[X] My Assistant

[Kye Goodwin]

MarsIsImportant, Re your 165, There is no necessity for impact-surge to explain the concentrated silica found at Gusev. It is a tricky mental exercise, I know, but try to consider the evidence of the layering in isolation from the chemical evidence. A hypothesis should explain all the facts but the facts are always in various degrees of doubt. It is a really solid fact that the layered structure at Meridiani and Homeplate dates from the time when the sediments were laid down. The present chemistry at the surface of these deposits (or near them in the case of the silica) may be the product of processes that have little to do with the original deposition. I don’t think that either theory should have to account for the present-day chemistry at Meridiani or Gusev primarily by invoking ancient processes, because we do not know how old that chemistry is or how deep it goes. Endurance Crater is being considered as if it were a fresh drill core but it has probably been part of the surface weathering environment for hundreds of millions of years. The deepest RAT hole hasn’t even reached through the material that is heated diurnally by the sun. The layered structure of the rock is certainly deep but the chemistry could be that of a weathering rind centimeters to meters thick. There is likely some of the original material present as reaction products but even considerable replacement is possible at the surface, which is all that we have seen. Kilometers of chemically complex dust has settled on these surfaces over Mars history in close contact with even more frost. In high-obliquity periods the surface may be covered with a thin layer of dirty snow that comes and goes annually.

The MERs have added very little to the STRUCTURAL evidence of bulk liquid surface water or near-surface groundwater. There are the ambiguous festoons at Meridiani, and the planar layering believed by the MER team to have resulted from many similar shallow flooding events on very level ground. The evidence of water CHEMISTRY, on the other hand, is overwhelming but the form in which the water acted is unknown as is the time period when the chemicals formed. I think that slow processes involving ice, vapor, adsorbed films, water of hydration and even possibly fleeting liquid capillary water from melting frost, might account for the chemical alterations. Without the chemical evidence, the structural evidence of persistent surface water would be completely unconvincing, even to the MER team I would guess. The two kinds of evidence should be considered separately because they are not equally reliable. The structural evidence of the layering tells us something about the process of deposition at Meridiani and Homeplate. The chemical evidence from the present-day surface may not tell us much at all about the time when the deposits formed.

[MarsIsImportant]

Kye that was a good reasoned response. That fact is that we don't know all the circumstances in which this near surface water chemistry took place. We don't know for sure (not 100%) whether this was a very local occurrence or evidence of water chemistry in bulk. But the mere fact that we found this evidence within a few kilometers from the landing site suggests that it is rather common on the surface of Mars.

It certainly does not rule out impact surge as an explaination; but it certainly is not helpful to that hypothesis either. It proves that additional processes have altered parts of the surface since these layers were laid down. It certainly gives me confidence in what the MER team has suggested as the explanation at Meridiani too, simply by implication.

Yes, much of the MER team's evidence for their hypothesis is circumstantial. Yet, the circumstantial evidence keeps building. To rule out impact surge or confirm it, we need a prediction that can be tested by the rovers. I know science is not exactly a court of law; but many people are convicted upon mere circumstantial evidence. If the circumstantial evidence has substantial weight, then chances of error become very low. When circumstantial evidence is backed up by empirical evidence, it becomes very strong of a case indeed! In my eyes, the evidence of some sort of water chemistry occurring on or near the surface combined with the circumstantial found so far makes the case almost conclusive. At this point, I see no reason to doubt the MER teams's conclusions.

In my opinion, the impact surge or brine splat hypothesis maybe a contributing factor to the complexity of the geology on Mars. I don't doubt that, given the enormous number of visible craters on the surface. I do doubt the impact surge hypothesis as an explanation of the evidence found at Meridiani so far. Although very thin deposits would be possible with an impact surge, many are not that thin. In fact, we should have already seen evidence of some thick layers at Meridiani, if the impact surge explanation was even plausible. At this point even if we find thick layers laid down much deeper, it still will not rule out the current MER team consensus. It will only place constraints upon time frames for the various processes involved.

[dburt]

How do you explain the near pure silica deposits found by Spirit? How would that fit in with impact surge?

Kye - Thanks for your thoughtful and philosophical reply to this post. Let me try to answer it much more directly (although I may have already stated this in a previous post): As far as I can tell from the images, the silica-rich rocks are loose pieces sitting on top of the surge deposits, along with various chunks of highly vesicular (gas bubble-filled) and totally non-vesicular (bubble-free) rock. Therefore they need have nothing to do with the surge deposits. They and the other loose surface rocks most likely are random pieces of impact ejecta, of unknown ages, from unknown sources. The silica-rich rock presumably indicates some sort of hydrothermal activity, but whether associated with water (or ice) interacting with a volcano or associated with water/ice interaction with impact melt, would be impossible to say. The silica-rich rock, as another conceptual possibility, might have formed at low temperature by silicate rock reaction with sulfuric acid formed during sulfide weathering (my mine dump model), as a result of acid leaching of the more basic constituents (MgO, CaO, etc.). Again, not necessarily anything to do with the surge process. If a hydrothermal system had interacted with the surge deposits themselves, they would all most likely have been turned almost instantly into clay-rich goo, owing to their highly reactive and porous and permeable nature. Good question - I hope this was a good answer.

--HDP Don

[MarsIsImportant]

Yes, that was a good answer Professor Burt. But much of these silica deposits are in the form of sand. So they cannot be ejecta pieces. A few silica rocks indeed have been found and could correspond to what you just stated. But the sand or dust like 90% pure silica require a longer term water based solution in my eyes.

I supposed it could simply be local because of hydrothermal activity; but that is similar to what the MER team has stated.

Your answer was helpful however. Thank you.

[dburt]

This thread seems to be decaying into the same non-specific objections by just a few people. Does this mean that everyone else reading this agrees with the reasonableness impact surge hypothesis, in light of Occam's Razor? In hopes of providing some fresh material for discussion, I attach, from March 2006, a detailed critique by Squyres et al. of our original 2005 Nature paper, and our point-by-point attempted rebuttal of that criticism. After sending the critique and our response out to two reviewers, Nature decided not to publish it (although they did publish a critique of the competing volcanic surge model). I therefore feel justified in "publishing" it here, as an attachment. This is particularly so because the Squyres critique (but never our response) was for a long time published on the Cornell website, here: http://www.astro.cornell.edu/~banfield/nature2.pdf (link since removed), without giving us any ability to respond. Also, the MER team has repeatedly used these highly dubious points (e.g. the spurious idea that Fe/Ni ratios in iron meteorites must match those in the spherules) in allegedly "refuting" the impact surge hypothesis in publications, even though, IMHO, we demonstrated these objections to be utterly without merit well over a year ago. Note: be aware that this pdf file (short and hopefully not too technical) is a time capsule from over a year ago. Our insights have evolved since then, in part owing to my past several weeks of highly useful discussions here.

One example of an insight not mentioned in the file is the simple fact that all the Meridiani spherules consist uniquely of the high temperature form of hematite - the blue/gray or "specular" (shiny) form. This, the main reason why Oppy landed in Meridiani, appears to provide unambiguous evidence against the low-temperature concretion hypothesis of the MER team. Although this point was as plain as the nose on my face, looking through my own eyes, I didn't see it for years, until I had discussed things on this forum (a form of mirror). Thanks.

Please enjoy or feel free to mock.

--HDP Don

Attached File(s)

 1_reviewer_attachment_2_1143837763.pdf ( 41.2K ) Number of downloads: 268

 

[dburt]

But much of these silica deposits are in the form of sand. So they cannot be ejecta pieces. A few silica rocks indeed have been found and could correspond to what you just stated. But the sand or dust like 90% pure silica require a longer term water based solution in my eyes.

Um, to what sand or dust of 90% pure silica are you referring? All I recall seeing in images are broken rocks (some of them quite tiny pieces, but no more than you would expect from breaking a weak piece of impact ejecta as it landed or grinding it under the rover wheels). None of the high-silica rock seems to be "in place." Thanks (and all for now).

--HDP Don

[MarsIsImportant]

...Very interesting arguments Professor. Many of the issues brought up in the attachment have been discussed already, sometimes at length. It does give us a better idea of why you seem frustrated. Some of your counter-arguments are good.

I still have not changed my mind. But this discussion may be useful.

I must point out that silence does not indicate affirmation. Despite the common rumor among salesmen, many people simple don't want to argue. That does not mean that the customer actually agrees; they just don't want to create confrontation.

[MarsIsImportant]

Um, to what sand or dust of 90% pure silica are you referring? All I recall seeing in images are broken rocks (some of them quite tiny pieces, but no more than you would expect from breaking a weak piece of impact ejecta as it landed or grinding it under the rover wheels). None of the high-silica rock seems to be "in place." Thanks (and all for now).

--HDP Don

http://marsrovers.jpl.nasa.gov/gallery/pre...39_L257F_br.jpg

Sorry but there is no way that the rover could possibly crush these deposits to such fine grains. There are image examples where the rover has crushed some silica rocks. They don't resemble this at all.

[dburt]

Sorry but there is no way that the rover could possibly crush these deposits to such fine grains. There are image examples where the rover has crushed some silica rocks. They don't resemble this at all.

Um, again (I came back online). Are those really silica rocks? They sure look like crushed subsurface sulfate efflorescences to me - something that Spirit has been seeing more and more of since it broke its wheel. Please don't confuse high-albedo soft sulfates with high-albedo hard silicates. (Or is it I who am confused?) Thanks.

--HDP Don

[MarsIsImportant]

Um, again (I came back online). Are those really silica rocks? They sure look like crushed subsurface sulfate efflorescences to me - something that Spirit has been seeing more and more of since it broke its wheel. Please don't confuse high-albedo soft sulfates with high-albedo hard silicates. (Or is it I who am confused?) Thanks.

--HDP Don

http://marsrovers.jpl.nasa.gov/gallery/pre.../20070628a.html

It is clearly marked as fine grained silica. This was the biggest and best discovery of the entire mission and it was very recent. I am correct.

[dburt]

I still have not changed my mind. But this discussion may be useful.

I must point out that silence does not indicate affirmation.

Thanks for your kind comments. I am not asking you to change your mind - only open it.

As someone who has been teaching for a while, I am well aware that a class full of sleeping students does not indicate that they all agree with me. Sometimes I have to give them a quiz to wake them up. Inasmuch as you seem to have appointed yourself spokesman for the sleepers, how would you, for example, explain 1) the high-temperature (blue/gray) hematite in the blueberries, if they are actual low temperature sedimentary concretions, not to mention 2) why they are strictly size limited to 5 mm diameter, unlike actual concretions, 3) why they are not shaped like actual concretions, being generally perfect spheres, 4) why they never clump together like actual concretions (just the odd doublet or triplet of small spherules, explainable by natural stickiness and growth in a cloud), 5) why their distribution is apparently never controlled by fluid migration paths, inasmuch as fluid migration and mixing is how actual concretions form, 6) why they are characteristically nickel-enriched (keeping in mind the arguments in our Nature rebuttal), or 7) why similar appearing (albeit smaller) spherules are locally abundant at and near Home Plate, if that outcrop has an entirely different origin? Take as much time as you need, and feel free to work in groups. This is a take-home exam for the entire class. Good luck.

--HDP Don

[dburt]

http://marsrovers.jpl.nasa.gov/gallery/pre.../20070628a.html

It is clearly marked as fine grained silica. This was the biggest and best discovery of the entire mission and it was very recent. I am correct.

You are quite correct about the quote (and my impression about sulfates, based on the image alone, was apparently wrong). I am still apparently correct that it is a loose soil, of unknown origin, sitting on top of whatever rocks lie underneath. If future investigations show that it is part of an outcrop, and not just an orphaned bit of soil, then its parentage will have possible genetic significance. Otherwise it's just another Mars mystery. But defininitely score one for your team.

--HDP Don

[MarsIsImportant]

Well, I cannot speak for those who sleep. However, let me again state that the limited and consistent size of the spherules is your best argument--that in the absence of any known nearby volcanic source. That has bothered me from the beginning of the mission.

I cannot really explain it. My mind has been open all along.

Knowing how big some of the volcanoes on Mars are, I wonder whether the less gravity would allow surge clouds to travel much further than expected. But I cannot explain the distribution of the berries either. It doesn't seem to make sense--whether it is volcanic surge, impact surge, or concretions. I think there is a missing major piece of evidence that opportunity needs to find. Whether it will ever find such a piece of evidence is hard to say. Spirit did not find the silica until very recently, and only because of a broken wheel. Maybe Opportunity needs to lose a wheel in a place that it can dig deep too. I don't know where that could possibly be. Perhaps when it is inside Victoria.

[djellison]

Does this mean that everyone else reading this agrees with the reasonableness impact surge hypothesis,

Absence of objection doesn't mean evidence of support.

Doug

[ngunn]

Well, I for one find all this very interesting, even though my lack of relevant knowledge means I'm a million miles away from actually holding an opinion.

Sadly, however, I note that we are now 'students' who have been given 'homework' and even for the brighter ones an occasional 'mark' !!! This is an excellent strategy for deterring rather than encouraging intelligent comment. Why oh why do you do it, dburt?