MINUTES OF MARS CRATER MORPHOLOGY WORKSHOP III
US GEOLOGICAL SURVEY, FLAGSTAFF, ARIZONA
OCTOBER 10-11, 2000
Attendees: Nadine Barlow (UCF), Charles Barnes (NAU), Joe Boyce (NASA), Bob Craddock (NASM/SI), Trent Hare (USGS), Ruslan Kuzmin (ASU), Susan Sakimoto (NASA GSFC), Jennifer “Fred” Ramstad (NAU), David Roddy (USGS)
Monday morning: Updates Since Last Meeting.
Nadine Barlow updated everyone on the status of the ejecta nomenclature paper which was submitted to JGR-Planets. The paper has been accepted for publication and the typeset copy was submitted in September. Publication in JGR-Planets is expected in November. Copies of reprints will be provided to each of the authors.
Joe Boyce provided information about the status of the MDAP program, including a new addition for Mars Global Surveyor Data Analysis. The announcement for this new program will appear by mid-October, with proposals due in January.
Susan Sakimoto presented the latest information about crater morphology as deduced from MOLA. Susan and Jim Garvin now have digital elevation models (DEMs) for over 5000 craters. They are finding a considerable amount of layering in the interiors of high-latitude and polar craters. The transition between craters displaying these interior deposits and those which do not occurs at about 65° to 70° latitude. High latitude craters also typically have greater depth-diameter ratios than lower latitude craters and the simple-to-complex transition diameter is greater for the higher latitude craters than for mid-latitude craters. MOLA continues to reveal ramparts at the edges of all fresh ejecta blankets and the rampart is typically a substantial fraction of the total rim height. The combination of using MOC and Viking imagery together with the MOLA topography is proving to be very powerful in trying to understand martian impact craters. The pictures provide information on the processes associated with crater formation while the MOLA data provide clues to the physics involved.
Questions that Susan and Jim asked the Crater Morphology Consortium to consider:
1) Do we need the pancake morphology since all layered ejecta blankets seem to display a distal rampart?
2) Since all layered ejecta morphologies seem to display a distal rampart, do we need to specifically state “rampart” in describing the morphology?
3) What was the original structure of the crater and how does subsequent modification affect this structure?
Nadine Barlow led the discussion of the nomenclature which has been used to describe interior features in martian impact craters. After much discussion, the meeting participants suggested the following nomenclature:
Simple Craters (“Bowl-shaped”)
Complex Craters (“Flat-floored”). Modified by the following terms, when applicable:
· Interior Peaks
· Single Peak
· Summit Pit
· Multiple Peaks
· Peak Ring
· Wall Terraces
· Single Terrace
· Multiple Terraces
· Central pit(s)
There was considerable discussion as to whether to include a transitional category to represent those craters in the simple-to-complex transition zone where some, but not all, of the complex crater interior features are seen. It was decided to avoid the “transitional” category and instead allow researchers to simply list which features are seen in a particular crater.
Bob Craddock provided an overview of the work he has done on crater degradation. He has divided craters into five degradation stages, based on the appearance of the rim and the degree of infilling (i.e., changes in d/D). The photoclinometry technique of Davis and Soderblom was used to obtain topographic profiles. Bob and his colleagues then used computer modeling to simulate how a crater changes from a fresh appearance to its final profile. He has found that the slope change between the wall and the floor seems to be a strong indicator of the modification process(es). Interior features and ejecta blankets appear to be lost very quickly as a crater experiences degradation.
Bob has recently compared the photoclinometric profiles with the topographic results from MOLA and has found the two to be consistent. A discussion ensued that focused on developing a “typical” fresh crater profile using MOLA data and continuing computer simulations to better constrain the modification processes acting on craters in specific regions of Mars.
Jennifer “Fred” Ramstad gave a summary of her MS thesis work (NAU) on pedestal craters. She looked at small pedestal craters (Viking resolutions of 100 m/pixel and better) and compared their characteristics to those of layered ejecta morphologies (mainly single layer and double layer) in the same areas. She primarily compared the ejecta mobility and lobateness values for the different ejecta types. She separated the results by terrain (highlands vs lowlands vs transition region) and by appearance of the pedestal morphology (6 classes). Fred was in the process of recalculating the ejecta mobility ratios, but the trend is for pedestal craters to have higher mobility and lobateness values than layered ejecta craters. The only exception seems to be for the serrated pedestal class, which has the highest values for both ejecta mobility and lobateness.
David Roddy lead the discussion on the terrestrial experiments which have been conducted to study the formation of impact craters. David showed slides (before and after) of a chemical explosion crater, with particular emphasis on the features seen in the interior and within/around the ejecta . Of particular interest to the morphology people was the presence of a circumferential (and to a lesser extent radial) fracture pattern at the outer edge of the ejecta. Also of interest was how the passage of the shock front through a ground water aquifer produced mud volcanoes both within the crater interior and along the exterior faults. These was some discussion of whether such features could be visible in MOC images. Are they not seen or is it just a case that we have not been looking for such features?
David then presented the results of computer simulations of a Chicxulub-type impact into continental crust versus an ocean. He provided copies of the published report:
Roddy, D. J., S. H. Schuster, M. Rosenblatt, L. B. Grant, P. J. Hassig, and K. N. Kreyenhagen, Computer simulations of large asteroid impacts into oceanic and continental sites—Preliminary results on atmospheric, cratering and ejecta dynamics, Int. J. Impact Engng., 5, 525-541, 1987.
The calculations emphasize how quickly the crater actually forms. Interestingly, despite the target differences, both impacts form similar-size transient craters at the same rate. Transient rim uplift in the ocean environment was much larger and decayed rapidly to develop tsunami conditions. About 70% of the ejecta formed massive ejecta blankets within about 3 crater diameters of the rim. The rest of the ejecta was injected into the atmosphere.
The discussion turned to the formation of ejecta, particularly formation of the moat seen in the MOLA results and the distal rampart seen in most MOLA profiles of layered ejecta blankets. Does the moat increase with increasing crater diameter until we see it as a rampart? Are debris flows good analogs for producing a rampart? David knows of a code that could perhaps provide insights into these questions, but it is very expensive.
Trent Hare gave an update about the combining of the crater databases into the ArcInfo database. He noted that the biggest problems he has encountered in combining the databases (Barlow, Boyce, and Roddy databases) are the different formats and the control net differences. The rest of the morning session was devoted to a discussion about the status of the MDIM 2.0 update, which is based on the control net derived from MOLA data. Trent asked Randy Kirk to join us to discuss the MDIM 2.0 update—Randy agreed to talk to us after lunch.
Randy Kirk discussed the status of the MDIM 2.0 update. He noted that there is no easy (i.e., automatic) way to correlate the old and new control nets and that has been the source of much of the delay in releasing MDIM 2.0. There was discussion about possibly hiring a student to manually update the catalogs to the MDIM 2.0 coordinate system. Trent and Randy noted that a unique ID for each crater would greatly facilitate the transformations between the old and new systems.
Joe Boyce led the discussion about possible collaborative projects between the consortium members. Several projects were discussed:
· The effect of latitude and topography on ejecta mobility (Kuzmin, Sakimoto/Garvin, Barlow)
· Obtaining information about subsurface properties from crater morphology (Barlow, Kuzmin)
· The moat-rampart connection identified during the crater mechanics discussion (Sakimoto/Garvin)
· Obtaining standard crater profiles for ejecta types by terrain (Barlow, Sakimoto/Garvin, Hare)
· Emplacement mechanisms of the distal ramparts (Roddy, Boyce, Craddock)
· Ring fault vs moat location (Boyce, Roddy, Barnes)
Joe Boyce and Nadine Barlow led the discussion about future plans for the consortium. Susan Sakimoto offered to host a workshop at Goddard (tentative date: August 2001) so that people can see first-hand how the MOLA processing is done. Susan would be the local host; Nadine Barlow and Bob Craddock agreed to serve on the organizing committee. People from the crater mechanics community (such as Richard Grieve, Jay Melosh, Pete Schultz, and Olivier Barnouin-Jha) would also be asked to be a part of the organizing committee. Susan will check with LPI regarding logistics support. She will also submit a proposal to MDAP to cover not only some of the costs of the meeting but also travel funds for terrestrial crater/explosion experts.
At the next meeting we need to decide on formats for combining databases, discuss resolution issues, and address the issue of providing a unique ID for each crater (perhaps the latitude and longitude, if a standard control net is available).
We also need to obtain input from the crater mechanics community regarding the formation of the layered ejecta deposits and their associated features. The biggest question remains “How do subsurface volatiles affect the formation of impact craters and their associated interior and ejecta features?”