Mars Crater Consortium VII Meeting

October 7-8, 2004    USGS Flagstaff


Attending:  Nadine Barlow (NAU), Dan Berman (PSI), Joe Boyce (U. HI), Clark Chapman (SWRI), Albert Haldemann (JPL), Trent Hare (USGS), Keith Milam (U. Tenn), Pete Mouginis-Mark (U. HI), Gordon Osinski (U. Az), Jamie Phillips (U. Tenn), Betty Pierazzo (PSI), Alexis Rodriguez (U. Tokyo), Sarah Stewart (Harvard), Ken Tanaka (USGS), Livio Tornabene (U. Tenn), Shawn Wright (ASU).


Minutes provided from Nadine Barlow’s notes.


Thursday AM Session:

There has been a recommendation that the Mars Crater Consoritum (MCC) meet in conjunction with the annual Planetary Geologic Mappers (PGM) meeting to encourage greater participation.  This would put our meetings into summer rather than fall and we would only meet in Flagstaff every other year.  Participants agreed that an important part of our meeting was the interaction with the computer group at USGS.  Final decision was that we will investigate meeting in conjunction with PGM during the years they meet in Flagstaff but on the alternate years we will meet at our normal October time period at USGS. 


July workshop: 

Barlow and Stewart provided updates on the current status of our July 11-14, 2005, Workshop on the Role of Volatiles and Atmospheres on Martian Impact Craters.  Meeting will be held at APL (Olivier Barnouin-Jha, LOC chair) and consist of invited, contributed, and poster talks.  Possible field trips in association with meeting:  Reston USGS (would require all day); National Air and Space Museum, tour of Goddard, tour of APL.  Perhaps have USGS—Reston put up display of Chesapeake Bay cores?  Stewart will follow-up.


Participants encouraged that the workshop include talks on field observations of craters from MER.  Perhaps also an exobiology session?


What about a proceedings volume related to workshop?  Do we want to publish as a special issue of some journal or as a special proceedings book like the AGU monographs/GSA special papers?  The Feb. 2003 Bridging the Gap workshop went with MAPS because it got out very rapidly--second announcement announced that the manuscript deadline was 3 months after meeting.


We also need to be sure that researchers in all areas of interest to this meeting know about the meeting.


Other Related Meetings:

Mouginis-Mark:  Mouginis-Mark and Barlow are organizing a special session on the Role of Volatiles on the Geology and Geomorphology of Martian Impact Craters at the fall AGU meeting.  We received 23 abstracts, which will give us 2 oral sessions and 1 poster session.  Question about whether we want to have a proceedings volume associated with this session—decided we want only one proceedings volume to cover both the AGU session and July workshop. 


Pierazzo noted that there will be an impact geology session at the fall GSA meeting. 


Osinski noted that the Society of Sedimentary Geologists will be meeting in May in Missouri and will include a session on impacts into sedimentary rocks.


Stewart:  The American Physical Society will host a shock compression of condensed matter topical meeting next August in DC.


Hare:  Geospatial Consortium was just held in Chicago.  The group hasn’t included planetary datums yet, so Trent is working in this direction.  Problem:  Planetary datums don’t have standard formats.  Trent will start an ad-hoc Planetary Datum Commission.


Stewart:  India is having a conference about the Moon in November


Impact and Explosion Literature: (Stewart) 

Stewart has a compilation of technical crater data on her web site.  Barlow will put link on MCC web site.  Pierrazo is trying to do something similar as a result of the LPI conference—is currently trying to find funding to do it.  Stewart and Boyce suggested Jim Garvin might be interested.  Where should such a literature database eventually be stored?  LPI?  USGS?  Boyce:  LPI has funding and/or interest to do such things so perhaps we could approach them—it is simply a matter of them prioritizing it. We will ask Paul Schenk to pursue this.  What about PDS—Barlow will ask Mark Sykes who is chair of the Planetary Data System Working Group. 


Computer updates (Hare):

USGS is digitizing Lunar Orbiter data and registering it to Clementine.  MDIM 2.1 is now available at the USGS Astrogeology web site and will be available through PDS eventually.  JMars (available through ASU) uses MDIM 2.1.  MCC can provide feedback to USGS about what products are of most importance/interest, which USGS can then use when proposing projects to PGG.


Hare maintains a Planetary Dataset Discussion Group at webgis.wr.usgs.gov with lots of good information about ISIS, GIS, etc.


Consensus:  Would be useful to have an instructional session on using JMars/ISIS in association with MCC.


USGS ISIS has been shying away from a Windows version.  ISIS 3 is supposed to be released this year.


PIGWAD on-line maps are now available.  Can download JMars and view GIS maps.   There is a proposal to JPL to do on-line serving.   Crater databases are downloadable from the FTP site.  Intermediate crater database also allows extraction of data.


ArcMap 9 has solar system definitions built in and can read ISIS files.  Gdal_translate takes ISIS 2.0 and converts to other format conversion (Linux, UNIX, Mac, and Windows).  ArcMap is now being used for publishing geologic maps.  An advantage of using ArcMap is that it is quick with animations.


High priority targets (Boyce):

Malin (MOC) and Christensen (THEMIS) accept target requests.  Do we have high-priority targets that we want data for?  Mouginis-Mark’s experience with MOC is that images are obtained typically within 3-4 weeks of one’s request.  It is recommended that you let Malin/Christensen know that you need the info for a particular project so they know time is of importance.  Stewart commented that it would be useful to have list of what has been requested so we don’t do a bunch of duplication.  Hare will serve as contact and put list on PIGWAD (Lat/Long, Requester name, date of request, resolutions, etc.).  People commented on the difficulty in using the MOC map system to find a specific image, particularly since it only shows each 3 month release one at a time, not cumulatively.  Hare has the ability to put footprints on background GIS maps, but they may be offset from the actual location.  Wright noted that you can also do this using JMars.  For MOC, submit requests individually.  Wright will check with Christensen about submitting requests to THEMIS.  Boyce reminded everyone to be sure to acknowledge and thank the THEMIS team when the data are published since this is what NASA uses to continue funding the instrument/mission.


Tornabene noted that right now daytime IR are taken too late in day so we are not getting good thermal distinction.  Wright noted that MGS TES is on its last leg and that THEMIS only gets ~30 minutes per day for mapping.


Thursday afternoon:

Following discussion of posters, Randy Kirk joined us for discussion of the Mars Express High Resolution Stereo Camera (HRSC).  HRSC has ~12m/px resolution at the equator.  Mars Express is in a very elliptical orbit which is precessing.  We have a tradeoff of larger coverage area vs higher resolution.  DLM is doing stereo processing with automatic routines--any problems with software have to be adjusted using smoothing routines.  USGS expects to eventually produce digital terrain models (DTMs) from the HRSC data. 


Can public set targets?  Officially probably no.  But can probably email PI with requests. DLM has basically spent the first ½ year learning how to run the mission and doing little science, but they are now working more on systematically adding up coverage.  First images will go to PDS by early 2005.  By early 2006, USGS will be able to submit a proposal to do mapping.


ISIS photoclinometry project status report:  Not much new since last year.  Paper is about ready to be submitted.  Next step is to start using thermal models to go to the next stage of thermal shading photoclinometry.


Topography issues:  (Boyce)

Boyce warned everyone to keep in mind that MOLA, THEMIS, etc. data have limitations.   In particular, keep in mind what the MOLA measurements are representing, which may be much less than the level of crater detail that one would like.  Much of the data are interpolated—keep in mind that gaps between actual tracks can be as much as 12 km near the equator.  What is the minimum number of shots needed across a crater to get an accurate representation of crater?  Depends on what you are looking for in the crater.  Depth-diameter is a big thing and easy to measure.  Rim heights, ejecta morphology measurements, and anything related to fine detail are more difficult.  Issue comes down to resolution vs size of the topographic feature. 


Thermal photoclinometry (the procedure that Soderblom and Kirk are developing) could help fill in MOLA gaps.  Problem is you need MOC, THEMIS IR and THEMIS VIS covering same feature, which is not always available.


Stewart noted that one does better when working with the individual tracks than with the interpolated data.  North-south track data is generally better than the east-west data, which is fine for extrapolation to circular features but can cause major problems with noncircular craters.  Pierazzo noted that terrestrial analysis suffers from a similar problem since data are taken from only specific locations.  Stewart commented that if one requires fine detail, one should use the individual tracks.  If you only need gross measurements such as radius or depth, one can usually use the interpolated data.


Another issue is how to deal with central peaks, central pits, etc. as we measure crater depths.  What exactly are we measuring when we measure crater depth?


Mouginis-Mark reviewed the IMPACT topography program developed at University of Hawaii, which is a Windows-based program to quickly determine elevations using MOLA.  Paper on the technique is in press in JGR.  Anyone requiring additional information about the program can contact Mouginis-Mark. 


Stewart argued that we need to save background surface information for future use when we will be combining datasets.  Everyone’s “background level” for topography measurements will likely be different.  The largest uncertainties that this could produce would be in volume measurements.


Haldemann brought up the additional problem of lack of image calibration from different instruments.  As an example, the diameter of Endurance crater has been measured from MOC, THEMIS, MER descent imager, etc.  Diameter estimates using these different data sources range between 130 and170 m, with a probable best estimate of 156 m.  The MCC strongly recommends that people discuss the various measurement uncertainties when quoting a specific value for a feature.


Friday, October 8, AM:

Future Directions: 

We need to be more proactive in enhancing the interactions between the different communities studying impacts (numerical simulations, laboratory experiments, terrestrial field studies, morphologic/morphometric analysis, etc.).  In particular, we need to get information out about the July workshop.  Pierazzo will pass the word around at GSA Wright and Millam will announce the workshop at IFG, and Millam will pass information around with Meteoritical Society.  Pierazzo will work with Boris Ivanov, Dieter Stöffler, and Alex Deutsch to advertise the meeting among the modeling community.  Stewart will work with Horton Newsom to contact David Crawford and others in the DOD community.  We also need to contact NASA Program managers.  Barlow will send information to MEPAG for distribution on the Mars Calendar. 


MCC participants suggested we include tutorial talks at the July workshop.  We also need to include someone to give an invited talk on climate perturbations caused by impacts. 


Boyce:  Other topics which we might want to focus on at the July workshop and/or in future workshops: 

1)      Chronology—in light of new databases, secondary crater issues, etc.

2)      Stratigraphy—deposition, erosion, exhumation

3)      Climate change


MCC participants suggested that we plan on having a focused meeting/workshop approximately every 2 years.  Alternatively we could have a meeting in conjunction with a larger meeting like LPSC, but most participants felt that was too much—we all get “saturated” at LPSC alone.  We could request a topical session at a meeting like LPSC, or suggest one of these issues as a topic for the Brown-Vernadsky Microsymposium.  Cratering issues have many applications:  one example would be how interpretation of cratering chronologies constrains the timing of climate change epochs.


How can we encourage more terrestrial studies that would benefit Mars studies?  NSF and NASA MFRP could both be funding possibilities.  DOD, Army Corp of Engineers, DOE all have previously funded crater research.  National Geographic is another possibility. 


It is necessary to emphasize the importance of cratering studies (all aspects) to Program Managers and to make sure cratering issues/implications get into the MEPAG documents.


One area of cratering studies that needs more attention is basin forming impacts.  We really do not understand how basins, or even complex craters, form.  Understanding basin formation has implications to early Martian climate and climatic history, possible imprints on the martian mantle, basin effects on magnetic remanant signatures, and the general thermal history of Mars.


Many of these crater questions might be able to be addressed by a Scout mission.  What types of measurements would we need to make?


Research Reports:

Osinski:  A Search for Terrestrial Analogues to Martian layered ejecta structures:

No pristine ejecta blankets preserved for craters larger than ~1 km diameter on Earth.  Oskinski has focused on Houghton and Ries (both ~24 km in diameter).  These are complex craters (peak/peak ring).


Ries displays two layers of ejecta:  the Bunte Breccia and Suevite.  The Bunte Breccia is widely distributed while suevite is much more localized.  Sands and other sedimentary material are found to the south of the crater in the same location where Bunte Breccia is primarily found. Suevite occurs both inside and outside the crater.  Suevite lies above Bunte Breccia.  Bunte Breccia is interpreted as ballistic ejecta with some interaction with the surface though ballistic sedimentation.  Suevite originally was interpreted as fallback material, but Osinski interprets it as a ground-hugging flow.  Suevite infills valleys in the Bunte Breccia—expect suevite would form an even layer atop the Bunte Breccia if it were an airfall deposit.  Also the contact between the two is sharp—fallback would probably produce more mixing along the boundary.  Microscopic analysis of suevite shows a melt structure, not angular fragments as previously believed.


Houghton displays a large number of hydrothermal vents around the crater.   Microscopic analysis of impactites shows more evidence of melting than people originally thought.  See a lower layer of large fragment breccia which is overlain by a fine grained deposit.


Suevite originates below the center of the crater, then flows out and over the rim.  The suevite incorporates much material from the crystalline basement at Ries, while the Bunte Breccia is derived from higher-lying layers.  The Bunte Breccia was probably emplaced during the central uplift formation.  Stewart and Pierazzo noted that such an outflow from the central uplift is not seen in numerical simulations.  Mouginis-Mark compared the sequence of events with what is seen in martian double layer craters, where the outer layer is apparently emplaced after the inner layer. 


Wright:  Thermal infrared spectroscopy of basalt from Lonar Craters, India: Implications for Mars.

Bandfield’s work indicates that 99% of the martian surface is covered by type 2 (andesitic/weathered basalt) and type 1 (basaltic) compositions.  Wright is investigating the role that impact glasses might contribute to these compositions.


Kieffer and Simonds (1980) found that twice as much impact melt is generated in crystalline targets compared to sedimentary targets on Earth.  A global layer of ~60 m impact melt has been predicted for Mars (Newsom, 1980).  Impact glass composition is a product of the target rock (Bouska and Bell, 1993).


Experimental shock alters the thermal IR (TIR) spectra of anothosite, albite, basalt, and to a lesser extent orthopyroxene (J. Johnson).  Certain absorption bands change depth and position as plagioclase feldspar receives increased shock.  After 25-30 GPA, plagioclase converts to maskelynite.


Lonar Crater:  1.8 km diameter and ~52 ka.  1 of just 2 terrestrial impacts in basalt.  Emplaced in 5-6 flows of Deccan trap flood basalt, analogous to Mars basalt.  TIR spectral shape suggests this is similar to TES Surface Type 1.  The geochemistry of the target is similar to basaltic shergottites (high Fe, low Al).  However, TIR remote sensing of Lonar is not particularly useful since there is a lot of urbanization and vegetation.    The previous petrographic study (Kieffer 1976) divided Lonar basalts into 5 shock classes: <20 GPa, 20-40 GPa, 40-60 GPa, 60-80 GPa, >80 GPa.  The percentage of SiO2 increases as degree of shock goes up.  Naturally shocked basalts have different petrographic properties than experimentally shocked Deccan basalt, which is generally attributed to the duration of the shock wave.  This study is mainly concerned with shock effects on the nature of the clinopyroxenes.


Results of the Lonar study show that neither Type 1 or Type 2 compositional units are entirely composed of impact glass, but impact glass could contribute to both.


From deconvolution of impact melt, Wright gets (a) 10% pyroxene, 17% plag, 9% Si-K, 25% saponite; or (b) 10% pyroxene, 2% Si-K glass, 16% saponite, 47% shocked anorthosite (56.3 GPa).   Saponite is due to hydrothermal alteration of the basalt.


Deconvolutions of Deccan basalt give:  (a) 20% pyroxene, 4% olivine, 47% plag; (b) 25% pyroxene, 2% olivine, 51% plagioclase, 2% Si, 3% shocked anorthosite (29.3 GPa).  Library b deconvolutions provide lower RMS errors.


Currently the TIR spectral library does not include shocked rocks.  This would be a very useful addition for Mars studies since shocked minerals might constrain better fits to the observed data.


Haldemann:  Survey of craters by Spirit in Gusev Crater and Opportunity in Meridiani Planum

Hollows at Gusev appear to be small impact craters (probably secondaries) filled in with fine-grained material.  Round with generally angular clasts outside of crater.  83 hollows were counted on the traverse to Bonneville.  Size-frequency distribution of hollows is characteristic of crater size-frequency distributions.  Do not see interbedding of dust within the hollows, indicating that the hollows were filled in quickly.  Do not see most of the hollows on MOC views, which do show the rover track.  From Columbia Hills, Spirit can see hollows in all directions.


Bonneville is ~200 m in diameter and is filled with fine grained material.  It has a depth of ~9 m deep and walls have slopes of ~11°.  However, small blocks are seen poking up through the floor deposits.  It is thus far the “freshest secondary crater” seen on the martian surface.  Clast size increases as one approaches the rim of Bonneville, suggesting that the clasts are ejecta blocks.  Bonneville’s rim height is ~4 m above surroundings.


Basalt in Gusev Crater is very homogeneous, indicating it was emplaced in only one event.  The working hypothesis of the MER team is that any lakebed deposits within Gusev underlie the volcanic flow.  The top flow is what has been gardened and affected by secondary cratering.


No breccias have been seen yet by Spirit.  The team also did not note a “lip” when the rover traversed over the outer edge of Bonneville’s ejecta, suggesting no rampart exists.


Largest rocks near Spirit lander are ~50 cm in size.  The largest rocks near the Bonneville rim are >250 cm in diameter.  There is a factor of 5 increase in size as Spirit traversed the ejecta.  The size-frequency distribution of these rocks is generally consistent with impact fragmentation.  Largest fragments imply fragmentation from ~6 m blocks.  These large fragments would be difficult to transport in a fluid, suggesting they were not deposited by flowing water. 


At the Opportunity landing site, Eagle crater is 20 m in diameter with a 3 m high rim.  There are many fewer craters at Meridiani than in Gusev Crater and all are significantly modified.  Of the three craters studied in detail thus far, Eagle is probably the most modified.  There is lighter toned material outside Eagle compared to the inside—modified ejecta?  Freshest crater is Fram (D = 10 m).  Still see some ejecta outside the crater. 


Endurance crater is 160 m in diameter with a depth 18.9 m below the surrounding plains.  The rim-to-floor depth 23 m.  There is some debate as to whether the uppermost layer is ejecta or just an upper layer of fractured/comminuted material.  Endurance shows no evidence of an overturned flap, but this may simply be due to the degraded nature of the crater.  Chlorine concentrations go up by a factor of 3 with increasing depth.  Can use variation in sulfur concentration across the ejecta blanket as a proxy of sulfur variation with crater depth.  This observation confirms that the deposit that looks like ejecta around Endurance is in fact ejecta since rocks from lower down are more distant from the rim.


Stewart:  Validation of Martian impact crater geometry measurements

A shock wave facility is being installed at Harvard.  The core of this facility is a 40 mm single stage gun which can shoot either from gas or powder.  Associated instrumentation includes an interferometer (VALYN VISAR (Velocity Interferometer System for any Reflector)).  Embedded gauges can give pressure and particle velocity.)  With this facility, Stewart’s group will have the ability to do both embedded measurements as well as remote sensing.


The group will be conducting impacts into porous targets and measuring the decay of the hemispheric shock wave, which has applications to comets and asteroids.  They also plan to look at shock remnant magnetization.  Shock can actually increase the magnetization of peridotite.  Reasons for studying magnetization include: Mars geophysics (timing of dynamo field), Mars chemistry (composition of crust), Mars samples (peak pressures and T of meteorites).  What we really need though are terrestrial analogs!


Shock compression lab also has a cold room (walk in for preparation of ice samples).


Validation of Martian impact crater geometry measurements:  Use measurements of crater attributes to infer regional and temporal variations in surface properties and to test rampart ejecta formation hypotheses.  The validation technique is to use representative simulated craters to determine the accuracy and precision of the measurements, and to determine the statistical significance of crater geometry measurements of martian craters.  Largest source of measurement error for all craters is how you fit the background.  This technique excludes the gaps in data coverage.  Simulated craters with known attributes were added to real topographic backgrounds taken from target areas.  Information can be found at www.shock.eps.harvard.edu.  Craters were measured to determine uncertainty, systematic error, and sensitivity to MOLA track density.  Results show almost no systematic error.  For craters with D > 4 km, uncertainty is dominated by background topographic features, not by track density.  The conclusion from this study is that we can measure geometrical attributes of craters with high accuracy and precision.


MOLA tracks often miss the rim peak, so to get rim height measurements Stewart used the highest point obtained on the outer rim and the highest point on the inner rim, and extrapolated.  This interpolation works best on fresh craters.  As terrain becomes more hummocky, results become more uncertain, particularly for larger craters.  Volume measurements in general have higher errors because they are more sensitive to track coverage.


A sample analysis of craters on the lowland plains indicated that simple craters are more paraboloid than those on highlands plains.  Craters in the northern plains tend to have higher rims, especially in Utopia.  The difference is statistically significant (>2 s).  Small (strength-controlled) craters in the lowland plains display smaller cavity volumes than craters in the highlands.  Larger (gravity-controlled) craters follow single power-law.  In Utopia, volume above the background surface (i.e., the ejecta blanket volume) is larger than the cavity volume, with a mean ratio of 1.6.  In other regions, this volume ratio is ~1.  Thus in Utopia we see much more volume is contained in the ejecta than expected from analysis of the cavity volume


Pierazzo:  Characterizing starting conditions for hydrothermal systems underneath martian craters.

Motivation:  Many terrestrial craters have strong evidence for the presence of hydrothermal systems.  Hydrothermal systems are sites favorable for the evolution or growth of prebiotic organisms on Mars.


Pierazzo and her colleagues used the SOVA 3D code for early stage studies.  They looked at impacts in dry/wet target mixtures and the influence of water on shock propagation.  During the late stage events, they use the code SALEB 2D to get the final thermal profile of the impact and affected target.  They used both cometary and asteroidal impacts and impact angles vary between 90° and 45°.  The target is basalt (as given by the New Basalt ANEOS; MORB composition; melting occurs at ~1400K, but code gives no details of the solid-liquid transition) and the codes include the effects of the CO2 atmosphere.  The late stage thermal gradient is 13K/km.  During the early stage, ~2,000,000 traces are made which allow estimation of the shock levels at different stages of the impact process.


During the early stage events, the modeling shows the shock melting which occurs.  Comets tend to produce more melt material shocked at a particular level.  The shock levels range from 1.5x as high as asteroids at 30 GPa to 15x at 80 GPa.

Study of the dry/wet targets at the macroscopic mixing level shows that shock reverberation occurs at material interfaces (varying P-T shock levels).  Get different thermodynamic behavior when you have mixed material.


The microscopic mixing effects which occur in dry/wet targets is the area where much work remains to be done.  We are still seeking strategies for dealing with mixed material targets in hydrocode simulations.  The wet target mixed cells used in this study are composed 80% of basalt and 20% ice, requiring a mixed equation of state (EOS).  Unfortunately the model cannot separate water from basalt, thus only 1 equation of state can be used.  Is this the best approach?  Team is still not sure.


Results:  Mixed cells apparently give less melt.  Less volume is being exposed to the higher pressure.  What is the shock level for melting of wet basalt?  Don’t have the data yet.  S. Stewart is working on providing these data.


Boris Ivanov is starting to work on how to model the post-impact thermal state, but this requires a rock/ice strength model which is currently not well known.  How does strength vary with temperature? 


One thing these simulations show is that we end up with a column under the central uplift area where water and water vapor are concentrated in both asteroid and comet impacts.  This could provide a way for water to escape.  N. Barlow wondered if this is the mechanism by which central pits form.


Friday, Oct. 8, PM:

Tanaka:  Does Mars have a pristine -2 cumulative power law distribution for craters >5 km diameter?

What is the production curve for craters on Mars in the intermediate (~5 to 20 km) range?  Many workers use N(5) to define ages of Late Noachian to Early Amazonian surfaces, but there is significant disagreement on the form of the production curve for both the Moon and Mars in this size range.  Understanding the production curve will thus allow us to determine the role of surface materials and histories (e.g., lavas, sediments, volatiles, etc.).


Previous work suggests that the power law slope for craters >5 km on Mars range from     -1.2 to -2.5.


Tanaka and his colleagues looked specifically at the Vastitas units, which is base of the Amazonian.  The Vastitas units have been proposed to be heavily reworked ocean sediments.  They cover and area of 18.1 x 106 km2.  The team believes that the superposed crater population post-dates the formation of the Vastitas materials. 


Ghost craters (craters almost completely buried) are seen in this area, indicating a crater population buried by the Vastitas materials.  Statistics for craters in the 5-16 km size range show an approximate -2 slope.  For larger craters, the slope is closer to -2.5, which is similar to the results from Neukum’s group (Werner, 2004).


Why the paucity of large craters?

  • Large craters destroyed?  No!
  • Target material effects?  A weaker, volatile rich target material might lead to a loss of larger craters due to relaxation and mass wasting of crater walls.  (Werner et al., 2004)
  • Pristine production population?  Tanaka’s group thinks so.  There is good preservation of pristine craters, and the only other craters are “ghost” craters which predate the Vastitats units.  No significant latitude or elevation variations in the crater distributions are seen.


Why hasn’t a -2 slope been found previously? 

  • Requires a surface that effectively resets the cratering record.  Older craters must either be absent or easily distinguished.
  • Surface must be large and/or old enough for a good statistical sample (Vastitas units have >1000 craters >5 km)
  • Lava flows on Moon (maria) and Mars (Alba Patera shield) have been used by Hartmann and Neukum to establish the crater production function.  But these areas likely include multiple flows of different ages and may include preferential burial of smaller craters (5-km-diameter craters have ~200 m rim height; flows are typically tens of m thick).


Previously cited shallower slopes may be affected by resurfacing and obliteration of smaller craters.  New data sets for Mars and Moon may lead to refined counts and testing of our interpretations.


Boyce:  History of major degradational events in the highlands of Mars: Preliminary results from crater depth and diameter measurements

Boyce and his colleagues are looking at selected areas in the highlands with (1) Noachian and younger terrains and (2) Hesperian and younger terrains.


In mid-latitudes (±30°), we see 3 types of crater morphologies among the Noachian and younger population:  (1) Fresh, (2) slightly degraded (shallower), and (3) buried craters.  At higher latitudes (poleward of ±30°) there is no bi- or tri-modal distribution.  There is no obvious distinction between these two regions.  For Hesperian mid-latitude craters in highlands terrains, Boyce’s group again sees a bimodal distribution, but it is very different than what is seen in the Noachian population.  The high latitude Hesperian-age crater population shows much shallower depths.  See a nice bimodal population in Meridiani. 


These observations imply a distribution of depths but not necessarily ages.  Deepest buried craters correspond to Early Noachian or pre-Noachian.  The lower (shallower) population in Meridiani corresponds to late Noachian.  The higher (deeper) population in Meridiani is Amazonia/Hesperian in age.



  • All regions studied show highly subdued Early Noachian-age craters.
  • The Noachian-age mid-latitude regions include an intermediate mode.
  • Noachian-age high latitude regions do not appear to include this intermediate mode in their depth/diameter distribution.
  • The depth/diameter distribution curves of both Hesperian- and Noachian-age mid-latitude terrains include a mode of relatively deep craters, some of which lie nearly on the fresh crater distribution curve.
  • Neither the Hesperian-age nor the Noachian-age high latitude depth/diameter distributions include this deep crater mode.



  • A planet-wide volcanic resurfacing event occurred early in Mars history (early to middle Noachian).
  • During the Noachian, the mid-latitudes of Mars were degraded by a period of fluvial activity.  This study’s preliminary data show evidence that this episode only affected the mid-latitudes, not the high latitudes.


Tornabene:  Recognition of rayed craters on Mars using day and nighttime temperature images from the Mars Odyssey’s THEMIS

THEMIS dark streaks are now recognized as crater rays.  First such crater where dark rays were seen was 10-km-diameter Zunil, located at 7.7N 166E.  Rays are ~800 to 1600 km long.


No strong albedo contrast is seen between the rays and the martian surface, unlike the well-known examples from the Moon, Mercury, and Galilean satellites.  The rays are only seen in night-time thermal infrared imagery (nTIR).  Why can we see only some rays in the nTIR?  Where are the rest of them?  A study of nTIR from THEMIS is revealing additional examples.


“Crater A”:  17.8°S 202.02°E, D = 6.9 km, Rays ~540 km long and up to 13.8 km wide.


Reason you can’t see additional rays at Zunil is because of a temperature contrast.  Formed in an area that is predominantly cooler at night (dust-covered) thereby cloaking most of the rays.  We can see this same phenomenon occurring to the north of Crater A.  Rays contrast well with the surface with respect to temperature but not necessarily albedo.


Thermophysical facies of Crater A (TB » TI):  Crater facies consist of low thermal inertia (TI) intracrater deposits grading into high TI crater wall/rim.  Ejecta blanket facies:  Low TI continuous ejecta blanket, grading into High TI continuous ejecta blanket and low TI continuous ejecta (rocky/blocky ejecta).  Ray facies are predominantly low TI rays often with higher TI cores.


Bearing in mind the thermophysical facies defined, Tornabene’s group conducted a survey from 45°N to 45°S using a THEMIS nTIR global mosaic overlay in JMars.  The lower temperature poleward decreases THEMIS signal-to-noise (S/N).  This study has revealed three more rayed craters.


Crater B:  SW of Arsia Mons at 28.65°S 226.9°E, D = 3.3 km; Rays ~200 km long and up to 3 km wide.  This crater has very good day coverage in THEMIS TIR.  Some rays have better contrast in dTIR than nTIR.  Crater B shows the same thermophysical facies defined for Crater A (nTIR).


Crater C:  Southwest of Elysium, at 16.47°N 125.77°E, D = 7.4 km, rays up to 600 km long and up to 6.5 km wide.  Crater C displays curved rays, suggesting possible atmospheric interactions during emplacement resulting from the Coriolis effect on Mars  (Wrobel and Schultz, 2004).


Crater D:  East of Elysium at 13.28°N 157.21°E, D = 2.0 km, Rays ~50 km long and up to 2.7 km wide.


The study looked to see if there is a linear correlation between ray length and crater diameter.  The length of lunar rays follows a power law distribution with crater radius.  The martian rayed craters appear to show a linear correlation with crater radius—this is probably due to an under-sampled population, but may be due also atmospheric effects.


Comparing the THEMIS nTIR mosaic for Crater A to a TES-derived TI map: 

  • Nearly concentric pattern of the different thermophysical facies.
  • This concentric pattern can be also see in Craters B and C
  • Some rays are discernable (also seen at C and Zunil)
  • Possible detection of a forbidden zone (also seen for craters B and C)—may suggest that ray formation is somehow related to oblique impact events.


Observations from MOC:

  • Larger secondary craters are associated with low albedo deposits, which correlate with the higher TI ray cores (brighter).
  • Smaller secondaries are associated with high albedo deposits, correlating with lower TI ray exteriors (darker)



  • Prior to THEMIS, martian rayed craters were difficult to identify due to their weak albedo contrast.
  • To date, THEMIS TIR offers the best views of rayed craters on Mars.  Some features are now even recognizable in TES datasets.


Barlow:  Martian impact crater ejecta and interior morphologies: New insights from MGS and Odyssey data analysis

Barlow is updating her Catalog of Large Martian Impact Craters using MGS Mars Orbiter Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) data and Odyssey THEMIS data.  Among the changes being made are:

  • A new preservation system has been instituted, which uses a 0 to 7 preservation scale (0 = “ghost crater”; 7 = “pristine crater”).  The system assigns numeric values to depth of the crater relative to depth of a pristine crater of the same size, rim height of crater relative to rim height of a pristine crater of the same size, preservational state of the ejecta blanket (if any), preservational state of any interior features, and thermal inertia of ejecta blanket relative to surroundings.  The sum of these numeric values is used to assign the final preservation value (0 to 7) to the crater.
  • Ejecta morphologies are being updated using the nomenclature recommended by the Mars Crater Consortium (Barlow et al., 2000, JGR, 105, 26733-26738).


Analysis of the new data shows the following:

  • Approximately 25% of all craters in the original Catalog are having their ejecta revised into a different class due to improved image quality and insights from topography and IR analysis.  The single layer ejecta morphology still dominates.
  • The highest concentrations of double layer ejecta craters are found in topographic lows in Utopia, Arcadia, and Acidalia (primarily between 35° and 65°N latitude).  The current study is identifying increased numbers in the equivalent latitude zone in the south, but the overall percentages of double layer craters here are much lower than in the north.  Some characteristics, such as ejecta flow extent, are different between double layer craters in the north and south, with those in the north typically having larger ejecta extents for the outer ejecta layer.
  • Barlow’s current analysis supports the proposal by Costard (1989) that pancake craters are simply the inner ejecta layer of the double layer morphology.  The outer layer either was not discernable at Viking resolutions or has been destroyed.
  • Pedestal craters tend to be small craters (typically 2-3 km in diameter) where both the crater and ejecta are elevated above the surrounding terrain.  They display both single layer and double layer morphologies, with ejecta extent values approaching the highest values seen for the double layer outer ejecta layer.  They are found in the same regions where ice-rich fine-grained materials are proposed to exist.  Barlow proposes that these craters formed by impact into ice-rich materials from which the ice subsequently sublimates, lowering the surroundings.
  • Multiple layer ejecta craters are seen around larger craters (typically >20 km in diameter) primarily along the highlands/lowlands dichotomy boundary.
  • Many martian craters display a central pit.  Approximately 35% of all central pit craters also display a multiple layer ejecta morphology.  But many pits are also seen in craters with no remaining ejecta.  Since central pits are commonly believed to form from impacts into volatile-rich targets, the occurrence of central pits in craters of various preservational states suggests that near-surface volatiles have existed on Mars for a substantial period of the planet’s history.


Mouginis-Mark:  Morphology of Martian double layer ejecta craters and the speed of ejecta emplacement

How fast is ejecta material moving?  We can estimate the speeds based on deflection of the ejecta by previously existing topography.


One interesting observation made by Mouginis-Mark and his colleagues is that thus far they see no sign of secondary craters with any double layer ejecta (DLE) craters.


Radial striations are prominent in the ejecta layers of DLE craters and go right up to the rim crest.  The boundary between the inner and outer layers appears to have failed structurally.  A moat exterior to the rim crest is seen in MOLA data, but no distal rampart appears to be associated with DLE craters, even with the outer layer.


What causes the striations seen on the ejecta deposits?  Stewart suggested that it is caused by the collapse of a vapor plume (essentially a base surge).


Based on the highest obstacle that the ejecta tops and how the material is deflected around such pre-existing topography, the speed with which the material is emplaced appears to be a few 10’s of meters s-1.


See double layer structures that are very similar in the northern plains, Hesperia Planum, and in the southern highlands.  Found at elevations of up to ~1 km above datum.  This indicates that previous speculation that DLE craters only form in topographic depressions (possibly volatile-rich sediments) is not the whole story.


Rodriguez:  Control of exposed and buried impact craters and related fracture systems on hydrogeology, ground subsidence/collapse, and chaotic terrain formation on Mars

Rodriguez and his colleagues propose that groundwater stored and released from an extensive subterranean cavern system was responsible for carving the outflow channels.  Chaotic terrain was formed by an initial stage of gradual subsidence and warping of the surface, followed by a relatively shallow collapse of the deformed plateau materials.  Observations indicate that the caverns are the result of removal of subsurface materials along tectonically interconnected impact craters.  The association of their terminal regions with the head source of the outflow channels suggests a connection.


Hypothesis:  Significance of Noachian layered materials: Wet early Mars?

  • Layered materials have resulted from one or more geological processes.
  • Layered materials may have been deposited in extremely ancient paleobasins and sourced from surrounding topographically higher regions.
  • This would require that the putative source regions have been destroyed, suggesting that more Earth-like conditions prevailed during the Early and Middle Noachian, as theorized by Baker et al (2002).


Impact craters form surface depressions where water may have ponded during one or more wet Mars episodes.  The plateau layered materials reveal numerous buried impact craters, whose interior deposits are likely to be ice-enriched regions within the crysophere.


Crater fracture networks:

  • The group proposes that intermingling concentric and radial fracture systems from multiple impact crater events will result in complex crater fracture networks.
  • Periods of lesser/higher bombardment and/or changes in rates of deposition will result in the formation of layer sequences with a variable abundance of buried impact crater populations.
  • As a consequence, the crater fracture networks will be less/better well-developed in regions of the crust.  Referred to as low and high fracture density regions.


The group has conducted a mathematical simulation of buried impact crater populations during stages of irregular bombardment.  Highly fractured regions which surround impact craters do not extend more than 1/3 of the crater’s diameter outward from the rim.  Extensive connections may arise where craters are densely overlapping and/or closely spaced, resulting in high interconnectivity subterranean networks.  Eventually there was a transition of high interconnectivity subterranean networks into metastable cavernous systems filled with pressurized water. 


Hydrothermal activity results in the preferential heating of high interconnectivity subterranean networks.

  • Convergence of fractures radiating from each buried impact crater with interior ice-enriched layered deposits may result in higher heat flow to these regions.
  • Subsurface erosion may form cavities oriented along high interconnectivity subterranean networks, allowing subsequent storage of high volumes of water.


Observed linear trend in subsidence and collapse features may have resulted from interconnection of randomly distributed surface and buried craters that were within range of their respective highly fractured peripheral zones (~1/3 of crater’s diameter)


Impact-induced fracture systems dominate the basement structural fabric in ancient crustal regions except in regions influenced by magmatic-driven activity, such as Tharsis and Elysium.


Open discussion:

Attendees thought that the talks on the second day were really good and would have liked more time for discussion.  Thus it is recommended that we schedule talks on both days at future MCC meetings. 


There was additional discussion of future topical workshops that could be natural extensions of the July conference.  Everyone agreed that we will discuss this further after the July workshop.  In addition, Chapman will check with Jim Head about possibly having a crater chronology session at one of the future March Brown University Vernadsky Institute Microsymposiums.


There was additional discussion about MCC meeting in conjunction with the Planetary Geologic Mappers meeting.  It was decided that we should test it out one year (probably 2006 when PGM meets in Flagstaff) and see how it goes, then decide if we want to continue.  In any case, in the alternate years when PGM does not meet in Flagstaff, we will continue to meet separately in October.


Additional announcements:

  • Dan Berman:  Crater chronology/isochron description can be found at www.psi.edu.  A manuscript about this topic has been submitted to Icarus.
  • Sarah Stewart:  Topography measurement tools are available for Beta testing.  Contact Sarah if interested.