JOURNAL
OF GEOPHYSICAL
Martian
RESEARCH,
VOL. 97, NO. E6, PAGES 10,213-10,225, JUNE 25, 1992
Parent Craters For The SNC
P. J. MOUGINIS-MARK, T. J. McCoY,
Meteorites
G. J. TAYLOR, AND K. KEIL
Planetary Geosciences,Department of Geology and Geopttysics,Schoolof Ocean and Earth Scienceand Technology
University of Hawaii at Manoa, Honolulu
The young ages (~1.3 Ga) and the basalticto ultramaficcompositionsof the shergottites,
nakhlites,and chassignites
meteoritesseverelyrestricttheir potentialsourceregionson Mars. We
have usedthis age and compositional
information,togetherwith geologicdata derivedfrom Viking
Orbiterimages,to identify25 candidateimpactcratersin the Tharsisregionof Mars that couldbe
the source crater for these meteorites. None of these craters are close to the size (~100 km
diameter)implied by the dynamicalstudyof SNC ejectiondevelopedby Vickery and Melosh
(1987). The craters in our study were selectedbecausethey are >10 km in diameter, have
morphologies
indicativeof youngcraters,and satisfyboth the petrologiccriteriaof the SNCs and
the proposed1.3 Ga crystallizationages. Of these25 craters,only nine are found on geologic
unitsbelievedto be young(craterdensityis lessthan570 cratersof greaterthan 1 km diameterper
106km2).Nocrater
exists
to satisfy
wellthecriteria
of sampling
botha 1.3Gasurface
(nakhlites
and Chassigny)
and a 180 Ma surface(shergottites)
withoutat the sametime imposingsignificant
constraintson the chronologyof Mars as inferredfrom the cumulativecratercur•es. The relatively
young age (basedon their inferred position in the stratigraphiccolumn of Tharsis (Scott et al.,
1981)) of the SNCs impliesthat volcanicactivityon the plainsof the Tharsisregionextendedwell
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 ejectedthe meteorites. On the basisof 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 basisof 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 authorshavemadeattempts, someinterpretationsof the absolutechronologyof Mars.
based on various lines of reasoning, to locate the parent
crater(s) of these rocks on Mars [e.g., Wood and Ashwal,
1981; Nyquist, 1983, 1984; McSween, 1985; Jones, 1985;
Vickery and Melosh, 1987]. Here we addressthis problem
by using the extensivephotogeologicdata base providedby
the Viking Orbiter images, combinedwith informationon a
numberof key propertiesof the SNCs. These propertiesinclude their young ages and basaltic to ultramafic
compositions which, taken together, severely restrict
potential sourceregions on Mars. We also make use of the
present knowledge of ejection mechanisms[e.g., Melosh,
1985; Vickery and Melosh, 1987], which indicate that the
SNCs were most likely near-surfacerocks that were subjected
to low shock but high stressgradients,and that the material
was ejected in the form of relatively large fragments(>1 m
CONSTRAINTS
Below, we discussa numberof propertiesof the SNCsand
Mars in an attempt to constrainthe number of potential
parent craters for the SNC meteorites.
Petrologically Diverse Volcanic Terrain
The SNCs are a petrologicallydiverse group of igneous
meteorites that range in mineralogy from basalts to dunite,
sampling both extrusive and intrusive rocks. Numerous
attempts have been made to relate the SNCs to one another
through simple geologic processes such as fractionation
[Shih et al., 1982; Longhi and Pan, 1989]. When all of the
relevant data are considered, it appears that the SNCs
in size). Because SNCs are rare materials in the meteorite probably came from different initial magmas which
collection, it is also likely that they .were ejected by an experiencedvarying degreesof partial melting, fractional
unusualcratering event on;Mars.
crystallization, magma mixing and, possibly, wall rock
In this analysis, we first review the constraintsimposed assimilation. The parent crater is thereforeinferredto have
on the parent terrain and crater by our knowledge of the formed on materials from two different volcanic centers or to
petrologyand agesof the SNC meteorites. We then discuss have formed on a single volcanic center that had evolved
the geomorphicpropertiesof 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 cratersin the Tharsis region of Mars, which appears
Previousworkershave used a variety of age datingtechniques 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,
Papernumber92JE00612.
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.: MARTIAN 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•ldRb-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 singleimpactejectedall
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 ejectionof the SNCs.
characteristics
of young impact craterson Mars, as discussed
More recent calculationsby Vickery and Melosh [1987] have
below.
suggestedthat a crater >100 km in diametermay have been
required
to eject the SNCs. These theoreticalconsiderations
SingleImpactfor Ejection
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 lava 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
very small part of the surfaceof Mars (<5% [Greeley and
Spudis, 1981]). Indeed,we would have expectedall of the
SNCs to come from impactsinto the older Martian regions
diameter.
In this analysis we give preferential considerationto
unusual crater morphologies in order to help address the
such as the ridged plains, smooth plains, or in the cratered
uniqueejectionmechanismof 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
apparentlycoherentbouldersexist outsideof the young volincreasedlikelihood to escapeMars when comparedto nearcanicregionsof Mars, as evidencedby imagingfrom the two
vertical impacts. Laboratory experimentsgenerateelongate
Viking landers [cf. Mutch et al., 1977], so that it seems
craters from oblique impacts only at impact angles of 5ø or
likely that massiverocks could have been ejectedfrom these
areas had an appropriate impact event occurred. Thus the
range of cosmicray exposureagesobservedfrom the $NCs
must have resultedfrom the in-spacebreakupof large pieces
from a single site.
less [Gault and Wedekind, 1978]. Schultz and Lutz-Garihan
[1982] identified 175 cratersthat probably formed by oblique
impacts on Mars, but only 122 of these craters possesswellpreserved ejecta blankets (and are thus inferred to be
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[Schultzand Lutz-
of the parentcrater;this is a key factorthat wasomittedby 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 parentcraterssolelyon the craters in the list of Schultz and Lutz-Garihan [1982] (their
basis of the age of the target material as inferred from the
numberof superposed
impact craters. Whetherthe 180 Ma
age is the shockage or the crystallizationage, the SNCs
could not have been ejected from Mars prior to 180 Ma.
craters 33 and 37) that meet our selection criteria are also
includedin our study (craters2 and 5, respectively; Table 1).
CANDIDATE
TERRAINS
Thus we needto find impactcraterswhichare very young
Young volcanic surfaceson Mars are quite rare. The
(lessthan 180 Ma). We identifyyoungcraters(thosecraters Tharsis region is the only area on Mars with regionally
MOUGINIS-MARK ET AL.: MARTIAN PARENT CRATERS FOR METEORITES
extensive young volcanic flows. It is possible that other
areas of Mars 'may have young volcanic flows (specifically
the Elysium region [Plescia, 1990]), but such areasdo not
contain superposedcraters >10 km in diameter that could
eject the meteorites. Since the Tharsisregion appearsto be
the most likely source of the SNC meteorites,it is worth
consideringthe general setting and history of this area. For
the purpose of this investigation, our designationof the
perimeterof the Tharsisregion (Figure 1) includesthe lava
plains extending around the volcanoesOlympus Mons and
Alba Patera, as well as the Tharsis ridge volcanoes(Arsia
Mons, Pavonis Mons, and Ascraeus Mons). All of these
volcanoes are enormousby terrestrial standards,rising as
much as 27 km above the surroundingplains. At least six
10,215
phy of the Tharsisregion. The volcanicunits of Tharsiscan
be placed in stratigraphicsequenceon the basis of mapping
of lava flow units and the morphology of the lava flows
[Scott et al., 1981a].
Crater counts for the various units
were made by Scott and coworkers to verify these age
relations and to obtain some degree of correlation between
flows in widely separateareas,where overlap relationscould
not be established. Scott and Tanaka [1981c] identified six
stratigraphicevents in Tharsis that representmajor periods
of volcanism
that resurfaced
the basement
terrains.
Most
of
the volcanic flows evidently originatedfrom the summitsand
flanks of the volcanoes,althoughsome issuedfrom fractures
and fissuresin the surroundingplains. We have chosennot
to assign absolute ages to these units because of the
other smaller volcanoes can also be found in Tharsis [Carr,
uncertaintyin correlatingcumulativecrater 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 detailedgeologicmapsof the
Tharsis region to provide the relative chronology and
be its base, is ~10 km high at its center.
We use the lava flow mapsproducedby 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'
South
Fig. 1. Mapshowing
location
(dot)of eachcandidate
SNCp•rentcrater.
All 25 craters
larger
than10kmin diameter
that may be the parentcraterfor the SNCs are shown,but only the bestcandidates
referredto 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
1:25,000,000topographicmap of Mars preparedby the U.S. GeologicalSurvey[1976] and extendsfrom longitude50ø
to 170 ø and from latitude 50øN to 30øS.
longitudeat the equatoris 590 km.
Contours are elevations in kilometers above the Mars datum.
For scale, 10 ø
10,216
MOUGINIS-MARK ET AL.: MARTIAN PARENTCRATERSFOR METEORITES
TABLE 1. Locations,Diameters,Image Resolution,and Viking Orbiter Frame Numbersfor 25 CandidateSNC Craters
Crater
Latitude,
degrees
Longitude,
degrees
Diameter,
km
Resolution,
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
890A68
Aom2
4
5
6
7
8
9
26.3
25.2
22.2
17.8
19.5
22.7
98.1
97.6
98.0
111.0
99.8
92.0
201
200
200
189
190
249
516A23
516A24
516A24
516A53
516A45
857A48
Atm
Atto
Atm
Atm
Atm
Atm
AHvu/Atm
91
13.7
34.2 x 18.2
33.8
21.9
18.3
12.0
10
26.4
96.9
11.1
201
516A23
11
12
13
14
15
16
17
18
-18.7
44.9
46.0
-9.6
- 13.9
- 10.8
-13.3
-25.5
131.3
106.7
115.1
141.8
139.5
139.4
143.8
136.6
17.4
21.3
18.0
22.2
15.4
15.0
12.1
15.2
268
72
83
284
272
283
273
256
639A61
253S05
252S36
639A34
639A35
639A36
639A33
639A39
Aam4
Aap3
Aap3
Aam3
Aam3
Aam3
Aam3
Aam9.
AHvu
19
26.6
91.2
10.0
248
857A46
20
21
22
23
39.2
43.1
29.9
24.1
120.8
117.5
123.5
121.2
14.7
22.6
16.7
21.9
243
72
243
178
853A03
252S09
853A10
890A04
24
37.7
99.5
18.5
87
254S48
25
31.7
128.3
16.9
73
251S05
AHap:
AHap/Aap3
AHap:
AHap:
AHap2
AHap•
Unit designations
comefrom the mapspreparedby the U.S. GeologicalSurvey. Cratersare presented
in termsof the apparentrelativeage
of the units (youngestis 1).
underlying flows or dikes to provide petrologicdiversity at
should also often be found, and these should be well
other
preserved(relatively deep), forming pitted terrain aroundthe
parentcrater. Only thosecratersthat fit the C4 classification
are includedin our searchfor candidateSNC parentcraters.
We have carried out an investigation of the Viking
Orbiter images that have a spatial resolutionbetter than 300
m/pixel for the Tharsis region of Mars. The 300 m/pixel
cutoff was chosen so that the smaller morphologicfeatures
on the ejecta blankets (such as radial striations, distal
ramparts, and secondarycraters) could be seen and used as
criteria for the identification of the youngestcraters. At a
resolutionof 300 m/pixel or better, all areas of Tharsis can
be includedin our study. Our searchhas identified25 craters
larger than 10 km in diameter with well-preserved ejecta
blankets (Figure 1). Table 1 lists the geographiclocations,
diameters, image resolution, image frame numbers, and
geologicunits of each of thesecraters. Figure 2 placesthese
candidate SNC parent craters into their relative stratigraphic
craters.
CANDIDATE
CRATERS
We need to identify impact craterson Mars which are
<180 Ma, and we use 10 km as the minimum diameter of the
parent crater in order to consider a representativenumber of
cratersin Tharsis. Becauseof its young age the SNC parent
crater should have experienced comparatively little
modification (due, for instance, to eolian erosion or
meteorite bombardment)comparedto other impact craterson
Mars and should appear "fresh." Morphologicalcriteria for
the absoluteidentificationof young, fresh, Martian impact
craters do not exist, becausethe geometry and appearanceof
the crater is influenced by a combination of factors,
including projectile and target rock properties(stratification,
volatile content, and strength), the role of regional
weathering processes (eolian erosion and creep due to
subsurfacevolatiles), and the effects of subsequentcratering
events. However, as a general guideline to the interpretation
of the relative ages of Martian impact craters, we use the
U.S. Geological $urvey's criteria [e.g., Chapman et al.,
1989]. These criteria state that interior features of young
impact craters(the C4 craters)shouldinclude sharpand wellpreservedcomplete rims, steep walls, and deep rough floors.
Exterior ejecta deposits should be extensive and well
preserved,often have radial striations on their surfaces,and
commonly terminate in prominent distal ridges (or
ages.
Stratigraphic age is the secondimportant considerationin
the identification of the candidate SNC parent crater.
Although Tharsis is a geologicallyyoung area on Mars, 23
different stratigraphicunits have been identifiedby Scott and
coworkers on the basis of superpositionrelationships and
the cumulative size frequency distribution of impact craters
on each unit. These 23 units clearly represent different
stagesin the formation of Tharsis,which could have spanned
a time interval of hundredsof millions of years [Neukumand
Hiller, 1981]. From Figure 2 it is apparentthat many of the
"ramparts"). There shouldbe 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
CRATER
NUMBER
10,217
Ascraeus Montes (Atm). Obviously, if it were to transpire
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
VOLCANIC
FLOWS
ø
• 2). Conversely,
if unitAtmisyounger
thanexpected,
more
,,_a of the identifiedcratersshouldbe consideredas candidates
_•
•z
•
(i.e.,oneshould
select
morecraters
toward
themiddle
of the
stratigraphic
column).
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
11
•
•
included
in the setsselected
by WoodandAshwal[1981]or
by McSween[1985]. In theseearlierstudies,
craterswere
<
chosenonly on the basis of the approximateage of the
12-13
•
14-17
_•
•
u_ McSween [1985] from our list of preferredcandidates. The
•
two candidate craters (25 km and 27 km in diameter)
19
18
targetmaterial;
theneedfor veryyoung(<180Ma) craters
was not taken into account. This morphologicalcriterion
eliminatedall of the cratersof Woodand Ashwal[1981] and
o
• identified
byJones[1985]areontheplains
in Amazonis
,,_a Planitia (i.e., outsidethe boundaryof the Thatsisregion
•
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 unusual
elongate
shapeandrelativeyouth,crater
n:
•
o
20-
24
25
Fig. 2. Stratigraphiccolumnfor the lava flows in the Tharsis
region of Mars, showing the relative chronologyfor the 25
cratersidentified as candidatesfor the SNC parent crater. Unit
namesand relative chronologywere derived by Scott and Tanaka
[1981a, b] and Scottet al. [1981a, b, c]. Note that we placeunit
Aeu at the top even thoughas mappedthe unit is of uncertain
origin.
5 in our data set was also suggestedby Nyquist [1983] to be
a suitableparent crater for the SNCs.
We now discussthe specific characteristicsof the nine
craters which we believe are the most likely candidatesfor
the parentcrater of the SNC meteorites,concentratingon the
attributes which are the best and worst for satisfying the
criteria of a young crater (<180 Ma) on a petrologically
diverse,young (<1.3 Ga) terrain.
Crater 1:11.6
km Diameter, 10.8øN, 135.2øW
Best attributes: This crater was formed on a very young
surface(unit Aop of Scott and Tanaka [1981a]) to the south
of OlympusMons (Figure 3). The craterhas 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.
Worst attributes:From the mappingof the main lava flow
relative, it does imply that if the SNC parent crater formed
on some of the older rocks in Thatsis, then the 1.3 Ga age units in the Tharsisregion [Scott and Tanaka, 1981a], it apof 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 petrologipresent;it is hard to believe that all of Tharsis formed this cally diverseSNC 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 reducedby excluding all craters that out the possibility of a thin flow overlying a secondflow
did not form on stratigraphicallyrecent targetmaterials. 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
on both Tharsisand older terrains. On the
This correspondsto units that have less than 570 craters>1 craterssuperposed
km diameterper 106 km2. The rangeof unitsincludedin our basis of calculationspresentedby Wetherill [1983, 1984],
sample of craters encompassesthe youngest lavas in the one would expectthe delivery timesof ejectato Earth from
Tharsis region (Aop), depositionalmaterials associatedwith such craters to be extended over hundreds of millions of
the Olympus Mons 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 craterssuch as this one shouldbe high if they did
the late-stagelava flows extruded from Arsia, Pavonis, and indeedeject 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 becausethey 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 presentday surface.
Crater 3:14.8
Best
km Diameter, 18.5øN, 131.9øW
attributes:
This
crater is found at the summit of
OlympusMons (Figure 5), where it is possiblethat the low
atmosphericpressuremay 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
the Olympus Mons escarpment.High-albedo feature is a wind
streakassociatedwith eolian activity not part of the ray system
of the crater. Image resolution is 148 m/pixel. This is Viking
Orbiter image 888A15.
depositson the crater floor and a hummockyejecta blanket.
An asymmetryon the crater rim suggestseither a smaller,
preexistingcrater or a multiple impact event. Owing to its
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
typical circular craters should have the same ability to eject
volcanic
the SNCs
unusual characteristics when compared to other Martian
cratersof this size and suffersfrom the circular crater, singlelava-type, problem.
is a common
situation
for other
craters
in our
sample (specifically, craters 3, 4, 6, 7, 8, and 9). For
brevity in the discussionthat follows, we will refer to this
situation as "the circular crater problem." For craters 3, 6,
units
Worst
7, 8, and9, the lackof apparent
petrologic
diversity'allowsCrater
on Mars.
attributes:
4:13.7
Like
crater 1, this crater has few
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 volcanoUranius Tholus 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 elongatecrater 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 Stratigraphicrelationshipsof the target permit the identificapattern of its interior deposits and the numerous well- tion of Uranius Tholus as being older than the surrounding
preservedsecondarycrater 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 craterson 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. Numerouslandslidescan 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
[Lopeset al., 1982], and it is possiblethat this processmay certainly much older than the other, which is inconsistent
have placedmany 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 drasticallydifferent 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
Best attributes: This crater is the most obviouselongate
the inferred very young age of some lava flows on Olympus
Mons [Neukum 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 Ceraunius Tholus
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
asymmetricdistributionof secondary
cratersaroundthe crater,whichis suggestive
of an obliqueimpactevent. Image
resolutionis 198 m/pixel. This is a compositeof Viking Orbiter images512A45 and 512A46.
(Figure 7). The crater has a prominentbutterfly-wing ejecta
blanket and well-preserveddistal ramparts, and some radial
striations can be seen on the ejecta lobes at a resolution of
-200 m/pixel. Parts of the ejecta lobes are emplaced on the
lower flank of Ceraunius Tholus, and there is a prominent
central ridge in the middle of the crater. As a result of this
unusual geometry, this particular crater has been suggested
by Nyquist [1983] to be a suitable parent crater for the
SNCs.
In addition to its unusual geometry, this impact crater
probably excavated material from both the flanks of
Ceraunius Tholus (perhaps including both extrusives and
intrusives)and the surroundinglava plains; it may thus be a
good crater for sampling multiple rock types of different
ages on Mars, with the possibility that CerauniusTholus has
an age of 1.3 Ga and the surroundingplains having an age of
northern
flank
of Ceraunius
Tholus
that cuts the rim.
There
is also some material on the floor of the crater that appears
to be associatedwith this valley. While this stratigraphy
implies that the crater did not post-dateall activity on the
volcano, we do not believe that this is sufficient evidence to
discount crater 5. The origin of valleys of this type of
Martian volcano has been variously attributedto lava flows
[Carr et al., 1977], volcanic density currents such as
pyroclastic flows [Reimers and Komar, 1979], and fluvial
erosion [Mouginis-Mark et al., 1982, 1988; Gulick and
Baker, 1990]. The lack of lava flows and late-stage
pyroclasticdepositson CerauniusTholus suggeststo us that
the valley in question is probably fluvial in origin, perhaps
initiated by water releasedas a consequenceof the seismiceffects of the impact event. No other valleys on Ceraunius
180 Ma.
Tholus appearto have been active since the surroundinglava
plains were emplaced(i.e., all the other channelspredatethe
Worst attributes: Jones [1985] questioned the relative
youth of crater 5, becauseof the existenceof a valley on the
formation of crater 5). However, we note that other Martian
volcanoesthat have numerousvalleys on their flanks (such
10,220
MOUOINIS-MARK ET AL.' MARTIAN PARENT CRATERS FOR METEORITES
as Hecates Tholus [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
generationof 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 bombardment
of 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
(unitAtm) thatembayCeraunius
Tholusare
ß
.::
:
..
ß
"
.:
ß
.'.
180 Ma, then many other units within Tharsis such as the
youngerflows aroundAscraeusand OlympusMontes (which
both have lower superposedimpact crater densities) must
have absoluteages 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,
1981].
Crater 6:33.8
Fig. 5. Crater 3, 14.8 km diameter, which lies close to the
summitcalderaof OlympusMons volcano.Note the prominent
swirl pattern of material on the floor of this crater, and the
ballistically emplaced ejecta blanket. Image resolution is 91
m/pixel. This is Viking Orbiter image 46B13.
km Diameter, 22.2øN, 98.0øW
Best attributes: Immediately to the southof the volcano
Ceraunius
Tholus
are three craters that have been formed next
to each other on a young/mediumage target (Figure 7). The
largest of these craters (crater 6) is the oldest of the three
Fig. 6. Crater4 (arrowed),13.7,km diameter,
formedon the flankof thevolcanoUraniusTholusImageresolution
is
200 m/pixel.This is Viking Orbiterimage516A23.
MOUGINIS-MARKET AL.: MARTIANPARENTCRATERS
FORMETEORITES
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.:•
Fig. 7. Crater5 and crater6. Crater5 (34.2 x 18.2 km in diameter)is an elongateimpactcraterformedon the flanks
of the volcano CerauniusTholus (arrow labeled "CT"). Note the large channelthat originatesat the summit of the
volcanoand cutsthe wall of the impactcrater,indicatingactivity(possiblyfluvial) on the volcanoafter the formation
of the crater.Crater 6, 33.8 km diameter,is unusualbecausethereare two smallerimpactcraterssuperposed
on its ejecta
blanket("A" and "B"). Image resolutionis 200 m/pixel.This is Viking Orbiterimage516A24.
impact events (based on the superpositionof overlapping
ejecta blankets) but still possessesa well-preserved lobate
ejecta deposit.It has a small central pit rather than a central
peak in its interior, and the inner wall has prominent
terraces. Wood et al. [1978] suggestedthat central pits may
have been produced by the explosive release of subsurface
volatiles, making crater 6 somewhat unusual, but fresh
craters with central pits are relatively common on Mars
[Wood et al., 1978; Pike, 1980] so that the formation of a
central pit within a crater does not appear to be a likely '
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
of the preservationof their ejecta and rim deposits.
Worst attributes: The presenceof the two smaller craters
on its ejecta blanket implies that crater 6 is not very young.
In addition, this crater suffers from the circular crater, singlelava-type problem.
.
Crater 7:21.9
km Diameter, 17.8øN, 111.0ø14/
Best attributes: This crater (Figure 8) is located to the
northwestof AscraeusMons, just south of CerauniusFossae.
The crater has a prominent lobate ejecta blanket, a sharp
well-defined rim crest, and a small central pit implying that
it is a very young crater.
Worst attributes: Most likely, this crater was excavated
in a singlegeologicalunit and so couldnot be the sourcefor
the petrologicallydiverseSNCs. The cratersuffersfrom the
circularcrater,single-lava-type
problem.
Crater 8:18.3
km Diameter, 19.5øN, 99.8ø14/
Best attributes: This crater(Figure9) formedto the south
of Ceraunius
Tholus
volcano
on the lava
flows
that extend
northeastward from Ascraeus Mons.
Its young age is
indicated by a well-preservedinterior morphology,including
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
Fig. 8. Crater 7, 21.9 km diameter, located northwest of
AscraeusMons. This is Viking Orbiter image 516A53.
Worst attributes: Ejecta on the northernside of the ejecta
blanket have been faulted by a circumferentialfracturerelated
to Ascraeus Mons, suggesting either recent tectonic
deformationof the flank of the volcano or a greater age for
the impact crater. In the latter case, this would require
tectonic activity to have taken place on Mars since 1.3 Ga.
The crater also suffers from the circular crater, single-lavatype problem.
Crater 9:12.0
km Diameter,
22.7øN, 92.0øW
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, althougha central
pit can be identified.
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.
Tholus
that formed
on lava
flows
from
the northern
flanks
of
AscraeusMons. Note the circumferentialgrabenthat hascut part
of the ejecta blanket. This is Viking Orbiter image 516A45.
A secondfeature of this analysisis the lack of any craters
>100 km in diameter, superposedon young terrain. This
sizeof crateris requiredin the modelsof Vickeryand Melosh
[1987], but it is clear that craters of this size simply do not
exist on young volcanic regions on Mars. Even if the
criteria for identifying a fresh crater (which are necessaryto
explain the 180 Ma age of the shergottitesas either a shock
age or crystallizationage) are relaxed so that all cratersof
any degradationstate are includedin our sample,there are
only nine craters(including two that are listed in Table 1)
'",,..
";.----";;!ii
:i
DISCUSSION
Numberof 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 requirements
for 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 includedas a criterion, six of
these nine craters (craters 1, 3, 6, 7, 8, and 9) would also be
excluded. Considering the enormous number of craters
presenton diverse surfaceson Mars, the ability to narrow
the numberof potential SNC parentcrater candidatesto these
nine (or three) is in itself worthy of note.
Fig. 10. Crater 9, 12.0 km diameter, south of the volcano
UraniusPatera. Note that at an image resolutionof 250 m/pixel
it is difficult to be confident that this is a very young crater,
since texture on the ejecta blanket may be the result of small
superposed
impact cratersthat cannoteasily be seen. This is
Viking Orbiter image 857A48.
MOUGINIS-MARKET AL.: MARTIAN PARENTCRATERSFORMETEORITES
10,223
that are larger than 30 km in diameter. Only one of these
consistentwith the earlier hypothesisof Nyquist [1983].
However, if crater 5 is the SNC parentcrater, this places
at 8.5øN, 113.0øW,exceeds60 km in diameter,and only one severalrigorousconstraintson the absolutechronologyof
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 ejectablanketextensively chronologies
of the areabasedon the numberof superposed
craters, which lies to the north of the volcano Pavonis Mons
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)
suggeststhat this crater is older than 180 Ma. In terms of
its stratigraphic position relative to other craters listed in
Table 1, this crater formed on unit Atto, which is the same
Shergottite Ages
As mentioned earlier, the crystallization age of the
geologicunit as that of several other candidateSNC parent
shergottitesremains a question of much debate. While the
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
that a crater>100 km diametercompletelydestroyeda young
sequenceof lava flows in an area that currently shows no
morphologicevidence for recent volcanic activity (such as
the southern highlands), but the probability of this is so
small that we reject this idea. Thus our analysisshowsthat
ejection of material off of the Martian surface has to be
achieved during impact events that produce craters
significantlysmaller than the 100 km thresholdproposedby
Vickery and Melosh [1987].
selected to constrain the true ages of these rocks. If the
crystallizationage of the shergottitesis really 180 Ma, then
the SNC parent crater apparentlysampledmaterial that was
180 Ma and 1.3 Ga, with no materialsof intermediateage.
This of courseassumes
that all the SNCswere ejectedby a
single event, which we favor, as discussedabove.
Craters2, 4, and 5 are consideredto be the mostlikely
SNC parent craters, and they each sampled at least two
distinct lithologies. Crater 2 sampledthe OlympusMons
aureolematerial,which almostcertainlycomprisesa variety
of volcanicunits that may have been depositedover a long
spanof time. In this case, it may have beenpossiblefor an
Which Crater?
impact event to randomlysamplematerialsof only 180 Ma
We return now to the original question of the parent and 1.3 Ga, withoutsamplingmaterialof intermediate
age.
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
examinationsuggeststhat some of these craters are in fact ages of these surfacesare uncertain,it seemspossiblethat
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 youngage for the plainsunit 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 becausethey
On the basisof thesestratigraphicand absolutechronoloformed part of the ejecta blanket of an earlier crater; gies for geologiceventson Mars, it seemsless likely that
however, this geographic setting does not appear to be the 180 Ma age of the shergottites
is the crystallization
age.
unique on Mars, and the younger craterssuperposedon the No crater existsto satisfywell the criteriaof 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
significantconstraintson 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
asymmetric
ejectablanketindicativeof an obliqueimpact.
Craters4 and5 occuron the boundarybetweenthe flanksof Extent of Volcanism
a volcanoand surroundingvolcanicplains, 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 additionaldistinctionof beinghighly and may even continueuntil the present. This inferencehas
elongate(probablydue to a very obliqueimpact 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
impossible,to unequivocallydecide betweenthese craters,
and the other six leadingcraterscan certainlynot be ruled
out. However,thesethreecraters(particularly2 and 5) have
uniquefeatureswhich may explain why they could be the
only cratersto have delivered to Earth the retrievedMartian
samples.On the basisof the reasonsstatedabove,including
the uncertaintyplaced on crater 2 becauseof the unknown
would assign ages as old as 3.0 Ga to some of the lava
plainsin Tharsis. Althoughcrater1 formedon the youngest
units in Tharsis (Figure 2), all the other cratersformed on
slightly older units. If any of craters2-9 were the SNC
parent crater, volcanic activity continued after the 1.3 Ga
crystallizationage of the SNCs. For example, craters4-9
formedon older lava plains (Figure 2), mappedas unit Atto
by Scottet al. [1981a]. Five geologicvolcanicunits formed
originof the OlympusMons aureolematerial,our preferred after unit Atto, thusimplyingthat volcanicactivityoccurred
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-MARK
ET AL.:MARTIANPARENT
CRATERS
FORMETEORITES
crater 5 is the SNC parent crater, geomorphicactivity (in the Chapman,M.G., H. Masursky, and A.L. Dial, Geologicmaps of
sciencestudysite 1A, East MangalaValles, Mars, U.S. Geol. Surv.
form of renewed valley formation) must have taken place on
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,
Geochim. Cosmochim. Acta, 50, 955-968, 1986.
superposedcraters than the target rocks of crater 5, then
other areasof 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,Experimentalstudiesof obliqueimmuch 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 Greeley, R., and P.D. Spudis, Volcanism on Mars, R evs.
Geophys., 19, 13-41, 1981.
of the SNCs, must therefore be included not only in current
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
magma sourcesin Tharsis and the elasticlithosphereof Mars
[e.g., Solomonand Head, 1990].
CONCLUSIONS
Res., 82, 3099-3107,
1977.
Jagoutz,E., Sr and Nd isotopicsystematicsin ALHA 77005: age of
shock metamorphismin shergottitesand magmatic differentiation
on Mars, Geochim. Cosmochim. Acta, 53, 2429-2441, 1989.
Jagoutz, E., and H. Wiinke, Sr and Nd isotopic systematicsof
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
m.y. igneousactivity on the SPB (abstract), Lunar Planet. $ci.,
diameter) that formed on young terrainsin the Tharsis region
satisfy the petrologic criteria of the SNCs and the proposed XVI, 408-409, 1985.
1.3 Ga crystallization ages of the meteorites and have a
Jones, J.H., A discussion of isotopic systematicsand mineral
zoning in the shergottites: Evidence for a 180 m.y. igneous
crystallization age, Geochim. Cosmochim.Acta, 50, 969-978,
pristine morphologythat might be expectedfor a 180 Ma
1986.
crater. No craters of >100 km diameter exist on young
volcanic units on Mars, and only two degradedcratersare in Landheim, R., and N.G. Barlow, Relative chronolgy of Martian
volcanoes (abstract), Lunar Planet. Sci. XXII, 775-776, 1991.
the diameter range 40-70 km. This lack of large, fresh
Longhi, J., Complex magmatic processeson Mars: inferencesfrom
impactcratersimplies that previoustheoreticalstudiesof 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,Furtherevidencefor
2.
No crater location can be found where convincing
a mass movement origin of the Olympus Mons aureole, J.
evidenceexists for two adjacentgeologicsurfacesof signifiGeophys.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
evolution, Rev. Geophys., 23, 391-416, 1985.
time preserving the general characteristics of cratering
chronologies as inferred from the number of superposed Melosh, H.J., Ejection of rock fragments from planetary bodies,
craters on different
units.
3.
Volcanic activity on the plains of the Tharsis
region may extend well past 1.3 Ga.
Geology, 13, 144-148, 1985.
Mouginis-Mark,P.J., L. Wilson, and J. W. Head, Explosivevolcanism on Hecates Tholus, Mars: Investigation of eruption
conditions,J. Geophys.Res., 87, 9890-9904, 1982.
Mouginis-Mark, P.J., L. Wilson, and J.R. Zimbelman, Polygenic
eruptionson Alba Patera, Mars, Bull. Volcanol., 50, 361-379,
Acknowledgments.
This researchwas supported
by grant NAGW.437
1988.
(P.M.-M., PrincipalInvestigator)from NASA's PlanetaryGeology and
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 anonymous
reviewersfor theircomments
on an
Res., 82, 4452-4467, 1977.
earlierversionof the manuscript,
HaroldGarbeilandMark Robinsonfor
the preparation
of Figures3, 4, 6, 8, 9, and 10, andMarc Normanfor Nakamura, N., D.M. Unruh, M. Tatsumoto, and R. Hutchison,
Origin and evolutionof the Nakhla meteoriteinferredfrom the Smhis discussion
of the youngage of the shergottites.This is Planetary
Nd and U-Pb systematics and REE, Ba, Sr, Rb abundances,
Geosciences
publication678 and $OEST contribution2850.
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-frequencydistributionsand a revised
Martian relative chronology, Icarus, 75, 285-305, 1988.
Becker, R.H., and R.O. Pepin, The case for a Martian origin of the
shergottites: Nitrogen and noble gasesin EETA 79001, Earth
Nyquist,L.E., Do obliqueimpactsproducemartianmeteorites?,
Proc.
Lunar Planet. $ci. Conf., 13th, A785-A798, 1983.
Nyquist, L.E., The oblique impact hypothesis and relative
probabilitiesof Lunar and Martian meteorites,Proc. Lunar Planet.
$ci. Conf., 14th, B631-B640, 1984.
Planet. Sci. Lett., 69, 225-242, 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,
Sr and Nd isotopic systematics of EETA 79001 (abstract),
Geophys.Res. Lett., 4, 69-71, 1977.
Bogard,D.D., andL.E. Nyquist,39Ar/4øAr
chonology
of related
achondrites(abstract), Meteoritics, 14, 356, 1979.
Bogard,D.D., L.E. Nyquist, and P. Johnson, Noble gas contentsof
shergottites
and implicationsfor the Martian origin of SNC meteorites, Geochim. Cosmochim. Acta, 48, 1723-1739, 1984.
Carr, M.H., The Surfaceof Mars, Yale UniversityPress,New Haven,
Conn., 1981, 232 pp.
Carr, M.H., R. Greeley, K.R. Blasius, J.E. Guest, and J.B. Murray,
Some volcanic features as viewed from the Viking orbiters, J.
Geophys.Res., 82, 3985-4015, 1977.
Meteoritics, 19, 284, 1984.
O'Keefe,J.D., and T.J. Ahrens,Obliqueimpact:A processfor obtaining meteoritesamplesfrom other planets, Science,234, 346-349,
1986.
Papanastassiou,
D.A., and G.J. Wasserburg,Evidencefor late formation and young metamorphism in the achondrite Nakhla,
Geophys.Res. Lett., 1, 23-26, 1974.
Pike, R.J., Control of crater morphologyby gravity and target
type: Mars, Earth, Moon, Proc. Lunar Planet. Sci. Conf.. 11th,
2159-2189,
1980.
MOUGINIS-MARK ET AL.: MARTIAN PARENT CRATERSFOR METEORITES
10,225
Plescia, J.B., Recent flood lavas in the Elysium region of Mars,
Icarus, 88, 465-490, 1990.
Reimers, C.E., and P.D. Komar, Evidence for explosive volcanic
Solomon, S.C., and J.W. Head, Heterogeneitiesin the thicknessof
densitycurrentson certainmartianvolcanoes,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
record of lost satellites, Proc. Lunar Planet. Sci. Conf., 13th, Part
1, J. Geophys.Res., 87, suppl., A84-A96, 1982.
Schultz,P. H., and J. Singer, A comparisonof secondarycraterson
the Moon, Mercury and Mars, Proc. Lunar Planet. Conf., 11th,
2243-2259,
the elastic lithosphereof Mars: Constraintson heat flow and
internal dynamics, J. Geophys.Res., 95, 11,073-11,083, 1990.
1980.
19,
318-319, 1984.
Tanaka,K.L., Ice-lubricatedgravity spreadingof the OlympusMons
aureoledeposits,
Icarus, 62, 191-206, 1985.
U.S. GeologicalSurvey, Topographicmap of Mars, U.S. Geol.
Surv. Invest. Ser., Map, l-961, 1976.
Vickery,A.M., and H.J. Melosh,The largecrateroriginof the SNC
Scott,D. H., and K.L. Tanaka,Map showinglava flows in the south- meteorites, Science, 237, 738-743, 1987.
eastpart of the Amazonisquadrangleof Mars, U.S. Geol. Surv. Wetherill, G. W., Orbital evolution of impact ejecta from Mars
Invest. Ser., Map, 1-1280, 1981a.
Scott,D. H., and K.L. Tanaka,Map showinglava flows in the noaheast part of the Amazonisquadrangleof Mars, U.S. Geol. Surv.
(abstract), Meteoritics, 18, 420, 1983.
Wetherill, G. W., Orbital evolution of impact ejecta from Mars,
flows in the southwest
part of the Arcadiaquadrangle
of Mars, U.S.
achondritesChervonyKut, GovemadorValadares,and Allan Hills
Meteoritics, 19, 1-13, 1984.
Invest. Ser., Map, 1-1279, 1981b.
Wood, C.A., and L.D. Ashwal, SNC meteorites:Igneousrocks from
Scott, D. H., and K.L. Tanaka, Mars: Paleostratigraphic
restoration Mars?, Proc. Lunar Planet. Sci. Conf., 12th, 1359-1375, 1981.
of buried surfaces in Tharsis Montes, 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, Radiometricagesfor the
Geol. Surv. Invest. Ser., Map, 1-1278, 1981b.
Scott, D. H., G. G. Schaber, K.C. Hortsman,A.L. Dial, and K.L.
77005 (abstract), Lunar Sci.,X, 1380-1381, 1979.
Tanaka, Map showinglava flows in the northwestpart of the
K. Keil, T.J. McCoy, P.J. Mouginis-Mark,and G.J. Taylor,
Tharsisquadrangleof Mars, U.S. Geol. Surv.Invest.Ser.,Map, 1- Departmentof Geologyand Geophysics,PlanetaryGeosciences,
1266, 1981c.
University of Hawaii at Manoa, 2525 CorreaRoad, Honolulu,HI
Shih, C.-Y., L.E. Nyquist,D.D. Bogard,G.A. McKay, J.L. Wooden,
B.M. Bansal, and H. Wiesmann, Chronologyand petrogenesisof
young achondrites,Shergotty,Zagami and ALHA 77005: Late
magmatism on a geologically active planet, Geochirn.
Cosrnochirn. Acta, 46,
2323-2344, 1982.
96822.
(Received October 14, 1991
revised March 6, 1992;
acceptedMarch 13, 1992.)