kaylaiacovino^1.0

OK. Let's talk.

In Edgett's discussion of Victoria Crater, he suggests that the original rim is buried beneath the uppermost strata of the plains. By this, I assume he is referring to what I would call the Upper Burn's Cliff Fm, the light toned, sulfate-rich unit that we first encountered in Eagle Crater.

As I see it, we have traced this distinctive, slabby weathering unit (with some textural variations) laterally from Eagle Crater into Endurance Crater. And seemingly we have been driving over subcrops of the same unit all along the route to Victoria Crater. As suggested in MOC images of the area, and now HiRise images, this unit is laterally widespread over the landing site area but buried by the surface plains unit of aeolian drift materials that comprises the youngest unit in the area.

In both Endurance Crater and now it appears in Victoria, this upper light-toned unit is exposed near the rim, with large, flat slabs having slipped downslope so that they are now lying on the inner crater walls. If I have interpreted you correctly, you have raised the essential question: Is this light-toned, slabby unit younger than the two impact craters, having once extended above these craters, burying them. Or, is the unit older, pre-dating the impacts. Edgett thinks the unit is younger, but let's examine exactly why.

As near as I can tell, Edgett's claim of a younger age seems to be based on the fact that slabs of this unit are sitting on the inner crater walls, as though they have fallen in from above during erosion. And that's it!

The problems I see with this conclusion are:

1) the upper unit dips radially away from the rim of Endurance Crater, as would be expected if it were there before and was uplift during impact. Because the unit appears to be the same over the whole area (with admittedly some textural variations and differences in abundances of "blueberries"), I can see no reason to think we have crossed into another unit of a different age. In other words, I think the upper unit is correlative over the whole area traversed to date and at Endurance, I think the raised rim gives the answer.

2) While it is noted, there has been exhumation and extensive retreat of the rims of both craters. However, relative to Endurance, which still preserves raised (upturned) and faulted units that dip away from the crater in all directions, Victoria has clearly sustained much more erosional retreat, with removal of the expected impact upturned units. The beds exposed in the crater walls at Victoria are all pretty flat lying, consistent with removal of most, if not all, of the rim. While one might argue on this basis that Victoria crater is older than Endurance, it is probably not a lot older. I conclude based on lateral continuity implied in HiRise images and from the Opportunity traverse data, that the upper unit is older than Endurance, and probably Victoria as well.

3) My view is that the large slabs of this upper unit that have fallen into the crater and which are now sitting on the slopes of the inner crater walls tell us little about relative age relationships. These units are clearly resistant to weathering and would be expected to persist as the rim retreated, regardless of age. In other words, if the upper unit is older than the impact, I can see no reason (given the resistance of this unit to erosion) why slabs of it should not come to lie on the inner crater walls during slope retreat, just as they would if the units were younger and once extended over the whole area. For this reason, I think Edgett's claim is weak.

4) Edgett showed the "upper unit" of the Burns Cliff in this area overlaps in time with impact cratering events. Clearly many craters are buried, while others are being exhumed. In other regions of Meridiani, layered light toned rocks are actually seen in MOC images as crater filling. However, Edgett also notes that there is a great deal of lithological variation over Meridiani and while these units are likely sedimentary, the composition of units (and paleoenvironments represented) is still unknown (MRO CRISM may solve this problem) when moving from place too place. I think it is dangerous to assume that what is seen somewhere else in MOC images can be applied across the board at the Opportunity landing site. We need to figure out what is going on where we are, using high resolution imaging methods on the ground to map the crater wall units in detail. Given the size of the crater, that will take a while! However, this may actually provide some definitive answers if we persist.

If I understood you properly, you indicated that unraveling these age relationships is crucial for testing the impact surge hypothesis Don presented on Thursday. I assume this is based on the assumption that Victoria Crater, the largest impact crater in the area was the source for the local surge deposit as you have interpreted it. However, seems to me that the arguments for an impact surge origin you have formulated are based on volcaniclastic structures in outcrop and composition, not necessarily on inferred age relationships. True, it would be nice to have a large impact crater source to support your hypothesis, but it seems to me that the arguments to date have been wrapped around textures and composition. How has this changed? I think that given the latitude for potential burial of craters in the region, the tests should be based on what is seen in outcrops we are about to visit.

As you know, I feel most comfortable with the team's present interpretation of interdune playa (groundwater-mediated) sedimentary system to explain what we have seen so far, I would be happy to be proven wrong with new observations. Again, I see any important answers as coming out of the rocks exposed in the crater walls of Victoria, our next big adventure.

How's that for getting the ball rolling?