I was just thumbing through Squyres' "Roving Mars" again recently, and ran across something that I had thought I remembered.

For quite a good time (several weeks), Steve himself kept holding on to the notion that the layered rocks in the walls of Eagle crater were some type of welded tuff. As I understand it, Don is basically proposing that these rocks are, in fact, a form of welded tuff.

The vugs and especially the formation of concretions within the rock layers (as opposed to disturbing or displacing the rock layers) were the factors that changed Squyres' mind. That and the fact that the blueberries were of rather different composition (i.e., hematitic) than the rock in which they were embedded; the question that begs an answer is why *any* basal surge (volcanic or impact) would deposit simultaneously two populations of materials, each very different from the other. The compositional differences were sort of the nail in the coffin as far as Squyres was concerned.

Don, I don't think I've yet seen you address the question of why the blueberries would be so well distributed within the fine suplhate-rich layers and yet be of such different composition from them if these rocks were laid down by the same surge process. Wouldn't the mixing that occurs within the surge cloud, and the tendency for items of like mass to travel like distances (and of unlike mass to travel unlike distances) tend to sort out the gravels from the fines? An atmosphere would only tend to accentuate such sorting, I would think.

I just don't think we're seeing the kind of sorting one would expect between the heavy hematite-rich ferrous gravel and the sulphate-rich salt fines.

Also, how does the impact surge theory account for the apparent "feeder" formations into blueberries still in place -- small stalks of blueberry-like material leading in random directions from many of the in-place berries? These make sense if you picture the berries as concretions, formed by water flow within microfractures in the salty rocks. I don't have a feel for any inherent process in a surge that would account for them.

-the other Doug

CosmicRocker - Thanks for joining this discussion. Well, so far 3 people, including you, have chimed in agreeing with me that those look like "festoons" in the middle of the Cape St. Mary cliff, and none have disagreed, so apparently I wasn't, in my highly biased way, just seeing things. It has been claimed by a certain member of the MER team that these are an unambiguous record of little current ripples (like tiny sand dunes) that uniquely indicate water that was locally flowing vigorously ankle- to waist-deep across the flat surface of Meridiani. We have begged to disagree about the significance of these features, and even about their actual presence. Most look much more like a topographic artifact, caused by the downward viewing angle, as discussed in a previous post: Imagine yourself standing behind a person wearing horizontally striped trousers, and looking down at their posterior - you will see perfect "festoons" every time - UU.

The MER team, as far as I am aware, has been stating for the past 6 months or so that the cross-bedding in the Victoria cliffs uniquely indicates a record of aeolian (wind) deposition. Do you possibly see a consistency problem with having "festoons" in the middle? Does it in general sound reasonable to you that water (a sort of miracle brine) came gushing out of the ground, flowed ankle-deep to waist-deep across a level sandy interdune playa (an oasis among the dunes in the desert), leaving local little current ripples and absolutely no other record, and then sank back into that same reservoir? Can you explain why this artesian water flowing across water-saturated sand didn't simply form a braided stream deposit (dozens of tiny channels branching and rejoining as they flow leisurely across a plain), which is what 99.9999% of terrestrial streams would do under the same circumstances? Sorry for answering a single good question from you with so many more from me, but then I hope the answers to my questions are obvious.

The answer to your question is, no, I seen no problem with a huge variety of features, ranging from dune-like to ripple-like, occurring in a single cliff face that exposes surge deposits. The extreme turbulence (violence even) and rapidly changing conditions (including stickiness of particles) allow for far more variety than would be expected in a simple blowing-wind- or flowing-water deposit. The Victoria cliff exposures and their intricate, inconsistent bedding patterns therefore strike me as typical surge deposits.

Those so-called festoons are the only evidence ever cited for flowing water on this part of Mars. IMHO (as mentioned in an earlier thread), it's as unlikely to have such flowing water on a horizontal surface is it is was to have a puddle at a 20 degree inclination on Burns Cliff (as discussed prior to my participation here).

In deference to the other Don who started posting here before me, I'll henceforth refer to myself as

--HDP (for Herr Doktor Professor) Don

Like Tom, I've been lurking as this excellent discussion has progressed, but I now have to comment on what Don/"HDP" wrote in his last post regarding "festoons". As I've mentioned before, I never use this term, and it really has been out of fashion in clastic sedimentology for quite a while. We have better terms that more exactly speak to the scale and geometry of cross bedding and cross lamination.

What I see in the images in question of Cape St. Mary, at this distance, are large (meter) scale trough cross beds bounded by truncation surfaces. Other views of the other capes have also shown evidence of deflation or other more regional truncation.

What I interpret as having been called "festoons" in the past are either current or oscillation ripple cross lamination that is on the centimeter scale. Without better exposures that show more dimensions of these features, it will be difficult to definitively interpret their origin. The key features that I would like to examine would point to angle of climb, preferred current direction, foreset asymmetry etc. The clotty, recrystallized nature of the rocks in close-up photos of these features also makes "remote sensing" difficult.

Don Burt has provided some good references on the theory and some of the sedimentological features of base surge deposits, which include both cross lamination and cross bedding. However, bedforms are bedforms, no matter what the fluids and particles may be (channeled scablands to mud puddles). Allen's seminal work on the signal importance of flow separation phenomena on bedforms and resultant cross stratification is required reading for any and all interested parties, and Dave Rubin's visualizations are also very educational.

I am looking forward to even higher resolution close-ups of the Victoria cliff faces. If we start seeing grain-flow tongues and grain-fall drapes at the toesets of the large-scale trough foresets, an aeolian interpretation will certainly be preferred.

MarsIsImportant - Thanks for chiming in. You haven't asked a specific question, which makes me uncertain how to respond. Perhaps I'll blather a bit, like you. First, as I stated at the very beginning of this thread, please don't confuse me with Nick Hoffman and his "white Mars" scenario. As far as I'm concerned, you can have all the wind and water and climate change you like on Mars. Just be aware that surge deposits, in their considerable variety, can mimic nearly perfectly a huge variety of features generally described as being typical of (or even unique to) wind or water. Also be aware that, since the end of heavy bombardment (late or not), Mars scientists overwhelmingly agree that the dominant Martian climate has been very similar to that which we see today (with perhaps short excursions variously attributed to obliquity variations, underground brine outflow, volcanism, or impacts). The Meridiani "oasis" thus appears to be something of an anomaly, which is why it interests so many people. (Its high-temperature or "gray" hematite signature was an anomaly from orbit, which is why Oppy landed there.)

Our impact interpretation for the Meridiani (and Home Plate and, tentatively, other salty cross-bedded) deposits implies that they need not indicate either wind or water. No one argues that the soluble salts indicate water (a brine) in the past - the only argument is over when in the past. The impact hypothesis argues that the salts could be considerably older than the deposits in which they are found. After we had pointed out that the unlikely Meridiani mixture of salts that were both soluble and insoluble, and acidic and neutral was incompatible with a simple evaporation (playa or sabkha - marine salt flat) model, the MER team modified their model to include wind transport and mixing of older salts. (Then one of them, in what could be perceived as an attempt to heckle or intimidate me, vehemently denied having done so, in front of the entire Planetary Geology Division of GSA - as brought up by an eyewitness yesterday. A year earlier SS had argued to reporters that our Nature paper was worthless because we weren't aware of - and therefore didn't address - the latest changes to their model. Whom to believe? Did they make changes to their model or not?)

I won't argue with you that Mars is complex. Occam's Razor (or the K.I.S.S. principle) is useful only in deciding among competing hypotheses (preferably physical ones involving neither biology nor biochemistry), not as a general description of natural processes. You seem to be accusing me of misusing it. I don't think I am. Nevertheless, I'll freely admit that my personal prejudice is that Mars geology is probably considerably simpler than Earth geology (and more complex than lunar geology). The problem is that we all wear terrestrial "blinders" and cannot always recognize the stark, beautiful simplicity of Mars when we see it.

--HDP Don

Other don - Thanks for the exact reference to the Spirit reporting of sulfides. It sounds like you were lucky enough also to be an eyewitness to my very first scientific encounter with the MER team member you mentioned in your last post, only a year earlier. After his GSA talk (which dealt with the team's new "vanished playa" modification to its earlier "playa" model - so I was quite aware of this modification well before its publication in Earth and Planetary Science Letters, contrary to claims made afterwards by SS) I asked the speaker, quite reasonably I thought, why the possibility of brine freezing was apparently excluded from consideration, given that Mars is such a cold planet (i.e., why the "Death Valley/Persian Gulf Days on Mars" story, instead of, say, making analogs of the permanently frozen, highly saline lakes of the Antarctic Dry Valleys). He replied, as I recall, that "because the brines are eutectic, we don't need to worry about them ever freezing". I was astonished nearly speechless by this statement, inasmuch as, 1) as I blurted out, Knauth and I had co-authored the 2002 Icarus paper "Eutectic brines on Mars..." which seemed to freak him out a little - at that time we had never met, and 2) he didn't seem to have the least idea how eutectic brines actually form. As I explained to him and SS in a later e-mail, by definition, the only way to make a eutectic brine is by freezing (or melting) - it is the last tiny little bit of brine left after fractional crystallization of all the solids, including ice and various salts (hey Ma, can you tell who'd been teaching igneous and metamorphic petrology for the previous several years? ). For the record, no reference to "eutectic brines" has ever afterwards found its way into the Meridiani literature, not even in meeting abstracts or talks.

Sorry about telling that story (no Ma, really ). You gave me the obvious opening, and I couldn't resist. The utter misapplication of the term "eutectic" (I think he meant to say "all-salt saturated" or some such) certainly didn't inspire confidence in his infallibility as a sedimentologic super-authority. Nevertheless, rather than dissecting personalities (to which I'm as vulnerable as the next person), I'd really much rather keep this thread devoted to a semi-scientific dissection of that horrible, awful impact surge hypothesis, if you don't mind. As a starter question, are there any observed features of Meridiani that CAN'T be explained by that horrible, awful impact surge hypothesis?

BTW, Phoenix is supposed to get up to 116 F (47 C) tomorrow, for the holiday. Does that sound a lot like Mars to anyone?

--HDP Don

HDP Don,
It sounds like you are arguing that all the layers that Opportunity
has seen, and even that the entire 1km stack of layers, are likely
created by base surge events. Do you not allow for a significant
number of these layers being wind-blown deposits?

Going back to what you say in this post:
"All impact spherules are caused by vapor condensation in a hot turbulent cloud.
Specular (blue-gray) hematite typically forms in steamy volcanic fumaroles by
condensation and reaction of volatile Fe-chlorides or other volatile Fe species,
and this is a very similar environment to that in a steamy surge cloud. The
Meridiani difference is that some other sticky condensate must have caused
the hematite flakes to preferentially adhere to each other and other particles,
and grow as a snowball does..."

"[S]ome other sticky condensate must have caused the hematite flakes to
preferentially adhere to each other and other particles..." sounds vague and
wishful. Is there any candidate for this "other sticky condensate"? The
"volcanic fumeroles" example sounds like something that takes time. I still
have a hard time seeing hematite berries forming in a violent, sudden event.
Has this sort of differentiation by condensation been seen in base surge deposits
on Earth or are most or all berry-like features in Earthly base surges similar in
composition to the material surrounding them?

My problem is with seeing such an efficient differentiation of materials ocurring in
such a violent, rapid, and short-lived event. I don't have a problem with differentiation
based on size, but with that based on chemistry. How was virtually all of the hematite
removed from the sulfate materials with which it must have been thoroughly mixed
by impact and recondensed as spherules so rapidly?

Hi Kye,

A web search revealed you as our only defender on that other site a long time ago. Nice to meet you at last and thanks for holding the fort, although I'd be especially interested in discovering why you decided to defend our highly controversial idea at such an early date.

Regarding your specific question about "single particle layers," not being or claiming to be a sedimentologist, I'm probably not the best person to answer it (but that hasn't stopped me yet - why now?). I'll give my reply in two parts: 1) I doubt that such layers could be unambiguously be recognized on MI images of Meridiani sediments, given the amount of salt encrustation. 2) Early Meridiani papers referred, I believe, to "pinstripe lamination" as being diagnostic of eolian deposits. We therefore looked up the original 1988 paper by Fryberger and Schenk in Sedimentary Geology, v. 55, p. 1-15. This referred to pin stripe laminations as "distinctive" and concluded only that "They may prove useful in the recognition of eolian sediments" in the abstract and "may help in identifying rocks of eolian origin" in the conclusions.

Once Knauth had published his first LPSC abstract here:
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1869.pdf
in which his Fig. 3 was labelled "Pinstripe lamination and low angle cross-bedding in base surge deposits from Kilbourne Hole, NM" the MER team apparently stopped referring to "pinstripe lamination" as proving anything much at Meridiani. Note also his Fig. 2 that shows a Burns Cliff-like high-angle cross-bed (dune-like form) from a surge deposit in the Superstition Mts. about an hour out of Phoenix, and his Fig. 5 that shows Meridiani-like polygonal cracking from a bedding surface at the same locality.

A better answer about "single particle layers" could perhaps be given by Tim Demko. I'm pleased to discover, from his latest post, that we have a real honest-to-God HDP sedimentologist hiding here amongst the sheep. Baaa. Baaa. Baaa. (He doesn't seem to think much of the term "festoon" either, and agrees that cross-beds indicate scouring - something that surges are certainly capable of, far more than the wind.)

--HDP Don

Other Doug,

I'm running out of time to answer all of these questions tonight (my wife just called to find out where I am), so l'll try to be brief. Some types of tuffs are welded ("ignimbrites") usually from non-basaltic types of volcanoes. You wouldn't really expect welded tuffs on Mars. Volcanic surge deposits are characteristically non-welded (else they would never be mistaken for normal sediments), and impact surge deposits on Mars, where the target was probably frozen, could be expected to be even less so. The only welding is by salts, which encrust the grains during "diagenesis" (chemical and physical changes to a sediment that occur just after deposition) - that's also when the larger chloride crystals formed, later to be leached out (perhaps by frost). This encrustation does not require being immersed in water (in which case you would grow truly large crystals and segregate salts by solubility), only moisture and capillarity.

Regarding the hematitic blueberries, they theoretically could form in a number of ways (metal condensates, later oxidized, Fe-rich glass condensates, later oxidized, or Fe-rich accretionary lapilli, later oxidized). The only method that explains why they are the blue-gray or high-temperature form of hematite is if they actually grew in the surge cloud from Fe-chloride vapors and steam, as blue-gray hematite does in fumaroles. They probably condensed and accreted in the particle-poor, gas-rich top of the cloud, not the particle-rich bottom. Once they had reached a certain size, however, they gradually worked their way down through the turbulent cloud (still travelling at perhaps 100's of km per hour) and were deposited with the sand particles as a dissemination. Sorry if that sounds unreasonable to you. Make a better suggestion. Keep in mind that the conventional concretion story absolutely fails to account for the blue-gray nature of the spherules under any circumstances (or their strict size limitation, or their shape, or their failure to clump in masses) - and the blue-gray nature of the hematite in the spherules was the ONLY reason why Meridiani was picked as a landing site (other than being flat and equatorial). As I said in a previous post, picture yourself injecting BB's and sand in the turbulent exhaust of a screaming stationary jet engine (a crude approximation of a surge at some stage in its evolution). Do you really expect them to "sort out"? Read my earlier posts on this topic, and explain why a sedimentary concretion should consist of the high-temperature, blue-gray form of hematite.

As I also said in a previous post, don't confuse salt encrustations on top of spherules, or smoothing and shaping caused by wind, for the spherules themselves.

That's all for now before I have to get a divorce. Try to get to the rest later.

--HDP Don

Doctor Burt, Regarding your reply 108 and my 99: Why did I decide to support your outlandish theory early on? I guess I did something that few bothered to do. I found images of base-surge exposures on the net so I could see what you were talking about. The original brine-splat was not illustrated. Years later I am still answering variations on the objection that "An explosion couldn't produce regular layering", and that objection is apparently still a problem for some on this forum. I do not know HOW surge produces regular layering, and I suspect that this is not well understood by sedimentologists, but one can have no doubt that surge does often create regular stacks of layers if they just look at the pictures.

Also, I am a big fan of Occam. Your explanation isn't just a little simpler than the MER team's, it is an order of magnitude simpler.

I was not referring to pinstripe layering in my reply 99, but to single-particle layers be they thick or thin, that is, layers largely made up of particles with diameters that match the depth of the layer. I take your point that grains may not be reliably distinguished in MIs of the Meridiani sediments, but taking the MER scientists word for it, I have tried to find out more. Single-particle layers are apparently rare in sediments, aeolian sand-sheets being one of the few examples. I suspect that base-surge sediments and perhaps turbidites also hold single-particle layers but I have yet to read anything that discusses this question. I hope that we hear from sedimentologists. Could some of the regularity in thickness in some of the planar beds be contributed by a common large grain size? Are the layers shearing against each other at some time during deposition and being reduced to the minimum thickness that can hold the larger grains? It is a minor question perhaps with so much else to talk about, but the mechanism of surge deposition continues to intrigue me.

Centsworth - Thanks for the excellent question, which allows me to expound a little. No, I have no prejudice against eolian sandstones. In fact, some of my best friends are eolian sandstones. I've made their acquaintance in the Page area, AZ, over the past 3 years, and their names are the Page Sandstone and the lower down Navajo Sandstone. I've learned a lot from those friends, but not what the MER team claimed I might. See previous posts for some of what I've discovered about hematitic concretions, old water tables, flowing water, interdune playas, and so on.

On dry, cold Mars, as I see it, eolian sandstones have had a minor problem for the past 3.8 billion years at least. That problem is that it seems nearly impossible to form them. The basaltic sand forms with no problem (probably some or most of it was rock originally pulverized by impacts) but cementing it into a solid rock is the problem. That requires a long immersion in liquid water. Without that liquid water, the sand grains just blow around and and around, getting a little rounder in shape with each bounce, forming nice dunes, and filling craters such as Endeavor and Victoria partly or completely. The rounded grains act like little ball bearings (I credit Ron Greeley for this idea), making it easier for Oppy to get stuck, as already happened in Purgatory Dune and may happen shortly in Victoria (let's all hope not). If the sand gets buried deeply enough, or arrives near the poles, it may be incorporated into permafrost, but this sandy ice hardly a rock by conventional definitions - an hour in the lab and you'd have a pile of wet sand. Inasmuch as this is my own hypothesis, you will note that I am excluding from consideration the anomalous "Sahara desert oasis" hypothesized by the MER team for giant Meridiani (too many problems).

Well, you say, how do we make a sandstone then? Boom! of course. A big impact will dig up salts and vaporize ice, sending a super-hurricane force surge cloud out radially (with local little tornadoes leaving radial grooves, as described by Sue Kieffer and others for volcanic deposits). The cloud will efficiently scour away any sand or dust it encounters and incorporate it into the cloud - probably until the cloud eventually consists dominantly of such particles. The cloud will override mountains, efficiently scouring the highlands, and will eventually "pond" in a lowland, such as Meridiani or perhaps Gusev Crater, as it loses steam, literally. As salts and then steam condense the sticky sands are deposited (perhaps along with contained condensation-related spherules), and after a few post-depositional changes involving moisture and salts, voila, instant rock. The salt cement would never meet terrestrial construction standards - a little rain or snowmelt and it would rapidly fall apart, as happens to terrestrial impact deposits. On dry cold Mars these rocks seem to last for billions of years, however. In this regard, the famous Mars rampart craters were hypothesized to be erosion-resistant examples of abrupt steam condensation in relatively small and wet surge clouds by Wohletz and Sheridan (1983). The surge hypothesis thus accounts for the rounded sand grains imaged by Spirit's MI in the upper beds of Home Plate without requiring a separate eolian episode (mistaking surge for eolian deposits was the mistake made by Gene Shoemaker at Kilbourne Hole and other volcanic surge deposits in the late 1950's, before he had mapped Meteor Crater for his Ph.D. thesis at Princeton). The surge hypothesis also accounts for the many layered, salty deposits that fill old craters to above their rims (something seemingly impossible with water deposition).

I'm not saying that water- or wind-deposited sandstones might not occur in the lower levels of Meridiani (or in the Northern Plains) - I have no idea what torrential rainstorms or blizzards might have been condensing during the Late or other Heavy Bombardment, or what the transient weather might have been like. I'm just stating that in the 3.8 billion or so cold dry years afterwards, over the long term impact seems to me to be the only process capable of routinely depositing sandstones at higher elevations (absent drainage networks and outflow channels, that is, as at Meridiani).

I eagerly await HDP Demko telling me that there are a dozen other ways to make martian sandstones that I haven't considered. Please go easy on the technical vocabulary though - remember the audience.

BTW, Centsworth, why don't you tell your shy, insecure Australian buddy Jon Clarke to stop singing my praises on that other obscure site and get his two festoons ( UU ) over here ASAP (unless his enthusiasm has already caused him to be banned from this site). We HDP's have enormous egos that require constant stroking and I look forward to basking in his adulation. Tell him I'll give him a penny for his thoughts.

--HDP Don

HDP Don,
Tell me again why you're having trouble making friends
and influencing people in the scientific establishment?

You're still wrong; in fact, so wrong that trying to convince otherwise is a wasted effort.

I think some of you are being way too antagonistic. Do you not realize that we all have a special opportunity here to have a conversation with a scientist who publishes in major journals, and who is smack dab in the middle of a major planetary debate? How many other such researchers do you see coming out into the open, to publicly discuss their ideas so openly?

We are very fortunate that this forum encourages such communication. Personally, I disagree with a number of Dr. Burt's ideas regarding Meridiani, but others of his thoughts ring a bell with me. I think I will continue to sit in the back row and listen.

Agreed, CR. While I am nowhere near knowledgable enough to participate in this debate, I treasure the fact that UMSF is a forum of choice for this dialogue.

And, just to reset the baseline here, let's not forget that Mars is an alien planet--not Earth--and alternative hypotheses to explain our obviously biased early observations should therefore be explored to the fullest. (Come to that, look at the full spectrum of gradual vs. catastrophic hydrological/stratigraphic events in Earth's geological record...who's to say that Mars' history is any less complex? Definitely insufficient data at this time to state otherwise.)

Viva free thinking, viva free exchange of opinions! Maybe it's just the US 4 July spirit speaking here (or other spirits, I confess... ), but, boy, is it great to see this sort of debate free of meetings, inevitable personal grandstanding/filibusters & ancilliary agendas. After all, what does Prof. B have to fear from all us Internet ghosts in terms of professional issues? Press on, gentlemen, press on!

Whoa!! Careful, Bill. You're getting positively verbose! Sixteen words of dismissal, instead of just three. Next time you might even include an argument or two!

OK

--Bill

First, I like some of your ideas; but in the case of Meridiani, the evidence so far does not support your hypothesis. Another major problem is that the sandstones at Meridiani are MUCH harder deposits than what you claim would be created by an impact surge. The rover is fairly heavy. It has driven over these sandstone deposits many times. If they were as soft as you claim, then they would have crumbled. The only time the rover makes track marks is when it drives over loose or dune like crusty soil. The hard stuff underneath is a layered deposit that is not marked up unless the MER team uses the RAT tool. Your hypothesis does not account for its observed durability. If it were made as you suggest, then the RAT tool would have sliced through it like butter. The only possibility is that these hard layers were wet at some point.

In the second part that I quoted, you seem to be indirectly contradicting yourself slightly. If torrential rainstorms were possible, then why not directly at Meridiani? The Mars crust has shown through computer models to have significantly deformed with the shifting of the poles.

Low areas could easily gain altitude over the billions of years that passed. And if there was a lot of water, then capillary action could easily extend these dune like formations as they captured the blowing dust. Eventually the process would end and the formation of normal dunes on the surface would occur. Meters below the surface, the groundwater could easily morph the rocks further. Eventually the groundwater itself would either recede or freeze. With the water no longer in contact with the surface atmosphere, different types of brine could easily form over time depending on the circumstances. So far, we have an incomplete water record. We cannot be sure of all the circumstances. That's why it is so important for Opportunity to descend into Victoria as deep as possible. The best record should be revealed within days or weeks.

It's possible that that record will support your hypothesis; I just doubt it considering all that has been discovered up to now. I am willing to keep an open mind.

To change my mind, there must be a number of critical discoveries inside Victoria. The Pans of the inside of the crater suggests a similar story as the MER team has pictured. The story will likely change again slightly. But that is not a problem.

The MER team never stated how much water was on the surface. During interviews they stated the parameters were wide. When asked whether this body of water could have been a sea, they said it was possible but they didn't know. The press ran with the story that they found evidence of seas. Recently they simply narrowed the parameters. Their basic idea has not changed. You suggested that it had. That's why you got the confrontation. You seem to suggest that the MER team has not seriously considered the type of morphology that you currently propose. I would argue that they have and ruled it out. At times when discussing here you seem to have belittled their arguments--although I'm sure that was not your intent.

I appreciate your coming to this forum to discuss alternatives. Even if the evidence does not fit here at Meridiani, these ideas are still relevant. I particularly see possible evidence of what you are talking about at Gusev. The missing piece is Spirit finding a lot more examples of sulfides. Yet, even observed evidence at Gusev suggests further alteration after what appears Could have been impact surge.

Like I said, the story is complex. I personally don't think everything shut down after bombardment. I believe that Mars is dynamic even today. It is just not nearly as dynamic as Earth. Very recently and not necessary published, we are currently seeing the dynamic nature of Mars...the dust storms, the sudden change in the tracks with the sudden increase in wind (never before seen, until just days ago), the massive observed changes at Gusev with the impact of dust devils. Stuff moves around Mars a lot more quickly then we first thought. It just appears to happen in bursts.

It would be helpful to the rest of us if you would pick a point that the professor has made,
either to disprove the MER team theory or support his own, and make your case against
it. HDP Burt may not be convinced, but the purpose of this type of forum is to educate and
convince the rest of us.

Unfortunately, despite all the science, it is this which stands out the most. Going on to belittle this place and those that visit it and run it was also totally unnecessary. It is a pity HDP feels the need to pitch his hypothesis this way, whatever its merits might be.

Doug

I've been watching this with interest but in silence (almost) so far. Doug, I understand why you are concerned - the discourse is rather one way. I would like to ask dburt (no fancy handles here) if anything at all has been gained by taking your ideas out for a spin on this forum? Have you heard it all before? If so, I think you should do us the kindness of withdrawing. We are not just here as hired devil's advocates.

BUT (and I hope this is the case) if in your view the discussion has been productive, i.e. has thrown up some new ideas, then I'd like to hear that. We are here I believe an open-minded and at the same time sceptical community, and I'd like to think we don't exclude ideas that challenge the consensus of the day unless we ourselves have the refutation to hand (as in the case of the water puddles). We don't just accept 'authority' - this is a separate court to the court of peer review, but I think it deserves respect in its own right.

Hope I'm not speaking out of turn . . .

I'd agree with everything you've said there Nigel.

Doug

Doug - I much regret if I have been giving the wrong impression. I have nothing but the highest respect for this forum, its outspoken members, and you as an exceptionally gifted and successful moderator. I've been reading this forum for years and I have always admired its free-wheeling debate on the merits and significance of various observations and ideas, including my own. I also have nothing but the highest respect for the MER team and all of its individual members, as highly trained, professionally distinguished scientists (else they never would have been chosen by NASA to be members of the team). If the MER team developed an unforseen problem, it was only that some of its members were perhaps TOO distinguished in their chosen fields (i.e., they may have been real HDPs, not total phonies like me). Nevertheless, they more than compensated, in my eyes, by placing all of the MER images and preliminary data interepretations on the web, thereby allowing all of us to see what we see and do what we do in close to real time. "Letting it all hang out" as they have been willing to do is perhaps the greatest breakthrough in planetary science ever, and I cannot praise it highly enough. I just cannot.

That's all besides the point though. My main point, as made in previous posts, and as some of you have politely made to me, is that when it comes to Mars, we all arrive wearing huge blinders. That is, we all have our biases and blind spots, derived from our professional and personal experiences, all of them so far on this planet, not Mars. My co-author Paul Knauth likes to say that Mars is nothing but a giant Rorschach ink blot, and that we all reveal ourselves when describing it. My biases are obviously drastically different from those of anyone on the MER team, owing to my background with volcanic surge deposits, and a long period of hopeless contamination with the ideas of ASU's late Robert F. Dietz, an iconoclast who spent the latter part of his life (once his ideas on plate tectonics had become scientifically acceptable) trying to convince other geologists of the importance of occasional large meteorite impacts in Earth history. My research specialties, other than the scientific study of mineral deposits, relate to the joys of exceedingly complex phase equilibria and crystal chemical interactions among minerals, so when it comes to Mars, many "pros" choose to dismiss me as an amateur, comparable to most of you. (And yes, I do have a mineral named after me.)

One other apology - science making, like sausage making, law making, and family feuding, is an extremely rough and tumble business, very different from its depiction in press releases, news stories, and freshman textbooks. It's also incredible fun, if you let it be. The way I have been speaking to you is no different (actually, far nicer - no use of "idiot" or "moron" for example) than the way we all speak to and of each other in the hallways of scientific meetings. I am deeply sorry if I have offended anyone with my unedited remarks or ironic sense of humor, or sounded patronizing, because that was not my intent. In compensation, I hope you have enjoyed this unique opportunity to witness and perhaps participate in a little of the real rough and tumble (and humor) of science. If you never think about a scientific press release or news headline in the same way again (e.g., "Scientists find cure for cancer"), then this little experiment in scientific communication will have succeeded. (Now in unison, class, all shout, "Show me the evidence!")

If I am allowed to continue on this forum, perhaps through mutual blinder removal we can arrive at original insights about what once happened on that utterly alien little planet, and have some fun doing it. Amateurs have always had an essential role to play in scientific discovery, and the modern science of geology itself is usually said to have been started by an amateur (Hutton, a Scottish physician). If not, it's been a real pleasure, and you all have inspired me to tremendous insights already. Thanks to you all, and thanks especially Doug for the chance (I was going to say opportunity, but ... ).

--HDP Don

Centsworth - Vague and wishful it is, you got me. I wasn't there. We're just observing the possible results, and trying to explain them (we don't get to do this experiment in the lab). My candidate for "other sticky condensate" would probably be some sort of early-condensing salt or concentrated brine, but I'm open to suggestion. A large variety of spherically-shaped condensates and accretionary lapilli characterize both impact events and volcanic surge deposits, and they all are believed to form in a manner similar to hailstones in a thunderstorm - that is, by condensation. They aren't necessarily the same composition as the rock around them. Tiny native iron spheroids (condensates) surround Meteor Crater, AZ, for example, and that wasn't even close to the size or potential chemical variety of a good-sized Mars impact. What may or may not have happened at Meridiani is still open to debate, as you know. My "innocent" starting point was that those suckers sure don't look anything like real concretions, but they do look an awful lot like tiny hailstones (i.e., condensates).

Most people, including me for years, have real problems with surge deposition doing so much work in such a short time. Our geological training biases us in exactly the opposite direction (processes that take place slowly over millions of years). Nevertheless, high temperature gas-phase processes can occur extremely rapidly, and condensation is one of those processes. Volcanic fumaroles can develop and change extremely rapidly too (they are basically roaring steam jets). To separate the tiny hematite flakes from the rest of the solids, an earlier post hypothesized that hematite condensation occurred in the hotter, gas-rich upper part of the cloud, and that the "hematite hailstones" worked their way down to join the rest of the solids once they had reached a large enough size. Also, condensation really only had to happen in one narrow zone of the expanding, cooling cloud, and after that hurricane-force movement could scatter spherules over the whole of Meridiani. But again, I wasn't there, and am open to suggestion.

Great questions, as usual.

--HDP Don

Kye - I appreciate your doing your own investigation of surges. Also I'm always glad to meet another fan of Occam. Regarding single particle layers, still not my field, sorry. Perhaps Prof. Demko can enlighten us? If not, I doubt if such claims for Meridiani are important to the big picture. (When all is said and done, those wonderful MER machines are still somewhat limited investigative tools. For example, their Pancams can't bend down to look at things sideways...)

--HDP Don

MarsisImportant - That's a quite long quote, and perhaps it should get edited out, but I wasn't sure where to begin - you have a lot of interesting comments to make. Regarding the apparent hardness of the Meridiani rocks, remember that Mars gravity is weak, the Meridani cliffs are visibly crumbling, and the wind appears to have carved them like a knife. So they can't be all that hard. The RAT isn't necessarily a good indicator, because it's going to be stopped by the hard sand, not the soft cement (would you want to rub your face with butter-cemented sand, as opposed to butter?) Also, I've walked across plenty of volcanic surge deposits that formed in the proposed manner (steam condensation), and they were all about as hard. For strength, the rock cement, in addition to the salt mixture, probably contains much of the dust in the cloud. That makes it relatively strong, so long as you don't add much liquid water. Remember that - almost no liquid water since it formed. (Magnesium sulfate, apparently the dominant salt at Meridiani, is the most water soluble of all the common sulfates.) By our surge hypothesis, everything was once moist (or "wet" if you will), but never more than that - it never saw large quantities of liquid water, or it would have fallen apart. The MER team gets around this objection by special assuming that all of their hypothesized brines fortuitously were saturated in all of the salts present, but they apparently forget that even then, all of the soluble salts should recrystallize into large crystals (put some fine table salt in jars, add various amounts of water, and come back in a month - let me know what you see). I may have mentioned this in a previous post - I'm losing track now...

Regarding self-contradiction, in terms of the possiblity of torrential rains or blizzards, I was talking about the height of the great bombardment, that could be sampled very deep in the section, not the very tail end, that has perhaps been sampled at Meridiani at the top. (Because layered rocks are always deposited from the bottom up.) By then I think Mars had largely reached its present cold, dry state, as mentioned previously.

On Earth, low areas can easily gain great elevation, owing to plate tectonic processes, but on Mars, I doubt it, unless you intruded a huge body of molten rock underneath, and rafted things up. There is little evidence of such a process. You can erode off the top, to a certain extent, and fill in low spots, and build extremely tall basaltic volcanoes, which need to be gravitationally compensated via extemely broad and gentle warping elsewhere, and accumulate polar ice caps, but that's about it offhand (I'm probably leaving something important out, although I'm including glaciers and outflow channels in the above).

Capillary action of migrating salts combined with dune accretion on sticky surfaces might indeed eventually form and preserve sandstones - a thoughtful suggestion - but they would not contain spherules. It was the spherules that first made us think of impact. Also, in terms of "incomplete water record" I would instead argue at this point that we have "no water record" at all in terms of liquid water - and I have spent much of my professional life looking for indications of the passage of liquid water in mineral deposits. I agree that exposures deep in Victoria may change that, but I'm not optimistic.

The critical way that the MER team changed their story between 2004 and 2005, apparently in response to our impact hypothesis, was to admit that the highly improbable (in terms of evaporation) mixture of salts must have been transported from somewhere else. Why did one of them publicly deny that change in 2006? I don't know. Perhaps because the putative "playa" had by then logically lost its home at huge Meridiani, unless the depths of Victoria reveal some highly distinctive lake beds, not seen in any ejecta. In terms of what they originally considered, I have been led to believe, possibly erroneously, that they considered only volcanic surge, not impact surge, as a possibility for the cross-bedding and spherules (in part because a person they consulted was my co-author Ken Wohletz). I freely admit that I am trying to poke holes in (or "belittling") their arguments - that's the only way science can advance. Please feel free to belittle mine - that's what I'm here for. I won't take it personally. (And even if I did, what could I do about it except get banned from this forum?)

I fully agree with you that today's Mars is dynamic and geologically active. That's part of its excitement. My personal prejudice, which I have certainly made no secret of , is that most of the really "heavy lifting" got done billions of years ago by impacts. Most Mars scientists agree that impacts and wind are the dominant processes affecting the entire surface today, and seem to have been for a very long time. And impacts may provide the real "bursts" to which you refer in terms of active processes.

Thanks for helping me clarify some of my ideas. I much appreciate it.

--HDP Don

Dburt,
I've had a few nagging questions on the impact surge hypothesis:

1. How can an impact surge explain berry multiples where the berries are not the same size? See, eg

http://marsrovers.nasa.gov/gallery/press/o.../20040312a.html

My thoughts are that if the berries meld together on the ground, then they should have all fallen at about the same time, and therefore should be the same size.

If the berries melded together in the air, they should also be the same size, for the above reason, except moreso.

Also, the chance of them forming a straight line is very slim by impacting one another in a cloud or on the ground.

2. The ground is currently covered in blueberries, at least around Eagle and Endurance. If the berries were caused by multiple impact surges, where is the record of the previous lag deposits? IE, there should be occasional layers of berries in the outcrop, corresponding with time between impacts where erosion extracted them from the deposits, yet we don't see any.

Yes, but... the rocks at Gusev are all dense basalts (as can be observed empirically and also inferred from dozens of different sensor measurements), and *they* have also been carved by the wind, like a knife through butter.

Doesn't matter how hard the rocks are or how thin the air is -- if you blow on rocks with wind for long enough, you get aeolian erosion. Doesn't matter how dense or light the rocks are.

-the other Doug

Good point "the other Doug".

I'd also extend this idea to point out that the cliffs at Victoria are still there after billions of years. The mere fact that they are still there is a good indication that these layers are hard indeed.

The point that the Professor made about not being able to tell the hardness from the RAT tool is not accurate. Yes, you can determine the relative hardness from the rate of the RAT digging into the rock (at least to within a certain parameter, not exact but we don't need exact--just relative). And the MER team can determine a relative standard of comparison from Gusev. Spirit has found a variety of hard and soft rocks. The soft rocks at Gusev may correspond well with what the Professor has talked about; but the layered rocks at Meridiani are much harder than those soft rocks at Gusev.

As for the weight of the rover on Mars being a lot less because of less gravity, that point is well taken. But I had accounted for the less gravity already. The rover is still relatively heavy and should have made some kind of mark on the underlying layered deposits, if they are as soft as he suggests. Given the uncertainty factor, I pointed out the RAT tool evidence as confirmation. But if that was not enough, then the view of Victoria crater should be. Either the soft rocks are much younger and Victoria is NOT billions of years old; or the relatively HARD rocks are old and Victoria is billions of years old. You cannot have it both ways. If the layered rocks were soft, then billions of years should have eroded the crater to look more like Erebus. The possibility that the rocks are soft and Victoria is relatively young has drastic implications that destroy many of the assumptions that the Professor holds dearly. For that matter, it would also destroy many of the assumptions that the MER team has operated with too. My point is that I don't think any of us "want to go there".

dburt - A repentant Prof Burt !! A tear just rolled down my cheek. Where was this quality when you were grading Igneous Petrology papers back in our youth ?

I agree with your statement about personal bias, you’ll see what your comfort level allows you to see. For me the bias is groundwater/aquifer geochemistry. So from a 50,000 foot view the MER team account of a paleo-regional groundwater system at meridiani with an extended diagenetic history has some merit. Obviously the story gets complicated as the resolution increases but that may be a hindrance at this time - we see only the tree in the proverbial forest in terms of rock geochemsitry. Granted, the devil is in the detail, but given our inexperience with >3 byr old terrain, I suspect we have overlooked a detail or two.

If water was retained within the sediments for even a fraction of the rocks proposed age ( >3 byrs) the potential for diagentic alteration is endless - interaction or mixing of different aquifers, dewatering of the aquifer, aquifer recharge with different water type, volcanic gas flux, impact, surficial water infiltration, etc. In other words, throw out those mass balance models for now and you’ll sleep better at night. Finally, the geographic coverage of meridiani is great but certainly in line with the areal extent of the High Plains aquifer that underlies parts of eight western U.S. states. Size alone should not an obstacle for an exhumed aquifer at meridiani.

Details, details. A major reason why I have become convinced that the impact-surge hypothesis is realistic is that the rough fit with Mars is so excellent. Huge impact craters are the single most prominent feature of the Southern Highlands from space. They are clearly ancient and yet have been preserved, a fact that makes it very likely that the inter-crater plains are still covered by their ejecta. Land in the highlands and you are very likely to see ejecta. Outside the volcanic areas impact ejecta should be the commonest sort of material. Mars craters over a certain size (roughly 5 km diameter) often produce a continuous ejecta blanket that suggests fluidized emplacement, and the indications of fluidization generally increase with crater diameter as does the trend to produce double and multiple layers of ejecta.

The large-scale regularly-layered landscapes visible from orbit have been especially difficult to understand. Really there has never before been an explantion that seemed better than barely possible. The brine-splat authors have made this connection, but only in the most general way. I am glad to read that Doctor Burt is not shy about conjecturing and I am hoping that he will be willing to write something more for us about how impact-surge might explain the widespread large-scale layers.

Well, since conjecturing is in play. Another possibility is that the Late Heavy Bombardment is not the only evident heavy bombardment that occurred. Just conjecturing and brain storming but (and I have my doubts about this) maybe Mars records multiple and even later bombardments which we have not accounted for yet. Perhaps our solar system was relatively recently much more like Tau Ceti is now. Then the question becomes what event changed things here. It's a huge "can of worms" that crosses many disciplines.

Perhaps instead, many craters were secondary and tertiary products from a late planetary type impact. Or possibly the large Volcanoes on Mars created such large explosions that many projectiles created crater impacts all over the planet. If that is the case, then terrain that seems very old may not be nearly as old as it first appears. Again, 'a monkey wrench' is thrown into our original thought processes. And again, I have my doubts about these issues I'm bring up in this post.

The problem with positing a series of heavy bombardments, dating up until fairly recent times, is that we don't see a record for such a bombardment on Earth or Luna. While it is possible, I suppose, for bombardments to be set into motion that only affect a certain portion of the local neighborhood, I'd imagine that a heavy bombardment which would affect Mars would also affect Earth/Luna, and probably Venus and Mercury as well.

There have indeed been models put forth to explain the LHB in terms of migration of major outer planets -- the one I can remember best suggests that Jupiter originally formed farther from the Sun than Saturn did, and that as it migrated closer to our star it tossed Saturn further out and disrupted whatever rocky body (or bodies) occupied the orbital neighborhood that now lies between Mars and Jupiter. This disruption caused a great deal of the mass located in what is now the Asteroid Belt to fall towards the inner system... and smash into any inner rocky planet or moon that stood in its path.

Yes, there could have been other less major disruptions that could have caused a lot of the debris in the current Asteroid Belt to have meandered in towards Mars' orbit. But even without detailed mathematical calculations, my gut feeling is that anything that would have caused a major bombardment on Mars should also extend itself in towards the rest of the inner system. And the problem with that is that the most recent of Luna's large craters seems to be on the order of 100 million years, and those are very scarce. The shoulder-to-shoulder large crater structures on Luna are all of an age, and that age is somewhere around 3by ago.

Remember, the LHB resurfaced much of Luna, and from the looks of it, resurfaced much of Mars, as well. The same thing happened on Earth, of course, but Terra has effectively resurfaced itself many times since then and the crater remnants of the LHB are almost all gone by now.

I just don't see any way for a major disruption to pull enough mass in towards Mars to create a major bombardment and yet not toss enough of that mass in to the inner system to leave a record on Luna. I guess I'd say it's possible but extremely unlikely.

-the other Doug

That's why a more local event particular to Mars is more probable, such as one huge planetary impact. Debris could have been thrown into orbit and it fell to the surface much later. Perhaps the moons of Mars are remnants of such an impact.

One more big 'monkey wrench' that is not conjecture has been reported in the last couple of days. Mars Express has found a lot of clays on Mars that could Not form on the surface with much CO2 in the atmosphere. That highly suggests that the ancient Mars atmosphere was far different than anything we so far assumed.

http://space.newscientist.com/article/dn12...early-mars.html

The headline of the article makes an assumption. And that assumption is one of many possibilities. First of all, these clays were made. And they must have been made in the presence of water without CO2. So some major event or events changed things substancially on Mars.

Hey, along with a childlike curiosity about the wonders of the universe goes a childlike tendency to mock any authority that is perceived as foolish or pretentious. Call it a dangerous occupational hazard of scientists. In any case, feel free to mock my ideas all you want.

--HDP Don

Remember they laughed at Galileo, they laughed at Albert Einstein and they laughed at Bob Hope.

Good questions. 1) Growth in a turbulent cloud does not mean they all have to be the same size - they just should not exceed a certain size (more mass than the cloud can support). If three spherules happen to stick together during growth, and are spinning about a common center of gravity, then they should tend to line up, and also be smaller than the largest spherules (so that their combined mass does not exceed what the cloud can support). That seems to be the case here. (BTW, can you imagine how tired I am of seeing that one triplet photo as "typical" of all the millions of spherules out there?) Actual concretions growing in rocks tend to merge together in nodular masses at random orientations, semi-random sizes, semi-random shapes, and random numbers of berries, up to dozens or many hundreds. I am still waiting to see such a feature (that is, wherever the spherules are densely packed together, they are still entirely separate entities). As you mention, another way to line berries up might be after deposition, if they land lined up along a pre-existing crack, and then develop salt encrustations that "glue" them together. No evidence of that here, though.

2) I have no idea at this point how many impact surges might be represented in rock exposures at Meridiani, or what the time interval was between impacts. If there were a Meridiani-like lag deposit on the ground when the second impact surge arrived, it would almost certainly be scoured away and incorporated into the new surge cloud. In that manner you could spread spherules across a much wider area that that originally covered, and that may have happened across parts of Meridiani (just look at all the spherules spread around by Victoria Crater).

Thanks for the great questions. Keep'm coming!

--HPD Don

MarsIsImportant - What can I say? I've never hammered on a Mars rock. The terrestrial surge deposits I've examined vary considerably in their hardness, but you generally need a rock hammer to break them. Nevertheless, they're softer than basalt (many are kind of like adobe). If the wind has only eroded 10 meters or so of Meridiani outcrops over 3.8 billion years, that probably has more to say about the ineffectiveness of wind erosion on near-vacuum Mars than it does about the intrinsic hardness of the rocks. Also, the impact surge hypothesis, unlike the vanished playa lake hypothesis, does not require Mars to be any different than it is today, so the topmost surge deposits could be considerably younger than 3.8 billion years, and still look the way they do. At Gusev, the various larger basalt pieces on top could well be impact ejecta of almost any age. Without any time calibration, further speculation trying to relate hardness to amount of erosion would be pointless, IMHO. Thanks for the discussion, though.

--HDP Don

I appreciate your honestly. You don't know how many impact surges were needed to create Meridiani.

If the new impact surge scoured the ground and incorporated the previous deposits, then many layers at Meridiani would be missing. Also, the layers themselves should be a lot thicker than appear.

Instead, Meridiani shows thousands upon thousands of very thin layers. In my mind that suggests more like seasonal dust deposits. Doesn't it make more sense that the dust storms that we see every two years on Mars deposits the thin layers? When water was available, then the dust was cemented. That process stopped a long time ago; but the seasonal dust storms continue. If this layering process occurred over a very long time, then it is possible that surge clouds of some sort might have created the berries--I don't know. But I still think that the groundwater significantly altered these rocks.

Regardless, your hypothesis does not account for the vugs found at Eagle crater. You asked where were the crystals. But they found the cavities where crystals once were. Then, the crystals later dissolved again in solution and the salts carried away by some presumed liquid. It's possible that all the evidence is really from groundwater or brine of some sort at various times in the distant past. Perhaps aeolian processes excavated large parts of Meridiani. But that would suggest Victoria would need to be more recent of an event (I doubt it).

When we have a known process for deposition--the dust storms--why would we need another one, especially when the known one would create the fine thin layers that we see? I don't understand why we need another process when lithification of these layers through groundwater is perfectly plausable.

You point out the concretions are not what they seem to be. Ok. They might have been formed by Volcanic surges tool Couldn't they have? You stated yourself that we have no good examples of impact surges. Even you are using Volcanic surges as a standard. If memory serves me well, even the MER team has been open to the possibility of some sort of volcanic activity nearby. Perhaps the berries are not concretions but a result of periodic volcanic activity. But I don't think the dust layers are a result of the surge mechanism you propose. A series of surges would simply not make such fine layers.

You haven't hammered on the rocks at Meridiani; but NASA has. You are forgetting that parts of the space craft slammed into the Meridiani surface and the heat shield was examine by opportunity. The impact barely made a dent into the surface. We can easily calculate all the forces involved. In does not take much to realize that the surface rock layers are very hard. They must have been cemented together by water over a long period of time.

Other Don - That's all well and good, but many things are theoretically possible (including little green men, perhaps). Just show me the evidence, please. More to the point, are there any features at Meridiani (or Gusev, or any of those many other salty layered deposits all around the Mars highlands) that CAN'T be accounted for by the impact surge hypothesis?

Thanks,

--HDP Don

Forgive me if this has been asked & answered earlier, DB, but how fast would blueberries be expected to form in your scenario? I have a hard time imagining that the 'splat' conditions would persist long enough to let them form via accretional processes, and frankly they just don't look like tektites or another form of melt product to me because of their symmetry and those weird little bumps some of them have (which seem to be more common on berries in protected locations; maybe they get weathered off of the surface berries due to rolling around by the wind?) The spectrum of sizes at different locations is odd as well, and seems to argue for a more gradual formation process. Has there been any sort of detailed morphological analysis of them?



Yeah, but for truly enduring comedy you just can't beat Hoaxland... ...how the hell does he get out of those straightjackets so fast?

Sorry, I have trouble quantifying "very hard" and rocks generally don't "dent" unless they're squishy mud or hit at hypervelocity, forming an impact crater. Also, if the grains were cemented together by water over a very long time, were there any cements available other than the abundant sulfate salts that we see? If not, how would that salt cement make the rocks harder than the surge hypothesis would account for? In fact, why weren't the basaltic sand grains simply altered to "soft gunk" (crystalline clays) if they were soaked for so long in liquid water? None of these problems seem to occur with "Boom!" (at least as I see it in in my admittedly biased way). Thanks for the good discussion.

--HDP Don

MarsIsImportant - I'd really prefer it if you could give me one question per post, rather than this stream of consciousness type of questioning. Much easier for me to try to answer, and much easier for people to read. Thanks.

Regarding missing and thin layers - the exposures in Endeavor are very limited, and haven't been matched yet with exposures in Victoria. Besides, the scouring in a given area would probably be fairly uniform. Each cross-bed represents a scouring episode, but probably just owing to turbulence in a single surge cloud. Surges have no problem building up numerous fine layers in a single episode, as covered in previous posts. Also, the Meridiani material seems to be mainly sand, not dust.

Regarding the crystal-shaped vugs - the impact surge hypothesis accounts for them just fine, if they represent a former chloride mineral. Chlorides are much more soluble and also subject to frost leaching (owing to much greater freezing point depression) than the surrounding sulfates. Minor post-depositional drainage or frost leaching also accounts for why the chloride content seems to increase with depth. This was covered in our original Nature paper from 2005 (attached to a previous post).

Regarding possible volcanism - as covered in a recent post, the MER team early on dismissed volcanism as a causative agent for the cross-bedding and berries, and we have never disputed that finding (although others have).

Remember, not so many different questions per post, please. Thanks for them though.

--HDP Don

Ok. Hardness doesn't matter. I only keyed on that issue because you stated that the layered rocks at Meridiani were soft. So it is Not a problem for either theory or hypothesis (whatever you want to call them).

I still have a big problem with the thickness of the layers. The thin layers observed, support the MER team's hypothesis.

Nprev - Good question, but I can't answer it. I would expect a time of something from minutes to hours, depending on the size of the impactor and where in the cloud they condensed. If they grew in a mushroom-shaped cloud directly over the impact site, that might give them longer and allow them to grow larger, but then a second nearby impact might be needed to distribute them over wide areas in a surge cloud. On the other hand, the mushroom cloud itself might eventually gravitationally collapse as it condensed and cooled in the very thin atmosphere (like a downburst in a thunderhead cloud), and form a surge deposit on top of an earlier blast surge. Pure speculation at this point, I'm afraid.

Rolling along on the ground owing to the force of the surge cloud certainly seems possible, as mentioned in a previous post. Rolling owing to the wind, not observed. For me the only key observation about the spectrum of sizes is the strict size limitation at about 5 mm. Condensation-related impact spherules are entirely different from the shaped splash droplets called tektites, as covered in a previous post. I don't offhand know of a detailed morphological analysis, but would be surprised if one had not been done, perhaps by someone in another forum.

Keep'm coming, but I may not get to them for awhile.

--Don

Howso?

--HDP Don

P.S.: Sorry, I'll take pity on you - I think the thinness of the layers has already been discussed in numerous previous posts, as not being particularly diagnostic of either of the suggested processes. I just wanted to see if I could actually make a one-word reply, for once I dood it! HDP Don

The MER team ruled out Volcanism as a causative agent for the cross-bedding and the berries. The wording is important. This does not rule out the possibility of volcanism providing the heat necessary to have the liquid water given their hyposthesis.

One of the problems you seem to have with the MER team hyposthesis is that Mars has been a cold dry place for billions of years. Well, we now know that there was plenty of H2O around. But was it in the correct state? The current atmosphere suggests "No". But now we know that the atmosphere must have changed drastically over the billions of years, because the massive amounts of clays found by Mars Express cannot be formed with CO2 predominant in the atmosphere. The ancient atmosphere of Mars was not of CO2. As far as we know, the atmosphere could have been a lot thicker and supported liquid water on the surface at one time. We cannot be sure one way or another yet. But if we believe the MER team, the answer is highly suggested to be "Yes". Liquid water on or near the surface is the basis for their hypothesis.

From the beginning, I have not ruled out the possibility of impact surge contributing to the geology of Mars. I just don't believe it was predominant like you suggest at Meridiani. I don't know how you could convince me otherwise.

Your best argument seems to deal with the berries themselves. But their differing shapes over most of the terrain observed by Opportunity seems to be a problem (frankly, for both hypotheis). They have been completely round until we approached Victoria...yet not exactly round near Victoria. Why? How does the shape or change in shape fit with your impact surge hypothesis?

...because of the seasonal dust storms creating the mechnism for such layers. I already explained that.

Just one last comment on the relative softness of the rocks on Mars:

There is one other measure of the hardness of the rocks that hasn't yet been mentioned. The RAT on Spirit wore out considerably faster than has the RAT on Opportunity. The RAT cutting edges were identical on MER-A and MER-B, so there is a quantifiable and measurable amount by which the rocks at Gusev are harder than the rocks at Meridiani. (I believe some estimate of Mohs scale was made for the various rocks that have been RATted, based on the amount of electrical power required to make the observed cuts. I don't know where I read that, though, and so I can only offer it as a piece of potential apocrypha.)

My own take on it is that the Meridiani rocks are probably pretty friable when you apply pressure cross-layer. But the way those layers have cemented give the rocks a fairly decent load-bearing strength when you apply pressure along a layer's plane. In other words, it's like plywood -- you can easily crumble off the edges, but sit on a slab of it and it won't tend so much to crack and crumble. Or, to sound like I know more than I do, the material's axis of greatest strength is planar and alined with the layers...

The very few surviving blocks of ejecta made of this material show preferential erosion cross-layer, as well. An ejecta block with a flat, contiguous layer for much of an exposed face appears to erode selectively along its non-planar faces. This would tend to support that the material is stronger (i.e., more erosion-resistant) when force is applied perpendicular to the plane. However, the rather noticeable lack of extant ejecta blocks from this material, as well as the "ground-down" condition of what now look like flat pavement slabs at various places in this unit, also speaks to material that is soft and relatively easily eroded.

-the other Doug