecraters superimposed over Athabasca Valles originatedfrom this single impact event. This may be the young-est crater on Mars of this size class, perhaps < 106 yrsold [14]. Mars meteorite EET79001 (basaltic glass) hasan ejection age of less than 1 Ma and could have origi-nated from this crater, but more rocks from this cratershould arrive at Earth over the next 20 Ma [9].

The small (15-100 m) bright-rayed craters in theCerberus region (Fig. 1) had been puzzling. They ap-pear very fresh and well-preserved, with bright ejectaand fine rays extending up to distances of ~10 craterdiameters. They are strongly clustered both locallyand globally (most are in the Cerberus region). Thecrater rims are generally circular, but those imaged atthe highest resolution appear more angular and unusu-ally shallow (compared with primary craters). Some ofthem consist of very tight clusters of craters. Theyrange in size from the limits of MOC resolution (~10 mdiameter) up to ~100 m; a few are slightly larger.

THEMIS IR images (day and night) have revealedexquisite detail in well-preserved impact craters [15].There are strong variations in thermal inertia (TI) andalbedo, apparent from early morning and late afternoontemperatures and visible images. The fresh craterstypically have 3 facies: (1) very high-TI, low-albedocrater rims and interiors; (2) moderate-/high-TI andmoderate-albedo continuous ejecta, and (3) low-TI andhigh-albedo outer ejecta and fine rays. The high-TI,low-albedo material is rocky, as expected from lunarand terrestrial craters. The low-TI ejecta facies appearsunique to Mars. The TI and albedo of this material issimilar to that of the ubiquitous Martian dust, but thefine ejecta may be lightly sintered or cemented. Apossible origin for the outer facies is atmospheric win-nowing of fine particles from the expanding ejectacurtain, producing a turbulent cloud that collapses toproduce radially-directed density currents. This ideawas proposed by Schultz and Gault [1979, ref. 16] toexplain large fluidized ejecta blankets, but might betterapply to the dusty outer ejecta facies described here.This fine outer facies must be easily eroded by eolianprocesses, so it is likely to be present only on veryyoung craters, and is likely to disappear more rapidlyfrom small craters than from larger craters.

Daytime and nighttime THEMIS mosaics of theAthabasca Valles region were acquired and assembledto support the study of this region as a candidate land-ing site for MER [17]. The nighttime mosaic revealedroughly east-west trending streaks of cold material su-perimposed over diverse terrains (Fig. 2). Small bright

(warm) spots could be resolved in some cold streaks.Comparison to MOC images revealed a 1:1 correspon-dence between these streaks and the small bright-ejectacraters. These are like crater rays composed of clus-ters and streaks of secondary craters, much like thoseobserved by Ranger imaging of rays from Tycho anddescribed by Shoemaker [1].

A larger-scale nighttime IR mosaic of the Cerberusregion was assembled and showed a regional pattern tothe cold streaks: they radiate from a position ~400 kmsoutheast of Athabasca Valles. THEMIS daytime IRimages revealed a fresh 10-km crater at this centrallocation (7.7° N, 166° E), surrounded by swarms, ra-dial streaks, and clusters of secondary craters. ATHEMIS visible image at 18 m/pixel shows this pri-mary crater to be pristine, with no superimposed cra-ters; it includes flow ejecta.

Unlike typical secondary crater fields [1, 12], thiscrater does not have obvious chains of irregular secon-dary craters within a few crater diameters of the rim.The great majority of the resolved craters formed fromblocks thrown 10 to 80 crater diameters, impactingwith velocities sufficient to produce at least crudelycircular craters. Nevertheless there is a strong radialpattern pointing to this crater as the source. The sec-ondary craters extend in all directions from the pri-mary, although they are more difficult to detect to thenorthwest and west of the primary where the dust man-tle may be thicker and/or eolian processes appear to beredistributing the fine ejecta. The rays are roughlysymmetric, not a pattern suggesting a highly obliqueimpact.

From crater classifications we concluded that ~70%to 80% of the craters superimposed over the younglavas and fluvial deposits on the floors of the Atha-basca Valles are secondaries from this single impactevent [13]. From crater counts over these and otherimages in the Cerberus region we conclude that thereare at least 105 and probably no more than 107 secon-dary craters from 30-100 m diameter produced by thisevent. There may be many more craters smaller than30 m diameter, not all identifiable at typical MOCresolutions (~6 m/pixel) and lighting angles (about 60°above the horizon at equatorial latitudes).

A global search of the THEMIS and MOC data hasrevealed other primary craters that may have producedlarge numbers of distant secondaries, but none withpreservation of the fine outer ejecta facies. This resultsuggests that the Cerberus crater is the youngest crater≥10 km diameter at least in the equatorial half of theMartian surface. Polar processes may more rapidlyeliminate or hide the outer ejecta facies.

Morphology of Small craters on Mars. Manysmall craters with sharp rims on Mars have very differ-ent morphologies than small craters on the Moon, ex-cept in lunar rays of Copernican age. The typical craterseen in NA-MOC images is shallower and less circular

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than fresh lunar craters, and similar to the morpholo-gies of very recent secondary craters in the Cerberusregion. For example, see Malin and Edgett [2001, ref.18] for images of the Pathfinder landing region in AresValles (their Fig. 7) and part of Tiu Valles (Fig. 8).The surfaces appear saturated with small craters, butonly Big Crater (~1.5 km diameter) has a bowl shapelike a fresh primary crater on the lunar mare. All oth-ers appear to have much smaller depth/diameter ratios.There is no obvious correlation between crater depthand presence of eolian bedforms (e.g. Figure 6 of Ma-lin and Edgett); if they are shallow due to eolian fill,then there must have been a very recent filling eventbecause few or no bowl-shaped craters have subse-quently formed.

If all of the small craters in the Ares and Tiu re-gions are primaries, then this region must have experi-enced impact "gardening" to depths of 3-14 m [Hart-mann et al. 2001, ref. 19]. The presence of a regolithseveral meters thick is not consistent with the Path-finder team’s interpretations that the surface appearsvery similar to what would have been expected soonafter catastrophic floods ~2 to 3.5 b.y. ago [20, 21]. Inparticular, a series of troughs and ridges of 1-2 m am-plitude, visible throughout the Pathfinder scene, havebeen interpreted as ancient flood features [22] or asyounger transverse dunes [23]. MOC images showthat they are confined to channel floors and interruptedby the larger craters, supporting the fluvial interpreta-tion [18]. However, ancient 1-2 m high features couldnot be preserved if the surface has been uniformly gar-dened to depths of 3-14 m.

An alternative explanation is that the craters seen inthese regions are largely secondary craters. The shal-low morphology would be the original form of theselow-velocity (<5 km/s) impacts, and many of themcould have formed simultaneously. Impact gardeningstill must have occurred, but to shallower depths andwith less uniformity, perhaps allowing preservation of2-3.5 b.y. old meter-scale fluvial features in many ar-eas. However, it is very difficult to understand howimbricated boulders seen by Pathfinder could have re-mained in place since the catastrophic flooding [21].The surface rocks seen here and at VL1 are most likelythe result of much more recent impacts, regardless ofwhether the impacts were primaries or secondaries. Inaddition, deflation around the Pathfinder rocks mustdate back to the cratering that emplaced the rocksrather than the flooding, so the extremely slow erosionrate estimates from the VL1 and Pathfinder sites [24,25] may be orders of magnitude too low.

Crater Age Dating of Young Surfaces: Hartmannand Neukum [7] have each published model isochronsfor Mars; the two models are nearly identical for cra-ters smaller than 1 km diameter. Several studies [e.g.,26, 27] have compared crater counts to these models,concluding that some lava flows and flood channels

have very young ages, probably less than 10 Ma. Fur-thermore, the absence of any craters superimposed onmorphologies such as high-latitude gullies and flowlobes suggest ages of less than ~5 Ma, according to theHartmann or Neukum isochrons. These results havegarnered much attention because they indicate that flu-vial and volcanic activity and climate change are on-going, but the age constraints must be reconsidered ifthe abundance of secondary craters is greater than orcomparable to that of small primaries.

If the Hartmann/Neukum isochrons are still validfor the production function, then secondaries from theCerberus crater could be eliminated from counts in thisregion. The result would be to reduce the model age ofthe floor of Athabasca Valles from ~4 Ma [26] to ~1Ma or less. However, secondaries must have contrib-uted to the Hartmann and Neukum functions, to an un-known degree. If secondary cratering were uniform inspace and time, then the isochrons would remain validfor age estimates. But secondary craters are highlyclustered spatially and temporally.

What are the uncertainties in these estimates due tothe nonuniformities of secondary cratering in space andtime? The Cerberus plains include extensive individuallava flows (large areas at a constant age) that we havemapped from MOLA shaded relief images [28]. MOCimage M13-01528 crosses both heavily and lightlycratered regions on one well-defined lava flow; craterdensities differ by a factor of 17 over areas with sig-nificant numbers of craters (>100) and give model agesranging from ~200 Ma to ~10 Ma. We conclude fromthis simple test that the age uncertainty here is at least afactor of 20, independent of counting statistics. Thus,the maximum age for areas free of craters in MOC im-ages (e.g., gullies) may be ~100 Ma rather than ~5 Ma.Conversely, surfaces with many small craters could beyounger than a few Ma (or <1 Ma in the Cerberus re-gion). But it gets worse for crater counters. If Head et al.[9] have the right explanation for the age variations ofMartian meteorites, then impacts into young lava plainswith little regolith must produce many more distantsecondary craters than impacts into older regolith-mantled terrains. Hence the secondary cratering ratein the southern highlands must be less than that in thenorthern plains. We may need different isochrons fordifferent regions of Mars if secondaries are significantto the statistics.

Future Studies. Much can be accomplished withMGS and Odyssey data to better understand the issuesdiscussed here. Full-resolution MOC images can beused to better constrain the SFD of the Cerberus sec-ondaries, and MOC and THEMIS data can be used toidentify and map out other secondary crater fields.Perhaps the greatest future need is for meter-scale to-pographic data such as that expected from HiRISE [29]to better distinguish primaries from secondaries and to

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constrain regolith thicknesses. One of the Mars Explo-ration Rovers is expected to land in Gusev crater,which contains a few small bright-rayed craters that areprobably distant isolated secondaries from the Cerberuscrater, so we could get a close-up look at a fresh sec-ondary crater on Mars.

References:[1] Shoemaker, E.M. (1965), Chapter 2 in The Nature

of the Lunar Surface, edited by W.N Hess et al., JohnHopkins Press, Baltimore.

[2] Wilhelms, D.E. et al. (1978) Proc. Lunar Planet.Sci. Conf. 9th, 3735-3762.

[3] Soderblom, L.A., et al. (1974) Icarus 22, 239-263.[4] Neukum, G., et al. (1975) Proc. Lunar Planet. Sci.

Conf. 7th, 2867-2881.[5] Neukum, G. and Ivanov, B.A. (1994) In Hazards

due to Comets and Asteroids.[6] Chapman, C. et al. (1996), Icarus 120, 231-245.[7] Hartmann, W. and G. Neukum (2001) Space Sci-

ence Rev. 96, 165-194.[8] Ivanov, B.A. et al., in Asteroids III, 89-101.[9] Head, J.N., et al. (2002) Science 298, 1752-1756.[10] Lorenz, R. (2000) Icarus 144, 353-366.[11] Bierhaus, E.B., et al. (2001) Icarus 153, 264-276.[12] Melosh, H.J. (1989) Impact Cratering.[13] McEwen, A.S. et al. (2003) LPSC abstract.[14] Ivanov, B.A. (2001) in Chronology and Evolution

of Mars, Space Science Reviews 96.[15] Christensen, P.R., et al. (2003), Science, in press.[16] Schultz, P.H., and Gault, D.E. (1979) J. Geophys.

Res. 84, 7669-7687.[17] Christensen, P.R., Ruff, S., et al., in preparation.[18] Malin, M.C., and Edgett, K.E. (2001) J. Geophys.

Res. 106, 23,429-23,570.[19] Hartmann, W.K. et al. (2001) Icarus 149, 37-53.[20] Golombek, M. P. et al. (1997) Science 278, 1743-

1748.[21] Smith, P.H. et al. (1997) Science 278, 1758-1765.[22] Parker, T.J., and J.W. Rice (1997) J. Geophys.

Res. 102, 25,641-25,656.[23] Greeley, R. et al. (2000) J. Geophys. Res. 105,

1829-1840.[24] Arvidson, R.E. et al. (1979) Nature 278, 533-535.[25] Golombek, M.P., and N.T. Bridges (2000) J. Geo-

phys. Res. 105, 1841-1853.[26] Burr, D.M., et al. (2002) Icarus 159, 53-73.[27] Hartmann, W.K., and Berman, D.C. (2000) JGR

105, 15,011-15,026.[28] Lanagan, P., and McEwen, A., this conference.[29] McEwen, A.S. et al., this volume.

Figure 2 (right). THEMIS nighttime mosaicshowing dark (cold, low thermal inertia) streaks withinterior bright spots. These are rays if secondary cra-ters like those in Figure 1. (Image ~50 km wide.)

Figure 1 (below). Part of MOC image M2-00581 inAthabasca Valles, showing part of a ray composed ofsecondary craters. Scene is 3 km wide.

F

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