sign in sign up
<<
Home , Ceraunius Fossae, Ceraunius Tholus, Tholus, Uranius Tholus

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. E6, PAGES 10,213-10,225, JUNE 25, 1992

Martian Parent Craters For The SNC Meteorites

P. J. MOUGINIS-MARK, T. J. McCoY, G. J. TAYLOR, AND K. KEIL

Planetary Geosciences,Department of and Geopttysics,School of Ocean and Earth Scienceand Technology University of Hawaii at Manoa, Honolulu

The young ages (~1.3 Ga) and the basalticto ultramafic compositionsof the shergottites, ,and chassignitesmeteorites severely restrict their potentialsource regions on . We have usedthis age and compositionalinformation, together with geologicdata derivedfrom Viking Orbiterimages, to identify25 candidateimpact craters in the Tharsisregion of Mars that couldbe the source crater for these meteorites. None of these craters are close to the size (~100 km diameter)implied by the dynamicalstudy of SNC ejectiondeveloped by Vickery and Melosh (1987). The craters in our study were selectedbecause they are >10 km in diameter, have morphologiesindicative of youngcraters, and satisfyboth the petrologiccriteria of the SNCs and the proposed1.3 Ga crystallizationages. Of these 25 craters,only nine are found on geologic unitsbelieved to be young(crater density is lessthan 570 cratersof greaterthan 1 km diameterper 106km2). No crater exists to satisfywell the criteria of samplingboth a 1.3Ga surface (nakhlites and Chassigny)and a 180 Ma surface(shergottites) without at the sametime imposingsignificant constraintson the chronologyof Mars as inferredfrom the cumulativecrater cur•es. The relatively young age (based on their inferred position in the stratigraphiccolumn of (Scott et al., 1981)) of the SNCs impliesthat volcanicactivity on the plainsof the Tharsisregion extended well past 1.3 Ga.

INTRODUCTION to be the only area on the planet that meets both the The SNC (shergottites,nakhlites, chassignite)meteorites petrologic and young age constraints and possesses are a group of nine rocks thought, on the basis of their relatively large superposed impact craters that may have young age, basaltic composition, and noble gas ejected the meteorites. On the basis of argumentspresented concentrations,to be impact debris ejected from Mars [e.g., below, we choose craters >10 km in diameter as candidate Wood and Ashwal, 1981; Shih et al., 1982; Bogard et al., craters. Finally, on the basis of the constraintsimplied by 1984; Becker and Pepin, 1984; Swindle et al., 1984; the identification of the candidate source craters, we make McSween, 1985]. A numberof authorshave made attempts, some interpretationsof the absolutechronology of Mars. based on various lines of reasoning, to locate the parent crater(s) of these rocks on Mars [e.g., Wood and Ashwal, CONSTRAINTS 1981; Nyquist, 1983, 1984; McSween, 1985; , 1985; Below, we discussa numberof propertiesof the SNCs and Vickery and Melosh, 1987]. Here we addressthis problem Mars in an attempt to constrainthe number of potential by using the extensivephotogeologic data base providedby parent craters for the SNC meteorites. the Viking Orbiter images, combinedwith informationon a number of key propertiesof the SNCs. These propertiesin- clude their young ages and basaltic to ultramafic Petrologically Diverse Volcanic Terrain compositions which, taken together, severely restrict The SNCs are a petrologicallydiverse group of igneous potential source regions on Mars. We also make use of the meteorites that range in mineralogy from basalts to dunite, present knowledge of ejection mechanisms[e.g., Melosh, sampling both extrusive and intrusive rocks. Numerous 1985; Vickery and Melosh, 1987], which indicate that the attempts have been made to relate the SNCs to one another SNCs were most likely near-surfacerocks that were subjected through simple geologic processes such as fractionation to low shock but high stressgradients, and that the material [Shih et al., 1982; Longhi and Pan, 1989]. When all of the was ejected in the form of relatively large fragments (>1 m relevant data are considered, it appears that the SNCs in size). Because SNCs are rare materials in the meteorite probably came from different initial which collection, it is also likely that they .were ejected by an experiencedvarying degrees of partial melting, fractional unusualcratering event on;Mars. crystallization, mixing and, possibly, wall rock In this analysis, we first review the constraintsimposed assimilation. The parent crater is thereforeinferred to have on the parent terrain and crater by our knowledge of the formed on materials from two different volcanic centers or to petrology and ages of the SNC meteorites. We then discuss have formed on a single volcanic center that had evolved the geomorphicproperties of impact craters on Mars in the this petrologic diversity through time. context of identifying relatively young examples. These constraints are then applied to identify probable SNC YoungTerrain ejection craters in the Tharsis region of Mars, which appears Previousworkers have used a variety of age dating tech- niques to derive the ages of the SNC meteorites. Copyright1992 by the AmericanGeophysical Union. Crystallization ages on both whole rocks and mineral separatesfor the nakhlites (Nakhla, Governador Valadares, Paper number92JE00612. and Lafayette) and Chassigny are well constrained at 0148-0227/92/92 JE-00612505.00 approximately 1.3 Ga [Papanastassiou and Wasserburg,

10,213 10,214 MOUGINIS-MARK ET AL.: PARENT CRATERS FOR METEORITES

1974; Bogard and Husain, 1977; Bogard and Nyquist, 1979; designated as C4 craters [e.g., Chapman et al., 1989]), by Wooden et al., 1979; Nakamura et al., 1982]. Shergottites their sharp and well-preserved rims, steep walls, deep and have whole rock Sm-Nd ages of 1.27 Ga [Nyquist et al., rough floors, and extensive and well-preserved ejecta 1984], but internal mineral isochronsyie•ld Rb-Sr, Sm-Nd deposits. and U-Th-Pb ages around 180 Ma [Shih et al., 1982; Jagoutz and Wi•nke, 1986; Chen and Wasserburg,1986]. Plagioclase Crater Size and Geometry shock melts and associatedcrystallization productsin ALH As notedabove, it appearsthat a singleimpact ejected all A77005 record an age of ~15 Ma [Jagoutz, 1989], of the SNC meteorites. Thus some feature of this unique synchronouswith the cosmic ray exposureage for this rock. crateringevent causedit to eject materialfrom Mars, while This geochronologyhas traditionally been interpretedas other craters did not deliver meteorites to Earth. The SNC crystallization of shergottites at 1.27 Ga with shock and parent crater appearsto be even more unusualwhen we ejection of large boulders at 180 Ma and breakup of these consider that any impact event on Mars which ejected boulders around 15 Ma. Some authors [e.g., Jones, 1986; material in the last 300 m.y. would still be delivering Jagoutz, 1989; Longhi, 1991] disagree with this material to Earth [Wetherill, 1983, 1984]. Thus the SNC interpretation, arguing that the shergottitescrystallized at parent crater appearsto be the only crater to have ejected 180 Ma with shock and ejection at around 15 Ma. material from Mars in the last 300 m.y., requiring even more Regardless of this age debate, all authors would agree that unusual circumstances.This has prompted us to consider the SNC ages imply that their parent terrain on Mars is craters which are larger than most other craters or which relatively young. It is also implicit that if the SNCs are have unusual characteristics(i.e., the crater was formed by a young, then the parent crater that ejected the rocks also has highly oblique impact). Melosh [1985] argued that craters to be young and should show all of the morphological >30 km in diameter were necessaryfor ejection of the SNCs. characteristicsof young impact craterson Mars, as discussed More recent calculationsby Vickery and Melosh [1987] have below. suggestedthat a crater >100 km in diameter may have been SingleImpact for Ejection required to eject the SNCs. These theoreticalconsiderations of SNC ejection are based on impact events that produced Cosmic ray exposure ages for the SNCs cluster in three circular craters,rather than oblique impacts. However, when groups: 11 Ma (Nakhla, GovernadorValadares, Lafayette, the above petrologicand age constraintsare appliedto Mars, and Chassigny), 2.6 Ma ($hergotty, Zagami, and ALH no crater larger than 100 km diameterfits all of the boundary 77005), and 0.5 Ma (EETA 79001). Some investigators conditions. Indeed, as we discussbelow, there are only two have argued that these groupings might record separate craters >40 km diameter (57 km and 69 km) of any impact eventson Mars [e.g., Wetherill, 1984; Vickery and degradational state that are preserved on flows in the Melosh, 1987]. This seemsunlikely, becauseit is unclear Thatsis region. In order to consider a larger number of why three random impact events would deliver young craters, we therefore choose to relax the requirement of a volcanic samplesto Earth, when these terrainsmake up a large crater, limiting our candidate craters to >10 km small part of the surfaceof Mars (<5% [Greeley and diameter. Spudis, 1981]). Indeed, we would have expectedall of the In this analysis we give preferential considerationto SNCs to come from impacts into the older Martian regions unusual crater morphologies in order to help address the such as the ridged plains, smooth plains, or in the cratered uniqueejection mechanism of the SNC parentcrater. Nyquist highlands. We have considered the possibility that only [1983, 1984] and O'Keefe and Ahrens [1986] have evaluated samplesfrom the young volcanic regions would be coherent oblique impacts as a mechanismfor ejection of the SNCs, enough to survive the passage to Earth. However, large, concluding that the ejecta from such an event have an apparentlycoherent boulders exist outsideof the young vol- increasedlikelihood to escapeMars when comparedto near- canicregions of Mars, as evidencedby imagingfrom the two vertical impacts. Laboratory experimentsgenerate elongate Viking landers [cf. Mutch et al., 1977], so that it seems craters from oblique impacts only at impact angles of 5 ø or likely that massiverocks could have been ejectedfrom these less [Gault and Wedekind, 1978]. Schultz and Lutz-Garihan areas had an appropriate impact event occurred. Thus the [1982] identified 175 cratersthat probably formed by oblique range of cosmicray exposureages observedfrom the $NCs impacts on Mars, but only 122 of these craters possesswell- must have resultedfrom the in-spacebreakup of large pieces preserved ejecta blankets (and are thus inferred to be from a single site. relatively young craters), and only six are found on young volcanic terrain. Furthermore, all but two of the 175 craters YoungCrater believed to have been producedby grazing impactsare either Constraining the age of the ejection of the SNC outside the area that we use to define young Thatsis lava meteoritesfrom Mars allows us to specifythe maximumage flows (Figure 1) or are <10 km in diameter[Schultz and Lutz- of the parentcrater; this is a key factorthat wasomitted by Garihan, 1982] and thus would not meet our selection criteria Woodand Ashwal[1981] andby McSween[1985] whenthey for the identification of the SNC parent craters. The two tried to identify candidateSNC parentcraters solely on the craters in the list of Schultz and Lutz-Garihan [1982] (their basis of the age of the target material as inferred from the craters 33 and 37) that meet our selection criteria are also numberof superposedimpact craters. Whether the 180 Ma included in our study (craters2 and 5, respectively; Table 1). age is the shock age or the crystallizationage, the SNCs could not have been ejected from Mars prior to 180 Ma. CANDIDATE TERRAINS Thus we needto find impactcraters which are very young Young volcanic surfaceson Mars are quite rare. The (lessthan 180 Ma). We identify youngcraters (those craters Tharsis region is the only area on Mars with regionally MOUGINIS-MARK ET AL.: MARTIAN PARENT CRATERS FOR METEORITES 10,215 extensive young volcanic flows. It is possible that other phy of the Tharsis region. The volcanic units of Tharsiscan areas of Mars 'may have young volcanic flows (specifically be placed in stratigraphicsequence on the basis of mapping the Elysium region [Plescia, 1990]), but such areasdo not of lava flow units and the morphology of the lava flows contain superposedcraters >10 km in diameter that could [Scott et al., 1981a]. Crater counts for the various units eject the meteorites. Since the Tharsisregion appearsto be were made by Scott and coworkers to verify these age the most likely source of the SNC meteorites, it is worth relations and to obtain some degree of correlation between consideringthe general setting and history of this area. For flows in widely separateareas, where overlap relationscould the purpose of this investigation, our designationof the not be established. Scott and Tanaka [1981c] identified six perimeterof the Tharsisregion (Figure 1) includesthe lava stratigraphicevents in Tharsis that representmajor periods plains extending around the volcanoesOlympus Mons and of volcanism that resurfaced the basement terrains. Most of Alba Patera, as well as the Tharsis ridge volcanoes(Arsia the volcanic flows evidently originatedfrom the summitsand Mons, , and ). All of these flanks of the volcanoes,although some issuedfrom fractures volcanoes are enormous by terrestrial standards,rising as and fissuresin the surroundingplains. We have chosennot much as 27 km above the surroundingplains. At least six to assign absolute ages to these units because of the other smaller volcanoes can also be found in Tharsis [Carr, uncertaintyin correlatingcumulative crater curvesand surface 1981]. The Tharsis ridge volcanoesand these smaller con- ages on Mars [e.g., Neukurn and Hiller, 1981; Barlow, structsall lie on the Tharsis bulge, centeredat approximately 1988]. Our approach for selecting candidate craters is 10øS, 110øW, which is a broad upwarped region that is similar to that taken by Wood and Ashwal [1981] and -5000 x 6000 km in size and, dependingon what is taken to McSween [1985] but usesthe detailedgeologic maps of the be its base, is ~10 km high at its center. Tharsis region to provide the relative chronology and We use the lava flow mapsproduced by Scott and Tanaka stratigraphy. We give preferenceto craterswhere petrologic [1981a,b] and Scott et al. [1981a,b,c] to define the stratigra- diversity can be readily demonstrated,but we cannotrule out

North

170' •0' lS0- lS0' •30' 120' 110' 100' •0 o •0' 70' •0' Fig. 1. Mapshowing location (dot) of eachcandidate SNC p•rent crater. All 25 craterslarger than 10 km in diameter that may be the parentcrater for the SNCs are shown,but only the bestcandidates referred to in the text are numbered. The solid line shows the boundary of the relatively young lava flows in the Tharsis region. Base map is the 1:25,000,000topographic map of Mars preparedby the U.S. GeologicalSurvey [1976] and extendsfrom longitude50 ø to 170 ø and from latitude 50øN to 30øS. Contours are elevations in kilometers above the Mars datum. For scale, 10 ø longitudeat the equatoris 590 km. 10,216 MOUGINIS-MARK ET AL.: MARTIAN PARENTCRATERS FOR METEORITES

TABLE 1. Locations,Diameters, Image Resolution,and Viking Orbiter Frame Numbersfor 25 CandidateSNC Craters

Latitude, Longitude, Diameter, Resolution, Crater degrees degrees km m/pixel Frame Unit

1 10.8 135.2 11.6 148 888A15 Aop 2 24.8 142.1 29.2 198 512A45 Aeu 3 18.5 131.9 14.8 91 890A68 Aom2 4 26.3 98.1 13.7 201 516A23 Atm 5 25.2 97.6 34.2 x 18.2 200 516A24 Atto 6 22.2 98.0 33.8 200 516A24 Atm 7 17.8 111.0 21.9 189 516A53 Atm 8 19.5 99.8 18.3 190 516A45 Atm 9 22.7 92.0 12.0 249 857A48 Atm 10 26.4 96.9 11.1 201 516A23 AHvu/Atm 11 -18.7 131.3 17.4 268 639A61 Aam4 12 44.9 106.7 21.3 72 253S05 Aap3 13 46.0 115.1 18.0 83 252S36 Aap3 14 -9.6 141.8 22.2 284 639A34 Aam3 15 - 13.9 139.5 15.4 272 639A35 Aam3 16 - 10.8 139.4 15.0 283 639A36 Aam3 17 -13.3 143.8 12.1 273 639A33 Aam3 18 -25.5 136.6 15.2 256 639A39 Aam9. 19 26.6 91.2 10.0 248 857A46 AHvu 20 39.2 120.8 14.7 243 853A03 AHap: 21 43.1 117.5 22.6 72 252S09 AHap/Aap3 22 29.9 123.5 16.7 243 853A10 AHap: 23 24.1 121.2 21.9 178 890A04 AHap: 24 37.7 99.5 18.5 87 254S48 AHap2 25 31.7 128.3 16.9 73 251S05 AHap•

Unit designationscome from the mapsprepared by the U.S. GeologicalSurvey. Cratersare presentedin termsof the apparentrelative age of the units (youngestis 1). underlying flows or dikes to provide petrologic diversity at should also often be found, and these should be well other craters. preserved(relatively deep), forming pitted terrain aroundthe parentcrater. Only thosecraters that fit the C4 classification are includedin our searchfor candidateSNC parentcraters. CANDIDATE CRATERS We have carried out an investigation of the Viking We need to identify impact craterson Mars which are Orbiter images that have a spatial resolutionbetter than 300 <180 Ma, and we use 10 km as the minimum diameter of the m/pixel for the Tharsis region of Mars. The 300 m/pixel parent crater in order to consider a representativenumber of cutoff was chosen so that the smaller morphologicfeatures cratersin Tharsis. Becauseof its young age the SNC parent on the ejecta blankets (such as radial striations, distal crater should have experienced comparatively little ramparts, and secondary craters) could be seen and used as modification (due, for instance, to eolian erosion or criteria for the identification of the youngest craters. At a meteorite bombardment)compared to other impact craterson resolution of 300 m/pixel or better, all areas of Tharsis can Mars and should appear "fresh." Morphologicalcriteria for be includedin our study. Our searchhas identified25 craters the absoluteidentification of young, fresh, Martian impact larger than 10 km in diameter with well-preserved ejecta craters do not exist, becausethe geometry and appearanceof blankets (Figure 1). Table 1 lists the geographiclocations, the crater is influenced by a combination of factors, diameters, image resolution, image frame numbers, and including projectile and target rock properties(stratification, geologicunits of each of thesecraters. Figure 2 placesthese volatile content, and strength), the role of regional candidate SNC parent craters into their relative stratigraphic weathering processes (eolian erosion and creep due to ages. subsurfacevolatiles), and the effects of subsequentcratering Stratigraphic age is the second important considerationin events. However, as a general guideline to the interpretation the identification of the candidate SNC parent crater. of the relative ages of Martian impact craters, we use the Although Tharsis is a geologicallyyoung area on Mars, 23 U.S. Geological $urvey's criteria [e.g., Chapman et al., different stratigraphicunits have been identified by Scott and 1989]. These criteria state that interior features of young coworkers on the basis of superpositionrelationships and impact craters (the C4 craters) shouldinclude sharpand well- the cumulative size frequency distribution of impact craters preservedcomplete rims, steep walls, and deep rough floors. on each unit. These 23 units clearly represent different Exterior ejecta deposits should be extensive and well stagesin the formation of Tharsis,which could have spanned preserved,often have radial striations on their surfaces,and a time interval of hundredsof millions of years [Neukumand commonly terminate in prominent distal ridges (or Hiller, 1981]. From Figure 2 it is apparentthat many of the "ramparts"). There should be a lack of small superposed candidate SNC parent craters formed on geologic units that primary craters on the ejecta blanket. Secondarycraters are intermediate or old when compared to other geologic MOUGINIS-MARK ET AL.: MARTIAN PARENT CRATERSFOR METEORITES 10,217

CRATER VOLCANIC Ascraeus Montes (Atm). Obviously, if it were to transpire NUMBER FLOWS that unit Atm has an age older than expected,fewer candidate craters should be included here (i.e., one should select only craters toward the top of the stratigraphiccolumn in Figure ø• 2). Conversely,if unit Atm is younger than expected, more ,,_a of the identifiedcraters should be consideredas candidates _ • (i.e.,one should select more craters toward the middle of the •z stratigraphiccolumn). • A total of nine craters listed in Table 1 fall into our >- category of forming on young geologic units in Tharsis. ua However, none of our nine preferred candidate craters were 4-10 • includedin the setsselected by Woodand Ashwal[1981] or • by McSween[1985]. In theseearlier studies, craters were 11 < chosenonly on the basis of the approximateage of the 12-13 • targetmaterial; the need for veryyoung (<180 Ma) craters _ • was not taken into account. This morphologicalcriterion 14-17 • eliminatedall of the cratersof Woodand Ashwal[1981] and u_ McSween [1985] from our list of preferredcandidates. The 18 19 • two candidate craters (25 km and 27 km in diameter) o• identifiedby Jones [1985] are on the plains in Amazonis ,,_a Planitia (i.e., outsidethe boundaryof the Thatsis region shownin Figure 1). These two cratersare not includedin our sample, because the target material (unit Aps of Scott and Tanaka [1981b]) is of uncertainorigin due to the mantle of • windblown material that covers the basementrocks. As a o, resultof its unusualelongate shape and relative youth, crater n: 5 in our data set was also suggestedby Nyquist [1983] to be • a suitableparent crater for the SNCs. o We now discuss the specific characteristicsof the nine 20- 24 craters which we believe are the most likely candidatesfor the parent crater of the SNC meteorites,concentrating on the 25 attributes which are the best and worst for satisfying the criteria of a young crater (<180 Ma) on a petrologically Fig. 2. Stratigraphiccolumn for the lava flows in the Tharsis diverse, young (<1.3 Ga) terrain. region of Mars, showing the relative chronologyfor the 25 craters identified as candidatesfor the SNC parent crater. Unit names and relative chronologywere derived by Scott and Tanaka Crater 1:11.6 km Diameter, 10.8øN, 135.2øW [1981a, b] and Scottet al. [1981a, b, c]. Note that we placeunit Best attributes: This crater was formed on a very young Aeu at the top even though as mappedthe unit is of uncertain surface(unit Aop of Scott and Tanaka [1981a]) to the south origin. of OlympusMons (Figure 3). The crater has two pronounced discontinuities in its rim crest, suggesting a somewhat unusuallayering in the target. In addition, the crater seems to be very young becauseit has a prominentswirl textureon its floor and appears to have a hummocky ejecta blanket, units in Tharsis. While this age classification is only suggestiveof ballistic (rather than fluidized) emplacement. relative, it does imply that if the SNC parent crater formed Worst attributes:From the mappingof the lava flow on some of the older rocks in Thatsis, then the 1.3 Ga age units in the Tharsisregion [Scott and Tanaka, 1981a], it ap- of the SNC meteoriteswould compressa considerableamount pears highly likely that the surfacematerial in this part of of volcanism into a period of time from 1.3 Ga until the Thatsis comprisesonly one lava type. Obtaining petrologi- present;it is hard to believe that all of Tharsis formed this cally diverseS NC meteoritesfrom the surfacewould therefore recently. The number of candidate parent craters for the be difficult from this site, although we cannot entirely rule SNCs can therefore be reduced by excluding all craters that out the possibility of a thin flow overlying a secondflow did not form on stratigraphicallyrecent target materials. For that is now totally buried. In addition, this crater is this further selection of candidate craters, we include only relatively small and has a typical geometryfor fresh impact fresh craters >10 km diameter that formed on the units craterson Mars. If this particularcrater, which is commonin classified as Aop through Atm in the stratigraphiccolumn. every way, ejectedmaterial from Mars, so shouldmany other This correspondsto units that have less than 570 craters >1 craterssuperposed on both Tharsisand older terrains. On the km diameterper 106 km2. The rangeof unitsincluded in our basis of calculationspresented by Wetherill [1983, 1984], sample of craters encompassesthe youngest in the one would expectthe delivery times of ejectato Earth from Tharsis region (Aop), depositionalmaterials associatedwith such craters to be extended over hundreds of millions of the aureole (Aeu), lava flows originating years, so that the probability of receiving ejecta from from crestal areas and flanks of Olympus Mons (Aom2), and typical craters such as this one shouldbe high if they did the late-stagelava flows extruded from Arsia, Pavonis, and indeed eject material from the surface. This "problem"of 10,218 MOUGINIS-MARK EF AL.' MARTIAN PARENT CRATERS FOR METEORITES

Worst attributes: The origin and age of the Olympus Mons aureole is unclear, and several different models have been proposedto explain the origin of this enigmaticfeature [e.g., Harris, 1977; Lopes et al., 1982; Francis and Wadge, 1983; Tanaka, 1985]. Because of this uncertainty in the origin of the target rocks, it is possible that these materials may not even be volcanic. In addition, their age is also uncertain; they could appear to be young because they are unconsolidated and are continually being rejuvenated by eolian erosion. The target rocks for crater 2 could thus be very old (>2-3 Ga?), since they may come from basal layers of Olympus Mons rather than the lava flows at the present- day surface.

Crater 3:14.8 km Diameter, 18.5øN, 131.9øW Best attributes: This crater is found at the summit of OlympusMons (Figure 5), where it is possiblethat the low atmosphericpressure may have aided SNC ejection. The crater has a morphologymore typical of fresh lunar impact craters than that associated with Martian craters, with swirl Fig. 3. Crater 1, 11.6 km diameter, located ~120 km south of depositson the crater floor and a hummockyejecta blanket. the Olympus Mons escarpment.High-albedo feature is a wind An asymmetry on the crater rim suggestseither a smaller, streak associatedwith eolian activity not part of the ray system of the crater. Image resolution is 148 m/pixel. This is Viking preexistingcrater or a multiple impact event. Owing to its Orbiter image 888A15. location and low number of superposedimpact craters, the target surface appears to be one of the very youngest receiving sampleson Earth from only one crater when many volcanic units on Mars. typical circular craters should have the same ability to eject Worst attributes: Like crater 1, this crater has few the SNCs is a common situation for other craters in our unusual characteristics when compared to other Martian sample (specifically, craters 3, 4, 6, 7, 8, and 9). For cratersof this size and suffers from the circular crater, single- brevity in the discussionthat follows, we will refer to this lava-type, problem. situation as "the circular crater problem." For craters 3, 6, 7, 8, and9, the lack of apparentpetrologic diversity'allows Crater 4:13.7 km Diameter, 26.3øN, 98.1øW us to refer to "the circular crater, single-lava-typeproblem." Best attributes: This crater formed on the westernedge of the Uranius and may thus have sampledboth Crater 2:29.2 km Diameter, 24.8W, 142.1øW the surrounding lava plains and the flanks of the volcano Best attributes: This is an elongate crater interpretedto (Figure 6). The sharp edges of the rampart lobes on the have been produced by an impact into a morphologically ejecta blanket, which in places rise up the lower flanks of fresh segmentof the Olympus Mons aureole material (Figure the volcano, demonstrate the relative youth of the crater. 4). The crater appearsto be very young, basedon the swirl Stratigraphicrelationships of the target permit the identifica- pattern of its interior deposits and the numerous well- tion of as being older than the surrounding preservedsecondary crater chains that extend more than 80 plains, since the basal flanks of the volcano are embayedby km away from the rim of the crater. These secondarychains the lavas. Although numerousvalleys can be seen on the are very long comparedto the size of the crater [Schultz and flanks of the volcano [Reimers and Komar, 1979], none of Singer, 1980], and possess a marked asymmetry to their these valleys extend onto the lava plains; rather, the lower distribution that indicates that the crater was formed by an portions of the valleys appear to be buried by the plains oblique impact event. No fluidized ejecta lobes exist around materials. the rim of this crater. Worst attributes: Uranius Tholus has a number of fairly In the context of the SNC samples,a key attributeof this large impact craters on its surface, indicating that it is likely target material is that it may have originatedas a landslide to be old compared to the surroundingplains, even though deposit associated with failure of the lower flanks of the the crater statistics are too poor to draw any firm volcanoOlympus Mons. Numerouslandslides can be seento conclusions. While it seems likely that two different originate from the basal escarpment of Olympus Mons geologic units were sampled, one of these units is almost [Lopes et al., 1982], and it is possiblethat this processmay certainly much older than the other, which is inconsistent have placed many different rock typesclose to the surfaceof with the similarity in ages of the SNCs. Also, this crater this single deposit. Becauseof this possibility for sampling suffersfrom the circular crater problem. rocks of two drastically different ages (180 Ma and 1.3 Ga), crater 2 is the leading candidatefor the SNC parent crater if Crater 5:34.2 x 18.2 km Diameter, 25.2øN, 97.6øW the shergottiteshave a crystallization age of 180 Ma due to the inferred very young age of some lava flows on Olympus Best attributes: This crater is the most obvious elongate Mons [ and Hiller, 1981; Landheim and Barlow, impact crater on lava flows in the Tharsisregion and formed 1991]. on the northern lower flank of the volcano MOUGINIS-MARK ET AL.: MARTIAN PARENT CRATERS FOR METEORITES 10,219

Fig. 4. Crater 2, 29.2 km diameter,located on a segmentof the OlympusMons aureolematerial. Notice the asymmetricdistribution of secondarycraters around the crater,which is suggestiveof an obliqueimpact event. Image resolutionis 198 m/pixel. This is a compositeof Viking Orbiter images512A45 and 512A46.

(Figure 7). The crater has a prominentbutterfly-wing ejecta northern flank of Ceraunius Tholus that cuts the rim. There blanket and well-preserved distal ramparts, and some radial is also some material on the floor of the crater that appears striations can be seen on the ejecta lobes at a resolution of to be associatedwith this valley. While this stratigraphy -200 m/pixel. Parts of the ejecta lobes are emplaced on the implies that the crater did not post-dateall activity on the lower flank of Ceraunius Tholus, and there is a prominent volcano, we do not believe that this is sufficient evidence to central ridge in the middle of the crater. As a result of this discount crater 5. The origin of valleys of this type of unusual geometry, this particular crater has been suggested Martian volcano has been variously attributedto lava flows by Nyquist [1983] to be a suitable parent crater for the [Carr et al., 1977], volcanic density currents such as SNCs. pyroclastic flows [Reimers and Komar, 1979], and fluvial In addition to its unusual geometry, this impact crater erosion [Mouginis-Mark et al., 1982, 1988; Gulick and probably excavated material from both the flanks of Baker, 1990]. The lack of lava flows and late-stage Ceraunius Tholus (perhaps including both extrusives and pyroclasticdeposits on CerauniusTholus suggeststo us that intrusives)and the surroundinglava plains; it may thus be a the valley in question is probably fluvial in origin, perhaps good crater for sampling multiple rock types of different initiated by water releasedas a consequenceof the seismicef- ages on Mars, with the possibility that CerauniusTholus has fects of the impact event. No other valleys on Ceraunius an age of 1.3 Ga and the surroundingplains having an age of Tholus appearto have been active since the surroundinglava 180 Ma. plains were emplaced(i.e., all the other channelspredate the Worst attributes: Jones [1985] questioned the relative formation of crater 5). However, we note that other Martian youth of crater 5, becauseof the existenceof a valley on the volcanoesthat have numerousvalleys on their flanks (such 10,220 MOUOINIS-MARK ET AL.' MARTIAN PARENT CRATERS FOR METEORITES

as [Mouginis-Mark et al., 1982]) may have been volcanically active in the very recent geologichistory ß of Mars, and so late-stage explosive volcanism (with the generation of relatively small pyroclastic flows) may be a characteristicof this type of Martian volcano. The two implied ages (1.3 Ga for CerauniusTholus and .•:: 180 Ma for the surroundingplains) also representproblems .' if crater 5 is the parent crater for the SNCs. Landheim and Barlow [1991] interpret the volcano to have formed during the heavy bombardmentof Mars about3.8 x 109years ago, so that the 1.3 Ga age is very young compared to that . inferred from the cumulative crater counts. Second, if the '• plainsmaterials (unit Atm) thatembay Ceraunius Tholus are

ß .:: 180 Ma, then many other units within Tharsis such as the

: younger flows aroundAscraeus and Olympus Montes (which .. ß both have lower superposedimpact crater densities) must have absolute ages of less than 180 Ma. Such an absolute chronology for volcanism in Tharsis is also very different

" .: from that inferred from crater counts [Neukum and Hiller, ß.'. 1981].

Crater 6:33.8 km Diameter, 22.2øN, 98.0øW Fig. 5. Crater 3, 14.8 km diameter, which lies close to the Best attributes: Immediately to the southof the volcano summitcaldera of OlympusMons volcano.Note the prominent Ceraunius Tholus are three craters that have been formed next swirl pattern of material on the floor of this crater, and the ballistically emplaced ejecta blanket. Image resolution is 91 to each other on a young/mediumage target (Figure 7). The m/pixel. This is Viking Orbiter image 46B13. largest of these craters (crater 6) is the oldest of the three

Fig. 6. Crater4 (arrowed),13.7, km diameter,formed on the flankof thevolcano Uranius Tholus Image resolution is 200 m/pixel.This is Viking Orbiterimage 516A23. MOUGINIS-MARKET AL.: MARTIANPARENT CRATERS FOR METEORITES 10,221

•-,"'::•'i. '::..• :..... ',-•... .:..W.:•...... ,:...... •:.... ?..•..• ..... :..-- •..... :.: .... '-. ;. :?',...... •:?.•:::::....:--: ":'"'..... • -. ..•. ;.--. .. : •- '-:•--• ::*:. .-'•::?".:-..::::•.-, '...... 4•. ...,'.::• ...:.;• ...." ....• • • •*':-:.•' '•.- •":-•.•-

• ,:....:-•...... :•-.: ..•.-.?...... :: '.•.:., ...... • •::.• :.:.•-. •...•-.'"-....:-.:-...-.....:>.. •::%?:•...-.: . ':•:'[,. : :::•..-*•-:.... :-;:...... ß .: ...... '•:.*.:::":':*.::*.****':•*:,.-,;::•;;' :'*'".. •e*"...... ':*;'**:...":.**.,....'•:,' '.:'•**-* .:;'::.. '• ,"::: -:;•./•' .:.;: , a:::.'-•...... ;' *:;::-,.....a:.'"',:*..:*:;.•:.':'-: --:-':•":...... ::;:•:-.:* ::.. '* '•. .:• ,}* . •'/-,. ß- "';: •: ':--:,..:.f.:•....< : •:-.•.::,

,.... r '" .' :*;'--' : C'T'*. : :• ...... •*'•':'. -,....;., .. ¾.ß

....•.,.:...•.... :.'•.};:... :}.. "*'-:..:::.x'•:.?..:.•.• •:.,. ;•: , ,: :•:':"..•:•:;:?.'-"::...'..>•:...'....

...... ß'.;:•':*•;**'::•'.;: ?,''""*...... •.....*/Xa*;'• '•:*•;•:•*...... *...... *...... ***-::.*.:*.*:*P'::½;•:**' "*•$:•.... ::-**.4"'•;•:: *.....

'"*•*•:...•?'...::½,;:,0,..';'::::;.*****...... ----.,- ,,-:.**--...... :......

'*• :,;½--•...... ,...•:•r,:• ,:,s...... "' ,;**:...... a• ....•. <•a..:.:.:..?:'"'; :•"-:::: :-:;-:,½,-.-'½:...... -.; **:::**::'-:**;.**- • .....'".:;a• ....,:*:•*:::*::• ..... *. ,,:* •.; ...:*:'"-::'-':':-:::::½;:,•'$a..... ,.• :'•:'•,:•:•::•:?,•:•_.•;:;.••:•' '"?• .....•:':' ' %: ...... ::•' ...... ::: :.•:•;• ...... •:• :•r•,.:4•:.:• *.:.'*.-..- .'-:- ' ,:':u.-: '... ..---::*-...:.'::, ...... '-- ..:...$::'*'"'.'-' -•::•,•:. -:::--..•a...--.**c-'.•.,'•- •, •:::----':...... *-•-.'.'•....' '*:'--.•--• '""...... :a•:,•:.•,s :•--• •c?•q,•.•ß ..... •;a*:;:;•* •*; ;;/.E"'"'* "'%?* ;:.'"::":•..:*a•c•:/'" •:•:..•;¾...:'-..-.*:<'•.,.:•.•' .a;...**•;-:; , , ...... ,"-'•...... -.**...... ,...... • , ...... :. ,,,...... ;...... :•

Fig. 7. Crater5 and crater6. Crater5 (34.2 x 18.2 km in diameter)is an elongateimpact crater formed on the flanks of the volcano CerauniusTholus (arrow labeled "CT"). Note the large channel that originatesat the summit of the volcanoand cutsthe wall of the impactcrater, indicating activity (possibly fluvial) on the volcanoafter the formation of the crater.Crater 6, 33.8 km diameter,is unusualbecause there are two smallerimpact craters superposed on its ejecta blanket("A" and "B"). Image resolutionis 200 m/pixel. This is Viking Orbiterimage 516A24.

impact events (based on the superpositionof overlapping Crater 7:21.9 km Diameter, 17.8øN, 111.0ø14/ ejecta blankets) but still possessesa well-preserved lobate Best attributes: This crater (Figure 8) is located to the ejecta deposit.It has a small central pit rather than a central northwestof AscraeusMons, just south of CerauniusFossae. peak in its interior, and the inner wall has prominent The crater has a prominent lobate ejecta blanket, a sharp terraces. Wood et al. [1978] suggestedthat central pits may well-defined rim crest, and a small central pit implying that have been produced by the explosive release of subsurface it is a very young crater. volatiles, making crater 6 somewhat unusual, but fresh Worst attributes: Most likely, this crater was excavated craters. with central pits are relatively common on Mars in a singlegeological unit and so couldnot be the sourcefor [Wood et al., 1978; Pike, 1980] so that the formation of a the petrologicallydiverse SNCs. The cratersuffers from the central pit within a crater does not appear to be a likely ' circularcrater, single-lava-type problem. ejection mechanism.The two smaller craters (7.9 and 10.3 km in diameter) formed in the ejecta blanket of the larger crater, but all three craters are inferred to be young because Crater 8:18.3 km Diameter, 19.5øN, 99.8ø14/ of the preservationof their ejecta and rim deposits. Best attributes: This crater(Figure 9) formedto the south Worst attributes: The presenceof the two smaller craters of Ceraunius Tholus volcano on the lava flows that extend on its ejecta blanket implies that crater 6 is not very young. northeastward from Ascraeus Mons. Its young age is In addition, this crater suffers from the circular crater, single- indicated by a well-preserved interior morphology,including lava-type problem. a prominent terrace and central pit. 10,222 MOUGINIS-MARK ET AL.: MARTIAN PARENT CRATERSFOR METEORITES

Fig. 9. Crater 8, 18.3 km diameter, located south of Ceraunius Tholus that formed on lava flows from the northern flanks of Fig. 8. Crater 7, 21.9 km diameter, located northwest of AscraeusMons. Note the circumferentialgraben that has cut part AscraeusMons. This is Viking Orbiter image 516A53. of the ejecta blanket. This is Viking Orbiter image 516A45.

Worst attributes: Ejecta on the northernside of the ejecta A secondfeature of this analysisis the lack of any craters blanket have been faulted by a circumferentialfracture related >100 km in diameter, superposedon young terrain. This to Ascraeus Mons, suggesting either recent tectonic size of crateris requiredin the modelsof Vickeryand Melosh deformation of the flank of the volcano or a greater age for [1987], but it is clear that craters of this size simply do not the impact crater. In the latter case, this would require exist on young volcanic regions on Mars. Even if the tectonic activity to have taken place on Mars since 1.3 Ga. criteria for identifying a fresh crater (which are necessaryto The crater also suffers from the circular crater, single-lava- explain the 180 Ma age of the shergottitesas either a shock type problem. age or crystallizationage) are relaxed so that all cratersof any degradationstate are includedin our sample,there are Crater 9:12.0 km Diameter, 22.7øN, 92.0øW only nine craters(including two that are listed in Table 1) Best attributes: This crater (Figure 10) formed on the lava plains to the south of the volcano Uranius Patera. At -•250 m/pixel resolution, little detailed information on the morphologyof the crater can be gained, although a central pit can be identified. '",,..";.----";;!ii :i Worst attributes: Most likely, this crater was excavated in a single geological unit and so could not be the sourcefor the petrologicallydiverse SNCs. The crater suffersfrom the circular crater, single-lava-typeproblem.

DISCUSSION

Number of Potential Parent Craters One feature that this analysis brings to light is the surprisingly small number of craters which satisfy our constraintsfor the SNC parent crater. After considerationof the requirementsfor a large (•10 km diameter),young (i.e., morphologicallyfresh) crater sampling a young, volcanic terrain, only nine craters satisfy these criteria well. If petrologicdiversity is also included as a criterion, six of these nine craters (craters 1, 3, 6, 7, 8, and 9) would also be Fig. 10. Crater 9, 12.0 km diameter, south of the volcano excluded. Considering the enormous number of craters UraniusPatera. Note that at an image resolutionof 250 m/pixel it is difficult to be confident that this is a very young crater, present on diverse surfaceson Mars, the ability to narrow since texture on the ejecta blanket may be the result of small the numberof potential SNC parentcrater candidatesto these superposedimpact cratersthat cannoteasily be seen. This is nine (or three) is in itself worthy of note. Viking Orbiter image 857A48. MOUGINIS-MARKET AL.: MARTIAN PARENTCRATERS FOR METEORITES 10,223 that are larger than 30 km in diameter. Only one of these consistentwith the earlier hypothesisof Nyquist [1983]. craters, which lies to the north of the volcano Pavonis Mons However, if crater 5 is the SNC parent crater, this places at 8.5øN, 113.0øW,exceeds 60 km in diameter,and only one severalrigorous constraints on the absolutechronology of other crater is >40 km in diameter. This largestcrater has a Mars (as described above) that are contrary to the diameterof 69 km and has had its ejectablanket extensively chronologiesof the area basedon the numberof superposed modified by the debris aprons on the northern flanks of impact craters [Neukum and Hiller, 1981; Landheim and PavonisMons. The rim crest of this crater is still complete, Barlow, 1991]. but the removal of the majority of the ejecta lobes (some fragmentary lobes can be seen to the north of the crater) Shergottite Ages suggeststhat this crater is older than 180 Ma. In terms of its stratigraphic position relative to other craters listed in As mentioned earlier, the crystallization age of the Table 1, this crater formed on unit Atto, which is the same shergottitesremains a question of much debate. While the geologic unit as that of several other candidateSNC parent exact age makes little difference in our selection of SNC craters but is older than than several of the other candidates parent craters, we may be able to use the set of craters (Figure 2). It is possible to make the ad hoc assumption selected to constrain the true ages of these rocks. If the that a crater >100 km diametercompletely destroyed a young crystallizationage of the shergottitesis really 180 Ma, then sequenceof lava flows in an area that currently shows no the SNC parent crater apparentlysampled material that was morphologicevidence for recent volcanic activity (such as 180 Ma and 1.3 Ga, with no materialsof intermediateage. the southern highlands), but the probability of this is so This of courseassumes that all the SNCs were ejectedby a small that we reject this idea. Thus our analysisshows that single event, which we favor, as discussedabove. ejection of material off of the has to be Craters2, 4, and 5 are consideredto be the most likely achieved during impact events that produce craters SNC parent craters, and they each sampled at least two significantlysmaller than the 100 km thresholdproposed by distinct lithologies. Crater 2 sampledthe OlympusMons Vickery and Melosh [1987]. aureolematerial, which almostcertainly comprises a variety of volcanicunits that may have been depositedover a long Which Crater? spanof time. In this case, it may have been possiblefor an impact event to randomlysample materials of only 180 Ma We return now to the original question of the parent and 1.3 Ga, without samplingmaterial of intermediateage. crater of the SNC meteorites. Although all nine of the most Craters 4 and 5 each sample two types of material likely candidate craters were initially ranked as having a (volcanic plains and the flanks of a volcano) and, high probability of being the SNC parent crater, detailed potentially, rocks of two different ages. While the exact examinationsuggests that some of these craters are in fact ages of these surfacesare uncertain,it seemspossible that more likely candidates than others. Craters 1, 3, and 7-9 the two surfacessampled differ in age by 1.1 b.y., even seem to be less likely candidates because they probably thoughthe young age for the plains unit Atto would assigna sample only one flow unit. Crater 6 offers the potential for very young absoluteage to many of the surfaceunits in the ejecting blocks that were originally at depth but that were Tharsis region. lying on the surface at the time of impact because they On the basisof thesestratigraphic and absolutechronolo- formed part of the ejecta blanket of an earlier crater; gies for geologicevents on Mars, it seemsless likely that however, this geographic setting does not appear to be the 180 Ma age of the shergottitesis the crystallizationage. unique on Mars, and the younger craters superposedon the No crater existsto satisfywell the criteria of samplingboth ejecta blanket are both quite small (--,10 km diameter), a 1.3 Ga surface (nakhlites and Chassigny)and a 180 Ma reducing, we feel, the probability of the ejection of these surface (shergottites)without at the same time imposing blocks. This leaves us with craters 2, 4, and 5. Crater 2 significantconstraints on the chronologyof Mars as inferred formed in a unique geologic setting, on the unconsolidated from the cumulative crater curves. aureole material of Olympus Mons, and also has an asymmetricejecta blanket indicative of an obliqueimpact. Craters4 and5 occuron the boundarybetween the flanksof Extent of Volcanism a volcano and surroundingvolcanic plains, consistentwith The most intriguing implication of this work is the the petrologic diversity of the SNCs. Crater 5, the one possibilitythat volcanic activity has occurredin the recent originallysuggested by Nyquist [1983] as the parentcrater past within parts of Tharsisother than on OlympusMons of the SNCs, has the additionaldistinction of beinghighly and may even continueuntil the present. This inferencehas elongate(probably due to a very oblique impact event), so significant implications for the interpretationof absolute that an unusualejection mechanismmay be more easily Martian chronologies[e.g., Neukumand Hiller, 1981], which ascribed to this crater. It would be difficult, if not would assign ages as old as 3.0 Ga to some of the lava impossible,to unequivocallydecide between these craters, plainsin Tharsis. Althoughcrater 1 formedon the youngest and the other six leadingcraters can certainlynot be ruled units in Tharsis (Figure 2), all the other craters formed on out. However,these three craters (particularly 2 and 5) have slightly older units. If any of craters 2-9 were the SNC uniquefeatures which may explain why they could be the parent crater, volcanic activity continued after the 1.3 Ga only cratersto have delivered to Earth the retrieved Martian crystallizationage of the SNCs. For example, craters4-9 samples.On the basisof the reasonsstated above, including formedon older lava plains (Figure 2), mappedas unit Atto the uncertaintyplaced on crater 2 becauseof the unknown by Scottet al. [1981a]. Five geologicvolcanic units formed originof the OlympusMons aureolematerial, our preferred after unit Atto, thusimplying that volcanicactivity occurred choice for the SNC parent crater is crater 5. This choice is on the plains of Tharsismuch more recentlythan 1.3 Ga. If 10,224 MOUGINIS-MARKET AL.: MARTIANPARENT CRATERS FOR METEORITES crater 5 is the SNC parent crater, geomorphicactivity (in the Chapman,M.G., H. Masursky, and A.L. Dial, Geologicmaps of form of renewed valley formation) must have taken place on sciencestudy site 1A, East , Mars, U.S. Geol. Surv. Invest. Ser., Map, 1-1962, 1989. CerauniusTholus more recently than 1.3 Ga. Since several Chen, J.H., and G.J. Wasserburg, Formation ages and evolution of lava flow units in Tharsis have significantly fewer Shergotty and its parent planet from U-Th-Pb systematics, superposedcraters than the target rocks of crater 5, then Geochim. Cosmochim. Acta, 50, 955-968, 1986. other areas of Tharsis (such as the lava flows to the southof Francis,P.W., and G. Wadge, The OlympusMons aureole:Formation the summits of Pavonis and Ascraeus Montes and to the by gravitational spreading, J. Geophys.Res., 88, 8333-8344, 1983. south and east of the Olympus Mons escarpment)must be Gault, D. E. and J.A. Wedekind,Experimental studies of obliqueim- much younger than 1.3 Ga. Recognition of this relative pact, Proc. Lunar Planet. Sci. Conf., 9th, 3844-3875, 1978. youth for several surface units in Tharsis, based on the age , R., and P.D. Spudis, , R evs. of the SNCs, must therefore be included not only in current Geophys., 19, 13-41, 1981. interpretations of the relative [e.g., Barlow, 1988] and Gulick, V.C., and V.R. Baker, Origin and evolution of valleys on martian volcanoes, J. Geophys.Res., 95, 14,325-14,344, 1990. absolute [Neukum and Hiller, 1981] crater curves for Mars but Harris, S.A., The aureole of Olympus Mons, Mars, J. Geophys. also in the thermal models for the evolution of discrete Res., 82, 3099-3107, 1977. magma sourcesin Tharsis and the elasticlithosphere of Mars Jagoutz,E., Sr and Nd isotopicsystematics in ALHA 77005: age of [e.g., Solomonand Head, 1990]. shock metamorphismin shergottitesand magmatic differentiation on Mars, Geochim. Cosmochim. Acta, 53, 2429-2441, 1989. Jagoutz, E., and H. Wiinke, Sr and Nd isotopic systematicsof CONCLUSIONS Shergotty meteorite, Geochim. Cosmochim. Acta, 50, 939-954, 1986. 1. Only a few (nine) cratersof sufficientsize (>10 km Jones,J.H., The youngestmeteorites: III. Implicationsof 180 diameter) that formed on young terrainsin the Tharsis region m.y. igneousactivity on the SPB (abstract), Lunar Planet. $ci., satisfy the petrologic criteria of the SNCs and the proposed XVI, 408-409, 1985. Jones, J.H., A discussion of isotopic systematicsand mineral 1.3 Ga crystallization ages of the meteorites and have a zoning in the shergottites: Evidence for a 180 m.y. igneous pristine morphology that might be expectedfor a 180 Ma crystallization age, Geochim. Cosmochim.Acta, 50, 969-978, crater. No craters of >100 km diameter exist on young 1986. volcanic units on Mars, and only two degradedcraters are in Landheim, R., and N.G. Barlow, Relative chronolgy of Martian the diameter range 40-70 km. This lack of large, fresh volcanoes (abstract), Lunar Planet. Sci. XXII, 775-776, 1991. Longhi, J., Complex magmatic processeson Mars: inferencesfrom impact cratersimplies that previoustheoretical studies of the the SNC meteorites,Proc. Lunar Planet. Sci. Conf., 21, 695-709, ejection mechanismfor the SNCs need to be reassessed; 1991. ejection of material may occur during impact events that Longhi, J., and V. Pan, The parent magma of the SNC meteorites, form craters a few tens of kilometers in diameter. Proc. Lunar Planet. Sci. Conf., 19th, 451-464, 1989. Lopes,R., J.E. Guest,K. Hiller, and G. Neukum,Further evidence for 2. No crater location can be found where convincing a mass movement origin of the Olympus Mons aureole, J. evidence exists for two adjacentgeologic surfacesof signifi- Geophys.Res., 87, 9917-9928, 1982. cantly different ages (180 Ma and 1.3 Ga), while at the same M½Sween,H.Y., Jr., SNC meteorites:Clues to martian petrologic time preserving the general characteristics of cratering evolution, Rev. Geophys., 23, 391-416, 1985. chronologies as inferred from the number of superposed Melosh, H.J., Ejection of rock fragments from planetary bodies, Geology, 13, 144-148, 1985. craters on different units. Mouginis-Mark,P.J., L. Wilson, and J. W. Head, Explosivevolcan- 3. Volcanic activity on the plains of the Tharsis ism on Hecates Tholus, Mars: Investigation of eruption region may extend well past 1.3 Ga. conditions,J. Geophys.Res., 87, 9890-9904, 1982. Mouginis-Mark, P.J., L. Wilson, and J.R. Zimbelman, Polygenic Acknowledgments.This researchwas supportedby grant NAGW.437 eruptionson Alba Patera, Mars, Bull. Volcanol., 50, 361-379, (P.M.-M., Principal Investigator)from NASA's PlanetaryGeology and 1988. GeophysicsProgram, and by grant NAG 9-454 (K.K., Principal Mutch, T.A., R.E. Arvidson, A.B. Binder, E.A. Guinness, and E.C. Investigator)from NASA's Planetary Materials and Geochemistry Morris, The geology of the Viking Lander 2 site, J. Geophys. Program. We thanktwo anonymousreviewers for their commentson an Res., 82, 4452-4467, 1977. earlierversion of the manuscript,Harold Garbeil and Mark Robinsonfor the preparationof Figures3, 4, 6, 8, 9, and 10, andMarc Normanfor Nakamura, N., D.M. Unruh, M. Tatsumoto, and R. Hutchison, his discussionof the youngage of the shergottites.This is Planetary Origin and evolution of the Nakhla meteoriteinferred from the Sm- Geosciencespublication 678 and $OEST contribution2850. Nd and U-Pb systematics and REE, Ba, Sr, Rb abundances, Geochim. Cosmochim. Acta, 46, 1555-1573, 1982. Neukum, G., and K Hiller, Martian ages, J. Geophys. Res., 86, REFERENCES 3097-3121, 1981. Barlow, N.G., Crater size-frequency distributionsand a revised Nyquist,L.E., Do obliqueimpacts produce martian meteorites?, Proc. Martian relative chronology, Icarus, 75, 285-305, 1988. Lunar Planet. $ci. Conf., 13th, A785-A798, 1983. Becker, R.H., and R.O. Pepin, The case for a Martian origin of the Nyquist, L.E., The oblique impact hypothesis and relative shergottites: Nitrogen and noble gases in EETA 79001, Earth probabilitiesof Lunar and Martian meteorites,Proc. Lunar Planet. Planet. Sci. Lett., 69, 225-242, 1984. $ci. Conf., 14th, B631-B640, 1984. Bogard, D.D., and L. Husain, A new 1.3 aeon-young achondrite, Nyquist, L.E., J. Wooden, B. Bansal,H. Wiesmann,and C.-Y. Shih, Geophys.Res. Lett., 4, 69-71, 1977. Sr and Nd isotopic systematics of EETA 79001 (abstract), Bogard,D.D., and L.E. Nyquist,39Ar/4øAr chonology of related Meteoritics, 19, 284, 1984. achondrites(abstract), Meteoritics, 14, 356, 1979. O'Keefe,J.D., and T.J. Ahrens,Oblique impact: A processfor obtain- Bogard, D.D., L.E. Nyquist, and P. Johnson, Noble gas contentsof ing meteoritesamples from other planets, Science,234, 346-349, shergottitesand implicationsfor the Martian origin of SNC mete- 1986. orites, Geochim. Cosmochim. Acta, 48, 1723-1739, 1984. Papanastassiou,D.A., and G.J. Wasserburg,Evidence for late forma- Carr, M.H., The Surfaceof Mars, Yale UniversityPress, New Haven, tion and young metamorphism in the achondrite Nakhla, Conn., 1981, 232 pp. Geophys.Res. Lett., 1, 23-26, 1974. Carr, M.H., R. Greeley, K.R. Blasius, J.E. Guest, and J.B. Murray, Pike, R.J., Control of crater morphologyby gravity and target Some volcanic features as viewed from the Viking orbiters, J. type: Mars, Earth, Moon, Proc. Lunar Planet. Sci. Conf.. 11th, Geophys.Res., 82, 3985-4015, 1977. 2159-2189, 1980. MOUGINIS-MARK ET AL.: MARTIAN PARENT CRATERS FOR METEORITES 10,225

Plescia, J.B., Recent flood lavas in the Elysium region of Mars, Solomon, S.C., and J.W. Head, Heterogeneitiesin the thicknessof Icarus, 88, 465-490, 1990. the elastic lithosphereof Mars: Constraintson heat flow and Reimers, C.E., and P.D. Komar, Evidence for explosive volcanic internal dynamics, J. Geophys.Res., 95, 11,073-11,083, 1990. densitycurrents on certain martianvolcanoes, Icarus, 39, 88-110, Swindle, T.D., M.W. Caffee, C.M. Hohenberg , G.B. Hudson, and 1979. R.J. Rajan, Noble gasesin SNC meteorites(abstract), Meteoritics, Schultz, P.H., and A.B. Lutz-Garihan, Grazing impactson Mars: A 19, 318-319, 1984. record of lost satellites, Proc. Lunar Planet. Sci. Conf., 13th, Part Tanaka,K.L., Ice-lubricatedgravity spreadingof the OlympusMons 1, J. Geophys.Res., 87, suppl., A84-A96, 1982. aureoledeposits, Icarus, 62, 191-206, 1985. Schultz,P. H., and J. Singer, A comparisonof secondarycraters on U.S. Geological Survey, Topographicmap of Mars, U.S. Geol. the Moon, Mercury and Mars, Proc. Lunar Planet. Conf., 11th, Surv. Invest. Ser., Map, l-961, 1976. 2243-2259, 1980. Vickery,A.M., and H.J. Melosh,The largecrater origin of the SNC Scott,D. H., and K.L. Tanaka,Map showinglava flows in the south- meteorites, Science, 237, 738-743, 1987. east part of the Amazonisquadrangle of Mars, U.S. Geol. Surv. Wetherill, G. W., Orbital evolution of impact ejecta from Mars Invest. Ser., Map, 1-1280, 1981a. (abstract), Meteoritics, 18, 420, 1983. Scott,D. H., and K.L. Tanaka,Map showinglava flows in the noah- Wetherill, G. W., Orbital evolution of impact ejecta from Mars, east part of the Amazonisquadrangle of Mars, U.S. Geol. Surv. Meteoritics, 19, 1-13, 1984. Invest. Ser., Map, 1-1279, 1981b. Wood, C.A., and L.D. Ashwal, SNC meteorites:Igneous rocks from Scott, D. H., and K.L. Tanaka, Mars: Paleostratigraphicrestoration Mars?, Proc. Lunar Planet. Sci. Conf., 12th, 1359-1375, 1981. of buried surfaces in , Icarus, 45, 304-319, 1981c. Wood, C.A., J.W. Head, and M.J. Cintala, Interior morphologyof Scott, D. H., G. G. Schaber,K.C. Hortsman,and A.L. Dial, Map fresh martian craters: The effects of target characteristics, Proc. showinglava flows in the northeastpart of the Tharsisquadrangle Lunar Planet. Sci. Conf., 9th, 3601-3709, 1978. of Mars, U.S. Geol. Surv. Invest. Ser., Map, 1-1267, 1981a. Wooden, J.L., L.E. Nyquist, D.D. Bogard, B.M. Bansal, H. Scott, D. H., K.L. Tanaka, and G.G. Schaber,Map showinglava Wiesmann,C.-Y. Shih, and G.A. McKay, Radiometricages for the flows in the southwestpart of the Arcadiaquadrangle of Mars, U.S. achondritesChervony Kut, GovemadorValadares, and Allan Hills Geol. Surv. Invest. Ser., Map, 1-1278, 1981b. 77005 (abstract), Lunar Sci.,X, 1380-1381, 1979. Scott, D. H., G. G. Schaber, K.C. Hortsman,A.L. Dial, and K.L. Tanaka, Map showinglava flows in the northwestpart of the K. Keil, T.J. McCoy, P.J. Mouginis-Mark, and G.J. Taylor, Tharsisquadrangle of Mars, U.S. Geol. Surv.Invest. Ser., Map, 1- Departmentof Geology and Geophysics,Planetary Geosciences, 1266, 1981c. University of Hawaii at Manoa, 2525 Correa Road, Honolulu, HI Shih, C.-Y., L.E. Nyquist,D.D. Bogard,G.A. McKay, J.L. Wooden, 96822. B.M. Bansal, and H. Wiesmann, Chronologyand petrogenesisof young achondrites,Shergotty, Zagami and ALHA 77005: Late (Received October 14, 1991 magmatism on a geologically active planet, Geochirn. revised March 6, 1992; Cosrnochirn. Acta, 46, 2323-2344, 1982. acceptedMarch 13, 1992.)