http://marsrovers.jpl.nasa.gov/mission/sta...tml#opportunity


"Sol 741: Opportunity drove 34.5 meters (113 feet) closer to the Payson outcrop after rover drivers set the current limits back to nominal values. Motor currents at the start of the drive were a bit higher than normal but dropped closer to normal values as the drive progressed."

Looks like not moving for a long time allowed the lubricants to settle noticably. Keep that puppy moving!

1.4.5 (the next one) will probably have the de-vignetting feature... but it may take me a while to finish it yet. I've got to experiment with it more, and I don't have any time to work on it... yadda yadda yadda. Glad you like it though, lyford.

I'm inclined to agree with you. I don't think there is a general appreciation of the destructiveness of a hypervelocity impact. Very little in the immediate vicinity of the impact point retains its original fabric or orientation. Although Erebus is a very small crater compared to the ones forming the basis of current cratering models, one couldn't really expect to see intact much of the surface layering present at the time of impact, right on the crater rim. That would be shattered and disrupted into impact breccia.

Er... ...no, it's not that simple!

My understanding is that, generally, the pre-existing layers are deformed and may be turned back over themselves, and that the blocky debris closest to the crater rim is generally from deepest within the pre-existing structures. I'm sure that somebody else will have a more cogent technical description of the process, but suffice it to say that it's *not* just an almighty wallop - there's a formal structure to impact events of any decent scale. And then the erosion starts...

Bob Shaw

Oh don't say the exploartorium is offline again already... images are down but nothing showing up on the site.

Well now that you have officially retired from actively developing MMB, you will have plenty of time left to work on MMB!
zeke - I have it now- thanks - though I am running out of HD space - time to delete all those BeeGees MP3s

Happy to ear this, Michael!

Well, I wouldn't get too excited about it yet. MichaelT's anti-vignetting tool will probably remain a better choice when creating non-MMB panos. I just got tired of the ugly joins in MMB views, and am trying to see if I can do anything about it without too much effort.

I don't entirely agree. First off, Erebus is several times larger than Endurance. It would not have had a small rim when it formed. It would have had a very pronounced rim, a bowl shape, and a deep floor. As time went on, it would have suffered collapse along the outer rim, as the very steep faces of the inner walls of the crater are not stable over long periods. You get a characteristic scalloped rim as a result of the edges of the rim falling into the crater, widening and shallowing it.

According to my understanding of lunar crater development and aging, it takes the better part of a billion years for a crater the size of Erebus to become as wide and as shallow as mass wasting is going to make it.

Note, too, how Endurance has been partially filled with dust, but is nowhere near "filled" to the extent that Erebus is. And Endurance may well be more than a billion years old.

The problem with Erebus spending eons being worn down to a nub of a crater *before* the evaporites were layed down over it is that, according to curren theory, Mars was only wet enough for the large mass of evaporites we observe to have been laid down for a fairly short period of Mars' early history. My best guess is that there was simply not enough time for an Erebus-sized crater to have been worn down to nearly planar before Mars froze up and standing water was no longer possible.

I would amend your timeline as follows:

First - Deposition of some layered beds (mostly from dune formations)
Second - A shallow sea forms cyclically in the area, laying down layer after layer of evaporite
Third - A cluster of impact craters (including Erebus and Terra Nova) form after some of the local evaporite is laid down
Fourth - Deposition of the rest of the layered Meridiani beds and subsequent groundwater alteration, filling the Erebus-Terra Nova complex with evaporite shot full of hematitic concretions
Fifth - General raising of the land by volcanic uplift of Meridiani Terra south and east of Meridiani Planum, putting an end to standing water by changing the landform and raising it above the then-current "sea level" where seas were able to form
Sixth - Erosion of evaporite, formation of hematite-enriched soils, and dune formation
Seventh - Differential subsidence along old crater rim which offsets evaporite layer along old rim

I just don't think there was time for Erebus and the cluster it's associated with to form and degrade to a filled-in status and *then* have a Mars warm and wet enough to cover that over with evaporite. I think there had to have been evaporite formation both before and after the creation of Erebus.

-the other Doug

I like your scenario, neb. Closer examination of what lies before us (Coming soon to a JPL website near YOU!!) ) might suggest alternatives, but even by Martian standards Erebus is a genuine antique. There seems to be little of the original structure accessible for us to look at.

I'm aghast, Bob, if anyone got the impression from my 'thumbnail summary' that impact crater formation is "simple". I would hardly call it that, although there has been enough progress made over the last 20 years with Hydrocode models of cratering on size scales that are of geological interest, to permit some reasonable quantification of the parameters, given reasonably representative impactors and target lithologies. For anyone seeking a good basic understanding of the process, I can recommend reading Jay Melosh's textbook: Impact Cratering: A Geologic Process from Oxford Monographs, 1989. Though obviously 15 years out of date, and probably out of print, it should be found in most decent university libraries, and is a good starting reference. The math, of course, is a bit daunting, but even an informed layperson will be able to read around that and get the gist.
Since then there has been continuing progress in the models, so a thorough updating will require some reading in the primary and secondary literature. Topics such as acoustic fluidization, shatter cone formation, and improved equations of state governing vaporization, are much better defined now, and make such a foray rewarding. I'm happy to provide a selected reference list by email to anyone who wants it.

Doug, I have to agree with you, I like your time-line better. I think it fits more in line with how long water was available on Mars.
The lack of any large blocks that would have formed from the impact events that Bob mentions would have been eroded away during time point 'six' in your time-line above.

I hate to come in as a contrarian, especially considering how difficult it seems to remotely interpret these busted-up rocks. But I am not convinced that the top layer is draping. Looking at the pavement above the escarpment and going off to the left of Opportunity, I see bedding planes coming to the surface and striking off, across the top surface of the outcrop. If I follow some of them back to the vertical section, they seem to dip off to the left. It's a tough call, since the rock is so broken up. I wouldn't bet more than a six-pack or it's equivalent until we get closer.

The dipping beds I think I see also have implications for the dark strata we have been waiting so long to see up close. If those dark beds are actually coming to the surface as such dips would suggest, they do not appear as dark stripes on the surface. I'm thinking the dark stuff is only evaporite-cemented sandstone that has collected some dust or other stain.

I made a crude animated gif to illustrate what I think I am seeing. It alternates between a view of the outcrop, and one displaying a few possible dip lines leading to strike lines on the surface.

Sol 743:

http://origin.mars5.jpl.nasa.gov/gallery/a...unity_p743.html

Welcome to Homeplate 2.